DYNAMICS OF A HIGH-MACROTIDAL SALTMARSH TIDAL CREEK

A Thesis

Presented to

The Faculty of Graduate Studies

of

The University of Guelph

by

LAURA ELIZABETH SCHOSTAK

In partial fulfilment of requirements

for the degree of

Master of Science

Augusî, 1998

@ L a m Schostak, 1998

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DYNAMICS CF A HIGH-MACROTIDAL SALTMARSH TIDAL CREEK

Laura Elizabeth Schostak University of Guelph, 1 998

Advisor: Professor R. Davidson-Arnott

This thesis investigates sedimentary processes in a temperate high-macrotidal

saltmarsh tidal creek in the Bay of Fundy. The hydrodynamics and the sediment

dynamics within the creek were measured with electromagnetic current meters and

Optical BackscatteranceTM probes, temporally and spatially over tidal cycles of various

heights. The measured velocities within the creek were low, rarely exceeding 0.10 m s",

and there was a general decrease in the suspended sediment concentration from the

beginning of the flood tide to the end of the ebb tide. These results indicate little active

erosion in the channels, implying limited creek growth. A comparison of this study to

others suggests that tidal range is not the pnmary control of charnel dynamics, but rather

saltmmh topography and creek network structure.

This degree has provided me with new insights and experiences that were far

beyond what 1 had expected when 1 fïrst came to Guelph. I have corne this far only by

the grace of God and with the support of so many individuals who were there for me

every step of the way. First 1 must thank my advisor, Dr. Robin Davidson-Arnott.

Thank-you so much Robin for continuously being a source of support, encouragement,

and humour when 1 needed it most. Your enthusiasm for my project was so helpful and 1

am grateful for your willingness to literally 'go down into the trenches' for me! 1 must

also thank Dr. Ray Kostaschuk and Dr. Jeff Ollerhead for their continued patience and

guidance.

To Wayne Boulton and Danika van Proosdij, 1 will always remember your advice,

patience, and he$ throughout this entire process. 1 would like to thank Dr. Brian

Greenwood for allowing us to use his facility for instrument calibration. Thanks also to

Mario Finoro for his unlimited technical help and to Jaime Dawson and Becky Rush who

were two incredible and tireless research assistants and have my full appreciation!

1 could not have completed this project without the support of my close family

and fkiends. Thank-you so much Donna-Mae, Jaime, Jemifer, Dad, and Mom. Your

understanding, prayers, humour, and encouragement will never be forgotten.

Finally, 1 would like to acknowledge the financial support of the Natural Sciences

and Engineering Research Council, a Latomell Travel Grant, and a University of Guelph

Graduate Scholarship, which funded much of this academic pursuit.

**

II

TABLE OF CONTENTS

ACKNO WLEDGEMENTS

TABLE OF CONTENTS

LIST OF TABLES

LIST OF FIGURES

CHAPTER 1 RESEARCH CONTEXT, PURPOSE, AND OBJECTIVES

INTRODUCTION Saltmarsh Tidal Creeks Saitmarsh Tidal Creek Networks: Evohtion and Growth Sediment and Sediment Sources

SALTMARSH TIDAL CREEK FLOW D Y N M C S Saltmarsh Tidal Creek Hydrodynamics Saltmarsh Tidal Creek Sediment Dynamics

RATTONALE FOR THE RESEARCH

THE RESEARCH PROBLEM

RESEARCH OBJECTIVES The Field Work

CHAPTER n STUDY SITE, RESEARCH DESIGN, AND METHODOLOGY

STUDY SITE Saltmarshes in the Bay of Fundy Cumberland Basin The Allen Creek Marsh

RESEARCH DESIGN AND METHODOLOGY Field Mapping Erosion within the Study Channel The Measurement of Flow and Sediment Dynamics

Vertical Anay Instmment Arrangement Spatial Array Imtrument Arrangement

Data Recording Flow Velocity Sampling Laboratory Analysis

OBS Probe Calibrations Grain Sire Anabsis Pressure Transducer Calibrations

Instrument and Data Recording Problems

CHAPTER m RESULTS

MORPHOLOGY The Allen Creek Marsh Saltrnarsh Tidal Creek Network Morphology

EROSION PIN RESULTS Sediment Level Fluctuations and Trends Factors Controlling Sedirnent Level Change

Tidal Heigh t Precipitution

Spatial Sediment Level Variations within the Channel

INUNDATION OF THE SALTMARSH

VERTICAL ARRAY EXPERIMENT Signal Fluctuations for Individual Instruments Vertical Amy Hydrodyna?iics

Flow Direction Vertical Amy Sediment Dynarnics

SPATIAL ARRAY EXPERIMENT Spatial Array Hydrodynamics

Temporal Pattern in the Flow Dynomicr Spring, Transitional, and Neap Tides Flow Pattern Variation with Location

Reference Station Hydrodynamics Reference Station: X- and Y-Axis Patterns

Spatial Amy Sediment D ynamics Temporal Sediment Dynarnics Spring, Tronsitional, and Neap Tides Suspended Sediment Concentration Variation with Location Velocity and Suspended Sediment Concentrations

CHAPTER IV DISCUSSION, RESEARCH OPPORTüNITIES, ANI> CONCLUSIONS

INTRODUCTION

SEDIMENT DYNAMICS

COMPARISONS AMONG TIDAL RANGES

HYDRODYNAMICS Flow Directions Flow Velocities and Patterns Low Flow Implications on Channel Evolution

RESEARCH OPPORTUNITIES

CONCLUSIONS

REFERENCES

LIST OF TABLES

Table 3.1 Characteristics of the main tribubries in the Allen Creek Marsh. 44 The slopes are the linear regression line slopes for the selected thalweg sections.

Table 3.2 Channel bank composition. 46

Table 3.3 . Erosion pin measurement schedule, including tide and precipitation 51 characteristics. Tide type: Sp. = spring tide, Tm. = transitional tide, and Np. = neap tide

Table 3.4 Environmental conditions for the experiments. 64

Table 3.5 Vertical array expenment schedule and characteristics. (EMCM = 65 electromagnetic current meter, PT = pressure transducer.)

Table 3.6 Spatial array experiment schedule and characteristics. (EMCM = 83 electromagnetic current meter, PT = pressure transducer.)

Table 4.1 Cornparison references for discussion. Creek dimensions Iist 116 largest cross-section if multiple locations were studied. When creek lengths were not stated, they were roughly approximated fî-om the provided maps.

LIST OF FIGURES

Figure 2.1 Location of the Allen Creek Manh in Cumberland Basin.

Figure 2.2 The Allen Creek Marsh (elevation is in metres above datum NAD83).

Figure 2 J Erosion pins across cross-section C 1 (facing east). Height of closest pin is approximately 0.32 m.

Figure 2.4 Vertical array instrument positioning at station C2 along the study creek.

Figure 2.5 H - h e set-up of instruments for the spatial array (station C4).

Figure 2.6 Exarnple of an x-axis record showing the submergence of a curent meter.

Figure 2.7 Calibration curve for OBS probe 2.

Figure 3.1 Maximum forecast tidal heights for Peck's Point near the Allen Creek Marsh for June and July, 1997 (Canadian Hydrographie Services, 1 997).

Figure 3 3 Thalweg profiles of the main tidal creeks in the Allen Creek Marsh creek network. The direction is seaward and O m indicates the head of the trïbutary. S w e y points range from 5 to 30 m apart. Elevation in metres above datum NAD83.

Figure3.3 Profiles of the cross-sections chosen for the spatial array experiments, facing upstream (north). Elevation in metres above daturn NAD83.

Figure 3.4 V-shaped cross-section of the main creek.

Figure 3.5 Location of erosion pins across Cl md C2. Elevation in metres above datum NAD83.

Figure 3.6 (a) Daily average change in the sediment level. (b) Cumulative average change in the sediment level. Averages included rneasurements for al1 of the erosion pins. A negative change indicates sediment loss (erosion).

vii

Figure 3.7 Daily average change in the sediment level with tidal height changes. Indicated tide is that previous to measurement.

Figure 3.8 Daily average sediment level change vs. change in tidal height. Indicated tide is that previous to measurernent.

Figure 3.9 Daily average sedirnent level change with precipitation previous to measurement,

Figure 3.10 Daily average sediment level change vs. precipitation previous to measurement,

Figure 3.11 Cumulative average sediment level change with precipitation previous to measurement.

Figure 3.12 Average net sediment level erosion vs. elevation fiom thalweg for erosion pins at (a) C 1 and @) C2 (confidence level = 95%).

Figure 3.13 Cumulative change in sediment level for each erosion pin on the: (a) Cl nght bank; (b) Cl left bank; (c) C2 right bank; and (d) C2 left bank. 'Right' and 'lefi' refer to channel banks when facing upstream (north).

Figure 3.14 Raw data for the instruments in the vertical anay: (a) pressure transducer; @) V3 curent meter; (c) V2 current meter; (d) V3 OBS probe; (e) V2 OBS probe; and (f) V1 OBS probe. SSC = suspended sediment concentration. Only the x-axis data are shown for each current meter.

Figure 3.15 June 5 (day) vertical array data averages: (a) tidal stage above thalweg - BF = bankfull elevation; (b) velocity; and (c) suspended sediment concentration.

Figure 3.16 June 5 (night) vertical array data averages: (a) tidal stage above thalweg - BF = bankfull elevation; (b) velocity; and (c) suspended sediment concentration.

Figure 3.17 June 6 (day) vertical array data averages: (a) tidal stage above thalweg - BF = bankfull elevation; (b) velociây; and (c) suspended sediment concentration.

Figure 3.18 June 6 (night) vertical array data averages: (a) tidal stage above thalweg - BF = bankfull elevation; (b) velocity; and (c) suspended sediment concentration.

Figure 3.19 Vertical array velocity averages for V2 and V3.

Figure 3.20 Vertical array flood tide velocity pattems. Elevation above bed (m) vs. velocity (m s-').

Figure 3.21 Vertical array ebb tide velocity pattems. Elevation above bed (m) vs. velocity (m s-').

Figure 3.22 V2 and V3 flow direction vectors relative to north (O degrees). The vectors were derived using the mean values of the x- and y-axis records for each run. Only the direction of fiow is indicated, not the flow magnitude.

Figure 3.23 Vertical array suspended sediment concentration averages for VI- v3-

Figure 3.24 Vertical array: examples of the suspended sediment concentration patterns for the June 5 (D) and June 5 (N) experiments. Elevation above bed (m) vs. suspended sediment concentrations (mg 1-').

Figure 3.25 Instrument station positions dong the study creek and main creek thalweg profile, from C4 (study creek) to CS (main creek). Elevation in metres above datum NAD83.

Figure 3.26 Cornparison of the measured (a) x-axis speeds and (b) velocity vectors within the study creek (June 18 data).

Figure 3.27 June 8 (spring tide) spatial array data averages: (a) tidal stage above thalweg - BF = bankfûll elevation; (b) velocity; and (c) suspended sediment concentration.

Figure 3.28 June 18 (spring tide) spatial array data averages: (a) tidal stage above thalweg - BF = bankfûll elevation; (b) velocity; and (c) suspended sediment concentration.

Figure 3.29 June 9 (transitional tide) spatial array data averages: (a) tidal stage above thalweg - BF = bankfill elevation; (b) velocity; and (c) suspended sediment concentration.

Figure 3.30 June 19 (transitional tide) spatial array data averages: (a) tidal stage - no data; @) velocity; and (c) suspended sediment concentration - no data.

Figure 331

Figure 3.32

Figure 3.33

Figure 3.34

Figure 3.35

Figure 3.36

Figure 3.37

Figure 4.1

June 13 (neap tide) spatial array data averages: (a) tidal stage above thalweg - BF = bankfill elevation; (b) velocity; and (c) suspended sediment concentration.

June 17 (neap tide) spatial array data averages: (a) tidal stage above thalweg - BF = bankhill elevation; (b) velocity; and (c) suspended sediment concentration.

Spatial array velocity averages for stations: (a) Cl, (b) C2, (c) C3 (speed), and (d) C4.

Reference station velocity averages for: (a) spnng tide; (b) transitional tide; and (c) neap tide.

Reference station speed averages for the thalweg-parallel x-axis and the thalweg perpendicular y-axis on a: (a) spring tide - June 18; (b) transitional tide - June 9; and (c) neap tide - June 13. Arrows indicate approximate time when bankfull was reached.

Spatial array suspended sediment concentration (SSC) averages for stations: (a) C 1, (b) C2, and (c) C4.

Cornparisons between average velocity and suspended sediment concentration (SSC) for (a) June 6 (N) and (b) June 8. Positive velocity magnitudes represent the flood and ebb tide flows.

Average suspended sediment concentration patterns for stations C 1 and C2 (excluding C 1 data for June 9).

Figure 4.2 Average velocity patterns for stations Cl, C2, and C4.

1

C-R 1

RESEARCH CONTEXT, PURPOSE, AND OBJECTIVES

INTRODUCTION

Coastal saltmarshes are flats high in the intertidal zone that are colonized by salt-

tolerant vegetation. They exist between mean sea level and tidal spring high water, and

often grade towards the sea into mud or sandflats. They are located in sheltered

embayments and estuaries, and in the lee of spi& and bmier islands (Mitsch and

Gosselink, 1986; Allen and Pye, 1992; Lutemauer et al., 1995). Saltmarshes are

important features of the coastal zone and are known for their buffering effect against

storms, ability to naturally filter pollutants nom the water, high biological productivity,

and shorebird and waterfowl habitat.

Saltmarsh development depends upon tides, sediment supply, freshwater inputs,

shoreline morphology, and vegetation. A saltmarsh consists of key components

beginning with a mud or sandflat which grades inta the rnarsh or is separated from it by a

steep slope or cliff a few metres hi&. Landward of this rnargin is a low manh area with

salt-tolerant (halophytic) vegetation which grades into a high marsh area that supports

more salt-sensitive vegetation. These areas are dissected by a network of tidal creeks

carrying water and sediment into and out of the marsh. Further inland, there may be a

change in vegetation to forest or another fom of terresûial, non-halophytic vegetation.

Marshes evolve by vertical accretion on a flat or sloping sub-tidal surface.

Sedimentation and accretion processes are greatly affecteci by vegetation. Vegetation

2

enhances sediment accumulation because gras baffies wave energy and stems and leaves

obstnict cment flow, decreasing velocities so sedimentation cm occur (Stumpf, 1983;

Pethick, 1984; Frey and Basan, 1985). Plants act as depositional surfaces and c m

enhance sedimentation by chemically creating a 'micro-environment' that encourages

clay flocculation when salt is secreted from the stem (Frey and Basan, 1985).

The study of saltmanhes has increased over the past 20 years because global

climatic change may cause a sea-level increase. Such an increase threatens saltrnarshes

because sea-level nse may exceed the rate of saltmarsh growth, leading to vegetation

submergence and die-back (Eteed, 1990). n ie capacity of a marsh to maintain its

elevation with respect to sea level is important for its survival and relies on sedimentation

processes within the rnarsh (Wang et al., 1993). Marsh sediments have a substantial

organic component, but silt and clay must also be imported at a sufficient rate to keep up

with a rise in sea level (Gardner et al., 1989; Pillay et al., 1992).

Throughout the last three decades there has been a growing amount of research on

the various aspects of saltmarsh development like accretion and erosion (e.g., Harrison

and Bloom, 1977; Letzsch and Frey, 1980a; Pethick, 1981; Wood et al., 1989; French and

Spencer, 1993), along with studies on the development of saltmanh tidal creeks (Gardner

and Bohn, 1980; Steel and Pye, 1997). These creeks are important because they bring

water and sediment into the marsh system and the transport of suspended sediment

through marsh tidal channels influences the spatial patterns of sediment accumulation

(French et al., 1993).

Saltmarsh Tidd Creeks

Saltmarsh tidal creek networks direct a flow of water, sediment, and nutrients into

the marsh These creeks are often an extension of the tidal creeks that traverse the

mudfïats, but because marsh environments are highly varied, the pattern of these systems

can differ. The saltmanh tidal creek system also acts as a drainage network and it

develops in mponse to the tide as it dissipates the energy of the tide that floods the creek

network. The channel geometry is also influenced by the steepness of the manh dope

(Steel and Pye, 1997).

The type of creek network that develops is dependent on the tidal regime, the

topography of the area, and the source and type of sedirnent that feed the manh. Factors

such as marsh morphology and the presence of vegetation also play a role in the

cornplexity of creek patterns and migration (Pestrong, 1972; Frey and Basan, 1985).

Steel and Pye (1997) distinguish six kinds of planirnetric tidal drainage networks that are

controlled by the morphology of the saltmanhes: linear, linear dendritic, dendritic,

meandering dendritic, reticulate, and complex/superirnposed networks. On macrotidal

coasts, the creek system typically consists of straight, steep channels that are

perpendicular to the shoreline, whereas marshes with micro- and mesotidal ranges have

more complex creek patterns that may shift or migrate over time (Lutemauer et al.,

1995).

As it was mentioned, tidal regime is an important influence in the establishment

and maintenance of marsh creeks (Frey and Basan, 1985). Tidal range varies globally

fiom a few centimetres in the Baltic and Mediterranean (Ranwell, 1972) to 16 m in the

Bay of Fundy (Gordon and Cranford, 1994). Coastal regions are classified by tidal range.

4

A range of less than 2 m is microtidal, 2 to 4 m is mesotidal, and fiom 4 to 6 m is

macrotidal (Davies, 1980). For this paper, a tidal range of 4 to 6 m will be referred to as

a low-macrotidal range and a range of more than 6 m as high-macrotidal.

A saltmarsh tidal creek channel has a variety of different cross-sections dong its

length as a result of changes that occur b r n low to high marsh. For example, the mouth

of the channel would be exposed to the higher tidal energies of the oncoming flood tide

than the rest of the charme1 and vegetation root density and configuration impacts bank

erosion, potentially affecting cross-sectional shape (Frey and Basan, 1985; Steel and Pye,

1997). Throughout the channel there can be a range of width-to-depth ratios depending

on the tidal regirne, vegetation, substrate type, and elevation of the marsh (Steel and Pye,

1997). If there is abundant vegetation present (e.g., in the mid to high marsh) one may

find narrow, erosion-resistant cross-sections as the vegetation roots bind the bank

matenals (Frey and Basan, 1985).

SaItmarsh Tidal Creek Networks: Evolution and Growth

Saltmarsh tidaI creeks are a landward continuation of the tidal flat creeks (Steel

and Pye, 1997). A tidal creek originally forms on a mudflat as overland flow initiates

erosion. Sheet erosion first occurs on a newly exposed tidal mudflat during ebb flow as

rills and mal1 gullies form parallel to the slope. Eventually, d l divides break down and

new systems form perpendicularly to the initial nll, developing prirnary drainage

channels. The heads of these creeks slowly migrate upslope by headward erosion during

the ebb tide, while the channel gradients increase overbank deposition during flood tides

5

(Pestrong, 1965). Tidal creek channels decrease in width exponentially in a landward

direction. Most upper mid-marsh creeks are so n m w that they are covered by

vegetation, although they may be more than a metre deep (Pethick, 1992).

Lateral migration of the creeks occun when the water course shifts pathways, but

once vegetation has establishcd, root networks hinder migration (Pestrong, 1965). Banks

beneath dense stands of vegetation are generally steep and deeply undercut because root-

bound sediments are resistant to erosion, but channel migration occurs through

undercutting and slumping processes. Channels migrate as they become more deeply

incised and large blocks of soi1 and vegetation slump into the main channel, forcing fiow

around the obstacle (Pestrong, 1965; Letzsch and Frey, 198Ob; Frey and Basan, 1985; Shi

et al., 1995). In addition, burrowing organisms can make deep cracks in the channel

walls and water circulating through these burrows may weaken the substrate (Letzsch and

Frey, 1980b; Frey and Basan, 1985). In general, the degree of meandering depends upon:

1) the type of vegetation growing dong the channel bank; 2) the vertical distribution of

that vegetation; 3) the sediment type; 4) the ebb and flood tide characteristics; and 5) the

position of the channel within the marsh (Garofalo, 1980).

Sediment and Sediment Sources

Marsh sediments have a landward fining trend and the sediment is generally fine-

grained (fine sands to silts) with little clay because in many coastal regions the water

velocity is too high for clay to settle out of suspension (Allen and Pye, 1992). The most

irnmediate sources of marsh material are tidal waters (inorganic material) and marsh

plants (organic material). Many marshes also depend on riverine or terrestrial sources,

6

longshore drift, barrier washover, wind-blown sediments, organic aggregates, and the in

situ production of biogenic material, although this organic contribution is relatively small

(Frey and Basan, 1985). The type of minerai matenal depends upon the regional marsh

location.

SALTMARSH TIDAL CREEK FLOW DYNAMICS

Saltmarsh tidal creeks act a s drainage channels and conduits for the transfer of

matter and energy between the marsh and the adjacent body of water. The dynamics

within the channels are ofien dependent on surrounding topography; for example, during

flooding the levees along channel banks may act as barriers that cause a pulse in the flow

once their height is exceeded. Following the high tide, the water above the marsh surface

lowers at approxirnately the same rate as it does in the smaller channels because flow is

controlled by the sea-level fa11 rate. As the water level falls to the rnarsh surface during

ebb flow, the available cross-sectional area decreases faster than the volume of water that

must exit through the creek network, therefore, there is an increase in channel velocities

(Pestrong, 1965). Gravity enhances the flow as the ebb is fed by surface ninoff and

seepage (Bridges and Leeder, 1976).

Saltmarsb Tidal Creek Hydrodynamics

Studies of tidal creek hydrodynamics usually involve empirical methods whereby

a cross-section or series of cross-sections are chosen within one or a few rnarsh creeks

and cwent meters are utilized to detemine velocity. Techniques for velocity

measurement Vary fiom using a single curent meter to calculate velocity within the

7

channel cross-section to deploying an array of a number of current meters within the

cross-section-

Tidal creek discharge and velocity depend upon the tidd prism, which is

controlled by the morphology, elevation, and vegetation charactenstics of the marsh, and

creek hydraulic geometry. The channel network is responsible for the position and

strength of velocity surges and pulses associated with increased discharge rates in the

channels as over-bank tides spi11 ont0 the manh surface (Pethick, 1980; Steel and Pye,

1997).

Surges are caused by an initial velocity pulse when the channel first becomes

wetted, followed by deceleration over the flood tide and a second acceleration just above

the bankfull stage. This results because of a sudden tidal prism increase that occurs when

most of the flow is still confined to the channel system and because there is a differing

fictional resistance between the creek system and the manh surface. This pulse is

important for the transfer of sediment fiom the creeks onto the adjoining rnarsh surface

(Bayliss-Smith et al, 1 979; French et al., 1 993; S tee1 and Pye, 1 997).

Typically, as the water flows onto the marsh, a deceleration occurs before high

water is reached and this velocity approaches zero until the tide reversal. AAer the ebb

tide begins, there is once again a flow acceleration, often associated with ebb flow

cordinement to the channels (Ward, 198 1 ; Green et al., 1986; Leonard et al., 1995b ).

This acceleration may continue as a result of the marsh-surface vegetation which causes a

lag on the flow of water because it is retained on the surface by vegetation-induced

hydraulic roughness. When the surface has been drained, the flow may slow before there

is a final velocity peak caused by gravity drainage (French et al., 1993).

8

The asymmetxy of velocity in tidal creeks has been well established, although

some studies have observeci that tides that do not reach bankfull have a symmetncal

velocity distribution (e.g., Stoddart et d , 1989). The velocity pattern is 'asymmetrical'

when a higher maximum velocity occurs on either the flood or the ebb tide. Time

asymmetry occurs with velocity asymmetry if the peak velocities on the flood and the ebb

tide do not occur during mid-tide on either side of high water (Pestrong 1 965; Bayliss-

Smith et al., 1979; Ward, 198 1 ; Ashley and Zeff, 1988; Leonard et al., 1995b).

Ward (1979) found that peak velocities occurred 1-2 houn before and 2-3 hours

after high slack water. Many studies (e.g., Stoddart et al., 1989) have s h o w that there is

a higher velocity peak on the ebb tide and maximum velocities may exceed 1.0 m s-' as

surface water drains into the creeks (Green et al., 1986). In Bass Creek (South Cardina),

Ward (1981) found a net displacement of suspended material in the seaward (ebb)

direction, resulting f?om a strong velocity differential between the flood and ebb maxima.

There, the mean ebb current was 50% stronger than the mean flood current. Similar

results have also been found in other studies (e.g., Pestrong, 1965; Ward, 1979; Wells et

al., 1990; Wang et al., 1993), although Ashley and Zeff (1988) and Leonard et ai.

(1995b) found that the flood flows were stronger. Al1 marsh environments are different,

based upon the wide variety of factors that control the marsh environment, fiom their

tidal regime to their sediment sources. When combined with specific climatic and

seasonai conditions, there may be wide variations in the results of studies, making it

difficult to compare the processes between marshes of different regions.

Saltmarsh Tidai Creek Sediment Dynamics

It is ofien useful to measure suspended sediment concentrations in addition to the

velocity in a given tidal channel, siuce the two variables are often closely linked and may

be used to caIculate the sedùnent flux when the flow is confined to the creeks. Ward

(1979, 1981) and Ashley and Zeff (1988) monitored hydrodynamic processes within a

tidal creek systern to investigate their effect on suspended material transport. They

monitored the tides, currents, and suspended load within the tidal channel, then computed

mass budgets to find the total suspended load for the systern. The movement of sediment

through the drainage network is influenced by flow magnitude, velocity asymmetry,

biota, water temperature, and storm processes (Pestrong, 1965; Leonard et al., 1 995b).

The highest suspended sediment concentrations have been found to occur dunng

or slightly after peak curent velocities, when fine-grained sedirnent from the channel

bottom is re-suspended by incoming and outgoing tides (Ward, 1981; Green et al., 1986;

Ashley and Zeff, 1988). Current velocity, tidal stage, suspended load, and meteorological

data indicate that sediment transport processes are strongly infiuenced by the velocity

asymmetry of the tidal currents, springheap variations in the tidal cycle, and storm

activity. Under fair-weather conditions, the net direction of sediment transport is

dependent on the velocity asymmetry of the tidal currents, for example, when the currents

are ebb-dominant the result is a net export of material (Ward, 1979; Pillay et al., 1992;

Leonard et al.. 199%; Hemmlliga et al., 1996). It is important to note, however, that a

percentage of the material that is retuming fiom the marsh on the ebb tide includes some

of the sediment that entered the creek with the earlier flood tide.

10

As the tide rises, flow remains within the creeks, but when the tide reaches the

elevation of the marsh surface, the charnel banks are over-topped and the tide washes

over the rnarsh, flooding it with sediment-rich water (Luternauer et al., 1995). When the

creek overfiows its banks, deposition first occurs on the levees of the channels because

the velocity of the fiow is quickly slowed by vegetation on the levee, facilitating rapid

sediment deposition before sheet flow across the marsh begins (Frey and Basan, 1985).

Sedimentation patterns show a considerable range of spatial variability over the marsh

surface, but some key trends have been identified. Stoddart et al. (1989) found that

deposition rates in the marsh decline away fiom creek banks suggesting that sediment

availability for deposition on the manh surface is not only controlled by flooding

frequency, but also by proximity to the sediment source. Leonard et al. (1995a) also

found that there was a decrease in suspended sediment concentrations with increased

distance fiom the creek.

Flow velocity affects sediment transport by controlling the amount and type of

sediment that can be carried. Velocity pulses on spring tides within the creek system not

only transport considerable volumes of water and sediment ont0 the marsh, but also

mobilize sediment fiom within the creek that has collected when velocities were low, for

example, during neap tides (Ward, 1981; Leonard et al., 1995b). This sediment may be

re-suspended by spring tides and moved over the rnarsh surface, limiting accretion to

sediment availability and the oppomuiity for mobilization and transport within the marsh

system (Stoddart et al., 1989).

11

The level of suspendecl sediment in marsh channels usually increases during

spring high tides, or elevated stom tides, when a larger part of the rnarsh is being flooded

or drauled. Higher amounts of inorganic sediment have been found on the flood tide

because it has a large variety of sediment sources compared to the ebb tide which is only

supplied with a mal1 organic component (produced in the hi& manh) and the remainllig

sediment h m the flood tide (Ashley and Zeff, 1988). Initial ebb concentrations are

followed by lower values for the rest of the ebb because of deposition in higher parts of

the creek system. Sediment camîed in on the flood may also be trapped by the rnarsh

vegetation, although a portion of the material may be re-suspended fiom vegetation

surfaces on the ebb tide (Pestrong, 1965; Reed et al. 1985; French et al., 1993).

RATIONALE FOR THE RESEARCH

Saltmarsh research has often concentrated on micro- or mesotidal marshes

because it is easier to set up instruments and gain access to a site with a lower tidal range.

There have been many studies perfomed in micro- and mesotidal saltmarshes, while

relatively fewer have been done on macrotidal coasts. This is, in part, a resuit of the

relatively smalier nurnber of macrotidal areas, as well as the increased difficulty of

conducting experiments in regions with a larger tidal range.

There is a limited amount of information available on macrotidal saltrnarsh tida1

creek hydrodynarnics and sediment transport through the creek system. Most of the

available data are fkom the United Kingdom (UK) for example, Bayliss-Smith et al.

(1979), Green et al. (1986), and Stoddart et al. (1989). These studies explain the

12

processes occming within marshes of a greater tidd range ihan micro- and mesotidal

marshes, but there is relatively Iittle information about the flow and sediment dynarnics in

high-macrotidal saltmarshes (tidd range > 6 m).

Saltmarshes with tidal ranges that are greater than 6 m include those in the Bristol

Channel (üK) and the Bay of Fundy. Marshes with tidal ranges of this magnitude can

have flows up to 5 m deep move into and out of the creeks within hours. The transfer of

such a large volume of water could affect the sedimentary processes in the sdtmarsh tidal

creeks and across the marsh.

The pattern of water flow and sediment t r aqor t into and out of the rnarsh creek

controls marsh surface sedimentation processes and the evolution of the tidal creek

system. Deeper flows over the marsh surface and greater relief of the tidal creek system

are associated with a higher tidal range. It was expected that flow and sediment transport

characteristics in macrotidal saltmarsh tidal creeks would differ from those found in

micro- and mesotidal systems. There may also be differences between the low- and high-

macrotidal saltmarsh tidal channel dynamics. A study of the hydro- and sediment

dynamics in a high-macrotidal saltmarsh creek facilitated these cornparisons and

completed the data collection related to saltmarsh tidal creek processes within al1 of the

tidal ranges by adding data for an upper high-macrotidal range.

TICIIE RESEARCH PROBLEM

There has been little research in the high-macrotidal saltmarshes of the Bay of

Fundy (New Bnmswick and Nova Scotia). The Bay of Fundy has been well researched

by coastal geomorphologists, but the lack of research on the bay's saltmarshes is in part a

result of its renowned high tidal range, which makes the set up of instruments and

experimentation difficult. The majority of rnarshes in the bay have been altered by diking

during the last few centuries, so there are few rernaining 'natural' saltmarshes that may be

studied,

In addition, very little data have been gathered on the local processes of erosion

and deposition that occur on the tidal creek bank or bed surfaces. Much of the work has

focused on one creek cross-section and little has been done on the spatial variation of the

hydro- and sediment dynamics of a channel as the flow moves fiom the headwaters near

the mouth of the creek or at the marsh margin into the upper, vegetated regions of the

marsh. Such work would contribute to the literature about sediment transport pathways,

providing a basis on which to study the exchange of sediment between the saltmarsh tidal

creeks and the adjacent marsh surface where sedimentation supports marsh growth.

lhere is generally a larger availability of idonnation on sediment transpon and

exchange mechanisms between manh creeks and the marsh surface for saltmarshes with

lower tidal ranges (Le., micro- and mesotidal marshes). The present lack of this

information for macrotidd saltmarshes makes it difficult to determine the differences in

such processes in areas with higher tidal ranges. The goal of this work was to perform a

field experiment to measure the variation of the velocity and suspended sediment

14

concentrations over a number of tides, both vertically at one location and dong the

channel at a few locations within a saltmarsh tidal creek.

RESEARCH OBJECTIVES

The purpose of this study was to conduct a preliminary examination of the

hydrodynamics and the sediment dynamics in a high-macrotidal saltmarsh tidal creek in

the Allen Creek Marsh, Bay of Fundy. This was accomplished by addressing the

following objectives:

Measure the morphological components of the saitmanh tidal creek system.

Measure the dynamics of flow and sediment transport within the creek

network over a spring to neap tidal cycle.

Examine the temporal and spatial variation in the creek hydrodynamics and

sediment dynamics.

Compare the results to those reported in the literature for macrotidal,

mesotidal, and microtidal saltrnarshes.

The Field Work

This research was carried out in June, 1997, in the Allen Creek Marsh located in

Cumberland Basin on the northeastem Bay of Fundy coast, New Brunswick. The project

was part of a larger field study being done on saltmanh developrnent and sedimentary

processes by members of the Geography Departments at the University of Guelph and

Mount Allison University (Sackville, NB). The area of saltmanh that was chosen for this

15

çhidy is relatively small (approximately 0.06 h2) and it is part of one of the few

accessible, naîural sdtmarshes in the area The Allen Creek Marsh is one of several

saltmarshes in Cumberland Basin that contnbutes to the sediment budget, food chah,

carbon flux, and nutrient supply which sustain life in the Bay of Fundy.

16

C ~ R n

STUDY SITE, RESEARCH DESIGN, AND METEIODOLOGY

STUDY SITE

This research was carriecl out in a saitmarsh tidd creek in the Allen Creek Marsh,

located in Cumberland Basin in the upper Bay of Fundy region, New Brunswick (Figure

2.1). The Bay of Fundy is a northeastern extension of the Gulf of Maine, located on the

east coast of Canada between Nova Scotia and New Brunswick (Gordon and Cranford,

1994).

In the upper Bay of Fundy the tides are semi-dimal (every 12.5 hours) and the

average tidal range is approximately 1 1 m, although high spring tides can reach 16 m in

some places (Gordon and Cranford, 1994). The currents within the bay are controlled by

the tide. Generally, the strength of the flood tide is dependent upon the water surface

hydraulic gradient, while the ebb tide currents depend upon this and the seabed slope.

Within the hay. rhc flood tide is typically stronger, but not as long in duration as the ebb

tide (Canadian Hydrographic Service, 1966).

Three main sources of sediment within the Bay of Fundy are: 1) eroding cliffs; 2)

rivers; and 3) seabed reworking (Amos and Long, 1980). The Bay o f Fundy is supplied

with fine-grained sediment resulting fiom the in situ cliff erosion of Paleozoic sandstone,

siltstone, and shale (Gordon et al., 1985). Much of the bay's sediment is supplied by this

shoreline erosion and only a mal1 cornponent is fiom river input. Suspended sediment

concentrations found in the upper reaches of the bay range fiom 50 to 100 mg 1-',

Amherst .,

Figure 2.1 Location of the Allen Creek Marsh in Cumberland Basin.

18

however, higher concentrations may be found in the tidal creeks (Gordon and Cranford,

1994). The measured concentrations depend upon factors such as the conditions during

sampling (e.g., wind conditions) and the season during which the sampling occumed.

These variables can afkct the amount of sediment in suspension and lead to a

discrepancy among results, for example, Amos and Tee (1 989) found ~spended sediment

concentrations exceeding 200 mg 1-'.

Saltmarshes in the Bay of Fundy

Saltmarsh growth in the Bay of Fundy is continual because mean sea level is

rising at a rate of approximately 0.30-0.45 m per century and saltmarshes must adjust to

maintain their position with respect to sea level (Prouse et al., 1984; Gordon et oL, 1985).

Tidal characteristics, sedirnentation, and ice are important to the bay's saltmarshes. The

effect of tides in addition to saltmarsh morphology dictates how long the marsh is

submerged by saltwater each day. This, in turn, affects the type of vegetation that may

grow on the marsh and the location on the marsh where it c m suntive (i.e., low, mid, or

high marsh). Sedimentation on the marsh is dependent upon the sediment source and

existing vegetation. Deposition enhances the stability of the rnarsh surface and further

vegetation growth which occurs nom May to October. There are approximately 4-5

months during the year when there is no vegetation on the marsh surface. This is a result

of the shearing of the vegetation fiom the mmh surface by waves or ice after it has died.

The low marsh area is usually completely covered by shorefast ice between

December and March and ice can fkeeze to the substrate and later raft sediment and any

19

remaining vegetation when the ice is re-floated (Gordon and Cranford, 1994). This can

lead to mounds of sediment on the marsh surface that are deposited when the ice melts.

Ice may also scour some areas on the marsh or in its tidd creeks (Gordon et al., 1985).

Bay of Fundy saltmarsh vegetation may be divided into high- and low-marsh

zones (Prouse et al., 1984). The high marsh is above mean hi& water and is infiequently

flooded by high spring tides (Gordon and C d o r d , 1994). This area is rnainly composed

of the marsh grass Spartina patens (Gordon et al., 1985). The low marsh extends just

above mean hi& water to approximately 2 m below and has more relief. This marsh

zone is dominated by the marsh grass Spartina altemz~ora and is flooded twice daily; the

depth of flooding may reach several metres for up to 4 hours (Prouse et aL, 1984; Gordon

and Cranford, 1994).

Cumberland Basin

Cumberland Basin is a bedrock, dike-bound estuary that is 45 km long and up to 3

km wide (Amos and Tee, 1989). It has an area of 1 18 km' consisting of: saltmarshes

(17%), mudflats (40%), and sandflats (43%). The tidal pnsm in Cumberland Basin is

approximately 1 lm' and during the Iow spring tide, only one-third of the basin is

covered with water (Gordon et al., 1985). The maximum tidal range at the mouth of the

basin is 9 m and this range increases headward at a rate of 3.5% every 10 km (Amos and

Tee, 1989). Fluvial inputs and clin erosion contribute to the suspended sediment in the

basin (Amos, 1987). Once suspended, this sediment remains in the water column as it

travels up the mudflats of the basin on the flood tide and ont0 the saltmarshes.

20

The reclamation of saltrnarshes within Cumberland Basin has occurred since the

eighteenth century and shce this t h e at least 69% of the area for sediment deposition has

been removed. Since this period, there has not been any significant development of

saltmarshes or mudnats (Amos and Tee, 1989).

The Allen Creek Marsh

The Allen Creek Marsh is approximately 2-3 km long, extending east and West of

Allen Creek along New Brunswick's northem Coast of Cumberland Basin, however, it is

relatively narrow (approximately 0.20-0.30 km wide). Most of the marsh is natural,

meaning that it is one of the few areas within the basin that was not diked centuries ago

by settlers trying to increase the amount of land available for agricultural use. The entire

margin of this marsh consists of a cliff approximately 1.5 rn high, adjacent to the

extensive mudflats which extend into the basin. The section of the rnarsh that has been

chosen for this study is irnmediately east of Allen Creek and it is approximately 200 m

wide and 300 m long. For the purposes of this paper, this section will be called the

'Allen Creek Marsh' (Figure 2.21, although it is only a portion of the marsh.

The marsh's western border is bounded by Allen Creek which is a small river that

is under the influence of the tide for approximately 1 lan upstream and has very little

fieshwater input. The northem high marsh ends abruptly along a forest that has been

cliffed approximately 1.5 m above the marsh surface by storm and wave activity, and its

southerly, low marsh boundary is a cliffed margh that drops approximately 1.5-2.0 rn to

! - Boardwalk Instrument Platform Creeks and Instrument Stations -

390900 390950 391 000 391050 391100 391150 Easting (m)

High Marsh

Figure 2.2 The Allen Creek Mmh (elevation is in metres above d a m NAD83).

22

meet the tidal flats of Cumberland Basin. These tidal Bats are composed of fine-grained

sediments such as silt which eventually grade into sand and grave1 at the Iow water mark.

The Allen Creek Marsh margin is part of a 'marsh ami' (Figure 2.2). This a m

was fonned by the incision of a large saltmarsh tidal creek (the 'main' creek). This creek

is parallel to the rnarsh margin and is approximately 4 rn deep and 15 m across at its

confluence with AIIen Creek at the southwestem corner of the marsh. The main creek is

the channel that feeds the three trïbutary tidal creeks that are generally perpendicular to

the marsh margin and lead to the remaining, smaller tributaries of the mid marsh.

Growth of the marsh vegetation begins in late April or early May and the plant

life usually dies by the end of November, leaving a covering of hay-coloured stems. The

stems of the manh grass are quite firm and form a thick, bnttle stem embedded in the

marsh surface which may be broken and transported during the winter by waves, storm

events, or ice. This leaves the marsh quite bare until the period of growth the following

spring. The vegetation on the marsh is prirnarily Sparîina altemzjlora, with a small

percentage of Salicornia sp. on the low marsh and Spartina patens on the high marsh.

The high marsh has a larger variety of species within the vegetation canopy (van

Proosdij, 1997).

RESEARCH DESIGN AND METHODOLOGY

This research was conducted using an empirical, field-based approach. This

approach expanded upon previous efforts in the study of saltmarsh tidal creek dynarnics,

but it differed from them because it simultaneously measured flow velocity and sediment

concentrations using CO-located electromagnetic current meters and Optical

23

BackscatteranceTM probes. One salûnarsh tidal creek in the Allen Creek Marsh was

chosen to be the focus of this research and most of the flow measurements that will be

analyzed were carried out within this creek. This channel will be referred to as the

midde or 'study' creek throughout this paper (Figure 2.2).

To perform this research it was h t necessary to provide access to the manh and

channels. A boardwak was built for this purpose using 0.25 rn wide wooden planks that

were s~pported 0.50 m above the rnarsh surface. A wooden platform nsing 3 m above

the ground was located at the end of the main 100 m boardwalk that extended fiom the

high to the low marsh. A covered shelter was built on one half of the platform to house

the equipment and electronics that were necessary to power the instruments and

cornputers that were recording the data

Two bridges were then constructed across the study creek. Both of the bridges

were necessary to facilitate access to the channel. One was located across a nmow

section of the tidal creek in the mid rnarsh region and it was simply an extension of the

boardwalk that had been built as an arm fkom the main boardwalk. The second bridge

war approximately 9 m long in the low marsh area and it was built across a section of the

channel that was approximately 2 m deep, about 40 m fkom the mouth of the creek.

Field Mapping

D e W g the morphological components of the study tidal creek and its network

involved: 1) the creation of a plan-view map of the tidal creek system; 2) profiles of the

creek thalweg gradients; and 3) profiles of the cross-sections used for the experiments.

24

The banks of al1 of the saltmarsh tidal creeks within the network were mapped to generate

a plan view of the saltmarsh creek system. This was achieved using a theodolite and an

electronic distance measuring unit to determine a tnangulation network of control points

which were later tied into a New Brunswick Department of Highways benchmark

(#9658).

The thalwegs of the four larger creeks within the creek network were sweyed

using a theodolite to measure the level on a stadia rod that was carried down the thalweg

of each channel. This showed the elevation characteristics of the channe1 and allowed for

the calculation of the channel slope. Profiles of the cross-sections within the study creek

were also surveyed individually using the above-mentioned instruments so that the shape

of each could be illustrated and cornparisons made between the locations along the creek.

The thalweg and cross-section profiles were plotted using the Microsofi Exceln1

spreadsheet program.

After the field season, a survey of the marsh surface was performed using a

Geotracer 2000TM differential global positioning system, relative to the benchmark. This

system has a positional accuracy of 0.01 m and an elevational accuracy of 0.02 m. The

data were combined with those collected nom the summer survey to create a topographie

map of the marsh surface using the Surfer for Windowsm program. The positions of

each cross-section within the study channel were then indicated on the plan-view map.

Erosion within the Study Channel

An expairnent was carried out to measure the small changes occurring along the

banks of the study creek 'Erosion pins' were designed and uistalled to measure the

erosion and deposition of fine sediment within the channel between the large bridge and

the confluence with the main creek. Twenty-seven 0.60 m stainless steel bars were used.

Each pin had 0.015 m dots along the centre that were approximately 0.025 m apart.

These pins were pauited with altemating sections of red and white that were

approximately 0.10 rn long (each section encompassing 4 slots). The slots and painted

sections were necessary to facilitate easier reading of the pins nom a distance (e.g., from

the bridge or the bank) and to avoid the problem of a more detailed scale being obscured

by sediment deposits. The position of the sediment level of the bed or bank surface on

the pin was estimated with respect to the coloured section and slot on the pin.

Eighteen pins were placed at two main cross-sections near arrays Cl and C2

(Figure 2.2), stretching across the banks of the study channel (10 pins at Cl, 8 at C2).

They were inserted at an interval of 1 rn across the channel (Figure 2.3). n i e remaining

pins were placed along the channel between C 1 and C2. Each of the pins were assigned a

number for ease in recording the data.

The relative change in the sediment level for each pin was estimated with the use

of binoculars. To do this. a hand-held 'key' erosion pin was necessary. This pin was

painted in the same fashion as the others and had a scale fixed to the side for ease of

measurement, so that the position of sediment on the erosion pin with respect to a given

coloured section and dot could be matched on the key pin. Then, the scale could be

Figure 2.3 Erosion pins across cross-section C 1 (facing east). Height of closest pin is approximately 0.32 m.

27

referred to in order to determine a sediment level value. n i e higher the value was, the

lower the sedinient Ievel position on the pin, indicating a loss of sediment.

The Measurement of Flow and Sediment Dynadcs

The velocity of the 80w within the study creek was measured using bi-directional

Marsh-McBirney Mode1 5 12 electromagnetic current meters with a 0.05 m diameter

head. Electromagnetic current meten are well-suited to high intensity flows where active

sediment transport is expected to occur, as in tidal channels (Clifford and French, 1993).

The changes in suspended sediment concentrations were measured using Optical

Backscatterancenf probes (OBS probes) at the same locations within the cross-section

where the velocity was being measured. The OBS probes measure the suspended

particulate matter within a water colurnn and this matenal will be referred to throughout

this paper as suspended sediment, a term which includes organic and inorganic matter.

Each probe was rnounted at an appropriate distance f?om the other instruments on the

array so that the instrument signals would not interfere with each other.

There were two separate instrument arrangements for the research. Each set-up or

'array' required the CO-location of an OBS probe and a current meter (an instrument set or

pair). These instruments were held in position on a horizontal bras pipe that was

supported by two vertical posts on either end that were made of steel (vertical array], or

brass (spatial array). Every location where an instrument array was positioned was called

a 'station' and each of the stations had an alpha-numeric name, depending upon the

array's position with respect to the confluence of the study creek with the main creek.

28

The closest station was labeled Cl ('C' ref-g to an array within the creek) and that

which was farthest h m the confluence (mid marsh) was C4.

For both the vertical and spatial array experiments, two S h a e v i P pressure

transducers recorded water depth data. One of the transducers was fixed to one of the

vertical supports of the vertical array (at C2), while the other was mounted to the

reference station stand that had been placed within the main channel.

Vertical Array Imtrument Arrangement

For the k t part of the experiment, there were four sets of instruments arranged

on a ladder-like array at C2 near the bridge, so that the variation of dynamics with respect

to depth in the channel could be analyzed. Each horizontal cross-bar on which the

instruments were mounted with clamps was approximately 1.25 m wide and the sensors

of the instruments were at heights of 0.15 m, 0.75 m, 1.50 m, and 2.63 m above the

channel bed. These positions were named V1, V2, V3, and V4, respectively (Figure 2.4).

The intervals at which the instniments were placed were used to maximize the

distribution of the instruments within the channel, ensuring the maximum amount of data

collection over each tidal cycle. The spacing of the instrument pairs was initially based

on the possibility that a logarithrnic velocity profile could develop in the channel. The

arrangement also maximized the number of instruments in the lower portion of the

channel, thereby maxirnizing the length of time that would be represented in the data

record for each tide.

The x-axis of each current meter was aligned parallel to the thalweg of the creek

and the direction of the axis with respect to north was noted using a Brunton compass at

Vertical A m y Station C2 (Facing North)

O 2 4 6 8 10 12 14 16 18 20 22

Distance (m)

Figure 2.4 Vertical array instrument positioning at station C2 dong the study creek.

30

the time of instnunent installation. The positive sensor of the current meter was oriented

facing downstrearn so the flood flow would be positive, therefore, the negative sensor

wodd masure the ebb flow. The sensor of the OBS probe was oriented perpendicular to

the thalweg to allow for a proper reading during both the flood and the ebb tides.

Spatinl Array Imtrument Arrangement

The second part of the experiment involved setting up a different instrument

anangement, using the same sets of instruments. The vertical array was separated so that

each pair of instruments could be placed at different locations along the channel to

measure the spatial variation of the flow dynarnics along the creek. The bottom set was

left as a single anay at C2. Each array was H-shaped and the sensors of both the OBS

probe and the current meter were located 0.15 m above the bed (Figure 2.5). This was a

reasonable distance nom the channel bed for it allowed an ample proportion of the tidal

cycle to be represented; if the instrument was too high in the water column, the early part

of the flood tide and the latter portion of the ebb tide would be missing fiom the data set.

Once again, the width of the fbme was 1.25 m wide and the x-axes were aligned parallel

to the thalweg and their direction with respect to north was measured.

Reference Station

A reference station (CS) was installed in the main channel. This station was

cornposed of a Marsh McBimey Mode1 555 bi-directional current meter with a 0.10 m

diameter head, a ShaevitzTM pressure transducer, and an OBS probe. This station acted as

a control station throughout the field season, allowing for a general interpretation or

cornparison of flow dynamics in the main channel.

Figure 2.5 H-fkme set-up of instruments for the spatial array (station C4).

Data Recording

AU of the instrument cables were secured to wooden stakes on the rnarsh using

cable ties to prevent the creation of any noise that could occur if the cables were in

motion within the flow. These cables extended dong the marsh surface to the elevated

platfom where they were connected to the electronics which fed the signal to the

computea. The data were recorded on both a laptop and a desktop computer using the

EasyAGTM data-logging program. The computea and instniments were powered using

12-volt marine batteries. During the day these batteries were attached to a battery charger

which was connected to a generator to maintain the charge. The battery connections

decreased the interference that could have been caused by the generator. During the night

experiments, the two 12-volt marine batteries were run in parallel without the generator.

A wind vane and anemorneter were positioned on a mast at the top of the

platform's shelter (approximately 5 m above the manh surface). These instnunents

measured wind direction and speed and their battery-powered electronics were secured

inside the shelter. The measurements were recorded by StowAwaym data loggers which

were later down-loaded ont0 one of the computen using the LogBooknf program. An

effort was made to record this information for as many days as possible during the field

season, however, there were a couple days when the record was not available as a result

of problems with either the data-Iogging systems or the instnunents. Regardless of the

working order of the wind vane and the anemometer, a record was kept of visual

observations of the conditions each day. When there was no data record, these

33

obsenations were substituted, however, there was no visual record for the vertical array

experiment night nms.

Flow Velocity Sampiing

The flow velocity of the creek system was meanued during June, 1997 over a

number of tidal cycles, spanning the range of spring (maximum) to neap (minimum)

tides. The measurement of one full tide was called an 'experiment' and each

measurement interval during that tide was called a 'run'. There were two sets of data

taken for each tidal range (i.e., spring, transitional, and neap tides). Over an entire tidal

cycle, measurements were sampled at 2 Hz for approximately 8.5 minute intervals, with a

rest interval of approximately 10 minutes.

The rest interval was based upon the limitations of the data collection systern,

including the length of time required to check the data, save the file, and re-set the

program for the next m. Because the tide rose and fell so quickly (0.04 m min?), the

maximum number of nuis were desired. Following each run the data for each instrument

were graphed to ensure that it was functioning properly and that the electronic

connections were good. If this was not the case (if the chart appeared problematic), a11

possible steps were taken in an attempt to remedy the problem.

The analysis of plots of the individual time senes for each run was important for

exarnining average trends and for detecting anomalies and potential equipment

malfunctions. These data were also important for visually detennining when the senson

of an instrument had been submerged or had emerged from flooding and receding water

34

(Figure 2.6). These data pmvided a basis on which the m s that should be plotted as a

tirne series and averaged for M e r analysis could be detemiined. The charts could also

be referred to if the averages or average standard deviations appeared questionable.

Laboratory Analysis

OBS Probe Calibrations

The output of an OBS probe depends upon the size, composition, and shape of

suspended particles, therefore, it was necessary to calibrate the probes using in sitri

material @ & A Instruments, 1988). This procedure was carried out after the field season

at the University of Toronto ushg a re-circulating fa11 column and sediment that was

gathered from the study creek in the Allen Creek Marsh. Dispersed sediment was added

to distilled water and a stock solution of 15 g 1" was used to add increments of 50 ml to

the fa11 column. A re-circulating flow was necessary to ensure that the suspension of

sediment was homogenous and flow turbulence was reduced by a baming mechanism

that had been built into the system.

Each OBS probe was individually placed within the column for every suspended

sediment concentration. The instruments were mounted on aluminum rods and the

sensors were held 0.30 m below the surface of the water and onented to minimize

interference of the infrared signal with the clear walls of the fa11 column while each

calibration was performed.

Following each set of runs for a particular sediment concentration, two 0.5 L

boale sarnples were taken using an in silu suction filtration system that rernoved a sample

Submergence of V2 Current Meter X-Axis

-3.5 1 I Time (seconds)

Figure 2.6 Example of an x-axis record showing the submergence of a curent meter.

36

nom 0.40 m below the water level. These samples were filtered through 0.45 micron

Whatrnan filters to validate the suspended sediment concentrations within the fa11

column. Charts were created of the known suspended sediment concentrations against

the measured voltages for each instrument (Figure 2.7). The equation of the line was then

calculated and used to convert the raw voltage data that were collected in the field to

suspended sediment concentration values that could be averaged.

Grain Size Analysis

Suspended sediment sarnples were obtained in the field and stored for later

analysis to describe the grain size of the study creek bank sediment. The analysis was

performed on 20 g of sample using standard sediment sieving and pipetting techniques

for a sediment size range of 4-10 phi (Lewis and McConchie, 1994).

Pressure Transducer Calibra fions

The Shaevitzm pressure transducen were calibrated using a vertical fa11 column

at the University of Guelph. Each instrument was submerged at intervals of 0.10 m and

the resultant signal output at each depth was recorded. Al1 of the recorded voltages were

then graphed to provide a regression line so that an equation could be determined to

convert the voltages recorded in the field to the proper units of depth.

Instrument and Data Recording Problems

During the course of experimentation, a variety of weather conditions were

experienced that caused data recording problems. The humidity caused occasional

problems with the electronic circuits so the data for a particula. instrument became

Calibration Curve for OBS Probe 2

O w

O 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

OBS Probe Output (volts)

Figure 2.7 Calibration curve for OBS probe 2.

38

unusable for some of the nuis. Infiraquently, it would also intetfere with the proper

fûnctioning of the laptop cornputer. Al1 possible precautions were taken, however, a few

of the connections seemed to be more vulnerable to the elements. Before the end of the

experiments, none of the OBS probe connections were working.

Another problem met in the field was that one of the axes of a current meter was

not functioning during the vertical array experhnents and the OBS probe at this location

(V4) could not be properly calibrated, therefore, there are no data for this top position.

The current meter at position V1 (0.15 m fiom the bed) was also not working during this

period, therefore, there are only two positions on the spatial array that have current meter

data (V2 and V3) and three positions with OBS probe data (Vl, V2, and V3).

There are also no data for the OBS probe located at the reference station. This is a

result of an apparent change in the OBS probe voltage output fiom the period of data

collection during the field season to the day of the laboratory calibration of the

instrument. The objectives of this research focused on the flow and sediment

concentration dynamics withui the selected tidal creek which does not include the

reference station, therefore, this problern did not compromise the objectives of this study.

The current meter with only one working axis was used in the spatial array

experiments; the instrument was placed at station C3 with the fùnctioning axis parallel to

the thalweg. There is no converted OBS probe data for station C3, because by the end of

the field season there was a problem with the output of the probe and it could no longer

be calibrated. The data could, however, be used to analyze the trends of change in the

39

suspendcd sediment concentrations over the tidal cycle, which could be compared to the

trends recorded at the other stations dong the study creek.

40

CHAPTER III

RESULTS

MORPHOLOGY

The Men Creek Marsh

Saitmarsh tidd creeks have different fiow charactenstics fiom river or Stream

channels because they have bi-directional flow and typically have aimost zero discharge

during slack water (Bayliss-Smith et al., 1979; Steel and Pye, 1997). The saltmanh tidal

creek that was chosen to be the focus of this research was depicted in Figure 2.2, which

illustrated the locations of the cross-sections that were studied. Throughout this paper,

the tems high tide and slack water are used interchangeably, refemng to a time when the

tide reached its maximum elevation, and not necessarily when the flow was reduced to a

velocity of O m s-' (as could be inferred by the term 'slack water').

The spring high tidal range in this part of the basin is 12-13 m, while the lowest

neap tides range fiom 10.5-1 1.5 m (Figure 3.1). The Allen Creek Marsh is flooded hvice

daily and the second (night) tide is higher than the first. The difference between the day

and night tides is often maximized during the high spring and low neap tides (a difference

of approximately 0.40 m). The tides between the spring and neap tides will be called

'transitionai tides'. For the purposes of this paper, the tides have been categorized based

upon the maximum and minimum tidal heights during the research: 1) neap tides <

11.30; 2) transitionai tides fkom 11.30-1 1.80 m; and 3) spring tides > 11.80 m.

Maximum Tidal Heights (June - July)

l3 I Spring Tides

Neap Tides B .

'.* S . - . . .

Figure 3.1 Maximum forecast tidal heights for Peck's Point near the Allen Creek Marsh for June and July, 1997 (Canadian Hydrographie Services, 1997).

42

During a spring high tide, the water typically reaches a maximum depth of more

than 4 m h m the bed of the main channel. The water reaches the high marsh region

during a spring tide. During a low neap tide, bankfbll in the tbree tributary creeks is not

exceeded and the marsh surface itself is not flooded, however, the neap tide usually

exceeds bankfiill in the main channel.

There are often waves driven across the surface of the flood and ebb tides. The

effect of these waves is diminished with depth in the channels so that the creek beds do

not experience strong impacts of wave activity for the majonty of the flood tide. Another

effect seen during each tide is 'seiching'. In essence, this is a back and forth motion of

the flooding and ebbing body of water caused by amplification effects produced as water

in the Bay of Fundy is forced into volumetrîcally smaller basins. The waves in the

observed record ranged fiom c0.05-0.20 m in height, with the maximum height occuning

when the winds were the strongest for the research period - 4.6 m s-'. The prevailing

wind direction varied between northeast and southwest. The northeast wind was

prominent during the vertical array experiments while the southwest winds occurred

during the spatial array experiments.

The Coast to the east and West of Allen Creek consists of narrow saltrnarsh

(approximately 250 m wide) with cliffed marsh ma&m and tidal creek systems similar

to that found in the experiment site. These creek networks each have a main creek that

extends into the tidal mudflats of Cumberland Basin and landward there are small

branching tributary systems in the mid-marsh area. The creeks that lead nom the

mudfiats into the manh have the longest and widest channels which, in a few cases, lead

beyond the saltmarshes themselves (e.g., Allen Creek). Some of the marshes east of

43

Allen Creek were diked and used for agriculture, but have since been abandoned. These

marshes extend landward into fields of grass and bush and are relatively accessible by

foot. Those to the west are more n m w and are not as accessible, backing ont0 thickly

foresteci Iand.

The saltmarsh being studied had a relatively narrow width and the three main

areas within the studied part of the marsh had v-g slopes: 1) 0.014 - high manh; 2)

0.004 - mid marsh; and 3) 0.008 - low marsh. The marsh surface does not havea great

deal of topographical variation, with the exception of depressions around the creeks and

isolated patches of fine-grained sediment that were deposited as ice that rafled the

sediment melted during the spring thaw. The tidal creek network has relatively straight,

unvegetated channels near the marsh margin that are approxirnately 1.5-2.0 m deep.

Landward, in the mid marsh, the channels become shallower and narrower with increased

vegetation and steeper banks. In the upper mid manh, as some of the channels become

more shallow, the slopes become less steep and increasingly smaller until they are

negligible in the hi& marsh.

Saltmarsh Tidal Creek Network Morphology

As mentioned in the previous chapter, the shidy site contained a network of three

large tributary channels, ninning approximately perpendicular to the marsh margin.

These creeks branched fiom a main channel that lay parallel to the manh margin and the

Iast, or east, tributary creek was an extension of the main creek. The second of the three

channels is the shidy channel, chosen to be the focus of this research. The larger channels

are generally straight and relatively short with a smdl degree of meandering. The

tributaries of these creeks have occasional meanders, however, they form a reticulate

pattern, usually branching h m the other, larger channels at right angles (Steel and Pye,

1997). In the upper reaches of the creek network, the pattern becomes more dendritic.

The general characteristics of the main creek and its three tributaries are listed in Table

3.1. The gradient profile for the thalweg of each channel is graphed in Figure 3.2.

Table 3.1 Characteristics of the main tributaries in the Allen Creek Marsh. The slopes are the linear regression line slopes for the selected thalweg sections.

The thalweg profiles in Figure 3.2 depict the changing slope of each channel.

There are two interesting characteristics that may be noted about the profiles. It is

apparent that there are step-like features on each of the three tributary channels (the west,

middle, and east creeks). These features may be current or rernnant nick points (inhented

fiom the past mudflat), indicating where erosion actively worked landward in the

channels. They could also be a sign of underlying resistant strata or bedrock which is

known to exist a couple metres beneath the marsh surface. The second characteristic

noted in the figure was the increase in elevation along the east creek. This topographie

nse in the channel may once again be evidence of a resistant stratigraphie segment

beneath the channel, such as bedrock.

C'reek

West Middle (S tudy) East

1 Main 1 140.9 1 30.4 i -0.024 l t

S l o ~ e 1 a b 8 f

Le %th (ml 162.2 227.9 126.0

-0.02 1 -0.008

Confluence Width (m)

35.8 14.3 12.5

-0.0 16 -0.007

West Creek 6 ,

c a O = - 4 L ~ H e M c X x 8 " *; i

O , O 25 50 75 100 125 150 175 200 225

Distance along thalweg (m)

Middle (Study) Creek 6 -

c a 0 4 - u n S E 2 > --WH+--- b

- W

- O

O 25 50 75 100 125 150 175 200 225 Distance along thalweg (m)

East Creek 6 - a

E u Y C\ II

0 4 - - C i -

;'*- Y . . y C\, .

= O O 25 50 75 100 125 150 175 200 225

Distance along thalweg (nt)

O 25 50 75 100 125 150 175 200 225 Distance along thalweg (m)

Figure 3.2 Thalweg profiles of the main tidd creeks in the Allen Creek Marsh creek network. The direction is seaward and O m indicates the head of the tributary. Survey points range from 5 to 30 m apart. Elevation in metres above daturn NAD83.

The creek closest to Allen Creek, the West creek, had a V-shape similar to the

middle and east creeks, but it was subject to the highest wave energy. The marsh surface

adjacent to this creek abruptly ended in a small cliff that dropped approximately 0.50 m

to the creek bank. This cliffed bank underwent some dumping as a result of factors such

as the undercutting activity of the waves and drainage processes.

The tidal creek cross-sections within the marsh decreased in size as the elevation

and vegetation increased (Figure 3.3). As the transition fkom the unvegetated to the

vegetated channels occurred, the channels becarne more nanow and there appeared to be

a higher occurrence of slumping events along the steepened banks.

Typically, the larger channel cross-sections were V-shaped (Figure 3.4), however,

in the larger cross-sections, within approximately 0.10 m of the boaom, the channel took

on a small square or box-like shape. Along the channel walls nlls ied fiom the rnarsh

surface to the creek bed. The nlls directed the late ebb drainage water that had been

retained by the marsh surface and its vegetation or excess rainfall into the channel. The

channel surface was composed primarily of silt, clay fractions, and sand (Table 3.2). The

settling velocity for the silt was 1 .Dx l O-' m s-'.

Table 3.2 Channel bank composition.

Unaccounted (%) 0.3 1 1.88 1.21

Sample Location

C5 C2 C3

Silt (%)

95.14 93.21 95.19

Sand f%) 1 .O8 0.60 1.34

Clay Fractions (%) 3.47 4.3 1 2.26

Study creek cross-section profiles of stations C l 4 5 (facing north)

6

O 10 20 30

Distance (m)

Figure 3.3 Profiles of the cross-sections chosen for the spatial array experiments, facing upstream (north). Elevation in metres above datum NAD83.

Figure 3.4 V-shaped cross-section of the main creek.

49

Within the saltmarsh charnels, one method by which the channel morphology was

altered was by the process of slumping. This procas was observed where the channel

banks were steep with a more rectangular cross-section (e.g., mid marsh) and usually had

an adjacent marsh surface with dense vegetation that grew close to the edge of the bank.

Most of the seaward, deeper cross-sections that had V-shaped bank slopes leading

gradually into the vegetated marsh surface were not observed to experience this kind of

slope failure.

EROSION PIN RESULTS

The measurement of changes on the study channel banks involved the installation

of 27 erosion pins at 1 rn intervals across the banks of two cross-sections, Cl and C1

(Figure 3.5) and more randornly along the channel between the two cross-sections. The

sedirnent level or surface position of the bank or bed on each pin was recorded over a

period of 36 days using binoculars to measure the erosion and deposition along the

channel banks (Table 3.3). Measurements were made by refemng to a key erosion pin

which involved a possible error of 0.001-0.003 m. During the daily measurement of the

sediment levels, there was occasionally a small mass of seaweed or rnarsh grass caught at

the base of a pin, therefore, measurement could not take place at that pin location until the

next day.

Calculations of the averages uicluded al1 of the erosion pins, unless othenvise

stated, and the results include the analysis of both the average daily and the cumulative

average change over the perîod of measurement. These results also evaluate the

relationships of the tidal stage and the precipitation that occurred previous to a day's

C l Erosion Pins, Facing Upstream (North) 4 -

O 2 4 6 8 10 12 14 Distance (m)

C2 Erosion Pins Facing Upstream (North)

1 3 5 7 9 11 13 15

Distance (m)

Figure 3.5 Location of erosion pins across Cl and C2. Elevation in metres above datum NAD83.

Date

June 18 19 21 22 28 29 30

July 1 2 3 4 5 6 7 8 9

10 11 12 16 17 18 20 21 22 23

Table 3.3

-

Day # Previous Tide (m)

Previous Precipitation

(mm)

Unmeasured Pins

Erosion pin measurement schedule, including tide and precipitation charactenstics. Tide type: Sp. = spring tide, Tm. = transitional tide, and Np. = neap tide.

52

measurement with the sediment levels at the pins. This facilitated an attempt to

detemine a controhg factor for erosion within the saltmarsh tidal creek.

Sediment LeveI Fluctuations and Trends

There was usually a daily sedunent level fluctuation at each pin and when the

values for al1 of the pins were averaged, three relatively large erosion events were

observed when the average erosion for al1 of the pins exceeded 0.005 m (Figure 3.6a).

There were no accretion events of conespondhg magnitude, however, there were a

nurnber of occasions when several rnillimetres of accretion occurred. This accretion

fluctuated on a daily basis with similarly small erosion events, until the next major

erosional occurrence.

When the cumulative averages were calculated, it was f o n d that there was a net

decrease in the sediment along the surface of the channel's banks of 0.0203 m (Figure

3.6b). The overall pattern observed at the bed was dominated by the three main erosional

events, but there were periods of 3-5 days between these events when the accumulation of

a few rnillimetres took place.

Factors Controllhg Sediment LeveI Change

Part of the purpose of this analysis was to determine possible controls on the

erosion and accumulation of sediment dong the charnel. It was thought that two

potential factors were responsible for such change: 1) tidal range (a dominant force

within the basin); and 2) precipitation (this became a suspected control during the field

(4 Daily Average Change in Sediment Level

(b) Cumulative Average Sediment Level Change

Figure 3.6 (a) Daily average change in the sediment level. (b) Cumulative average change in the sediment level. Averages included rneasurernents for al1 of the erosion pins. A negative change indicates sediment loss (erosion).

54

season as measurements were being recorded). These controls were investigated with

respect to correspondhg gains or losses in the measured sediment levels.

TidaI Height

It was thought that the type of tide could affect the amount of sediment that was

eroded or deposited fiom the channel bed. The higher tides may have created higher

channel flow velocities that could cany increased volumes of sediment which could be

potentially deposited during slack water, enhancing sediment accumulation along the

beds. The increased speed could also have had the reverse effect, increasing the scour or

erosion at the bed and having a higher ability to transport this sediment.

The results ïndicated that tidal height did not have an effect on the erosion or

deposition processes along the channel, therefore, it was not considered to be a major

influencing factor. Both erosion and accretion events occurred throughout periods of

higher and lower tidal heights without any particular pattern (Figure 3.7). The calculation

of Pearson's correlation coefficient with a 95% confidence ievel showed that the

sediment level change was not significantly related to the daily maximum tidal elevation

(Figure 3.8).

Precipitation

Rainfall events were also investigated as a possible control on the erosion or

accumulation of sediment along the channel banks. It was thought that the impact of

raindrops could effectively loosen the grains, making them more susceptible to transport

by the tide. It was also possible that surface flow resulting from a saturated marsh

surface could travel down the banks, eroding some of the sediment.

Daily Average Sedirnent Level Change with Tide

Figure 3.7 Daily average change in the sediment level with tidal height changes. Indicated tide is that previous to rneasurement.

4-8

Daily Average Sediment Level Change vs. Tidal Height

02

- - +Change . Tidal Height

Tidal Height (m)

-1 - 8

D ~ Y

Figure 3.8 Daily average sediment level change vs. change in tidal height. Indicated tide is that previous to measurement.

Cornparisons of precipitation events with occurrences of erosion at the pins

displayed a différent result h m the tidal height investigation. It was found that the three

main erosion events identified earlier were each connecteci with a major d a 1 1 event

(Figure 3.9). Each event also caused erosion that was relative to the arnoun: of

precipitation that occurred. . Figure 3.10 indicates that there was a significant relationship

between the magnitudes of erosion and precipitation (confidence level = 95%). There

was a larger amount of average erosion within the channel when there was higher rainfall,

although the sediment level at every pin did not always demonstrate erosion with

precipitation. Some pins had the same sediment level or expenenced accumulation which

could have resulted fkom the addition of material by the previous tide, or the transport of

sediment down the channel slopes.

An analysis of the daily average sediment level variation with precipitation

showed that there was a significant loss of sediment following a rain event (Figure 3.9).

The cumulative average results showed an overall trend of erosion throughout the studied

channel section (Figure 3.11). The largest erosional events are seen as downward 'steps'

in the cumulative change. It can be seen that there is a generally slow accretion that

occurs between the major precipitation events, however, it would take a relatively long

period of time for the channel to regain what it had lost after the first rain event. If there

were no later events, this may be possible, but Figure 3.1 1 demonstrates the overall loss

that occurred as a result of subsequent precipitation.

Daily Average Sediment Level Change with Precipitation

Figure 3.9 Daily average sediment level change with precipitation previous to measurement.

Daily Average Sediment Erosion VS. Precipitation

O 10 20 30 40 50 60

Precipitation (mm)

Figure 3.10 Daily average sediment level change vs. precipitation previous to measurement.

Cumulative Average Sediment Level Change with Precipitation

2 - -

1.5 - -

-1.5 -- -2 --

-2.5 D ~ Y

Figure 3.1 1 Cumulative average sediment level change with precipitation previous to measurement.

59

Spatial Sediment Level Variations within the Channel

A study of the net change in sediment for each pin with respect to its elevation

above the channel bed showed that there was generally no relationship between the pin's

position above the channel thalweg and the loss of sediment at that location. An analysis

of the sediment level trends showed that there was a very weak relationship between

erosion and elevation at C 1 (Figure 3.12a), however, there was no such relationship at C2

(Figure 3.12b).

Plots of the measurements taken at Cl (Figure 3.13a and b) illustrated that over

tirne, more sediment eroded from the upper banks (pins 18,19,26, and 27), while there

was less erosion or some sediment accumulation closer to the bed of the channel (pins

21,22,23, and 24). This pattern of accretion occurrhg deeper within the channel was not

necessarily a result of the movernent of sediment fkom the upper banks to the bed because

the sediment loss near the upper banks did not always coincide with a gain of sediment

near the base of the creek.

BehKeen the two main lines of erosion pins, where pins were more randomly

installed, there were no consistent trends of erosion throughout the study period. As it

was mentioned, farther upstream, at the second line (C2) there did not appear to be a clear

trend of sediment loss (Figure 3 .13~ and d). Interestingly, for one pin in particular (pin 8)

there was a comparatively high amount of accretion over tirne and the precipitation

events did not have the same erosionai effect as that seen at the other locations. It was in

this position that there was the largest amount of vegetation, which slows water flow,

induces sedimentation, and protects the sediment surface nom irnpacting min drops. In

the other erosion pin locations, the splash of rain may loosen sediment particles, allowing

(a) Cl: Net Average Erosion vs. Elevation above Thalweg

3.5 ,

Carelatioci Coefficient = 0.674

50 100 150 200 EIevation (cm)

(b) CZ: Net Average Erosion vs. Elevation above Thalweg

O, t Corretatim Coefficient = -0.216

O 20 40 00 80 100 120 140 160 Elevation (cm)

Figure 3.12 Average net sediment level erosion vs. elevation fiom thalweg for erosion pins at (a) Cl and (b) C2 (confidence level = 95%).

(a) C l Right Bank

(b) C l Left Bank Ir

2

(c) C2 Right Bank n

2

E 1 -1

U - O +2 8 -1 + 3 8 -2

U *4

4 O 5 1 0 15 20 25 30 35

(d) C2 Left Bank 2 '-56

01 ,+6: Fg O 1 4 - 7 1 c s -2 U i*8 1

Figure 3.13 Cumulative change in sediment level for each erosion pin on the: (a) C l right bank; @) Cl left bank; (c) C2 right bank; and (d) C2 lefi bank. 'Right' and 'lefi' refer to channel banks when faang upstrearn (north).

62

them to be removed on the next tide or transporteci to another location within the channel

or onto the marsh d a c e .

LNUNDATION OF TEE SALTMARSH

As the tide nses at a rate of approximately 0.04 m min.", it floods the mudflats

and approaches the cliffed marsh margin. During this process, the tide may re-suspend

some of the h e sediment on the mudflats and cary it towards the marsh, although on

calm days this is more likely in the thalweg region where the flow velocity is higher. It is

expected that the majority of the suspended sediment was already present in the water

column before the onset of the flood

flows into Allen Creek. The water

Marsh, passing the reference station

The flooding water then fills the tidal

tide. Before the tide reaches the

then floods

as the flow

the main channel of

moves into the three

rnarsh margin, it

the Allen Creek

tributary creeks.

creeks and floods the marsh with a constant water-

level increase and there is often simultaneous wave activity.

The low marsh, closest to the margin, is slowly flooded even before bankfull by

water exceeding the main creek banks. Approximately 10-15 minutes before the water

level passes bankful in the study creek, it submerges the rnarsh arm, m e r flooding the

low marsh. As the water approaches bankfùll in the tributary channels, it extends towards

more shallow channels in the mid-marsh region that are branching fiom the three

tributary creeks. The topographie lows in the low marsh are submerged by this point and

the flow is moving into the mid and high marsh as sheetflow.

63

On a spring tide, the creek network will be completely submerged and the water

level will stop a few metres away fiom the boundary of the cliffed forest at high tide;

during the high spring tides, the water will reach this boundary. During a neap tide, the

channels will fill, however, bankfull will not be exceeded although the water may flood

areas of the low marsh d a c e closest to the main creek. The flood tide is approximately

2 hours in duration, followed by a 15-20 minute slack tide, then an ebb tide

approximately 1.5 hours long.

Once the main channel began to fill (or shortly thereafter), each experiment was

initiated, usually ending when the water level had fallen below the sensors of the

instruments in the study channel or at the reference station. The following pages display

and discuss the results of the hydrodynamics and sediment dynamics expenments that

were carried out fiom June 5 to June 19, 1997.

The environmental conditions that were experienced during the days these

experiments were carried out are shown in Table 3.4. Some of these charactenstics, such

as the wind conditions and precipitation events, will be referred to later in the results.

The quantitative data for wind speed and direction were determined by averaging the data

recorded fiom hKo hours before an experiment until ten minutes after the initiation of the

1s t m. The values before the beginning of the experiments were included because they

iniiuenced or generated the conditions experienced during each experiment.

Wind Direction (degr= from N)

Wave Directior in Basin

Tide Type D ~ Y

June 05

Junc O5

Junc 06

June 06

Time Maximum Tidal

Height (ml

Wind Spced

Wave Height in Basin (m)

Spring

Spring

Spring

Spring

moderate

moderate (4.2 mis)

moderate (4.2 m/s)

calm (0 mis)

NE

NE

E

SSE

S

S

S W

SE

Spring

S pring

ïransitional

ïransitional

Neap

Neap

June 08

Junc 18

June 09

June 19

June 13

June 17

Iow (3 -6 mis)

rnoderate (3.9 ds)

high (4.6 m/s)

rnoderate (4.0 mis)

modaate (3.8 m/s)

low

Table 3.4 Environmental conditions for the experiments.

65

VERTICAL ARlRAY EXPERIMENT

Four vertical anay experiments were conducted over June 5 and 6, 1997 during

spring tides. The anay was stationed at location C2 within the study creek and the

sensors were positioned at heights of 0.15 rn (Vl), 0.75 m (V2), and 1.50 m 013) above

the bed (Figure 2.5). The current meter located at V1 and the current meter and OBS

probe at V4 were not functioning. Table 3.5 s r n a r i z e s each experiment, including the

functioning sets of current meters and OBS probes for each experiment.

Experiment Date

June 05 @) June 05 (N) June 06 @) June 06 (N) kperiment T

- Functioning Instruments 1 EMCM (211 OBS (3) 1 PT (2) 1

Water Depth at C2 (m)

2.52 2.88 2.33 2.80

Time

tl:20 23:25 12:27 23:48

ight experiment (N). mes: Day experiment @) and b

Maximum Tide (m)

12.11 12.41 12.01 12.31

Table 3.5 Vertical array experiment schedule and characteristics. (EMCM = electro- magnetic current meter; PT = pressure transducer.)

Signal Fluctuations for Individaal Instruments

The pressure transducers, current meters, and OBS probes showed fluctuations

throughout each nui. Typical behaviour of these instruments is shown in Figure 3.14

which depicts the fluctuations of the instruments in the vertical array during the June 6

(N) experiment for a run near hi& tide. The pressure transducers located at C2 and CS

recorded a fluctuating signal throughout each run (Figure 3.14a). These fluctuations were

associated with the pressure changes, water motion, and wave activity that occurred

during the tidal cycle. Typically, there was also a trend throughout each run of an

(4 Pressure Transducer at C2

O 50 101) 150 MO 2% 300

Time (seconds)

(b) Current Meter at V3 (1.50 m)

Time (seconds)

(a Current Meter at V2 (0.75 m)

Time (seconds)

Figure 3.14 Raw data for the instruments in the vertical array: (a) pressure transducer; @) V3 cment meter; (c) V2 current rneteq (d) V3 OBS probe; (e) V2 OBS probe; and (f) VI OBS probe. SSC = suspended sediment concentration. Only the x- axis data are shown for each current meter.

(dl OBS Probe at V3 (1.50 m)

1 I 1 1

O sa 100 r s zm ~ S O 100

Tirne (seconds)

(el OBS Probe at V2 (0.75 m)

O 50 100 1 X, 200 250 300

Time (seconds)

(t) OBS Probe at V I (0.15 rn)

150 1 1 O 50 100 150 2(K] 250 300

Time (seconds)

68

increasing or decreasing mean, depending on whether the flood or ebb (respectively) was

being recorded.

The cumnt meter data (Figure 3.14b and c) show that the speed record along the

thalweg-paralle1 x-axis had a relatively consistent mean. There were two types of

fluctuations that occurred about the mean for each cunent meter. The first was a smaller

fluctuation that ranged fiom 0.010-0.035 m s". These changes were caused by mal1

waves that occuned during the experiments. This motion is seen as fluctuations

throughout the time series caused by the orbital mation of the waves that affected

velocities near the creek bed.

The second type of fluctuation was greater in magnitude (0.10-0.20 m s") and

there were two possible causes of this second type of fluctuation: pulses in the flow and

seiching. Pulses associated with the flooding and ebbing tides appear as 'spikes' in the

data record. Some of the oscillations are expected to have been caused by the occurrence

of seiching within Cumberland Basin, which in tum is seen in the saltrnarsh tidal creek

flows. These are seen in the record as an u p or downwards spike that has an opposite

counterpart that imrnediately succeeds the first.

The fluctuations shown by the OBS probes (Figure 3.14d-f) indicate the change in

the suspended sediment concentrations that occurred at 0.5 second intervals. These signal

fluctuations once again had a relatively consistent mean voltage throughout the run,

however, there were some instances when there was an overall trend of increase or

decrease in the voltage over an individual m. The same pattern of fluctuation was seen

when this occurred. Typically, there were no major events throughout the OBS probe

69

records (Le., events recorded over a number of seconds with an average exceeding two

times the value of the mean). The fluctuations seen throughout each nui were related to

those seen in the current meter record and were caused by random variations within the

suspended sediment concentrations.

Vertical Array Hydrodynamics

For the time series charts that will be shown in this chapter, the x-axis on the chart

represents the tirne in minutes before high tide (flood tide; negative values) and after high

tide (ebb tide; positive values). The zero minute mark along the x-axis represents the

approximate turning point of the tide or 'high tide', when the stage level was at its

maximum. The x- and y-axis speed data were resolved to produce the veiocity vectors.

The current meten at positions V2 and V3 on the array recorded similar flow

patterns (Figures 3.15b-3.18b). The highest tidal heights occurred dunng the night

experiments (1 2.3 1-12.41 m), however, the day-time experiments (1 2.0 1 - 12.1 1 m) had

similar velocities. Al1 of these experiments were classified as spring tides and had higher

tides than any of those recorded during the spatial array experiments. The mean velocity

was less than 0.10 m s*' for al1 of the experiments. There was a velocity of less than 0.05

rn s-' for the majority of the flood tide and this value rose to a peak of approximately

0.05-0.075 m s" at V2 and V3 on the ebb tide (Figure 3.19). Generally, similar velocities

were seen on the flood and ebb tides at both elevations.

The velocity magnitudes at each elevation above the bed showed a similar pattern

of a small increase in the velocity before high tide (Figures 3.15b-3.18b). This was

(a) Tidal Stage

-150 -la -80 -8Q 30 O 30 60 90 120 1 9

Time (min.)

(b) Velocity

p - 7

Time (min.)

(c) Suspended Sediment Concentration

-150 -120 90 80 30 O J) 60 90 120 150

Time (min.)

Figure 3.15 June 5 (day) vertical amiy data averages: (a) tidal stage above thalweg - BF = bankfidi elevation; @) velocity; and (c) suspended sediment concentration.

(a) Tidal Stage

-150 -120 -90 40 30 O 30 80 90 120 150

Time (min.)

(b) Velocity

Y..

Time (min.)

(c) Suspended Sediment Concentration

-150 -120 80 -60 30 O 30 60 90 120 15'0

Time (min.)

Figure 3.16 June 5 (night) vertical array data averages: (a) tidal stage above thalweg - BF = bankfûil elevation; (b) velocity; and (c) suspended sediment concentration.

(a) Tidal Stage

-150 -120 -90 ô0 30 O 30 6Q 90 120 150 Time (min.)

(b) Velocity

W..

Time (min.)

- -

( c ) Suspended Sediment Concentration

- 1 s -120 80 -60 30 O 30 80 90 120 150

Time (min.)

Figure 3.17 June 6 (day) vertical array data averages: (a) tidal stage above thalweg - BF = bankfiiii elevation; (b) velocity ; and (c) suspended sediment concent ration.

(a) Tidal Stage A C -*Y ,

(b) Velocity m 4 Y. 1

-- 4 V2

E U

g -1 -0- v3 - S

Time (min.)

(c) Suspended Sediment Concentration

-150 -120 -80 -60 3Q O 30 60 90 120 150

Time (min.)

Figure 3.18 June 6 (night) vertical array data averages: (a) tidd stage above thalweg - BF = bankfill elevation; @) velocity; and (c) suspended sediment concentration.

Position V2 (0.75 m) n 1

"-'l 0.075 ,

Y. l

Time (min.)

4 JnSn

Position V3 (1.50 m)

-. . T h e (min.)

+ JnSd

+ JnSn ' 4 J n M 1 : + Jn6n

Figure 3.19 Vertical array velocity averages for V2 and V3.

75

followed by a flow reversal in the seaward direction before high tide (in three of the four

experiments), which indicates that the ebb fiow had begun before the maximum elevation

of the tide was reached. This flow reversal ody appeared to occur early because the

water surface slope was not being measured; the direction of the tide could therefore

change before the maximum depth was reached at the pressure transducers. Following

hi& tide, there was an increase in velocity that led to a maximum value for the

experiments of 0.07-0.10 rn s", suosequently followed by another decrease in velocity

towards O m s-'. There was some indication of velocity asymmetry (occumng when

either the flood or the ebb current has a higher peak velocity). The instruments recorded

their maximum velocities at different times during the tide, however, the maximum was

reached during the ebb tide for most of the experiments.

In observing the change in velocity with change in elevation above the bed, it was

f o n d that the velocity differed relative to position on the array during different times

over the tidal cycle. It appears that at the beginning of each set of experiments, there was

a higher velocity closer to the bed (Figure 3.20). Within approximately 30 minutes

before high tide, when the instruments were submerged deeper within the water colurnn,

this changed and a higher velocity was recorded at V3. The patterns of velocity change

during the ebb tide did not show a clear pattern with respect to elevation above the bed

(Figure 3.21).

June 5 (D) Rune3 t - -42 min.

P i t l

Run 4 . - - min : ,

Y

- - -

June 6 (D) Run 3 -44 min.

Run 4 -26min.

Run 5 -10 min.

June 5 (N) Run 8 -7 min.

- 1

June 6 (N) Run 5 -71 min.

Run 6 -53 min.

Run 7 -34 min.

Run 8 -18 min.

Figure 3.20 Vertical array flood tide velocity patterns. Elevation above bed (m) vs. velocity (m s").

June 5 (D) Run 6 .. 22 min.

Y

'Run 7 42 min. 3-

Run 8 .)

60 min.

1

June 6 (0) Run 6 a

9 min.

E i T l

Run 7 29 min. 3 , 1

Run 8 45 min.

June 5 (N) June 6 (N) Run 10 Run11 a-

- ----

20 min.

z il 35 min.

- 1 gy-q - 1

O O O 0.05 O. 1 O 0.05 O. 1

(Ws) (mis)

Figure 3.21 Vertical array ebb tide velocity patterns. Elevation above bed (m) vs. velocity (m s").

Flow Direction

An analysis of the vector flow direction relative to noRh shows that the flow

occurx-ing at V2 showed a tum of the current direction fkom the flood to the ebb direction

(Figure 3.22). This pattern was generally parallel with the thalweg, although, as it was

mentioned, the turnllig of the tide appeared to begin a few minutes before hi& tide was

reached. The change in direction at this position aiways occurred towards the west. The

current meter at V3 followed a regular flood and ebb pattern towards and away fiom the

marsh and as high tide approached it also reversed in a westward tuming direction, or

counter-clockwise, as was noted for V2 (Figure 3.22).

Vertical Array Sediment Dynamics

The suspended sediment concentrations on the flood tide ofien began at a level of

300-450 mg P l near the bed, and decreased to approxirnately 150 mg 1-' or less by the end

of the ebb tide (Figures 3.15~-3.18~). At position V2 where the concentrations were the

lowest, the initial values were about 200-400 mg 1-', which were reduced to 50-150 mg 1-'

(excluding June 5 0).

The patterns of increase and decrease in the mean suspended sediment

concentration for one OBS probe position on the array was closely mimicked by the

patterns exhibited by the other probes (Figures 3.15~-3.18~). This indicates that the

suspended sediment concentration changed similarly throughout the water column. The

overall pattern seen throughout the experiments was a decrease in the suspended sediment

Figure 3.22 V2 and V3 flow direction vectors relative to north (O degrees). The vecton were derived using the mean values of the x- and y-axis records for each r u . OnIy the direction of flow is indicated, not the flow magnitude.

80

concentration over each tide (Figure 3.23), but for many of the experiments there was an

increase in suspended sediment just before high tide.

Plots of the sediment concentrations with respect to height above the bed

demonstrated that there was no consistent increase or decrease of suspended sediment

within the water colurnn (Figure 3.24). Mead, the patterns showed that the lowest

concentration was at V2, in the middle position, while there were higher and ofien similar

concentrations above and below this probe. Figure 3.24 only displays two experiments as

examples of this phenornenon, however, the pattern is seen in al1 of the experiments.

The middle position of the vertical array had the lowest suspended sediment

concentration in the channel over each tide. This pattern was generally consistent

regardless of any changes in the flow velocity or the elevation of the tide. It was also

seen in Figures 3.15~-3.18~ that V2 had an average suspended sediment concentration

that was almost always approximately 50 mg 1-' lower than the other positions. It is

suspected that the consistently lower sediment concentrations at V2 can be attributed to a

drift in the voltage output of the OBS probe that occurred between the time of data

collection in the field and when the calibration was performed in the laboratory. Because

the V1 and V3 positions often had similar suspended sediment concentrations and V2 had

a matching change in concentration throughout the tide, it is thought that the water

colurnn is well-mixed over most of the tide.

Position V I (O. 15 m)

-150 -120 90 -60 30 O 30 80 90 1 2 0 1 5 0 Time (min.)

Position V2 (0.75 rn)

- 1 9 -120 -a 80 30 O 30 60 90 120 1 5 0

Time (min.)

Position V3 (1 -50 m)

+ JnSd + Jn5n + Jn6d

-lm -1m -90 -60 30 O 30 60 90 120 150

Time (min.)

Figure 3.23 Vertical array suspendeci sediment concentration averages for V 1 -V3.

FLOOD TlDE June 5 (D) Run 2 -67 min.

Run 3 -42 min.

Run 4 -22 min.

June 5 (N) Run 6 4 1 min.

- 1

O O 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0

Run 8

EBB TlDE June 5 (0) Run 6 22 min.

Run 8 60 min.

Run 9 81 min.

June 5 (N) Run 10 #a

20 min.

- 1

O

Run II n

Figure 3.24 Verticai array: examples of the suspended sediment concentration patterns for the June 5 @) and June 5 (N) experiments. Elevation above bed (m) vs. suspended sediment concentrations (mg Tl).

SPATIAL ARRAY EXPERIMENT

This portion of the research involved measuring the spatial change of flow

velocity and suspended sediment concentrations that occurred along the selected

sdtrnarsh tidal creek. Four cross-sections were chosen along the study creek and these

cross-sections (or 'stations') were named Cl, C2, C3, and C4 (Figure 2.2). Their

locations along the thalweg of the study creek are shown in Figure 3.25. Once again, a

time of O minutes in the figures that display the velocity and suspended sediment

concentration averages indicates high tide.

A pair of instruments, composed of a current meter and an OBS probe, was

positioned at each station on an H - M e with the sensors of the instruments positioned

0.15 m above the channel bed to maximize the amount of data collected for each tide.

Measurements of flow velocity and suspended sediment concentrations were then

recorded over çpnng, transitional, and neap tides, as listed in Table 3.6.

Experiment (tide type)

1 - Sp. 2 - Sp. 3 - Tm. 4 - Tm. s -Np. 6 -Np.

Date - Time Maximum

Tide (m) Water ~ e p t h l ~ i c t i o n i n g ~nstruments

at C2 (m) 1 EMCM (5) Al1

CI, C4 AI1

None AI1 AI1

Table 3.6 Spatial array experiment schedule and characteristics. (EMCM = electro- rnagnetic current meter; PT = pressure transducer.)

Station Positions along the Study Creek and Main Creek Thalweg

Figure 3.25 Instniment station positions along the study creek and main creek thalweg profile, from C4 (study creek) to CS (main creek). Elevation in metres above datum NAD83.

Spatial Array Eydrodynamies

Before the data were analyzed, the vector was resolved between the x-axis and y-

axis speed data to detemine the actual velocity of the channel 80w. As done for the

vertical array data, the velocity values will be used to illustrate the results in this section,

except for the C3 current meter which only had one working axis. When compared to the

velocity vectors, it was found that the thalweg speed was very close to the velocity

magnitude (Figure 3.26). The velocity was usually less than 0.1 0 m s-' throughout the

study channel and less than 0.15 m s" within the main channel (CS). It must be noted

that the flow values closest to O m s-' could essentially be considered as O m 8' because

there may have been a small off-set value for each instrument. The same fluctuations as

those seen in the vertical array experiments were also seen in the spatial array

experiments. so an average velocity of O m s" does not indicate an absence of motion

within the water column.

Temporal Pnttents in the Flow Dynamics

For the first part of this analysis, the discussion of the results will focus on the

insîruments located within the study channel (Cl-C4), excluding the results of the

reference station (CS) in the main channel. The difference between bankfull at stations

Cl and CS was minimal. therefore, bankfull at Cl is indicated on the C5 tidal stage c w e .

The spatial anay experiments were carrieci out firom June 8 to June 19 and the figures are

in order of the highest spring to the lowest neap tide (1 1.9 1 - 1 1.1 1 m; Figures 3.2 7-3 .X).

During the spring tides, the velocity usually decreased as the slack tide

approached (Figures 3.2% and 3.28b). An analysis of the averaze flow values showed

(a) Study Creek Speed (x-axis) n a- V

-. . - Time (min.)

(b) Study Creek Velocity

Y- .Y

Time (min.)

Figure 3.26 Cornparison of the measured (a) x-axk speeds and (b) velocity vecton within the study creek (June 18 data).

(a) Tidal Stage a

-150 -120 -90 BO 30 O 30 60 90 120 150

Time (min.)

(b) Velocity

--. - Time (min.)

(c) Suspended Sediment Concentration

- 1 5 0 - 1 2 0 8 0 60 -30 O 30 80 90 120 133 Time (min.)

Figure 3.27 June 8 ( s p ~ g tide) spatial array data averages: (a) tidal stage above thalweg - BF = banldull elevation; @) velocity; and (c) suspended sediment concentration.

(a) TÏdal Stage

1 - ci j BFj

1 - - - - - l BF 1

-150 -120 8Q -60 30 O 30 80 90 120 150

Tirne (min.)

(b) Velocity

-- ." Time (min.)

(c) Suspended Sediment Concentration

-150 -120 80 4Q -30 O 30 60 90 120 150 Time (min.)

Figure 3.28 June 18 (spring tide) spatial array data averages: (a) tidal stage above thalweg - BF = bankfiill elevation; @) velocity; and (c) suspended sediment concentration.

(a) Tidal Stage

-150 -120 -90 -60 30 O 30 60 90 120 150 Time (min.)

(b) Velocity

--. - Time (min.)

(c) Suspended Sediment Concentration

-1s -120 -80 80 30 O 30 60 90 120 la, Time (min.)

Figure 3.29 June 9 (transitional tide) spatial array data averages: (a) tidai stage above thalweg - BF = bankm elevation; (b) velocity; and (c) suspended sediment concentration.

NO DATA

-

(b) Velocity

-. .- Time (min.)

NO DATA

Figure 3.30 June 19 (transitional tide) spatial array data averages: (a) tidal stage - no data; @) velocity; and (c) suspended sediment concentration - no data.

(a) Tidal Stage

-150 -120 -80 a 30 O 30 6D 90 120 150

Time (min,)

(b) Velocity

I n 4e W. .Y 1

Y. I "

Time (min.)

(c) Suspended Sediment Concentration

-150 -120 -80 -60 30 O 30 60 90 120 150

Time (min.)

Figure 3.31 June 13 (neap tide) spatial array data averages: (a) tidal stage above thalweg - BF = bankfiill elevation; @) velocity; and (c) suspended sediment concentration.

(a) Tïdai Stage

-150 -120 80 40 30 O 30 BO 90 120 1SO Time (min.)

(b) Velocity

-. . - Time (min.)

-150 -120 80 -60 30 O 30 Q 90 la) 15Q

Time (min.)

Figure 3.32 June 17 (neap tide) spatial array data averages: (a) tidal stage above thalweg - BF = bankfull elevation; @) velocity; and (c) suspended sediment concentration.

93

that there was evidence of a velocity pulse whai bankfull was exceeded. A velocity pulse

is the increase in velocity that occurs after the water elevation has exceeded the height of

the bank or a barrier, nich as a levee. The rate of the velocity hcrease with respect to

when bankfull was exceeded did not always appear to be immediate. This was, in part, a

function of the time intervals between nins and the fact that a topographie barrier was

absent which moderated the effect.

It was fond that at Cl, during the spring tides, the velocity decreased until

bankfull was reached, after which the velocity once again began to increase (Figures

3.2% and 3.28b). This was followed by a decrease just before slack water was reached.

M e r slack tide, the velocity increased then decreased until bankfull when the water

became confined in the channel and the velocity increased again as the tide receded. The

spring tides at C2 showed a less pronounced pattern, but there was a sirnilar effect. The

transitional tides did not show clear indications of a velocity pulse and dunng the neap

tides bankfiill was never exceeded.

The majority of the results for the transitional and neap tides showed the expected

pattern of a velocity decrease towards zero, before slack water, and an increase into the

ebb tide (Figures 3.29b-3.32b). The neap tides did not have as many fluctuations in the

velocity averages over the tide, but showed the expected decrease then increase in

velocity, before and after slack tide respectively.

The values of the maximum velocities reached during the tidal cycle varied with

tidal range - the spring tides had the greater values. Maximum velocities at Cl occurred

at the beguining of the experiments as the channels were first flooded, however, the

94

maximum velocities at C2 were experienced a few nuis after they had been submerged,

approximately haif-way between flood initiation and high water (Figures 3.27b and

3.28b). The flood currents typically had higher maximum velocities than the ebb currents

and these maximums often occurred early in the flood tide (the average velocity values

varied with each tide). The vertical array curent meters showed an opposing velocity

asymmetry (stronger ebb fiows), indicating a varying velocity asymmetry pattern in the

creek, however, those measurements were made higher in the water column. A

generalization cannot be made with respect to velocity asymmetry and depth because it

was found that there was no consistent pattem of velocity related to height in the water

column, especially on the ebb tide.

Spring, Transitional, and Neap Tides

The velocity analysis for the spring and transitional flows indicated that there was

flow during slack tide beyond the instantaneous fluctuations rnentioned earlier. This

apparent velocity was potentially affected by wind stress which may have contributed to

this flow during high tide. The flow in the main creek which could have been affected by

that in Allen Creek may have also been a factor.

The flow magnitude within the channels varied, depending upon the type of tide

and the average velocity variation. Throughout the tidal cycle the degree of variation

diminished nom spring to neap tides. Neap flow values were consistently less then 0.05

m S-', and the majority of the runs within a tidal cycle were 0.03 m s-' or less (Figure

3.31b and 3.32b). The transitionai tide had an intemediate velocity that was usually

withui the 0.05 m s-l range, while the spring velocity values were the highest (and most

varying), exceeding 0.05 m s-' and at times peaking to 0.12 m d. This pattern of increase

95

in velocity with increased tidal height was noticed when examining the individual charts.

A more detailed analysis which involved calculating correlation coefficients was

performed to measure the strength of the relationships between the average velocity for

an experiment and the tidal height and the maximum velocity for an experiment and the

tidal height, for stations Cl, C2, and C4. The results of this analysis indicated that the

relationships between those variables were not significant throughout the creek.

Flow Pattern Variation with Location

The velocity within the study channel was relatively low and, on average, less

than 0.05 m s-'. The instruments located at Cl were closest to the mouth of the study

channel and this station was subject to a flow that had experienced less darnpening than

the other stations (Figure 3.33a). C l typically had a greater amount of average flow

variation and had slightly higher flow velocities throughout an experirnent, wvhile C2 had

a lower range of variation and a lower average velocity during a tidal cycle (Figure

3 -33 b).

There was a much lower speed recorded at C3 (only one operational axis) which

was close to O m s*' throughout the tidal cycle (Figure 3 .33~) . The speed at C3 was very

low because the station had a deep, but narrow cross-section and as the tide rose the total

volume of space into which the fiow could expand was quite small in cornparison with

the space at other locations dong the channel. M e r the flow exceeded bankfull the

volume increased, however, most of the flow occtxred above the level of the channel and

the vegetation affecteci the fiow by sheltering the channel, effectively dampening the flow

within the cross-section. The results measured at this station illustrated how the flow

i Time (min.) l

I I n * P

W.. "

Time (min.) I

-- Y. v u

Time (min.)

Y. I V

Time (min.)

Figure 3.33 Spatial array velocity averages for stations: (a) C 1, (b) C2, (c) C3 (speed), and (d) C4.

97

could be dampend as it became more confinecl, especially in light of such low initial

velocities.

The last cross-section (C4) had a slight increase in the positive, flood-direction

velocity after it had been submerged, which was then followed by a decrease to O m s-' at

slack tide. Following high water, the velocity direction reversed and began to increase in

the ebb direction until it emerged from the receding sheet of water, as the flow had not

yet been completely confined to the channel network. This station expenenced a

relatively large range of flow velocities, although it was only submerged for a relatively

short period of time. The C4 curent meter showed somewhat of a different flow pattern

than that seen at the other stations, but one more consistent with the results of similar

studies. This station was not submerged on the neap tide, however, during the

transitional and spring tides, a common pattern emerged (Figure 3.33d).

This station was located in the upper mid-marsh region in a more exposed

location where there were low banks and vegetation in the channel. As a result of the

different environment in which the station was located, it experienced more of the Full,

unrestricted force of the tide and a larger range of 80w velocities than at C 1. This station

also became a shallow conduit for flow retreating kom the upper marsh regions, and

received flow fiom other srnalier tributaries.

Reference Station Hydrodynamics

The results thus far have focused on the spatial variation of flow dynamics within

the study channel. The reference station in the main channel warrants separate attention

because the CS current rneter experienced different fiow dynamics.

The spring tide experiments showed that the flood period began with a velocity

that decreased until the main channel reached banWull (approximately 10 minutes before

bankfull in the study channel). Once bankfull was reached, the flow increased in velocity

until approximately slack tide after which the velocity slowly began to decrease (Figure

3.34a). Once the flow was again confhed to the main channel, the velocity began to

increase again in the ebb direction, as the full influence of gravity took effect. This last

portion of the ebb tide experienced a strong seaward drainage flow. The transitional and

neap tides showed a similar pattern to that of the spring flood tides, but had a slightly

different ebb tide pattern. The transitional and neap tides illustrated the same initial

decrease, then increase in velocity towards high tide, however, the pattern differed fiom

that of the spring tide after slack water (Figures 3.34b and c). During, or soon after high

tide, the velocity decreased a small amount, then instead of m e r decreasing towards O

m s*', it once again increased until the instrument was no longer submerged.

Reference Station: X- and Y-Rris Patterns

The reference station current meter had one axis parallel and one axis

perpendicular to the thalweg (across the channel). This channel was bounded by banks

sloping towards the sdtmanh on the north bank and the ami or extension of the salûnanh

on the south bank. This rnarsh a m essentially separateci the creek network fiom the tidal

(a) Spring Tide Velocities I W.* I

A CI

I 1

ri& (min.)

(b) Transitional Tide Velocities

~ i & i (min.)

(c) Neap Tide Velocity

- - Yi6

Time (min.)

Figure 3.34 Reference station velocity averages for: (a) spring tide; (b) transitional tide; and (c) neap tide.

100

flats and protected the marsh h m direct exposure to the wind and waves in the basin

until the water level submerged this arm completely.

As the tide began to flood the main channel, this creek acted as the sole water

source for the creek network. Once the arm had become submerged, the main channel no

longer played a key role in supplying flow to its tributaries. (The thalweg current ofien

exceeded 0.10 m s" though, indicating that there was still notable flow within the

channel.) Once the immersion of the rnarsh a m occurred, there was a shift in the source

of the flow fkom the main channel to the tidal flats (directly fiom the basin). This shift

was represented in the data record as a decrease in speed approaching O m S-',

approximately an hour before high tide, followed by an increase in speed that was

coincident with a change in the flow direction (from a positive to a negative speed or vice

verso; see arrows, Figure 3.35). As the marsh arm was submerged, there was a reversal

of the cross-channel signal, from a negative offshore current to a positive onshore current.

This onshore component is indicative of the tidd flats beyond the marsh arm becoming

the main source of flow for the channels, however, there was no evidence of this change

in the study channel flow pattern.

Spatial Array Sediment Dynamics

The suspended sediment concentrations for each tide were in the range of

approximately 50-300 mg 1". It must be remexzbered that the OBS probes were located

0.15 rn above the bed, therefore, the rnajority of the rneasurements were taken near the

(a) Spring Tide Speeds n7

Time (min.)

(b) Transition Tide Speeds

Tirne (min.)

(c) Neap Tide Speeds n 7

I 0.15 -

Figure 3.35 Reference station speed averages for the thalweg-parallel x-axis and the thalweg perpendicular y-axis on a: (a) spring tide - June 18; (b) transitional tide - June 9; and (c) neap tide - June 13. Arrows indicate approximate time when bankfull was reached,

A O - E w

u g e V)

-150 -120 -90

Time (min.)

102

bottom of the water column. In light of the vertical array results, it may be assumed that

these values were relatively close to those within the rest of the water column.

Temporal Sediment Dynamicr

There was a general decreasing trend of approximately 50-75 mg P' for most of

the stations and most of the experirnents throughout the research period. It was not

unconunon, however, for there to be occasional fluctuations throughout an experiment or

a station that experienced a trend that opposes this generalization during one or two

experiments. The initial concentrations moving through the channel were usually the

maximum values measured for the tide. During most of the expenments, there was a

decrease in the suspended sediment concentration fiom the early flood tide towards high

tide, although it was not uncornmon for there to be a small increase just before slack

water, as was seen during the vertical array expenments (Figures 3.27~-3.32~).

Following the high tide, there was another decrease in the suspended sediment

concentration, however, there was often an increase in the concentration one or hvo nins

before the ùistruments emerged fiom the receding tide.

Spring, Transitional, and Neap Tides

An d y s i s of the transitional and neap tide expenments showed a tendency

towards an increase in the variation of sediment in suspension following high tide

(Figures 3.29~-3.32~). This could have been the effect of a sediment plume moving

down the channel. On the ebb tide, it was observed that there were mal1 sediment

plumes within the creeks. These plumes were concentrations of sediment that were lifted

by the cumnt from the creek banks as the water level lowered. It is possible that this

103

fine-grained sediment was deposited previously by the fiood tide. It could also have been

the result of marsh d a c e drainage d o m the banks.

n e r e was no apparent relationship between the suspended sediment

concentration and the type of tide, however, the number of experiments was smail and a

longer study may show such a relationship. The concentrations varied between the two

spring experiments and the average concentrations during those expenments were

approximately the same as those measured during the neap tide experiments. Al1 of these

experiments had relatively lower concentrations than the values recorded during the

transitional tide (Figure 3.29) and most of the concentrations were not as high as those

seen during the vertical array experiments. The precipitation data for these dates (Table

3.4) indicate that there was also no obvious co~ect ion between increased suspended

sediment concentrations within the creek and rain events.

An andysis of the wind conditions for the five days over which the suspended

sediment concentrations of the spatial array experiment were measured showed that there

appeared to be a relationship between the relative level of measured sediment in the water

and the wind conditions (Table 3.4). On June 9, the highest wind and wave activity was

experienced for the spatial array experimentation penod. The sediment levels on this day

were generaily high at each station for most of the runs, compared to the other spatial

array experiments. High winds causing heightened wave activity and more turbulent

water conditions could have suspended an increased amount of sedirnent. As the flood

tide entered the marsh system, it then introduced a higher sedirnent concentration to the

creek network and marsh surface.

104

Moderate winds during the vertical array experiments on the spring tides also

brought relatively high concentrations of suspended sediment into the study creek that

were comparable to those of June 9. The conmion factor s e a s to be the wind direction

which was northerly for al1 of the vertical array experiments (northeast or northwest).

The lower concentrations of the remainine experiments were measured during southem or

southwesterly winds.

Suspended Sediment Concentration Variation With Location

The C3 OBS probe could not be calibrated and the trends in the recorded voltages

occurring at C3 were inconsistent fkom experiment to experiment and compared to those

at the other stations. Stations C 1 and C2 showed similar patterns of a general decrease in

suspended sediment throughout a tide. There were no increases or decreases in the

sediment concentration patterns with respect to location along the channel (Figure 3.36).

A somewhat different pattern fiorn that of C l and C2 emerged at the vegetated C4

station for two of the three dates when the tide reached the array. For the June 18 spnng

and June 9 transitional tides, there was an increase in the suspended sediment

concentration towards high tide, then before slack water was reached a decrease began

which continued into the ebb tide. It must be remembered that the actual suspended

sediment concentrations rnay have been 50 mg 1-' higher than those shown for C4. The

OBS probe at this location was the same probe that was used at V2 and the vertical a m y

experiment results speculated that the output for this instrument may have changed so

that the actual values within the channel were higher than those reported in this chapter.

(a) Station C l

(b) Station C2

- - 1 - 1 -90 a 30 O 30 60 90 120 1so

Time (min.)

(c) Station C4

-150 -120 -90 60 -30 O 30 60 90 la0 150

Time (min.)

Figure 3.36 Spatial array suspended sediment concent ration (S SC) averages for stations: (a) Cl, (b) C2, and (c) C4.

106

Velocity and Suspended Sediment Concentrations

It is commonly found that an increase in the speed of a flow will result in an

increase in the amount of suspended material that the flow can carry. An analysis

comparing the flow patterns with the suspended sediment pattems showed that this was

not always the case in the study creek (Figure 3.37). While some of the experiments

showed that there was a similar trend between the velocity and the suspended sediment

concentration, others showed no similarity at al1 between the increases and decreases of

the two parameters,

(a) June 6 (N) (b) June 8

I -1 20 -60 O 60 120 i

Tirne (min.) !

4 -1 20 -60 O 60 120 / , Time (min.) ! I

1 -120 60 O 60 120 Time (min.)

Ln - -120 -60 O 60 120

I Tirne (min.)

Figure 3.37 Cornparisons between average velocity and suspended sediment concentration (SSC) for (a) June 6 (N) and @) June 8. Positive velocity magnitudes represent the flood und ebb tide flows.

108

CHAPTER IV

DISCUSSION, RESEARCH OPPORTZTNITIES, AND CONCLUSIONS

INTRODUCTION

This research involved the measurement of the hydrodynamics and the sediment

dynamics within the study creek in the Allen Creek Manh in accordance with the

objectives that were outlined in the hnt chapter. The morphological characteristics of the

study creek and its network were detemined and the flow velocities and suspended

sediment concentrations in the creek were measured. These results were analyzed with

respect to the temporal and spatial changes that occurred in the channel flow over spring,

transitional, and neap tidal cycles.

One of the objectives that remains to be fûlfilled is a cornparison of the results of

this research to the results of other studies that have been perfonned in saltmarsh tidal

creeks of microtidal, mesotidal, and macrotidal ranges. This is important because it puts

the findings of this study in context with those previously reported in the literature,

including those of other studies in a hi&-macrotidal envirorment. This discussion

considers the sediment dynamics and the hydrodynamics that were measured, how these

results compare with those found in other saltmarsh creek studies, and what their

implications are on ideas about the erosion and evolution of saltmarsh tidal creek

netwo rks .

SEDIMENT DYNAMICS

It was originally expected that there would be a decrease in the suspended

sediment concentration over the tidal cycle and that the concentration would Vary with

change in velocity. It was also expected that the higher velocities would entrain and

transport an increased amount of sediment throughout the creeks. The results have shown

that there was indeed a general trend of decrease over the tidal cycle, but there was no

relationship between the concentration of suspended sediment and the velocity in the

creek. There was also no clear pattern of suspended sediment increase or decrease with

distance fkom the mouth of the study channel. The initial suspended sediment

concentration was dependent upon extemal controls, outside of the marsh, within

Cumberland Basin (e.g., the wind).

For the majority of this discussion about the sediment dynamics, the focus will be

on the data for Cl and C2. These two stations better represent the creek concentrations

because they are located within unvegetated, relatively deeper channel sections and are

therefore providing Somation about the overall changes that occur before and after the

tide has covered the marsh. C4 is in a much shallower and vegetated channel that is more

representative of a marsh surface environment.

The overall trend seen throughout the experiments was a decrease in suspended

sediment concentrations over the tidal cycle (Figure 4.1). There was a decrease of

approximately 50-75 mg 1-' fkom the maximum on the flood tide to concentrations

measured near the end of the ebb tide at stations Cl and C2 (before any increase during

the last run). As the tide increased, the incoming concentrations decreased because the

Average Suspended Sediment Concentrations

-1 00 -50 O 50 100

Average Oh of Measuted Tide (before and after high tide)

Figure 4.1 -4verage suspended sediment concentration patterns for stations Cl and C2 (excluding C 1 data for lune 9).

11 1

Bats were then deeper beneaîh the water surface. making them less exposed to wave

energy or habulence associated with the floodhg, thereby contributing less to the

suspended sediment load. After high tide, the s a h g of sediment became a factor in the

decreasing concentrations as sediment fell to the marsh and tidal creek surfaces during

penods of lower velocity. The mean velocity measured at station C4 at high water was O

m s", which would facilitate the settling of suspended sediment, in addition to the

ba.£fiïng effect of the vegeîation on flow velocities throughout the marsh. Sediment may

also have been trapped by plant surfaces as it moved through the vegetation canopy.

The concentration pattern at C4 showed a trend different from that seen at the

other stations. An increase towards high tide occurred that could have been caused by the

rapid settling of sediment just before high tide which was not re-suspended following

slack water (accounting for the following decrease). Sediment deposited on the manh

surface is not usually re-suspended as a result of very low velocities within the vegetation

canopy. however, a proportion of the sediment that was deposited on vegetation surfaces

is more commonly re-suspended and transported on the ebb tide (Leonard et al., 199Sa).

The increase before slack water was also seen at Cl and C2 as well as high suspended

sediment concentrations at the beginning of the flood tide that were caused by the initial

channel wetting when the velocities were usually the highest (French et al., 1993).

The concentration increase observed at the end of the tidal cycle c m be attributed

to a few factors. Near the end of the ebb tide the probe was being re-introduced to the

upper portion of the water column and there was also drainage fiom the marsh surface

which could include sediment that had been re-suspended. This caused higher

112

concentrations near the end of the ebb, as smaller ûibutary contributions accumulated and

the cross-sectional area drainhg the channels became increasingly reduced Surface

wave activity was also being re-introduced to the lower portions of the channel banks,

which could have disrupted the channel d a c e and re-suspended some settled sediment,

contributhg to the concentrations (French and Stoddart, 1992).

A relationship between velocity and suspended sediment concentrations was

found in some saltmarsh creek studies (Ward, 1981; Wang et al., 1993; Leonard et al.,

199%). This study observed no relationship between velocity and suspended sediment

concentration and two of the mamotidal studies to which these results were compared

found a similar result (French and Stoddart, 1992; French et a l , 1993). A relationship of

this type could be expected as a result of the increased bed shear forces that are associated

with higher velocities and can increase erosion. Increased velocities would also increase

the potential for maintaining more sediment of a larger range of grain sizes in suspension.

Al1 of the measured velocities in this study were too 1ow to be expected to actively erode

sedirnent, therefore, it was not expected that there would be a relationship between

velocity and suspended sedirnent concentration.

The initial suspended sedirnent concentrations may not have been controlled by

velocity then, but by the tidal stage. It was found that as the flood tide progressed, there

were decreased sediment concentrations because the initial stirring effect that increased

the early flood tide concentrations had ended. As high water approached and the tidal

height increased, the mudfiats were deeper beneath the water surface, adding less

sediment to the flow, appearing as a concentration decrease towards high tide. Following

high tide, a M e r decrease was caused by sediment settling or trapping. The preceding

113

discussion descnied the concentration pattern associated with tidal stage. Velocity

patterns were also found to be associated with different stages during the tidal cycle. A

closer andysis linking velocity and suspended sediment concentration may have therefore

exhibited a misleadhg relationship because the two parameten should really be linked

separately to the greater control factor, tidal stage.

The lack of a relationship between velocity and suspended sediment concentration

uidicates that the majority of the suspended sediment in the flow is extemally derived.

Consequently, the horizontal advection of sediment is an important process of sediment

transport for the studied marsh, while the entrainment of channel sediment does not

supply a significant amount of sedirnent to the water column (French and Stoddart, 1992;

French et al., 1993).

There were no clear trends with respect to the suspended sediment concentration

and location along the channel. Cl and C2 typically had similar concentrations on the

flood tide and on the ebb tide one station usually had a higher concentration, but this

station varied among experiments. It would be expected that since there are well-mixed

concentrations throughout the water column and the stations are relatively close, that

there would be little difference between the Cl and C2 suspended sediment Ievels.

The trends at C4 illustrate the loss of sediment within the water coIumn and

indicate that the settling of sediment occurs before hi& tide, as has been found to occur

in other studies (French and Stoddart, 1992; French et al., 1993). As seen in the results, it

was only at this location that there was a consistent pattem of a velocity decrease towards

O m s-' into slack water, so that there would be an environment more conducive to

114

settling. This is also more representative of the sediment dynamics occurrïng higher in

the marsh where vegetation plays a much greater d e . Even if the sediment did not

directly settle onto the marsh, it may have appeared so as the sediment slowly settled past

the OBS probe to a greater depth within the water column.

The sediment concentration averages ranged from 50 to 300 mg lm' during the

experiments. Suspended sediment values are dependent upon the sediment source and

whether or not the marsh relies on the horizontal advection of sediment nom the basin to

the creek network. Some studies have found that increased suspended sediment

concentrations were associated with higher tides, indicating tidal height as a control on

sediment import to the rnarsh. This research found that a spring tide had the same

P

approximate sediment concentration as a neap tide, therefore, an alternative control on

sediment transport was sought.

A possible factor controlling suspended sediment concentrations was the wind.

As it was mentioned, high winds are expected to increase wave activity across a tidal Bat,

thereby enhancing the scouring action of waves on the mudflats and increasing the

sediment suspended w i t h the flow as the flood tide approaches the manh. The results

of this study found elevated sediment levels during a day that had reiatively high or

moderate winds ftom the northeast or northwest. This made wind speed and direction

factors which influenced the suspended sediment concentrations in the study creek. It is

expected that a strong landward wind blowing in the marsh direction would contribute a

larger amount of sediment on the tide. Wang et al. (1993) detennined that a strong

landward wind could increase the suspended sediment within a bay, increasing the

sediment transportecl ont0 the adjacent marsh The highest sediment concentrations in

115

this study were measured when the wind waç h m northerly directions, however, there

were no experimaits perfomed when there was a high marshward (southerly) wind to

which the other results could be compared.

These r d t s contribute to the fïndings of other studies that have concluded that

wind is a controllhg factor in sediment transport in saltmarsh tidal creeks (Ward, 1979;

Wang et al., 1993; Leonard et ai., 199%). The results of the study by Leonard et al.

(199%) show that when the winds were higher than fair-weather wind speeds, there was

an increase by up to three orders of magnitude in the total suspended sediment load. It

has also been found that there are not always increased velocities with higher wind

speeds, however, the wave activity can still have an effect by increasing the stress at the

channe1 bed (Leonard et al., 1995a).

COMPARISONS AMONG TIDAL RANGES

The results of a number of saltmarsh tidal creek studies with varying tidal ranges

were compared to the hdings of this research in an attempt to determine the sirnilarities

or differences between marshes of different hdal range classifications: microtidal (0-2

m), mesotidal (2-4 m), low-macrotidal (4-6 m), and hi&-macrotidal (>6 m). These

studies and some of their findings are Iisted in Table 4.1, which includes some of the

main characteristics of each marsh. Overall, it was found that there were similar velocity

patterns for al l of the marsh creeks, although the magnitude of the velocities varied. The

magnitudes were found to be somewhat based on the tidal range of the marsh, however, it

was detennined that the extemal environment and local manh characteristics are key to

the saltmarsh creek hydrodynamics. These characteristics also infiuence the suspended

Spring Tide (rn)

Approximate Creek Length

(m)

Refcrcncc Approximatc Creek

Dimensions (width x depth)

Marsh Arca

(km2)

Velocity As ymme t ry

Maximum Veloci ty

W s )

Wang et al . (1993)

Leonard et a l . ( 1995)

Ward (19811

Bayliss-Smith et al . ( 1979) 1 5-6

Low-Macrotidal

Low-Macrotidal

High-Macrotidnl

High-Macrotidal

High-Macrotidal

Green ei a l . (1986)

Reed ( 1988) Varies

French and Stoddart ( 1992) 1 6.4

French et nl . ( 1993)

Allen Creek Marsh Siudy 11-12 Varies

Table 4.1 Cornparison references for discussion. Creek dimensions list larges! cross-section if multiple locations were studied. When creek lengtlis were not stated, they were roughly approximated from the provided maps.

117

sediment concentrations in the creek network and the source of sedirnent that is being

transported, be it extemal or the creek itself.

Regardless of tidal range, al! of the marsh creeks generally experienced similar

velocity patterns, such as velocity asymmetry, velocity pulses, and patterns of increase

and decrease throughout the tidal cycle. This does not mean that there were not

exceptions, but that the exceptions appeared to result h m differing local rnanh

environments rather than tidal range-specific characteristics. Maximum velocities were

similar between the mesotidal and macrotidal marsh creeks (except for this research),

reaching peak velocities of approximately 0.80 m s-' (Bayliss-Smith et al-, 1979; Ward,

1981; Green et al., 1986; Reed, 1988; French and Stoddart, 1992; French et al., 1993).

The microtidal marshes had lower peak flows of approxirnately 0.40 m s-' (Wang et al.,

1993; Leonard et al., 1995b). Ward (1981), Wang et al., (1993), and Leonard et al.

(1995b) stated in each of their studies that increased tidal heights were accompanied by

increased flow velocities, which was ako found in this research.

In light of the findings of this study, one could conclude that the channel

velocities do not increase with an increase in the tidal range of the marsh. This study had

the lowest maximum velocity of approximately 0.10 m s", although it had the highest

tidal range. The crucial difference did not appear to be tidal range, but marsh and creek

network dimensions. This leads to the deduction that local marsh characteristics play a

key role in creek hydrodynamics, which was also suggested in the work of Leonard et al.

(1 995b). This finding is reinforced by a cornparison of the hydrodynamics results of this

study with those of other high-macrotidal marsh creek studies, which showed that creeks

in marshes with the same tidal range classification have experienced different dynamics.

118

The micro- and mesotidal range research stated that the suspended sediment

concentrations increased with an increase in velocity and tidal range. The macrotidal

shidies o d y agreed that there was an increased suspended sediment concentration when

the tides exceeded bankfull. Based on previous discussion, it cm be said that when

suspended sediment concentrations are affectecl by velocity, there is an increased

importance on local channel sediment entrainment within the creek flow. Although two

studies (French and Stoddart, 1992; French et al., 1993) found that increased tides

brought increased suspended sediment concentrations, they did not find a relationship

between velocity and sediment concentrations and stated that the horizontal advection of

extemal sediment on the tide was more important.

Al1 of these findings lead to the conclusion that the suspended sediment trend over

a tidal cycle is dependent upon tidal stage or channel hydrodynarnics and local sediment

conditions (including the initial concentration of suspended sediment). The type of

sediment within the channel would also affect the potential for sediment suspension on

any of the tides. Clearly, the dynamics of flow and sediment transport within the

salhnarsh tidal creeks are unique to each marsh based on its surroundings, both on a small

and broader scale.

NYDRODYNAMICS

Based on previous literature, it was expected that the patterns of fiow over a tidal

cycle would generally decrease in velocity towards high tide, when it would reach O m s-',

then increase throughout the ebb tide as the marsh drained into the tidal creeks. Figure

4.2 indicates that this generally occurred, however, the velocity did not typically decrease

Average Velocity Patterns

Average % of Measured Tide (before and after high tide)

Figure 4.2 Average velocity patterns for stations C 1, C2, and C4.

120

to O m s*' until &er the maximum tidal stage had passed and the expected pattern was

only clearly seen at C4, the shallow upper mid-marsh extension of the study creek. The

figure also indicates that the highest velocity in the creeks for a tide was usually

experienced near the beginning of the flood tide.

Expected patterns that were seen to some degree in this research were velocity

asyrnmetry and velocity pulses, but they were more prominent in the results of other

shidies. These patterns are not evident in Figure 4.2 as a result of the loss of detail that

occurs during the averaging process. In this study, the higher velocities were more

common during the spring tides. A higher variation in flow magnitude was seen at the

mouth of the study creek (Cl) and this variation diminished moving landward along the

creek. These, and the aforementioned hydrodynarnics results, will be addressed

following a bnef discussion about the flow direction changes that were measured during

the vertical array experiment.

Flow Directions

The results depicting the direction of flow during the vertical array experiment

showed that the change in direction fiom the flood to the ebb tide occurred in a westsvard,

or counter-clockwise direction. This tuniing effect corresponds with a larger-scale

circulation pattern that was observed within the basin fiom the platform on the marsh.

This circulation pattern showed that there was a flood cment Bow along the Nova

Scotian coast of Cumberland Basin as the flood began and an ebb tide retum circulation

towards the mouth of the basin dong the coast of New Brunswick.

121

This finding of a circulation pattem matches that which has been found previously

within the Bay of Fundy, where there is a regional tidal circulation in a counter-clockwise

direction, flooding the shore of Nova Scotia and ebbing along New Brunswick's shore

(Amos and Long, 1980). Within Chignecto Bay there is a net transport of sediment that

moves headward along the bay rnargins, and seaward through the centre (Amos, 1987).

When looking at the shoreline of Chignecto Bay and Cumberland Basin, it is evident that

a counter-clockwise circulation within the basin is possible as the Bow moves headward

along Nova Scotia's coast, then seaward along the New Brunswick coast and d o m the

centre of Chignecto Bay. Through the record of this turning pattern, the results of this

study have s h o w that the flow within the marsh creeks is affected by the general

circulation pattem of Cumberland Basin.

Flow Velocities and Patterns

Previous research has shown an increase in velocity with an increase in tidal

height (e.g., Bayliss-Smith et al., 1979; Ward, 1981). This Allen Creek Marsh study

concurs with that finding and has also shown that there was a decrease in the velocity

variation throughout the tidal cycle with increasing landward distance kom the creek

mouth. The flow velocities in this research were low, even compared to microtidal study

results, however, this reinforces the determination that saltrnarsh creek hydrodynamics

are dependent upon marsh topography and creek network characteristics. These low

flows are notable because they have implications on ideas about channel evolution.

These implications will be discussed following a more complete review of the

hydrodynamics measured in the study channel.

122

The finding of higher flows on higher tides was common among studies of this

type (e.g., Bayliss-Smith et al., 1979, Ward, 1% 1; Leonard et al., 1995b) although, for

this research the difference was only approximately 0.05 m s-' between the spring and

neap tides. Once again, this difference is relative to the maximum tide velocities. If the

fiow was almost an order of magnitude higher (as rneasured in other rnarsh creeks), then a

greater difference would be expected.

The rneasured flow over a tidal cycle initially showed a high velocity resulting

fiom a flow surge that often occurs at the beginning of the flood tide (Bayliss-Smith et

al., 1979). There was then a velocity decrease until bankfûll, followed by an increase

once the marsh level was exceeded dong the channel. This velocity increase, or pulse,

occurred once the flow exceeded bankfirll because the flow of water was no longer

restncted to the channel and could flow more fkeely across the m m h surface. A similar

increase occui-ied on the ebb tide once the flow becarne confined to the creeks. Such

pulses are not unusual, however, previous studies have found a more pronounced pulsing

effect (e.g., Green et al., 1986; French and Stoddart, 1992). A major factor that has been

found to affect the position and strength of a velocity pulse is the branching channel

network (Pethick, 1980). Creek network size and rnorphology changes fiom one rnarsh

to the next, therefore, the magnitude of the velocity pulse also changes.

It is likely that the more pronounced pulses that were recorded in other saltmarsh

creeks also resulted fkom higher flow velocities and are possibly a result of a higher

degree of surface topography than that seen on the Allen Creek Marsh. If there were

levees dong the study channel like those in other marshes, they could have provided a

greater obstacle to the flow. This would have resulted in an increased rushing of water or

123

pulsing effect in the data record, once the flow had exceeded the maximum elevation of

the levees.

This study found that the higher velocity typically occurred on the flood tide,

often near the omet of the flood in the creeks. Bayliss Smith et al. (1979) and Leonard et

al. (199%) also found higher flood tide velocities, but the Iiterature States many cases of

stroager ebb velocities (e.g., Ward, 198 1; Green et al., 1986; French and Stoddart 1992;

French et al., 1993). This asymmetry would Vary depending upon marsh topography,

tidal conditions which affect water surface slopes, and creek network characteristics; a

network with steeper creek bed slopes could also have higher creek drainage velocities.

n ie key hding with respect to location dong the study creek was that the farther

landward the instrument station was positioned, the more stable (or Iess varying) the

velocities were over a tidal cycle. Flows were also slightly darnpened as the water moved

through the creek (Cl to C2) and becarne more reduced as the creek became nairower

(C3). Bayliss-Smith et al. (1979) similarly found that the magnitude of the velocity

maximums decreased landwards. The C4 station experienced a large magnitude of

variation, as it was more exposed on the manh surface.

It must be remembered that in this study, the distance fiom Cl to C2 was only 40

rn and it was a relatively straight section of channel with quite Iow flows, so one would

not expect a great difference in velocity between the two stations. An implication of the

Gnding of a velocity decrease with increased proximity to the high marsh (Cl to C3) is

that even if the initial flood velocity had the capacity to erode the channels, such potential

could be diminished by the time the flow reached the mid marsh. Since the creek heads

124

are usudy in highiy vegetated regions, increased power would be necessary for erosion

to occur,

The results of this sîudy have shown that the average creek velocity throughout

the tidal cycle was 0.05 m s-' with peak velocities of approximately 0.10 m s-'. Some

studies of both meso- and mamtidal marshes have more commonly found higher

velocities reaching peaks of 0.80-0.90 m s" (e-g., Green et al., 1986; Reed, 1988; French

and Stoddart, 1992). Even microtidal marsh studies had velocities that reached 0.40 m s-'

(Wang et al., 1993; Leonard et ai., 1995b). It must be acknowledged that the mean

velocities within the other creeks on the Allen Creek Marsh may v v kom those

measured in the study creek. Factors such as local topography, creek bed slope, degree of

creek meandering, and position along the channel where the measurements are made

relative to the creek mouth can have an influence on the flow within the channel.

Saltrnanh area, creek network size, and study creek length are factors that affect

the hydrodynarnics in the marsh creek. Many of the saltmarsh creek dynarnics studies

that are being discussed in this chapter were carried out in marshes that have substantially

more area, therefore, a larger tidal prism. These larger rnanhes oflen have main creeks

that are 1-2 km long (whereas this study focused on a creek 0.225 km long). Marshes

with a greater area also usually have a larger creek network to drain that area. When

velocity measurements are made near the mouth an extensive main creek, they are

measuring the input of many kilometers of tributaria. This leads to much greater

recorded velocities than those seen on a marsh of smaller area with a Iess extensive creek

network like the Allen Creek Marsh study creek, which drained a relatively small area of

marsh quite quickly (having a lower lag time than larger networks).

125

Another factor that leads to higher mean velocities is the topography of a marsh

suface. Many saltmarshes do not have the abrupt, cliffed marsh margin seen on the

Allen Creek Marsh, but have embankments or elevated ridges which separate low marsh

fiom high marsh. The ndges create a 'terraced' cross-sectional profile down the marsh

and are fomed by the incomplete abandonment of channels and significant extemal

changes, such as erosion or progradation at the marsh edge (Steel and Pye, 1997). These

raised surfaces c m pond tidal water behind them in areas of lower topography, re-

directing the remaining water towards the creek network and increasing the volume of

water passing through the main tribu*, thereby increasing creek velocities.

On the Allen Creek Marsh, there is a generally uniform surface topography across

a relatively short high-marsh-to-margin distance and the creeks themselves do not have

any levees. This means that a large proportion of the water that floods the rnarsh directly

across the manh surface fiom the margin, leaves in a similar way without being re-

directed into the creeks. Once the water level is below the manh surface, the creeks only

have to drain what remains in the channels and the water that was detained by the

vegetation on the rnarsh surface. Some of the small depressions on the manh surface

retain water until evaporation occurs, or the next tide arrives. This al1 contributes to low

flow velocities in the channels, although the creeks may continue to drain with the

highest velocities occurring below the arrays in a srnall depth of water.

126

Low Flow Implications on Chiinnet Evolution

The relatively low channel velocities measured in the study creek would

expectedly provide an enWonment conducive to the settling of suspended particulate

matter. If there was an overall increase in the suspended sediment concentrations that

flooded the marsh, there could be a potential for the infilling of the saltrnarsh creeks. The

results of a number of saltmarsh studies have indicated that tidal creeks erode on the ebb

tide in a headward direction (e.g., Pestrong, 1965). Most creek heads are located in

highiy vegetated, root-dense sediments though, which consist of highly cohesive and

fine-grained sediments and these factors increase resistance to erosion.

The results of the erosion pin study indicated that precipitation was a controlling

factor in channel bank erosion - a result also found in the study of Ward (1981). Other

potential factors include wave action and tidal height, however, the design of the

experiment did not allow for a full investigation into the roles of these factors in creek

erosion, in combination with precipitation. When rainfall was absent, the cumulative

average throughout the channel showed a slow, general accumulation of sediment within

the creek. It was observed on the marsh that throughout the growing season, the

vegetation slowly expanded into the channel, onto the upper part of the banks. The

vegetation did not cornpletely cover the channel surface though, because the growing

season was not long enough. During the winter season the flood tide brings ice ont0 the

marsh, which c m erode the surface and tmcate the vegetation; a less extreme winter

season could also be a factor that le& to channel infilling. If the onset of the winter

season were delayed, the vegetation could cover more area in the channels. This would

also result in the steepening of channel banks because vegetation protects the surface

127

firom erosion and reduces channel flow, enhancing sediment deposition. This would

result in a positive feedback rnechanism, as this increased deposition encourages

vegetation establishment.

In the mid marsh, the creek dimensions greatly decreased. In the high marsh there

were only occasional shallow depressions where the channels may have previously

extended, however, most of the evidence for past creek incision had disappeared. This

observation contributes to the belief that the smaller channels will infill over time as

vegetation increases or there is a decreased need for the channels, and this could possibly

occur in more substantial creeks, under different conditions.

When considering the infilling of channels, the effect of precipitation on the creek

surface becomes important. The erosive effect on the channels that one rain event

(approximately 30 mm) had on the banks (Figure 3.9) indicated that a large stom could

have an effect that could last the entire season, especially in light of the cumulative trend

following later rainfdl events and the relatively slow rate at which accumulation occurred

over time (Figure 3.1 1).

Bayliss-Smith et al. il 979) commented that the processes associated with 'normal

tidal flows' are not likely to produce flows that have enough power to erode channels and

significantly impact creek morphology. High storm velocities could reach erosive

capacity though, and it could be inferred that a few major storm events (or an aggressive

winter season) could sustain a creek network or lead to the channel erosion normally

hindered by low Bows and the presence of vegetation. One must also consider the fact

that there is a winter season in the Bay of Fundy when wind and storm activity is

128

heightened, during which time there is not a dense vegetation cover protecting banks and

channels h m erosion.

The winter months can bring about a great deai of stem breakage and vegetation

loss as a d t of ice scouring and high wave energy. During this tirne, there may be

increased creek surface erosion and loss of sediment at creek heads, which c m only be

recovered once rnilder weather and the growing season begins. It must be noted that

there are ice blocks that corne to rest within the channels and can adhere to the banks,

thereby protecting the sediment surfaces. This ice also prevents sediment deposition

though, and through the melting process the sediment may be re-suspended and deposited

on the manh surface or completely removed by the ebb tide.

The above hdings have implications for the evolution of saltmarsh creek

networks with low flows, similar to those measured in this study. These results are in

agreement with the finding that tidal creeks are inherited features of past rnudflat drainage

networks that were maintained as the saltmarshes established and increased in elevation

(Frey and Basan, 1985; Steel and Pye, 1997). The observation and cornparison of the

Allen Creek Marsh and an extensive mudflat dong the same coastline showed that the

tidal creek networks wxe similar, however, the network on the mudflat was much more

extensive, reaching into the high-water region (where high rnarsh would be). It appears

that after the establishment of vegetation on a mudflat, the necessity of an extensive

drainage network is reduced over time and only the larger drainage channels are

preserved.

The infilhg of the drainage network on a salûnarsh is not ununial, for it has been

found to reflect the stage of marsh establishment within the coastal system. As a marsh

129

establishes on a tidal Bat and increases in elevation with the establishment of vegetation,

there is often an increase in the channel length and drainage density of the creek network.

Over tirne, however, the marsh creeks become more nmow and some of the channels in

the upper reaches of the network are abandoned and hfilled. This results in a decrease in

drainage density as the manh growth Ieads to a decreased tidai pnsm across the marsh

(Frey and Basan, 1985; Steel and Pye, 1997). Based on this infoxmation, it could be

deduced that the Allen Creek Manh has reached this stage of channel reduction,

especially considering the low active erosion within the channels that is suspected to limit

creek growth.

Theories of saltmanh creek extension through Iandward creek head erosion

clearly do not apply to al1 stages of saltmarsh growth. Instead of ongoing erosion, some

tributaries may be experiencing a namowing and seaward intilling in the mid marsh as

they become increasingly vegetated. In light of the discussion of the hydrodynamics and

sedirnent transport dynarnics within the studied saltmmh tidal creek, one is compelled to

consider the role of the tidai creeks within the marsh. These creeks are not always

conduits that convey sediment and nutrients to the saltmarsh, especially when the tidal

range is quite hi&. Some research has defined saltmarsh tidal creeks simply as ebb 80w

drainage channels and this seems to be the case in the Allen Creek Marsh, although to a

lesser extent because much of the water leaves the marsh without benefit fiom the creek

network.

In essence, the saltmanh tidal creek network on the studied portion of the Allen

Creek Marsh is a remnant of the pre-marsh tidal mudflats. It began as a drainage network

for ebbing flows and remained as such as the marsh established and evolved over time.

130

Present climatic conditions (e-g., rainfall and ice) and tidal energy prevent extensive

innlling of the creeks as the marsh maintains itself. In coastal environments where a

marsh is n m w with a s m d are . the topography is relatively uniform, and a large

proportion of the tide moves direcdy onto and off the marsh surface without the direct

necessity of the channels, the role of the saltmarsh tidal creek is diminished and becomes

ambiguous.

RESEARCH OPPORTUNTTIES

The results of this research have led to a better understanding of the flow

dynarnics and sediment transport within saltmarsh tidal creeks in a high-macrotidal

environment, such as the Bay of Fundy. Many of the ideas that surfaced as a result of this

study are speculative and their developrnent would benefit fiom M e r research in a

marsh environment similar to that of the Allen Creek Marsh, as well as in marshes of a

variety of tidal ranges.

The occurrence of erosion at the head of tidal creeks has been questioned in this

work. A more comprehensive study of the change in channel dimensions over tirne,

including a study on the effects of precipitation throughout the channel and on the flow

occurring at the head of the channel during ebb tides would help to define the potential

for creek erosion and extension. Such a study would require the measurement of velocity

closer to the bed and it may help to calculate the shear at the bed and how it changes

throughout the creek It would also be beneficial to study if and how the creek

morphology changes during the winter season and compare the results to those found

131

during the growth season to determine how the creek network is impacted by the changes

brought about by seasonal conditions.

To better comprehend the change in the suspended sediment concentration

throughout a tidal cycle, it would be necessary to measure the change in the

concentrations fiom the creek rnouth, beyond the creek and into the hi& marsh. It may

also help to position some instruments near the creek banks, at increasing distances from

the creek. This would facilitate an interpretation of the exchange of sediment between the

creek and the marsh surface and would allow for an analysis of the direct change that

occun over a tide as the flow encounten a vegetated surface.

This study attempted to measure the vertical change in the velocity and suspended

sedirnent concentration in the water column. Problems with some of the instruments

prevented the acquisition of enough data to make conclusions about the vertical variations

in the flow and the sediment dynamics over a tidal cycle. Future work with an increased

number of instruments in the array would be necessary to enabie the creation of profiles

depicting these dynamics within one or more channel cross-sections.

CONCLUSIONS

The main objectives of this research were to measure the hydrodynamics and the

sedirnent dynamics in a high-macrotidal saltmarsh tidd creek over a number of tidal

cycles fiom a spring to neap period. This included the examination of those dynamics

both temporally and spatially, and a cornparison of the results to the findings of other

studies of a similar nature in saltrnarshes of microtidal to hi&-macrotidal ranges. An

132

analysis of the &ta collected for this study has fulfilled the objectives of this research and

has r d t e d in some key conclusions.

The velocities mea~u~ed in the hi&-macrotidal saltmarsh study creek were very

low compared to those recorded for other studies in saltmarshes of various tidal ranges.

The patterns exhibited over tidal cycles were like those found in similar creek dynamics

studies with some differences in the magnitude of the patterns. These differences were

related to the low creek velocities, the less varying saltmarsh topography, and the smaller

size of the creek network, which affected the volume of ebb flow that exited the marsh

through the network. Logicaily, it could not be expected that such low flows would

contribute to creek surface erosion, implying that the present creek growth is limited.

The suspended sediment concentration in the study creek decreased fkom the

flood to the ebb tide. This was attnbuted to the increasingly lower concentrations

brought in over the flood tide and the seîtling or trapping of sediment on the marsh and

creek surfaces. The initial concentration of suspended sediment entering the saltmarsh

creek network depended upon an extemal source and the process of horizontal advection,

while sediment entrainment withui the study creek supplied an insignificant amount of

sediment to the flow.

The results of this study showed that there was an overall loss of sediment along

the channel where change on the surf'âce of the banks was measured. This erosion

occurred following precipitation, while the rate of accumulation that typically occurred

between rainfall events did not counterbalance the sediment loss. The apparent lack of

channel erosion resulting from regular tidal flows M e r implies that the channels do not

greatly contribute to the sediment being transported within the system. The time h n e of

133

the erosion pin study may not have permitted the identification of dl of the controlling

parameters. A study over a longer perïod of time would allow for the detedat ion of

the effects of tides, ice, wind, and storm events on the changes occurring dong the

channel surfaces.

The area of a saltmanh and its topography, in addition to the size of its drainage

network, have a large impact on saltmarsh creek dynamics. A cornparison of the results

of this research to those of previous studies indicated that the hydrodynamics and the

sediment dynamics of saltmarsh tidal creeks are unique to an individual saltmarsh and

depend upon the surroundhg topographie and hydrologie environments.

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