june, a.a. · 1.1 -2 basic flow structure ... plotting survey transects ..... 45 . list of figures...
TRANSCRIPT
TlDAL INFLUENCE ON FLOW STRUCTURE AND DUNE MORPHOLOGY,
FRASER RIVER ESTUARY, BRITISH COLUMBIA, CANADA
A Thesis
Presented to
1 he Faculty of Graduate Studies
of
The University of Guelph
by
JASON ANDREW ALLAN BLAIR
In partial fulfillment of requirements
for the degree of
Masters of Science
June, 2001
O J. A.A. Blair
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ABSTRACT
77DAl lNFtUENCE ON FLOW STRUCTURE AND DUNE AIORPHOLOGY,
FRASER RIVER E S N M Y , BRiTISH COLUMBIA, CANADA,
Jason A A Blair University of Guelph, 2001
Supewisor: Dr. R.A. Kostaschuk
This study examines the influence of tidal motion on veloaty flow structure
and dune morphology in the Fraser River Estuary near Vancouver, British
Columbia. An acoustic Doppler Profiler (ADP) and high resoluüon survey
echosounder were useâ to simultaneously record flow veiocity and dune fom
over the course of one tidal cyde along a 250 m reach of the Main Channel of
the Fraser River on June 20,2000. Eight large syrnmetric dunes were analyzed
During the tidal cycle rnean veiocity follows changes in tidal stage
alaiough deceleration on the flood tide is more rapid than acœleration during the
ebb tide. Due to continuous flow ditequilibrium in the estuary, a 'memory' of
previous flow conditions is retained next to the bed. This 'memory' resutts in
kinked velocity profiles that must be regarded with caution when calculating
shear stress estirnates. Dune height inmased on the falling tide and decreased
on the rising tide, indicating that the response time required for the adjusment of
dune height to mean velocity is considerabiy less #an one tidal cyde.
Acknowledgements
1 would fike to extend my appreciation to ail who helped me along as I
have progresseci through my academic career. It has k e n a long journey filled
with many trials and tribulations as well ennched leaming and many good times
that will never be forgotten. A special thanks to Dr. Ray A. Kostaschuk who
generously provided advice, academic guidance and professional expertise that
enhancecl this project and also for the opportunity to expand my knowledge,
experience and outlook on life. To Dr. P. Villard, thanks for al1 the technical
assistance and advice provided throughout this study and the hospitality you
extended during the field season. I would also like to thank Dr. R. Davidson-
Arnott for his objective outlook and advice which has always been appreciated
throughout my undergraduate and graduate degrees.
Additionai thanks is extended to Norm Rogers, Mario Finoro and Marie
Puddister for technical support extended throughout this project and to Dr. J.
Mersey for helping me clear my final GIS obstacles. Thank you io Arjoon
Ramnarine for assistance in the field and for piloting the UBC Oceanography
Launch with such precision. Further thanks to Dr. M. Church for providing
facilities and equipment. Financial support for this projed was supplied by
NSERC through Dr. R.A. Kostaschuk's Operating Grant.
Thank you also to my Mom and Dad for support when I needed it and for
letting me find rny own way when I didn't Lastly but certainly not least, I would
like to give special thanks to Jaime Dawson without whom this project and the
last two years would not be nearly as meaningful or fulfilling.
Table of Contents
Table of Contents i .............................................................................................................
List of Figures ................................................................................................................... iii
List of Tables v i .......................................................................................................................
List of Appendices ............................................................................................................ v i
List of Symbols ................................................................................................................. vii
INTR~DUCTION ..................................................................................................... 1
Estuarine Flows ..................................................................................................... 3 1 -1 Estuarine Flow Dynamics ..................................................................... 4 1.1 -2 Basic Flow structure ................................................................................ 7
Estuarine Dune Dynamics ................................................................................ II 1.2.1 Bedforms in Estuaries .......................................................................... 12 1-2.2 Dune M O ~ P ~ O ~ O ~ Y ................................................................................... 14 1 92.3 Lag Effects .... ........................................................................................... 20
Purpose and Objectives ................................................................................... 23
STUDY AREA: FRASER RIVER, BRITISH COLUMBIA ........................... 25
Fhw and Sediment D~namics ........................................................................ 26
Dune Morphology ................................................................................................ 30
rul~THODOLOGY .................................................................................................. 31
Channel Reconnaissance and Boat Setup ................................................ 32
Collection of Velocit~ Profiles ....................................................................... 36
Fbw Depth and Dune Geometry ...........................m... .... ...................... 39
Data Reductions ................................................................................................. 40 394.1 EchosoundiW Data ............................................................................... 40 3.4.2 Merging ADP and Echosounding ..................... .... ................ 42 3.4.3 Dune Statistics and Velocity Profiles .............................................. 43
Plotting Survey Transects ............................................................................... 45
List of Figures
Figure 1.1 Schernatic diagram of a salt-wedge estuary. Note the residual upstream flow in the salt-wedge. No turbulent mixing occurs across the halochne ......................................................... 5
Figure 1.2 Schematic diagram of a partially-mixed estuary. Residual upstream current induced due to turbulent mixing between sait and fresh water producing a vertical salinity gradient .--.........--... 6
Figure 1.3 Typical distribution of velocity within the boundary layer. Flow within the log-layer generally follows the relationship set out by the law of the wall and therefore shear stress within this region is constant ................................................................... 8
Figure 1.4 Deviation from logarithmic conditions induced by unsteady flow ..................................................---.......----.............................................. 11
Figure 1 -5 Sedform stability fields demonstrating bedform occurrence under different combinations of (A) mean depth averaged flow speed and median grain size and (B) rnean depth averaged flow speed and flow depth ..................................................................... 13
Figure 1 -6 Traditional dune rnorphology and conceptual flow regime 15
Figure 1.7 Cross sectional profiles of (A-E) srnall to medium dunes, and (F-J) large to VerY large dunes ...................................................... 17
Figure 1.8 Hysteresis plots for various dune dimensions and flow prope~ies ................................................................................................... 22
Figure 2.1 Study area, Main Channel Fraser River estuary, British c ~ l ~ m b h canada .................................................................................... 26
Figure 2.2 Hydrographs of Fraser River discharge (Qma in m3s-') showing (A) entire 2000 discharge and (B) discharge encompassing the study period between June 7 and June 20 ........................................................................................................ 27
Figure 2.3 Plot of tidal stage surrounding the June 20 measurements taken during this study ............................................................................ 29
Figure 3.1 Photographs of University of British Columbia Oceanography launch as outfitted for this study .--...-...---............................................... 33
Figure 3.2 Diagram showing study area with start and end points as well as channe! centeme .............................................................................. 34
Figure 3.4 Diagramatic representation of launch with experimental setup. including; SonTekTM 1200 kHz ADP . and 200 kHz echosou rider .............................................................................................. 38
Figure 4.2 Segmented velocity profile over Dune 6. Transect 1 . Profile is representative of velocity profiles observed within the first three transects ..................................................................................................... 51
Figure 4.3 Typical velocity profiles used to calculate u* over individual dunes ........................................................................................................... 52
Figure 4.4 Average mean velocity for each transect. calculated for Üt and 0"f ........................................................................................................ 54
Figure 4.5 Average coefficient of determination exhibited by velocity profiles over one tidal cycle ................................................................... 54
Figure 4.6 Summary of mean shear velocity values calculated over one cycle .................................................................................................... 55
Figure 4.7 Average roughness length (%) calculated for each survey transect during the course of one Ma1 cycle .................................... 56
Figure 4.8 Identification of dunes 1 . 8 along the survey line .............m.............. 57
Figure 4.9 Dune profile cornparison over one tidal cycle (June 20. 2000) ...... 58
Figure 4.1 2 Individual dune height (dune 6) over tidal cycle .............................S.. 62
Figure 4.14 Individual dune length (dune 6) over tidal cycle ................................ 63
Figure 4.16 Individual dune steepness ratio (dune 6) over tidal cycle 65
Figure 4.18 Individual lee face slope angle (Dune 6) over tidal cycle -------..--..-. 66
Figure 5.1 Absolute acceleration measured over the tidal cycle ---------.--.-..--..-.. 69
Figure 5.2 Distribution of average shear velocity over tidal cycle ------.------.-...-.. 72
Figure 5.4 Mean roughness length distribution over the tidal cycle -----.---.-.-..-- 76
Figure 5.5 Plot of dune height vs. mean velocity over one tidal cycle ----....-.--- 79
Figure 5.6 Conceptual model of the velocity profile evolution and dune morphological response over One Ma l cycle ................................... 83
List of Tables
Table 4.2 Mean dune characteristics calculated over one tidal C Y C ~ ~ .--.--.-.-..---..----. --.--.-....----.-.--....-.-..--- ** .-.--. * ...--......-.-..-.--.--.-------.--.---------- 60
List of Appendices
Appendix A Flow statistics over dunes with significant logarithmic vetocity profiles ...........*..........*..................--.....-.-.*-.-.-..-.--..-.-.-.*.-.-...--.*.-.-- 94
Appendix B
vii
List of Symbols
height above bed velocity mean velocity of the entire flow mean velocity of the upper flow s hear velocity von Karman's constant roughness length boundary shear stress water density dune height dune length dune steepness dune lee face angle
1
Chapter 1
1 .O INTRODUCTION
Macro-scale repeating bedforms found in aquatic environrnents,
commonly referred to as dunes, have been extensively shidied over the past
few decades. Dunes are ubiquitous features in sand-bed rivers and estuaries
and exhibit a dynamic relationship with the flow. Wherever flow W i n a
channel exceeds the threshold for dune development, dunes will continuously
migrate and alter their rnorphology to equilibrate with the water depth, sediment
sire, and flow conditions (Dyer, 1986). These bed features play an integral role
in the relationships between boundary layer flow structure, sediment transport,
and bedform development. Understanding the intricacies of this Row-sediment
transport-bedforrn trinity (Leeder, 1983) is fundamental to al1 aspects of modern
Fluvial Geomorphology including the prediction of sediment transport rates,
turbulent flow structures and dune response in unsteady flow conditions. The
complicated nature of these relationships requires comprehensive studies to
explore each facet of interaction between these processes in order that a better
understanding of the whote system rnay be achieved.
To a large extent the relationships between dunes and their controlling
environmental conditions have been quantified in laboratory Rumes (Bohacs,
1981 ; Rubin and Ikeda, 1990; Southard and Boguchwal, 1990; Onslow et al..
1993; Bennett and Best, 1995; McLean et al., 1994,1999a.b). Within a
laboratory setüng simplified flow conditions of unidiredional. steady, unifom
currents are frequently implemented. These simplifications have allowed many
relationships between the interaction of fiow, bedfom development and
sediment movement to be visualized. However, problems with scale and lack
of reproducibitity in natural environments have led to the questionability of these
relationships under field conditions. Conversely, studies that have been
undertaken in the field have resufted in site-specific relationships or
observations, but have lacked the resolution to properly illustrate the tme
process-response relationships between dune morphology and complex flow
conditions. Many field studies have examined dunes in estuanne
environments, where interaction between these dynamic bedforms and human
processes can be signifiant (Nasner, 1974, 1978; Black and Healy, 1985;
Aliotta and Perillo, 1987; Van Den Berg, 1987; Fenster and FitzGerald, 1996;
Kostaschuk and Villard, 1996a; Lyons, 1997; Villard and Kostaschuk, 1998;
Kostaschuk et al., in prep).
Large estuaries in particular are often the sites of major urbanized areas
and are hubs for marine transportation and industcy relying on the use of water.
Therefore, in these areas sub-aqueous infrastructure and major navigational
thoroughfares are increasingly sensitive to changes in channel morphology.
Migrating dunes can impede travel of oœan going vessets or put unforeseen
stresses on underwater power, communication and water intake lines
(McKenna et al., 1992; Kostaschuk et al., 1998; Wewetzer et al., 1999).
Despite the wealth of literature on dune relationships in estuaries little is known
about the precise nature of three dimensional flow structure and dune
morphodynamics during tidal fluctuations and salinity intrusions which
characterize estuaries. In order to describe and preâict sedirnentary processes
and to facilitate proper management of estuarine environments, a better
understanding of fiow structure and dune interaction must be achieved.
This study examines the influence of tidal movement on velocity flow structure
and dune morphology in the Fraser River estuary near Vancouver, Bntish
Columbia. The remainder of this chapter discusses the major properties of
tidally influenced estuarine flows and characteristic dune rnorphofogy,
concluding with the purpose and objectives of this study.
1.1 Estuarine Flows
Estuaries are semi-enctosed coastal bodies of water that extend to the
upper iimit of tidal infiuence, where seawater entering from one or more free
connections with an open saline body of water is significantly diluted with fresh
water de&& from land drainage (Perillo, 1989). Estuarine systems are highly
dynamic and encounter diverse fiow conditions over both spatial and temporal
scales. Unlike their fluvial counterparts, estuarine flows are particularly
unsteady as both fluvial and tidal currents influence flow. The remainder of
this chapter section will discuss the nature of estuarine flow dynarnics and how
they are both related to and distinct from unidirectional, steady. unifom
conditions.
1.1 -1 Estuarine Flow Dynamics
Over time the nature of flow dynamics within an estuary will Vary with the
size and shape of the estuary, extent of anthropogenic alteration of the estuary
and bed materiaf characteristics However, at any given instant estuarine flows
are ultimately govemed by interaction between seaward river discharge and the
constantly fluctuating tidal regime. The interaction or mixing between these two
flows will dictate the resuftant flow structure throughout the estuary. Dyer
(1973, 1986) proposes that based on mixing characteristics four main estuary
types exist: salt wedge, partially-mixed, welCmixed and fjords. This study
focuses on the Fraser River estuary which periodicatty behaves as both a salt
wedge estuary and a partially-mixed estuary, thus these two estuary types are
discussed below.
Salt wedge estuaries are estuaries where Iiffle mixing occun between
tidal and river flows. As illustrated in Figure 1.1. fiow in a salt wedge estuary is
highly stratifieci and is characterized by a relatively large river input and a
relatively small tidal flow. The motion of the river flow passing over the sait
water beneath will entrain some of the salt water into the flow above and create
a small residuat landward flow at the bed. This entrainment is a function of
Helmholtz instabilities initiated at the density interface, but there is little mixing
due to turbulent structures initiated at the bed (Dyer, 1986).
Partially-mked estuaries exist where turbulence created by the
interaction of bed roughness and appreciable tidal flows bewmes dominant
enough to extend into the fresh water layer and actively mix water in both
directions across the halocline (Dyer, 1989). This mking proœss leads to
upstream residual wnents at the bed (Figure 1.2). In some estuaries mixing
characteristics may var- seasonally, with fluctuating river discharge and over
neapspring tidal cycles (Dalrmyple and Rhodes, 1995). For example, the
Fraser River estuary remains partially r n k d throughout most of the year, only
becorning fully stratified during high river discharges associateci with the annual
spring snow melt freshet (Kostaschuk and Atwood, 1990). During this period of
high river discharge the degree of stratification and position of sait-water
intrusion are controlled largely by the tidal amplitude (Kostaschuk and
Luternauer, 1987).
Vecy little rnixing Dominant downstrcom of f r n h &mît watu f n s h wat8r flow
Tidal Input
Tip of salt wed*
salt water wcdge
Figum 1.1 Schematic diagram of a salt-wedge estuary. Note the residual upstrearn flow in the salt-wedge. No turbufent mixing occum a c r m the halocline (after Pethick, 1984, p. 179)
Mixing between soit 6 fresh wtw pmduces vertical Siight salinity gradient downstrearn tlow
Figum f.2 Schematic diagram of a partially-mixed estuary. Residual upstream cunent induced due to turbulent mixing between satt and fresh water producing r vertical salinity gradient (riftor Pethick, 1984, p. 180)
Tidal amplitude is a function of the gravitational forces of the moon and
the sun (Sleath, 1984). Higher (spring) tides are present with new and full
moons while lower (neap) tides accompany 1" and 3d quarter lunar positions
(Komar, 1998). Coastal conditions can result in diurnal or semi-diurnal tidal
cycles that are approximately 24 and 12 hours in length respectively.
In many coastal areas a salt-water intrusion is forced into an estuary
during the flood tide, effectiveiy reversing the flow upstream along much of the
bed (Kostaschuk and Atwood, 1990). Above the upstream extent of the salt
wedge inmision or in estuaries where no pronounced sait wedge foms due to
increased mixing characteristics, flow deceleration occurs due to the increased
resistance provided by the incoming tidal cunents. In this area of the upper
estuary little is known of the precise fiow dynamics occurring over an entire tidal
cycle. Certain measurements have been taken during low tide when fiows are
highest (Kostaschuk et ai, in prep), however the response of flow dynamics
over an enüre tidal cycle is poorly documenteci and not fully understood.
1.1.2 Basic Flow Structure
In steady, uniform flows with a fat bed the distribution of velocity within
the boundary layer will follow a characteristic pattern (Figure 1.3). In general,
velocrty increases away from the boundary as frictional influences induced by
the bed decrease toward the surface. The velocity distribution within the
boundary layer typically exhibits a logarithmic relationship close to the bed
(Soulsby, 1997). This section of the boundary layer is effectivefy tenned the
log-layer. Below the log layer is an extremely thin layer of stagnant flow called
the bed layer. Within the bed layer flow dynamics are dependent on the
relative importance of rnolecular viscosity and bottorn roughness (Bowden,
1978; Soulsby, 1983). Above the log-layer is a section of faster moving fluid
classified as the outer flow.
Outer Flow / 1 8O-9O0h Flow Depth
r
Flow Velocity (U)
Figura f.3 Typical distribution of velocity within #e boundary layer. Flow within the log-iayer genenlly follows the relationship set out by the 'law of the wall' and therefore shear stress within this region is constant
The isolation of conditions within the log-layer becomes increasingly
important as it provides a method by which estimates of bottom shear stress
might be obtained. This allows for visualization and quantification of the
momentum transfer that takes place between the flow and the bed. Within the
log-tayer a velocity profile is expected to adhere to the relationship outfineci by
the 'law of the wall':
where U is the mean velocity (averaged sufficienffy to eliminate turbutent
fluctuations), at a height z above the bed, K is von Karman's constant, assumed
to be 0.40 in clear water, Z, is the roughness length. U. is the shear velocity of
the fiow. In tum, boundary shear stress (ri) is related to u. by:
where p is the densrty of water in the flow. The utility of instituting the 'law of
the wall' lies in its ability to predict shear stress from velocity profiles.
Use of velocity profile data to determine shear stress within scientific
research has been widespread over the past few decades (Smith and McLean,
1977; Soulsby and Dyer, 1981 ; Villard and Kostaschuk, 1998; Lueck and Lu,
1998; McLean et al, 1999a; Carîing et al., 2000), despite the questionable
applicability of the 'law of the wall' retationship in geophysical flows that are
rarely steady and uniform. It is also exceptionally difficult to measure velocity
accurately near the bed where flow speed and depth, as well as inadequate
instrumentation inhibit such direct measurements. In areas with dunes, the
accurate measurement of velocity diredy adjacent to the bed is further
complicated by the influence of turbulent wake structures induced by the
bedforms that produce variability within the flow. Velocity profiles over dunes
are typically segmented into two logarithmic sections (Mclean et al., 1999a).
The logarithmic profile closest to the bed is often assurned to represent 'skin
friction' over the dune, although McLean et al. (1999a) suggest that this may
not be a reliable estirnate. The upper logarithmic profile, however, does
provide a measure of 'total stress' (form drag + skin frcüon) (Smith and
McLean, 1977; Zyserman and Fredsoe, 1994). Some variabitity in the shear
stress estimates will inevitably be retained but McLean et al. (1999a) argue this
as an improvement over past techniques and one that will allow better
visuakation of this process in areas subject to bedform development.
Despite the 'law of the wallf onfy being designed for steady uniform
conditions, the impacts of unsteady flow to this relationship are known. Figure
1.4 demonstrates deviation from the logarithmic relationship during accelerated
and decelerated flow. This plot reveals the influence of inertial foras which
impose a 'memory' of the preceding driving forces within the fiow (Dyer, 1986).
In acceferating flow this leads to a velocity profile that is concave upwards and
a current that will be srnaller than the logarithrnic value predicted by the 'law of
the wall'. Conversely, in decelerating flows the flow profile will be concave
downwards and flows away from the bed will be over estimated. Given the
above deviation from the 'law of the wafl' in unsteady flows, tidal influence of
estuarine currents wilt promote an increaseldecrease in shear stress that leads
the velocity tendency within the osciltating flow (Dyer, 1986).
Flow Velocity (U)
Figun, 1.4 Deviation fmrn logarithrnic conditions induced by unsteady flow (modified sfter Soukby and Dyer, 1981, pg. 8068).
i .2 Estuarine Dune Dynamics
Macro-sale bedforrns found in estuarine environments have been
referred ta as dunes, ripples, sandwaves, megaripples and bed waves (Ashley,
1990). This breadth of nomenclature has led to confusion and in some cases
misuse of the terrninology. It has been argued however that al1 flow-transverse
becifomis larger than curent ripples and smaller than barforms are sufficiently
similar in ternis of formative processes to be assigned a single name (Ashley,
1990). The collective terni 'dune" has thus k e n adopted in the description of
such bedforms and will be used exclusively throughout the remainder of this
study .
Aithough governed by similar formative processes. estuarine dunes are
much more morphofogiçally diverse than their fluvial counterparts. Highly
unsteady fiows, intermittent sediment movement and fluctuating water depth
produce dunes that are much different than the 'classic' dune forms found in
rivers. The remainder of this chapter section will sumrnarize and explain the
multifarious dune dynamics W i n estuaries.
1.2.1 Bedfonns in Estuaries
As fiow velocity at the bed increases, bedforms will develop through a
characteristic sequence. This sequence is largely a function of the sediment
grain sire, water depth and flow strength as measured by mean velocity, shear
stress or stream power (Figure 1.5) (Reid and Frostick, 1994). These sediment-
flow relationships result in bedform development from a Iower stage plane bed
through the following sequence:
Ripples --) Dunes -3 Upper Sîage Plme Bed ~-* Antidmes
10°C SPEED (m/s)
1.5 Bedform stability fields demonstrating bedform occurrence under different combinations of (A) mean depth averaged flow speed and median grain size and (6) mean depth averaged flow speed and flow depth. Boundaries are based on data from steady uniforrn conditions produced in a Iaboratory flume and standardued to 10°C to remove any impact in measurement due to changes in fluid viscosity. Fr = Froude Nurnber (After Datyryrnple and Rhodes, 1995).
In estuaries the threshold for ripple stability is frequently exceeded
leading to widespread dune development when sediment supply is sumcient.
Likewise, flow depth exceeds that required for super-critical flow conditions
necessâry to establish an upper stage plane bed. When flow in estuaries does
becdme critical, the elevated current speeds are seldom sustained long enough
to rework al1 sediment within a dune population into an upper stage plane bed
(Dyer, 1986). The dynamics of the estuarine flow regime therefore support the
development and maintenance of a variety of dune morphologies. Although
most stability relationships (e-g. Figure 1.5) were formulated in unidirectional,
steady, uniform, flow conditions, field research has shown that these
relationships are generaliy representative of the more cornplex conditions found
in estuarine environrnents (Boothroyd and Hubbard, 1975; Dalrymple et al,
1978; Rubin and McCulloch, 1980).
1.2.2 Dune Morphology
Ctassic Dune Morphology and Fto w
Throug hout much of the literature dunes are depicted as asymmetric,
two dimensional features with a longitudinal profile that is roughly triangular in
shape (McLean and Smith, 1986; Nelson et al., 1993; McLean et al., 1994;
Bennett and Best, 1995). Classic dune rnorphology exhibits a long, çhallow
sloping, slightly curved stoss side, a well-developed crest and brink point, and a
short steep lee slope (Figure 1.6). Dune lee dopes are seen to approximate
the angle of repose of the bed material, roughly 30" in sand sized sediment
(McLean, 1990). These classic dune forms develop over a large range of
sediment conditions; from well-sorted fine sand to coarse sands to poorly
sorted gravels and are generally viewed as bed-load-dominated features
(Bennett and Besf 1995). The 'classic' dune description is primarily based on
flume experiments (Raudkivi, 1963; Engle and Lau, 1980; Nelson et al.. 1993;
Mctean et al., 1994; Bennett and Best, 1995).
Flow Reuersal Point of R m h m e n t
Figun, 7.6 Traditional dune morphology and concepturf flow regime (modifieci afoer McLean, 1990)
Flow over a traditionat dune is shown in Figure 1.7. As fluid travels over
a dune it will converge and accelerate up the stoss dope of the bedform. At the
brink point of the dune the accelerated flow will separate, producing a layer of
high shear over the trough region and initiating a recirculation cell over the lee
slope (McLean et a/., t999a). This results in the formation of a decelerated
turbulent wake region that overlies the near-bed internal boundary layer of the
following dune and extends upward into the faster moving outer Row (Figure
1.6). The internal boundary layer resulh from the influence of local bed
tapography over the stoss of succeeding bedforrns, while the outer flow is a
product of the 'upstream history' of the fiow (McLean, 1990).
Although 'classic' dune morphology and fluid flow have been used to
produce models for dune evolution, migration and sediment transport (Einstein,
1950; Engle and Lau, 4980; Van Rign. 1994). flow structure and turbulence
(Smith and McLean, 1977; McLean et al., 1999), environments such as
estuaries often contain dunes with markedly different morphologies than that of
the classic dune fom. Despite being initiated by similar formative processes
(Ashley. 1990), estuarine dunes are subject to continuously variable flow
conditions which tend to produce a multitude of dune forms corresponding to
highly variable flow conditions generated by the interaction between river and
tidal flows. Given that dune form is ultimately dnven by sediment movernent
and flow conditions it is not surprising that many various dune forms are found
within estuarine environments.
Estuarine Dunes
In a comprehensive literature review Dalrymple and Rhodes (1995)
identify a nurnber of characteristic dune morphologies that are present within
estuaries (Figure 1.7). Although Mis summary of morphological shapes is
predominantly based on intertidal dunes with little or no fluvial influence, the
review is useful in illustrating the types of dune forms generated in estuaries
produced by varying strengths and directions of dominant and subordinate
currents.
Figun, 1.7 Cr- sectional profiles of (AS) small to medium dunes, and (F3) large ta very large dunes. (A) Repreientative of the 'classic' asymrnetrical dune form common in steady unidirectional flows. (6) Triangular dune with typicat tidal asymmetry, note the decmase in leeside angle. (C) Dune with c m t a l pktforrn characteristic of higher flow regimes. (O, E) Dunes afber weak (O) and strong (E) subordinats tides producing pronounced flow reversal and rubaequent reverse flow caps (dominant- flow st088 side dashed where buried). (F, G) Symmetric trochoidai profil- pressnt in amas with no net sediment b.n8port, with full bdded (F) and sediment staned (G) conditions. (H) Typbl Iarge to very Iarge asymmetric dune. (1, J) Profib demonstrating Iarge dune variations induced by crestai branching or the superposition of other dunes. (Modified after Dalrymple and Rhodes, f 995, pg. 378)
Given the ever-changing nature of the flows in estuaries, dune profiles
are continually modified by the combineci tidal and fluvial flow constituents. The
general shape of a dune at any instant will represent a quasi-equilibrium
adjustment to the relative strengths of the opposing flows. Where tidaily-
induced flow reversal occurs some displacement of a dune's crestal position is
likely to occur (Figure 1.7; D,E). This displacernent will be greatest in smafler
dunes where less sediment needs to be transported for a change in
morphology to be observed.
More commonly, estuarine dunes will remain asymmetrical over the
course of a tidal cycle, oriented with their lee faces in the diredion of the
dominant current (Dalryrnple and Rhodes. 1995). Dunes typical of an
imbalanced flow regime. whether it is the result of tidal disparity or impact of a
fluvial influence, will exhibit linear or slightly convex up stoss slopes sometimes
referred to as 'hump-back' forms. Lee slopes in these estuarine dunes will be
generally longer and less steep than the classic dune model for steady,
unidirectional flows would indicate (Dalryrnple and Rhodes, 1995). When found
in flows with slight flow reversal it is thought that the shape is a result of the
subordinate current moderating the lee slope, not altowing it to approach the
angle of repose. Where symmetric dunes are present in tidally infiuenced
unidirectional flows Kostaschuk and Villard (1996a) propose that their
symmetry and low angled leeside can be explained by the interaction of flow
and sediment. High near-bed velocity and bed load transport rates result in the
rounding of dune crests and the characteristic long low angled leesides are
produced by deposition of suspended sediment in dune troughs.
1, J).
stoss
Compound dunes are also commonly found within estuaries (Figure 1.7;
These large bedforms have smaller superimposed dunes along their
side. The smaller features are believed to be the result of a quasi-
equilibrium superposition related to the development of an internat boundary
layer along the stoss side of the larger dunes (Dalrymple and Rhodes, 1995).
in this case the superpositioned dunes will migrate along with. but generaliy
faster than, the underiying large dunes. Alternatively, when seasonal changes
bring about drastic changes in the fiow regime large relict dune features may be
present but not active and smaller superimposed features may develop
(Kostaschuk et al, 1989; Kostaschuk and Villard, 19Q6a).
Although shape is frequently used to identify various dune types, it is
often dimensional statistics such as height ( H , ), wavelength ( L , ) , steepness
ratio ( H d / L d ) and lee side slope angle ( P ) , that are used in the modeling,
interpretation, and identification of various dune morphologies (Terwindt and
Brouwer, 1986; Julien and Kiassen, 1995; Kostaschuk and Villard, 1 W6a;
Harbour, 1998). In estuarine environments dunes c m be found over a wide
range of lengths and heights ranging from the smaller dunes ( H , c i m, L, <Sm)
rearded by Dalrymple (1984) in the Bay of Fundy; to the large dunes (Nd E
5m. L, z 90m) found by Kostaschuk and MacDonald (1986) in the Fraser River
Estuary. Dune height and wavelength commonly fluctuate during spring-neap
tidal cycles and following seasonal changes in flow dynamics. However, it has
been found that over the duration of one tidal cycle, the wavelength of dominant
dune forms does not usually fluctuate (TeNvindt and Brouwer, 1986) because
not enough sediment can be moved dunng a single tidal period to significantly
alter dune wavelengtti (Dalrymple and Rhodes, 1995). Dune height is more
variable than wavelength and there is a cammon tendency for it to increase as
current speeds and flow depth increase from neap to spring tides. In a study
on the behavior of intertidal dunes Terwindt and Brouwer (1986) suggest that
variations in dune height over a single tidal cycle seldom occur, although mis
has not been tested where large fluvial influences are present within an
estuary.
1.2.3 Lag Effects
Under steady state equilibrium conditions, dunes take on dimensions
proportional to the flow regime responsibie for their genesis and maintenance
(Allen, 1976; Fredsoe, 1979). In estuaries steady state flow seldom exists,
therefore dunes in these environrnents are constantly evolving and adjusting
their rnorphology, striving to achieve a renewed equilibrium with the present
flow conditions. The morphologicaf changes that occur as dunes equitibrate
with the fiow, requires the movement of a finite volume of sediment, which
depends on the size of the bedform (Dalrymple and Rhodes, 1995). The
movement of this finite amount of sediment requires a finite amount of tirne,
which is calied the lag time of the bedform (Allen, 1976; Englund and Fredsoe,
1982; Gabef; 1993). A group of dunes witl therefore respond to changing flow
velocity by changing its size composition, with the rate of response or lag tirne
directly proportional to dune size ( H , and L,) and inversely proportional to
sediment movernent and current speed (Allen and Friend, 1976a; Bokuniewicz
et al., 1977; Allen, 1983). Thus, large dunes will lag further behind flow
changes than smaller dunes and for any given size of dune lag tirne will
decrease with increased flow speed.
The lag response of dunes to flow conditions ultimately leads to a
hysteresis relationship between flow velocity and dune size. Carey and Keller
(1957) noted that dunes in the Mississippi are srnaller than they should be on a
rising river, but larger than they should be during receding flows. Due to this
lag-induced hysteresis response, the extrerne limits of dune size, both minimum
and maximum, will not occur simultaneously with flow extremes. Essentially,
dunes wiH continue to grow/atrophy until the flow readjusts to a level that moves
the equilibration response in the opposite direction. Figure 1.8 illustrates this
response in an idealized representation of several hysteresis Ioops typical of
dunes. Where relationships between flow and morphological parameters are
directly related, movernent around the hysteresis loop will be counterclockwise.
Additionally, Allen and Friend (1976b) suggest that because the bedfoms are
never in equilibrium with extreme conditions, the extent of morphological
parameters observed in an unsteady flow should be less than predicted from
equilibrium relationships.
DEPTH (m) SPEEO (m/8)
Figura f.8 Hysteresis plots for various dune dimensions and flow properties. (A,B) illustrate the ideal hysteresis respon8e of dune length to changes in ffow depth and speed respectively. (C-0) illustrate the ideal hysteresis msponse of dune height to changes in flow depth and ipeed fespectively. Note direction of hysteresis is CCW in A, B, C, suggesting that bedformi are Irrger during decetemting flows than conesponding accelerating flows. (D) illustrates that dune height will begin to decrsase as critical flow conditions are neared, therefore, the hystaresis response in dune height with changes in flow speed may be either CCW or CW depending on conditions. (After Dalrymple and Rhodes, 1995, pg. 393)
A lagged response is commonly observed within a variety of changing
flow conditions in estuaries. Seasonal changes in flow (Nasner, 1978;
Kostaschuk et al-, 1 989; Kostaschuk and Illerçich, 1 995) generally produce
regular shaped hysteresis loops for both dune wavelength and height. In
contrast morphological change induced by neap-spring tidal cycles (Allen and
Frïend, 1976b; Terwindt and Brouwer, 1986) only appears to impact dune
height This is due to the shorter response time required to alter dune height
compared to length. Hysteresis response with respect to height is sometimes
difficult to discern within a neap-spring cycle as the loop direction is not always
positive (Figure 1.8D). This leads to increased variability within the hysteresis
plot where flow conditions reach a stage within the dune stability field where the
dune crests begin to be ptaned off with increased flow. Given a long enough
time interval and large enough range of conditions the hysteresis loop for dune
height would take the fom of a figure eight (Dalrymple and Rhodes, 1995)
(Figure 1 -8D).
Changes in dune morphology over a single Cdal cycle seem to be rare.
especially with larger dunes. This would suggest that the lag tirne of larger
dunes is greater than the time required for one half tidal cycle (6 houn in a
semi-diurnal environment). However, lime research has been conducted to
detemine wtiether this hypothesis is true in areas subject to significant tidal
induced flow accelerationldeceleration events.
1.3 Purpose and Objectives
The Fraser River Estuary near Vancouver, British Columbia has a strong
tidal influence and possesses large subaqueous dunes (Kostaschuk et al.,
1998). Previous studies of dunes and their associated flow interaction in the
Fraser have been undertaken primarily by point sampling of two dimensional
flow velocity, and by observation of bedform shape, size and migration using
echo-sounding (e.g. Villard and Kostaschuk, 1998). Much of this research has
been targeted at specific events involved with sediment movement such as kolk
and boil formation (Kostaschuk and Villard, 1999; Kostaschuk and Church,
1993) and to a large extent the effect of tidal inffuence has not k e n adequately
expfored. The purpose of this research is to examine tidal effects on flow
structure and dune morphology in the Fraser River Estuary. This study has
three objectives:
1. Measure changes in flow structure and dune rnorphology of a group of
large dunes in the Fraser River Estuary over a complete tidal cycle.
2. Determine the response of flow structure and dune rnorphology to tidally
induced unsteady flow.
3. Analyze the relationship between fiow structure and dune rnorphoiogical
response over a full tidal cycle.
The field work for this study was camed out on June 20, 2000 using an
acoustic Doppler profiler and digital survey echosounder to monitor flow
velocity and dune morphology over one tidal cycle.
2.0 STUDY AREA: FRASER RIVER, BRITISH COLUMBIA
The Fraser River is the largest river on the west wast of Canada and is
one of the largest un-darnrned rivers in North America (Milliman, 1980). The
Fraser exceeds 1200 km in length. originating in the Caribou Mountains of the
British Columbia intenor and empting into the Strait of Georgia just south of the
City of Vancouver (Figure 2.1). The Fraser River basin drains 250,000 km2 of
mountainous terrain. The Main Channel of the Fraser River estuary is a major
navigational corridor and represents the primary distributary within the Fraser
River delta. Much of the Main Channel's f o m and proœsses, including: bed
morphology (Kostaschuk and Illersich. 1995; Kostaschuk and Villard, 1996a),
sediment characteristics (Milliman, 1980; Church et al, l987), sediment
transportation (Kostaschuk and Villard, l986b; Kostaschuk et al, l9Q8), and fi ow
characteristics (Kostaschuk and Church, 1993; Best et al, in prep) have al1 been
researched and documented over a wide range of temporal, spatial and
environmental conditions. The study area chosen for this investigation was a
reach of the Main Channel of the Fraser River estuary located adjacent to
Steveston Harbour (Figure 2.1). Criteria for selection of this site are discussed in
section 3.1. This chapter will outline the general environmental conditions
present within the Main Channel and provide background information on the
sedimentary processes active in the study area.
Richmond
I
J 1
49"04'59.38"
Figum 2.1 Study area, Main Channel Fraser River estuary, British Columbia, Canada.
2.1 Flow and Sediment Characteristics
Discharge in the Fraser River is characterisücally low in fail, winter and
eariy spring, with minimum values approaching only 1 000m3s"(~ostaschuk et
al., 1989). In May wamer temperatures and spring rains result in the annual
snowrneit freshet and river discharge rapidly increases, peaking in eariy June
with values on the order of 6 000-12 000 m3s" (Figure 2.2). River discharge
steadily deciines until late August when it reaches lower consistent fiow values.
Mean annual discharge of the Fraser River is 3900 m3s" at the Port Mann
gauging station near Hope, 35 km upstrearn of Sand Heads. Approxirnately
27
eighty percent of the Fraser River's total flow is carrieci by the Main Channel
distributary through the delta and into the Strait of Georgia. At the study site the
Main Channel is approxirnately 1 -5 km wide and I O - 12 m deep at low tide. A
hydrograph depicting river discharge at Hope, British Columbia, illustrates the
fiow conditions present over the 2000 field season (Figure 2.2).
8000 - 7Oûû - 6000-
5000-
4000- -',
Study Period (June 7 - June 20) 3000 -
Figum2.2 HydrographsofFm~rRiverdischarge(Q,inmss~')showing(A)entirs 2000 discharge and (B) discharge encompassing the shrdy period between June 7 and June 20. Note red lino on (6) indicam study period.
The Strait of Georgia is a seml enclos&, high-energy marine basin
(Kostaschuk et al., 1998). Tides are mixed, mainly semidiumal (Thomson.
1981), averaging 3 rn near the mouth of the Main Channel. During spring tides,
tidal range will often approach 5 m. Each day mixed tides characteristically
exhibit a lower low tide, lower high tide, higher low tide and higher high tide
(Figure 2.3). Salt water intrudes from the Strait of Georgia upstream into the
estuary with each flood tide and recedes with each ebb tide. The tem 'salt
wedge' is frequently used to describe this intrusion of sak water (Pethick, i984)
and the feature is rnost prominently developed in the Fraser River during times of
high river flow (Kostaschuk and Atwood. 1990). During periods of low river
discharge, rnixing between the saltwater and river flow is increased and a
prominent salt-wedge does not develop (Hodgins et al. 1977). Salt-wedge
position within the Main Channel of the Fraser River Estuary is a function of river
discharge and tidal height (Kostaschuk and Atwood, 1990). Over one tidal cycle
(approximately 12h) river discharge will remain relatively constant therefore tidal
rnovement will dictate much of the flow variation within the estuary on a daily
basis. Low speed, unsteady flow conditions generally prevail during rising tides
when the landward tidal flow creates a resistance to river discharge. As the tide
falls, fiows become seaward directed. For a period around low tide,
approximately 2 to 3 hours, relatively steady state, higher speed flows occur
when river and ebb tidal flows are in the same direction.
Study Period
June 18 June 19 June 20 June 21 June 22
Figum 2.3 Plot of tidal stage aunounding the June 20 measuremenb taken during this study. Dashed r d line illustrates that tïdal stage was nearly identical at start and end points of the study period.
The Fraser River supplies on average 17.3 million tonnes of sediment
annually to the delta in the Strait of Georgia (Mclean and Tassone, 1991).
Unlike most large deltas, the Fraser's sediment discharge contains a high
proportion of sand (Orton and Reading, 1993). As a consequence of an
energetic river emerging from the mountains close to the sea, 35 percent of the
total load deposited in the Strait of Georgia is sand (Kostaschuk et al., 1998).
Bed sediment in the Main Channel has a median grain size of 0.25 - 0.32 mm
(Kostaschuk et al., 1989). Sand size bed sediment, significant flow velocities and
deep channels (= Zorn), provide an ideal environment for the formation of dune
bedforms.
2.2 Dune Morphology
Fraser Estuary dunes Vary in length frbm 4 m to greater than 100 rn and in
height from 0.3 m to greater than 5 m (Kostaschuk et al., 1 989a). According to
Ashley's (1990) classification, the Fraser River dunes would be mnsidered
medium to very large. Dunes in the Fraser are further classified as symmetric
and asymmetric forms (Kostaschuk and Villard, 4996). Symmetric dunes usually
are slightly larger and form in areas of higher flow velocities (Kostaschuk and
Villard, 1996). Asymmetric dunes are usually smaller and generally have
medium sized dunes superpositioned along their stoss slopes (Kostaschuk and
Villard, 1996). Multi-track surveys of bedforms greater than 10m in length reveal
a concave-downstream plan form with crests that are continuous for at least
300m across the channel (Kostaschuk and MacDonald, 1988). This pian f om
orientation suggests that the Fraser Estuary dunes are primarily two-dimensional
in Ashiey's (1990) classification. Dune morphology has been shown to Vary with
changing Rows associated wiai the annual freshet, however these changes lag
behind the seasonal variations in river discharge (Kostaschuk et a/., 1989).
31
Chapter 3
3.0 METHODOLOGY
This chapter outlines the field and laboratory procedures utilized for the
collection and analysis of data used in this study. In order to meet the objectives
of this study several factors were considered. Firstly the design of this
experirnent requires that measurernents be taken over one full tidal cycle. Due to
the nature of flow conditions in the Main Channel, a cycle from one high tide to
the next high tide provides the clearest picture of the processes during peak fiow
conditions surrounding low tîde. Secondty, measurements were undertaken
during the 2000 snowmelt freshet in order to ensure the presence of high river
discharges and large dunes. Larger dunes are of particular interest because they
can affect navigation and are easier to resolve with the survey equipment
available. Thirdly, data collection was timed as to correspond to days with large
tidal ranges. A large tidal range may lead to a greater contrast between channel
flow velocities at high and low tides. This range in tum leads to larger changes in
flow conditions and bed morphology. For the 2000 field season June 20 was
viewed as optimal in order to take advantage of the combination of high river
discharge and high tidal ranges.
3.1 Channel Reconnaissance and Boat Setup
The University of British Columbia Oceanography launch was used as the
platform from which al1 rneasurements within this study were taken. This vessel
is a 19' alurninum-hulled craft designed for experimental use. !t incorporated a
sturdy center console walk-around design that easily accommodated the
instruments used in this study (Figure 3.1).
A preliminary reconnaissance echosounding survey was undertaken along
a stretch of the Main Channel from Sands Head to East of Steveston Harbor.
The purpose of this reconnaissance was to locate a prominent dune field along
the centeriine of the channel to use as the setting for this study. It was essential
that a dune field transected by the centerline of the channel be located so that
the survey line could be easily retraced. Navigational aids set in place to ensure
safe travel of large ocean going vessels as well as stationary landmarks aided in
the reproducibility of the survey line (Figure 3.2). An appropriate dune field was
located in Main Channel just outside the entrance to Steveston Harbor. extending
neariy Ikm upstream. Channel marken and stationary objects on land were
used to determine start and end points for each transect along the chosen survey
line.
Three main instruments were mounted on the launch: a Trimble
AgGPS122 differential global positioning (DGPS) unit, a Oœan Data Equipment
Bathy 1500 Survey Echo Sounder and a Sonfek 1500 kHz 3-beam Acoustic
Figum 3.1 Photogmphs of University of Briüsh Columbia Oceanogmphy taunch as outfitted for thk study. Note locations of Bathy 1500, notebook PC, Bathy and ADP tranrducers, and DGPS antennae.
Figum 3.2 Diagram showing atudy area with rtart and end points as well as channel centeriine. Communication tower and marker outside Stieveston Harbor used to establkh start line. Channel marker and shore piling wed to establish end line. Centtedine markers used to keep suwey line stnight and on the same repeatable portion of the channel.
Doppler Profiler (ADP). The ADP and Bathy transducers were mounted on
opposite sides of the rear of the launch, rninimiùng both electrical interference
between the two devices and instrument displacement due to surface waves and
boat wake. The DGPS receiver was mounted high on the side of the center
console of the launch to ensure ciear reception of both satellite and ground
based position signals. The ADP and Bathy 1500 were both connected to the
DGPS unit, each receiving their own stream of NMEA navigation strings
containing identical positional information.
Data collection from the ADP was assembled through a notebook
cornputer equipped with a Sonterm terminal emulator that allowed for a reliable
interface with the instrument. Sontem is a DOS based teminai emulator
created by SonTek for use with its ADP instruments. Although newer Windows-
based software exists, in preliminary tests it proved to be neither as stable or
reliable as Sonterm. Depth information from the Bathy 1500 echosounder was
displayed in real time on a LCD display. This information was simultaneously
recorded in digital farm ont0 a mass storage device. The mass storage device
stores ASCII text data records of al1 information processed by the Bathy 1500.
The mass storage device can also be Iinked to a PC for further analysis or
cataloging of the data.
3.2 Collection of Velocity Profiles
Velocity profiles were collected along the survey line from the moving
launch using the ADP linked to the DGPS receiver. The ADP uses the Doppler
shift in the frequency of the acoustic signal reflected from scatterers in the water
to estimate the water velocity relative ta the instrument (SonTek, 1999).
Scatterers are srnall particles within the flow that are considered to be moving at
the same speed as the flow. These particles will then induce a frequency shift in
the reflected awustic signal, which is correlateci to flow velocity.
The ADP utilizes an intemal compas to define flow direction and a tilt
sensor to correct for ship pitch and roll (SonTek, 1999). The DGPS uses a
differentially-corrected signal from a navigation beacon located in nearby
Richmond (UTM: 491494.94, 5445771.13, LL: 4g01 O'N, lZ3°07'W 304/.305),
which allows for a spatial precision of less than 1 m. The three transducers of the
ADP (Figure 3.3) are set at 25 degrees from the vertical axis and are equally
spaced in the horizontal (1 20°) (SonTek, 1999), producing different orientations
relative to the flow (Kostaschuk et al., in prep.). Since the relative orientation of
the three transducers is known, combining these three dong-bearn velocity
profiles allows construction of a three-âimensional velocity field for the flow
relative to the instrument. Velocity data for each individual profile is automatically
rotated by the ADP in order to orient the resolved data in the direction of the
mean flow velocity. Measurement of three-dimensional velocity can be sampled
as fast as 1 Hz in up to 100 different depth incremenb, or bins, within the profile
(SonTek, 1999). Figure 3.4 illustrates the ADP deployment and beam sampling
configuration.
Figun, 3.3 SonTek three beam ADP (after SonTek, 2000).
200kHz Echosounder
Figure 3.4 Diagnmatic iepresentation of launch with expeiimental setup, including; SonTekN 1500 kHz ADP, and 200 kHz echosounder.
The static diameter of the ADP sampling area increases with depth to a
maximum of 0.93 depth at the bed (SonTek, 1999). This means that the velocity
rneasurements nearest the bed in dune troughs will be unreliable, because the
three ADP beams will encounter the bed at different depths. Kostaschuk et al.
(in prep.) suggest that a mean bed position from the ADP data can be
determined by a sharp increase in echo intensity, averaged over the three
beams. The infiedion point above the maximum echo intensity will represent the
transition between the bed and the water column, and therefore, the ADP bin
above the infiection point can be used to define the lower Iimit of unwntaminated
velocity measurements. Kostaschuk et al. (in prep.) determined that a sampling
interval of 5 s provides the best combination of low signal noise, staMe velocity
measurements and good spatial resolution over dunes. This sampling rate was
also used in this study.
Measurernents of Row data were recorded along successive passes of the
survey line identifieci during reconnaissance. On June 20, continuous passes
were made of the survey line for a total of 18 transects.
3.3 Flow Depth and Dune Geometry
Flow depth and dune geornetry were measured simultaneously with the
ADP data using the Bathy 1500. This device mets or exceeds the International
Hydrographic Organization (IHO) requirements pertaining to survey echo
sounder equipment (Ocean Data Equipment Corporation, 2000). Under the
conditions of this study (flow depth < 40m) the Bathy 1500 has a resolution of
7 cm and an accuracy of c 2.5cm (Ocean Data Equipment Corporation, 2000).
Echo sounding with the Bathy 1500 is perfomed using simple physical
principles. The transducer sends out an acoustic pulse in a narrow beam (3"),
which travels though the water column to the bed. Once the signal hits the bed it
is reflected back to the transducer. Considering the known value for the speed of
sound in water (- 1500 ms-') and the signal retum rate, the Bathy 1500 then
processes this information into a depth measurernent for the water colurnn.
During this study the Bathy 1500 recorded depth, time, and position at a
sampling rate of 9-1 0 Hz.
3.4 Data Reductions
Before calculations could be perfarmed, the raw data had to be modified
into a more useabte format. Severa! methods were used in the data reduction
process to produce a clear picture of flow structure and dune response to the
changes in the Main Channel's mean velocity over one tidal cycle. This chapter
section will outline in detail Me precise methods by which this data reduction took
place and the reasoning behind why it was done in this manner.
3.4.1 Echoaounding Data
The raw data recorded by both the ADP and the Bathy 1500, were
accompanied by a NMEA string of positional information relayed from the Trimble
DGPS unit. Within this string of data were several key pieces of information
including geographic position in degrees latitude and longitude (LL) and
Greenwich Mean Time (GMT). Although this positional information is highly
accurate it is difficult to determine the distance between points using LL
coordinates in degrees. Finding the distance between points is essential in
forming a length scale from which to derive dune measurements. Therefore it
was necessary ta convert al1 coordinats from LL to Cartesian based coordinates
within the Universal Transverse Mercator projection (UTM) NAD83- This
projection is frequently used in the design of nautical charts and also allows for
easy display of each individual transect on a digitized map. The LL data were
converted to UTM coordinates using Traline, a program produced by Mentor
Software Inc. Positional data recorded by the ADP was also convertecl to UTM
so that the ADP and echo sounding records could be more easiiy correlated later
in the analysis procedure.
Despite the precision of the Bathy 1500 there still remained significant
scatter within the raw data set. This scatter was primarily the result of the
acoustic signal of the echo sounder being reflected by suspended sand in the
water column. The outline of the bed was clearly visible, as scatter only exÏsted
above the bed, not below. To rernove this scatter and establish a useable digital
map of the bed a program was devised using the Matiab math programming
Ianguage. This program used a simple two-step procedure that allowed for the
identification of the bed. The fint step involved a manual onscreen digitization of
the bed outiine. The program then eliminates al1 points greater than 5cm from
this outline effectivety creating a smooth accurate representation of the bed.
By using a coordinate system that is based on a Cartesian plane with
identifieci axes in Meters North (MN) and Metres East (ME) it is possible to
determine the distance between points using the equation of a fine.
Once the distance between each point was calculated the total distance of the
transect was seen to be represented by the sum of all the other distance
intervals.
DTOd = (d, 9d2 .d3 -..--dn) 134
3.4.2 Merging ADP and Echo-sounding Records
Merging of the ADP and echo-sounding record was necessary in order to
correlate the two data sets. This correlation was an integral component of the
data reduction process, as the detailed positional information sent from the
DGPS unit was not completely recorded by the Bathy 1500. The positional
information recorded by the Bathy 1500 was inadequate as a means of spatial
representation of the bed because it had a 10 rn resolution. To alleviate this
problem the more detailed GPS information stored in the ADP record needed to
be combined with the depth information in the echo sounding record. The
problem with this merging process was that the ADP had sampled at 0.2 Hz and
the Bathy 1500 at Hz. Direct merging of these data sets would result in a loss
of much of the depth information contained in the echo sounding record.
In order to retain the integrity of the echo sounding record, a method for
expanding the positional information in the ADP data set was developed. In
order to accomplish this, a program was created to disaggregate the positional
information contained within the ADP record and produce a data set that
contained the same number of points as in the echo sounding record. Within the
intervals between the original points of the ADP record. evenly spaœd points
were generated to create an artificial sampling rate of $Hz. This procedure was
cornpleted with the assumption that the boat speed did not change significantly
over the 5s interval on the ADP record. The generated positional information is
only an approximation but is suitable for this research.
Once the positional information of the ADP record was expanded it was
combined with the echo sounding data. This was achieved utilizing spread sheet
and data base programs. Using the DGPS dock as a reference, the two sets of
data were combined using a simple query within Microsoft Access. The resultant
data sets were then imported into Microsoft Excel for further anatysis and data
organization.
3.4.3 Dune Statistics and Velocity Profiles
In order to acquire a detailed picture of flow structure and dune response
over the test period, several key parameters were requifed. The velocity profiles
recorded by the ADP are an excellent record of water movement within the water
column but do not estirnate the energy contained within the flow that is
responsible for sediment movement and dune rnorphologicaf response. An
understanding of the total stress evident within the flow was gained from shear
velocity (u.) estimates derived from the velocity profile information. McLean et
al. (1999a,b) observed that velocity profiles near the dune crest will
underestimate total stress and profiles in the trough will overestimate i t They
proposed that a profile between the crest and trough could be as accurate as a
spatial average over an entire dune in deterrnining the total stress within the
water column. Given these findings Kostaschuk et al. (in prep) raüonalized that
selected ADP velocity profites spatially averaged over the upper stoss and crest
areas of dunes wouid result in the best representation of total stress over dunes
in the Fraser. The profiiing procedures used in this study are similar to those of
Kostaschuk et al. (in prep) so spatial averaging over the upper stoss and crest
areas of the Fraser dunes was utilized in this study.
In order to attain a clear picture of the energy transfer from flow to the bed,
shear velocity (u, ) was calculated from the acquired velocity profiles. McLean et
ai. (1999a) note that velocity profiles are often segmented respect to their
logarithmic relationship with flow depth. Total shear velocity (u.,) provides the
most reliable indication of momentum transfer from flow to bed and is calculated
using the upper Iogarithmic portion of a velocity profile (McLean et al., 1999a).
Shear velocity was therefore calculated from the upper log-linear velocity profile
segments using the 'law of the watt' [i -11. Shear velocrty (u. ) and roughness
fength (2,) were calculated using linear regressions of the fom u = m ( h ) + cc.
Shear velocity is therefore:
13-31
and roughness tength is:
Shear velocity and roughness tength were calculated for each dune in
each transect. These values were then averaged to attain a single
representative mean value for each individual transect (G. ,z,). In addition to K.
and mean velocity values were also calculated using the entire water colurnn
(O,) and the upper log Iinear portion of the flow (Og) for each successive transect
line.
Several descriptive statistics are used to illustrate dune shape and form,
particularly height ( H , ), tength ( L, ), lee side dope angls (B) and steepness ratio
( H L ) Ail of these statistics are used within this study to illustrate and
document the morphological response of the Fraser dunes over a single fidal
cycle.
3.5 Plotting Survey Transects
Each transect in this study was run down the centerline of the Main
Channel in the Fraser River estuary. Despite the use of navigational aids to
remain on a consistent course, several factors rnay have led to periodic deviation
fom this path. Wind, waves, current and boat wake al1 impede the pilot's ability
to keep the boat on course. A Geographic Information System (GIS) was used in
this study to determine if the survey Iines were consistent and the same bed
features were continuousty monitored.
The first step in the procedure was to create a base map of the study
area, including al1 pertinent navigational aids and landmarks. This was
undertaken by the digitkation of a Ministry of Fisheries and Oceans Canada
(1997) nautical diart (Fraser River, Sand Heads to Douglas Island, scale
1 :20,000) of the study area using Atlas GIS. This digitization produced a detailed
vector-based image of the study area. The second step involves moving the
vector-based image of the study area as wetl as the DGPS coordinate
information from each individual transect, into Idrisi 32, a raster based GIS
program. Raster-based GIS programs allow for easier overiay analysis, which is
integral in the cornparison between transe*. By overiaying individual transect
lines on the digitized base map, iiiconsistencies and irregulanties wioiin the
transect lines becorne apparent.
3.6 Equipment and Experimental Limitations
Several unforeseen equipment problems occurred mer the course of this
experiment. The resolution of positional information recorded by the Bathy 1500
was not as high as what was provided by the Trirnble DGPS unit. Therefore,
positional approximations were necessary. Depth information recorded by the
Bathy 1500 became increasingly unretiabte as mean velocity in Main Channel
increased and sand was suspended. The increase in sedirnent movement
produced a false bottorn within the depth record due to the high density of
scatterers present in the water column. Transects 9 - 14 on June 20 were
impacted enough by this false bottom signature that no reliable information on
dune location or characteristics could be dsawn from these records and additional
dune measurements were acquired through manual interpretation of the Bathy
1500's digital paper trace.
47
Chapter 4
4.0 RESULTS
Throughout the study period favorable sampling conditions were
encountered within the estuary. Low winds produced on@ a small chop across
the surface of the estuary (wave heights generally 0.5m or smaller) allowing the
survey vessel to nin each survey line with little surface displacement. Low trafic
density within the Main Channel ailowed for continuous survey lines which were
only occasionally intempted by boat wake induced by large sea-going vessels.
This chapter presenh the results acquired from the fieid survey. Results
will be divided into three sections. The first section summarizes the results of the
GIS analysis of survey techniques, the second section describes Row conditions
and the third section outlines changes in dune morphology.
4.1 Consistency of Survey Technique
The alignment of the survey vessel for each transect line relied on the %ne
of sight' technique described in Chapter 3. Each separate transect could deviate
from its intended path due to natural or commercial interruptions, therefore GIS
analysis was used to examine the reproducibility of the survey line. Figure 4.1
displays the results of the GIS overlay analysis used to illustrate the location and
deviation of the transect lines. The area outlined in this figure represents the
complete spread in transect position over the course of measurement. Due to
software constraints the' resolution of the GIS analysis is in the 10 m range,
where the resolution of the positional information acquired by the Trimble GPS
unit was an order of magnitude lower. Despite the inflated resolution of the GIS
analysis Figure 4.1 clearly demonstrates that al1 transects fall within a band no
more than 60 m wide and most transects falf within a band 30 m wide. lncreased
deviation from the intended path is apparent along the upstream end of the
survey reach. However, measurements used in this study are derived from the
first 250 m of each transect where lime deviation occuned from the intended
w m e .
4.2 Flow in the Fraser Estuary
A total of 18 surveys of velocity flow structure were executed during the
field study on June 20, 2000. Table 4.1 displays a summary of flow variables
calculated from velocity profiles averaged over dune crests in each transect.
Estimates of ü. and r, are based on equations 13.31 and 13-41 respectively.
Resulta of the GIS analysh of survey line. Contours represent the nurnber of transect passes that occurred in each respective area ( represents 14 tmnsect passes, E represents 44 transect passes, repmsents 9+ transect passes).
Traosect
Table 4.1 Summary of average flow variables calculaoed over each suwey traniect. 0, is the mean velocity for the total flow, 0, is the mean velocity for the upper flow (part of profile from which % is derived), iZ k the mean shear
velocity, r2 is the mean coemcient of detemination. Zo is the mean roughness hm@.
Depending on dune length, either 4 or 5 velocity profiles were used to
assemble a single average velocity profile over each dune crest in every
transect. In transects 1-3, velocity profiles exhibited pronounced segmentation
with a distinct upper and lower loganthmic section (Figure 4.2). In these cases it
was the upper portion of the velocity profile that was used to calculate Z., as the
upper segment from the crest is thought to be representative of the total stress
active within the system (McLean et al, 1999). In transects 4-18 little
segmentation was found to occur within the velocity profiles and regression was
carried out on the portion of the profile which provided the most significant
regression relationship. In these cases the lower 1 Sm - 2.0m and upper 0.5m
were excluded from analysis. Examples of typical velocity profiles and
regression results are shown on Figure 4.3.
Figure 4.2 Segmented velocity profile ove? Dune 6, Tmnsect 1. Profile is representative of velocity profiles observed within the first three transects.
O 90 la, 150 Po 250 JX) U (crns-')
Figum 4.3 Typical velocity profiles used to calculate u- over individual dunes. (A) Iltustrates aie velocity profile over Dune 8, Transect 1. (B) Illuabates the velocity profile over Dune 2, Tmnsect 3.
Mean velocity (Ut) in the Main Channel fluctuates with the nse and fall of
the tide (Figure 4.4). Velocity is low at early high tide (07:30 - 09:00), and
inaeases as the tide falls. Peak values for Ut are reached near low tide (14:15)
and then dedine as the tide rises and a slight increase in Üt may occur as the
second high tide approaches (19:15). Figure 4.4 aiso cleariy illustrates that the
rate of flow deceleration during the tidal rise appean to be faster than flow
acceleration dunng the falling tide. The mean velocity of the upper flow (Od)
mirrors the pattern of mean velocity of the total fiow throughout the tidal cycle
(Figure 4.4). The magnitudes of 0~ are slightly higher than those of Ut, with the
largest disparity occuning around low tide when flow velocity is highest
The coefficient of determination ( r 2 ) derived from regression analysis of
velocity profiles are shown on Figure 4.5. Values of F' range from 0.223 to
0.540, but indMdual velocity profiles exhibit values up to 0.730. Values of F' are
highest at high tide (07:00 - 09:00) aien drop and follow the pattern of mean
velocity. Only profiles with significant (95%) r2 values (Shaw and Wheeler,
1985) were used in the averaging procedures utilized throughout this study. Out
of the 128 profiles recorded 21 were not significant and these were excluded
from subsequent analysis. A complete listing of significant velocity profiles
present in each transect and corresponding flow variables can be found in
Appendix A.
Mean shear velocity (2. ) (Table 4.1) for al1 the transect lines ranged from
4.6 to 13.3 cmd over the tidal cycle and follow a similar pattern to r 2 . Shear
velocity followed the pattern of tidal rise and fall after approxirnateiy 09:OO (Figure
4.6) but the highest values were measured during weak flows at high tide
between 07:00 - 09:OO.
350-
a 300-
1 250- g m m ~ g A A ~ A O Y 5 m a 200- 9 F L
3 3 150- ::
A 9
5 100- 9 ' 50-
O u I I 1 I 7:OO 9:24 1 1 :48 1432 16:36 19:OO
Tirne (h)
Figura 4.4 Average mean velocity for each tisrwect, calculated for 0, (A) and o,,,(m).
Time (h)
Figum 4.5 Average coefficient of chbmination (F' ) exhibiteci by velocity profiles over one tidal cycle.
Figum 4.6 Summary of mean sherrr veiocity values caicukted over one tidal cyck
14-
12-
n 70-
E 8- v v 6-
1% 4-
2-
O ,
Mean roughness lengths (5,) are highest during the weak flows at high
tide (07:OO - 09:00) and decrease to consistent values over the rest of the study
period (Figure 4.7). There appears to be no direct relationship between changes
in tidal Row conditions and <. Relatively small and constant values of Z, occur
just before and during low tide when current speeds are highest (1 1 :30 - 1500).
I
m m
m
I 1 1 i 1 7: O0 9: 24 1 1:48 14:12 16:36 1 9:OO
Time (h)
Time (h)
Figum 4.7 Average roughness length (i, ) calculated for each transeet during the course of one tidat cycle.
4.3 Dune Morphology
Eight distinct dunes were identified for analysis along a 250m downstream
section of the 1 km survey line (Figure 4.8). These dunes were selected because
they could be identified on al1 transects. Figure 4.9 illustrates dune profiles over
Me course of the tidal cycle. Recognition of dune fomi within the echosounding
record was straightfomuard h i l e fiow velocity was low. However, as flow velocity
increased more scatter was observed over each profile due tu refiectanœ of the
digital echo-sounder's acoustic signal by suspended sand (Figure 4.10). Plots
(A) and (8) (Figure 4.1 0) represent echosounding measurements recordeci
dunng low flow (08:30) and high flow (14:30) respectively. Each plot contains
simiiar amounts of data points Rom start to finish however (A) cleariy exhibits a
57
much üghter density of reflectance than (B). The increased variability in (8)
made it impossible for the Matlab bed finding algorithm to dissociate between
signal reflectance due to suspended sand frorn the bed. Therefore, during peak
Rows when sediment movement was greatest. (f 3:45 - 17:00, Transects 9-1 3) it
was not possible to identify bedforms on the digital depth records. Consequently,
al1 dune measurements in Transects 9-13 are based on the 'digital paper trace'
provided by the Ocean Data Equipment Bathy 1500w software package.
O 50 100 1 50 200 250 Distance ( m )
Identification of dunes - 8 dong the suwey lirte. Transect 2 (08:30) is used to illustrate these featu res.
I
Transect 1 S(1 W O )
- 6
Transect l5(18:52)
I
O 5 O 1 5 0 Z O O
Dune Length (m)
Rgum 4.9 A Cornparison of dune prof ik over one tidal cycle (June 20,2000). Note that flat portions of some dunes are due to suspended sediment scattering of acoustic signal and the inrbility of the bed finding algorithm to interpolate miuing depth values.
5000 10000 15000 20000 Number of Data Points
Figum 4.10 RPW echosounding mcord. Plot A reprewnts a 'cian' data aat before any further data reduction ha8 taken place. Plot B represenls a transeet near low tide whem sediment movernent (nota heavy scatter above bedfonns) i n h i b i the recognition of complede dunes.
60
At least 7 of 8 identified dunes were recognizabte inevery transect line.
Based on observations al1 eight identified dunes appear to rernain largely
symmetric in shape throughout the tidal cycle. Mean dune height, length,
steepness ( / zd ) and lee face slope angle are summarized in Table 4.2.
Table 4.2 Mean dune characteristics calculated over one tidal cycle. Where H, k
mean dune height, is mean dune length, N, Ird is mean dune steepness and B is the mean îee dope angle.
Mean dune height generally follows the pattern of mean velocity over the
tidal cycle (Figure 4.1 1). At the first high tide at 07:30, mean dune height was
1.04 rn, but by low tide mean flow velocity increased to 2.14 ms-' and average
dune height exceeded 2.0 m. During flow deceleration on the rising tide, dune
height begins to diminish towards values similar to those recorded a i the
beginning of the tidal cyde. However, the relationship between dune height and
tidal stage is less evident during deceleration than during the accelerating phase
of the tidal cycle.
Changes in individual dune height were also monitored over the study
period. Although relationships between individual dune height and tidal stage
exhibit more scatter #an the transect averages, evidence of height increasing
during the acceleration phase and then decreasing during the deceleration of the
fiow is apparent in most cases. Figure 4.12 shows individual dune response for
two of the eight dunes within this study. The response of al1 individuai dunes
over the course of the tidal cycle can be found in Appendix B.
The unsteady nature of flow during the experiment appeared to have little
effect on mean dune length (Figure 4.1 3) or individual dune length. Individual
dune length exhibits increased scatter when compared to transect averages.
The response of ail individual dunes over the course of the tidal cycle can be
found in Appendix B.
Time (h)
F@um 4.11 Mean dune height ove? one tidal cycle.
Rgum 4-12 Individual dune height (Dune 6) over tidal cycle.
3-
2.5- n
v 2- I 0 2 p 3.5-
a# = 1- 2
0.5-
O -
I
1 1 1 4 I 7:OO 9:24 1 1 :48 14: 12 16:36 19:OO
Time (h)
Tirne (h)
40 - 35 -
- 30- E
25- OI
5 20- d
g 15- a
I O -
Fi' 4.f3 Mean dune length over one tidal cycle.
m
m
Time (h)
5 -
O I 1 1 1 1
Figun, 4. f 4 Individual dune langth (Dune 6) over tidal cycle.
Mean dune steepness (Figure 4.15) reflects the changes in mean height
because dune lengths remain relaüvely constant. Mean steepness increases as
flow accelerates and decreases once the flood tide initiates the deceleration
phase of the tidal cycle. Changes in individual dune steepness are more diffÏcult
to determine due to increased scatter in the data set (Figure 4.16 and Appendix
B), though in rnost cases individual dunes foHow the pattern of mean steepness.
Tidal stage appears to have little impact on the tee face slope angle
measured during this study. No ciear relationship ernerges from the average
measurements (Figure 4.1 7) or fram individual dunes (Figure 4.1 8). Transect
average lee slope ranges from 7" to over 25" with the overall study average
being approximately 16". However, several individual dunes were recorded with
lee faces exceeding 50".
Time (h)
Figum 4.f5 Mean steepness ratio over one tidal cycle.
Figum 4.16 Individual dune steepness mtio (Dune 6) over tidal cycle.
0.15-
C4
2 = 0.1 - s - UJ V) Q, C 8 0.05- fi
O I 1 1 I 1
7:OO 9:24 1 1 :48 14:12 16:36 19:OO
Time (h)
Tirne (h)
Figun, 4.17 Mean lee face slope angle over one tidal cycle. (m) mpresents lee slopes calculated using digital record. (A) represents lee slopes calculated manually using Bathy paper trace.
Time (h)
Fjgurie 4. i8 Individual lee face slope angle (Dune 6) over tidal cycle. (i) represents iee dopes calculated using digital record. (A) repments lee dopes calculated manually using Bathy paper trace.
Chapter 5
5.0 DISCUSSION AND CONCLUStONS
This chapter explains the results presented in the previous chapter and
offers conclusions for the findings of this study. The first two sections interpret
the unsteady estuarine fiow observed over one tidal cycle and offer an
explanation of dune morphological response to tidally influenced unsteady flow
and the impact of dune morphology on the flow field. The third section
summarizes the discussion in a conœptuat modet tinking changes in Row during
the tidal cycle to dune morphological response. In the final section, the
conclusions of this study as welt as recommendations for future research are
out!ined.
5.1 Reaction of Estuarine Flow to Tidal Influence.
Although dominated by fluvial discharge, the Main Channel of the Fraser
River estuary is influenced significantiy by tidal forcing. The periodicity of the
ebb-flood action inherent in tidal motion leads to continuous adjustment of flow
variables over the cycle. Within the Results chapter several trends and
anomalies within the acquired ADP data set were found to occur over the course
of a tidal cycle. It will be shown that through careful analysis many of these
trends and anomalies can be togically exptained through current hydraulic
concepts.
5.1.1 Mean Velocity
When the flood tide opposes the direction of the river discharge the added
resistance to river flow will result in lower values of Ü,. Conversely, during low
tide when both river and tidal currents are in the same direction values of are
approximately two and a half times the magnitude of those recorded at high tide.
Although seemingly straightforward, the relationship between LI, and tidal
stage may not be as simple as it first appears. Along the falling limb of the tidal
cycle mean velocity steadily increases to a maximum value measured just after
low tide. Following peak velocity at low tide tide, Ü, begins to decrease. It also
appears that mean velocity rnay begin to increase again prior to high tide.
Although the tidal curent will still be directed against river flow, the upstream
propagation of the tidal wave, which is steepest early in the rising tide, decreases
as high tide approaches (Figure 2.2). Therefore, the river flow is impeded most
by the early rising tide, allowing Ü, to increase slightly as high tide is approached
and acceleration of the tidal wave subsides.
During the rising limb of the incoming tide E, decreases much more
rapidly than it increased during the previous half of the tidal cycle (Figure 5.1).
Several factors rnay a w u n t for this trend. The skewness observed in the cf distribution is probably due to tidal asymmetry over the observed tidal cycle.
Although the observed low tide was separated from each adjacent high tide by
approximately 7.5 hours, the amplitude of the tidal rise was nearly 1.5 m greater
than the proceeding tidal fal!. Therefore, it would be expected that during the
tidal rise, deceleration of flow would occur faster than the acœleration over the
same time period.
Mean velocity was always les than mean upper flow velocity because the
effect of bottom friction decreases away from the bed (Dingman, 1984).
However, the diRerence between Ü, and Üi is greater at low tide compared to
at high tide (Figure 4.3). This difference is probably due to changes in bed
friction and stratification due to suspended sediment, which occur over the tidal
cycle.
W . I I 1 1 I 7: 00 9: 24 1 1:48 14: 12 1636 19: 00
Time (h)
Figure 5.1 Absolute rate of change of mean velocity over the tidal cycle. Rate of change calculated as the average change in velocity per second within each interval plotted.
Fricüonai resistanœ in the Fraser River estuary is mntrolled primarily by
the fom drag produced by dunes (Villard and Kostaschuk, 1998). As suggested
by Dyer (1986) fomi drag will inevitably increase as dune height increases if
there is no change in dune lengai. Since dune height was greatest at low tide,
form drag would also be greatest at this time. Surface flows will respond more
slowly than mean flow to changes in f o n drag, therefore resula'ng in a greater
difference between ÜI and Üg at low ode.
In addition to changes in f om drag, the flow density field may be altered
over the course of the tidal cycle.
concentration near low tide is evident on
This increase in sediment concentration
reduce the flow velocity close to the bed.
An increase in near-bed sediment
the echosounding record (Figure 4.9).
will stratify the flow (Dyer, 1986) and
This will have an impact on Ü, more
than (If and therefore increase the difference between mean and surface
velocities.
5.1.2 Shear Velocity
Soulsby and Dyer (1981) have suggested that in accelerating fiows the
use of a logarithmic regression to obtain U. from velocity profiles will lead to an
underestimation of U. (Figure 1.5). Furthermore, Dyer (1 986) proposes that
away from the bed the relative importance of inertia to fnctional effects is greater
than near the bed and consequently accelerating currents wilt retain a 'memory'
of the preceding driving forces away from the boundary. These conditions imply
that the high values for U. at the beginning of the acceleration phase would be
even higher than calculated. However, there is no detectabte sediment
movement within the echo sounding record during this period and Ü, is very low.
This suggests that the calculated high u. values are not representative of the
actual shear stress present at the bed.
Explanation of the anomalous values calculated for U, appears to Iie with
the visibly kinked velocity profiles observed within the first three transects f Figure
4.3). Although Smith and McLean (1977) ascertained that velocity profites over
dune crests characteristically contain two separate logarithmic sections, the
lower profile in their rnodel represents flow conditions present directty adjacent to
the M. Throughout this study measurements within the lower portion of the
flow (- 1.5 m) are not sufficientfy detailed to define the lowermost logarithmic
section of the velocity profile and therefore the 'kinked' velocity profiles identifid
in the first three transects must have another exptanation.
The cause of the kinked velocity profiles relates to the 'memory' concept
outlined by Dyer (1986). However, in this case it appears that the frictional
forces next to the bed supplement the memory of previous flow conditions and
not the inertial forces of the upper fiow. At high tide river currents are
suppressed by the inland action of the flood tide (Pethick, 1984). As the tide
begins to ebb, both tidal and river flows are now in the same direction but the
higher energy river flows gain momentum quickly and push out overtop of the
slower moving fluid near the bed. The near-bed flow will retain a 'memory' of the
previous rising tide's flow conditions until the slower moving Ruid c m overwme
the frictional influence at the bed. Eventually, the upper profile will extend
downward due to mixing between upper and lower poroons of the water column
until no relic of the previous high tide flow suppression exists.
It is evident from analysis that the upper segment of profiles eariy in the
tidal cycle does not accurately refkct shear velocity at the bed. Figure 5.2
illustrates the distribution of U. over the tidal cycle using averages obtained from
the lower segment of the kinked profiles for the first three transe&. The lower
segment here represents the 'memory' of the velocity profile from the rising tide.
The lower segment U, values are similar to those from late in the flood tide and
correspond better with patterns of mean velocity.
Time (h)
Figun, 5.2 Distribution of average shear velocity over tidal cycle. (I) Shear velocity as calculateci from UM. (A) Shear velocity as calculated from the lower logarithmic portion of U*
Despite evidence of cornplex velocïty profile segmentation within this
study k i n g lirnited to the acœierating wrrent foliowing high tide, the influence of
prior flow 'memory' may extend throughout the tidal cycle. Velocity profiles
assernbled by Lueck and Lu (1997) also illustrate evidence of flow memory
impacts. It is probable that previous flow memory extends throughout the tidal
cycle following a pattern similar to the one outlined in Figure 5.3. As the tide
begins to ebb u., represents shear conditions of the higher river flow as it extend
over slower moving flow at the bed. Shear stress influencing the bed eariy in the
tidal cycle is represented by u., which is a memory of previous conditions as
indicated by u., . As the tidal cycle progresses, u., mixes down to become u.,
with u., still remaining as a remnant of u., . After low tide as flow becomes
unsteady and mean velocity declines, u., no longer dominates the water colurnn
and develops into u., with a small portion of the water colurnn next to the bed
(u,, ) still influenced by u., .
Mean Velocity Flow VectMs
Unsteady Accelerating Flow Steady Flow Unsteady
Decelerat ing Flow
Velocity (cms") 200 cms"
Figum 5.3 Chancterisblc velocity profile evolution as flow accehrates from high tlde toward low tide.
5.q .3 Coefficient of Determination (2)
Average values for r2, and in turn the quality of the loganthmic relationship
identified in velocity profiles are low compared to other flume (Mclean et al..
1999a) and field (Wlard and Kostaschuk, 1998) experiments. However. due to
the large number of points sampled in each velocity profile (- 40), the
regressions used to calculate U. were significant (%95) (Shaw and Wheeler,
1985), although weak. Kostaschuk et al. (in prep) used the same ADP in the
Fraser River estuary but their low tide profiles had higher 3 values than those
found in this study. Flow velocity in the Fraser was rnarkedly higher dunng their
measurements than those recorded during this study and may have resulted in
more rapid development of an equilibrium profile. Dunes were also larger in their
study, which could cause less wake influence between dunes and therefore less
scatter in the profiles.
In any estuary flow will be most steady when the river flow is dominant. In
the Fraser River estuary, river flow is most dominant at low tides during periods
when discharge is high. Therefore, the stronger flows at low tide result in higher
3 values. When flows are influenced by rising and falling tides, conditions are
increasingly unsteady and non-uniform. This relationship suggests that lower 3
correlations would be expected during the ebb and flood. In addition Lueck and
Lu (1997) advise that log linear profiles are rnuch more difticult to detect during
weak flows. Therefore, the proposed evolution of the velocity profile over the
tidal cycle (Figure 5.3) may be accurate even though poor $ correlations during
the latter portion of flow deceteration would not allow for the clear identification of
camplex velocity profile segmentation.
5.1 A Roughness Length
There does not appear to be any pattern to the distribution of 2, over the
tidal cycle. If the 2, values obtained in the first three transects are discarded and
replaced by values calculated using the lower segment of the velocity profile, Z,
appears to be reasonably constant around low tide with increased scatter on the
rising and falling tides (Figure 5.4). The increased scatter is a function of the
elevaied flow unsteadiness present a i these portions of the tidaI cycle as was
discussed above in relation to ? values. Mean values for Z, calculated around
low tide correspond with previous values of r, obsenred by Kostaschuk et al. (in
P=P)-
7: 00 9: 24 11:48 16: 36 19:OO
Time (h)
Figure 5.4 Mean roughness length distribution over the tidal cycle. (i) represents Z,,
as calculated by the upper logarithmic profile and (E) represents T, as calculated by the lower logarithmic profile.
5.2 Dune Morphologicat Response
Previous studies located in the Fraser River estuary have identified two
main dune types that occur within the Main Channel: a long, low asymmetnc forrn
and a larger, more rounded symmetnc f o m (Kostaschuk and Villard, 1996a;
Kostaschuk et al, in prep.). The dunes observed in this study are similar to the
symmetric forms found by Kostaschuk et al (in prep) only slightly smaller. River
discharge had been consistentiy high for at least two weeks pnor to
measurement on June 20,2000 (Figure 2.2) and lower energy asymmetnc dunes
would not be expected to occur until river discharge decreased significantiy
(Kostaschuk and Villard, lW6a).
Dunes showed no perceptible change in shape over the tidal cycle,
rernaining syrnmetric throughout this penod. Dune migration undoubtedty
occurreâ around low tide when flows were highest, but migration was too small to
accurately quantify .
5.2.1 Dune Height
Dune height (4) is the most dynamic component of dune sire (Dalrymple
an Rhodes, 1994), and as flow conditions Vary, 4 is the fint morphological
characteristic to adjust dunes toward a new equilibriurn. Several shidies (Allen
and Friend, 1976a, b; Dalrymple, 1984; TenMndt and Brouer, 1986) have noted
significant dune height adjustment over spring-neap tidal cycles, with H ,
increasing toward the spring tide and decreasing away from it. Akhough these
observations are lirnited to intertidal dunes influenced by tidally-induced flow
reversal, they provide a context to examine the changes in H , recorded in this
study. In the Fraser River estuary seasonal (Kostaschuk et al., 1989) and neap
spring (Kostaschuk and Ilersich, 1995) changes in dune morphology have been
welt documented. There is no known research, however, regarding short term
dune morphological response in areas with unidirectional Row subject to tidally
induced flow acceleration/deceteration events.
Large dunes subject to tidally induced flow acceleration equiiibrate most
closely with the highest steadiest flowç, usually occurn'ng around low tide
(Dalrymple and Rhodes, 1995). It is also assumed that because no significant
alteration in dune size occun frorn one low tide to the next, the lag time needed
for an adjustrnent in dune morphology must be greater han the duration of the
tidal cycle (Allen and Friend 1976a; Termindt and Brouwer, 1986). However. this
study has demonstrateâ that adjustment in dune height does occur over a single
tidal cycle and therefore the lag in dune height may be far shorter than originally
conceived. Dune height increased by nearly 112% from the fi& high tide to
Transect 7 (1.04m to 2.20m) then started to decrease again as flow velocity
declined. Although measured values of H , at the end of the rising tide
measurements were not the same as the beginning of the ebb, further reduction
in H, would continue to occur as the tidal rke suppressed flow for an additional
2 hours.
Despite the lag response in H , appearing to be much faster than
originally anticipated, evidence for some dune lag may be present within the
acquired data set. A weak hysteresis response could exist over the tidal cycle
leading to the conclusion that a small lag in H , must occur behind the fiow
conditions (Figure 5.5). However, it is apparent that Figure 5.5 shows a direct,
linear relation between mean flow velocity and dune height (r = 0.72). Figures
4.4 and 4.1 1 also indicate that height increases during the ebb to a maximum at
low tide when flow velocities are greatest, then decreases on the rising tide.
Terwindt and Brouwer (1986) reported that increases in the height of interüdal
dunes was a result of scour taking place in the dune troughs rather than
deposition on the crest. It is likely thaï scour in dune troughs is also the
mechanisrn responsible for the observed increase in H , in this study. This
interpretation is supported by the high shear velocities over dune crests at low
tide, which would make crest erosion more likely than deposition (e.g.
Kostaschuk and Villard, 1996). As aie tide rises, scour in the dune troughs would
decrease and deposition would occur, resulti ng in lowenng of dune height.
Figum 5.5 Plot of dune height W. mean velocity over one tidal cycle
5.2.2 Dune Length
Response times for dune length ( L , ) in unsteady flows are often several
tirnes longer than corresponding changes in H , (Dalrymple and Rhodes, 1995).
Tenfuindt and Brouwer (1986) found that even over a spring-neap tidal cycle no
appreciable change in dune length occurs. In the Fraser River estuary, dune
length changes with seasonal variations in river flows associated with the annual
snowmett freshet (Kostaschuk et al., 1989). This study has shown that dunes do
not adjust their length over the course of a single tidal cycle in the Fraser.
5.2.3 Steepness Ratio
Due to the stability of dune length over the tidal cycle, the steepness ratio
( H d I L d ) is predominantly controlled by H , . Steepness therefore increases on
the ebb tide and decreases on the flood. Dyer (1986) suggests that flow
separation will occur over dunes with steepness values approaching 0.067. In
this study mean steepness exceeds this criterion mainly at low tide. Steepness
values of individual dunes do frequently reach the critical value of 0.067 during
stronger flows and several transect averages achieve H, IL, values of 0.060 or
greater. It is therefore Iikely that intermittent flow separation is present over the
Fraser River estuary dunes around low tide (e-g. Best et al., in press), but it is
doubtful that significant flow separation occurs near high tide when flow velocity
is low. Unfortunately, the ADP is unable to resohe flow reversais in dune
troughs (Kostaschuk et al., in prep) so it is not possible to test this hypothesis.
Many surface boils were evident within the study area surrounding low tide
when flow velocity was highest. Boils indicate the presenœ of macroturbulence
structures generated by dunes at the bed (Jackson. 1976; Kostaschuk and
Church, 1993). Without ftow separation it is difficult to conceive the formation of
such macroturbulent structures and therefore separation must occur over some
dunes during these high flows. In addition without flow separation and
subsequent flow reattachment downstream, the scouring action required to
support the measured increase in dune height woutd be difficult. Reœnt flume
research by Besi et al. (in press) has shown that the flow field above dunes with
low angle leesides is considerably different than that found over traditional
asyrnrnetric dunes. Best et al. (in press) propose Mat a permanent region of flow
reversal is not present in the dune leeside, but intermittent separation and
generation of temporaliy-variable shear gradients in the dune leeside lead to
intermittent generation of large-scale. shear layer related turbulence. This
process will invariably lead to a complex series of 'stacked wakes' over a series
of dune forms, the most dominant of these turbulent features being sustained
long enough to reach the surface as boils. The combination of high flows and
higher average dune steepness at low ode in this study leads to fiow conditions
as described by Best et al. (in press). With lower flow velocity and lower dune
steepness around high tide, flow separation and in tum sediment suspension
events will become weaker and less frequent. When velocities are lowest it is
unlikely any flow separation takes place over these gently sloping dunes.
5.2.4 Lee Face Slope Angle
There was no obvious trend in lee face slope angle CB) with tidal stage in
this study and average dune lee face dope was approximately 1 6 O . An average
lee dope angle of 16" is in agreement with estirnates of P derived from Fraser
River estuary dunes previously measured by Kostaschuk and Villard (1996) and
Kostaschuk et al. (in press). The absence of any notable change in f l over the
tidal cycle suggests that despite an increase in Hd as ROW velocity increases, lee
slopes rernain reasonably constant. Therefore, any change of the fiow field in
the lee of each individuai dune will be wntrolted by changes in H , and H, I Ld .
In the case of asyrnmetnc dunes with lee hces less than the angle of repose of
the bed sediment, f i will generally increase toward the angle of repose as flow
speed and in turn bedload transport increases (Dalrymple and Rhodes, 1994).
This is due to increased bedload movement over the crest of the dune, which in
turn will eventually cause the slip face to fail by avalanching and consequenüy
approach the angle of repose. The lack of any change in p over the tidal cycle in
this study reflects the dominance of sand transport in suspension on symmetric
dunes and the reduced importance of bedload avalanching processes (e-g.
Kostaschuk and Villard, 1996).
5.3 Conceptual Model
The conceptual mode1 outlined on Figure 5.6 illustrates the characteristic
evolution of a velocity profile over the course of one tidal cycle and outlines the
major changes in bed morphology that accarnpany the continual adjustment in
flow velocity.
Ebb
Figum 5.6 Conceptual mode! of the velocity profite evolution and dune rnorphological response over one tidal cycle. (-) representa the developing velocity profile, (-) represents the profile 'memory' of the earlier flow conditions, (---) represents trough scour and (--) represents trough deposition.
Due to the ADP measurements k i n g inaccurate in the lower portion of the
flow (1 .O rn - 1.5 m), the profiles on Figure 5.6 do not represent the lower 'skin
friction' section of the velocity profile (e.g. McLean et al., 1999a). Figure 5.6
demonstrates that a 'memory' of previous flow conditions exists as the tide rises
and falls. Following high tide, the faster moving river current accelerates more
quickly than the underiying tidal flow, resulting in a kinked velocity profile. During
this acceleration phase dune height wiit begin to increase as enhanced scour
occurs in the dune troughs. At low tide a near equilibrium state is reached with
almost atl of the previous flow memory erased by the overiying high speed flow.
Following tow tide, flow begins to decelerate and trough deposition occurs as
sedirnent cornes out of suspension. Velocity decreases, causing low shear
stresses in the upper profile and continued deposition. Once high tide is reached
dune height has returned to a similar value as that of the previous high tide and
the cycle begins again.
5.4 Conclusions
(1 ) Mean velocity follows changes in tidal stage although deceleration on the
flood tide is more rapid than acceleration during the ebb tide. This pattern reflects
the relative influence of river discharge and tidal stage, the frictional influence of
the dunes and modification of the near bed f uid density at low tide.
(2) As the tide begins to fall, a 'memory' of previous flow conditions is retained
next to the bed, resulting in 'kinked' velocity profiles. The upper segment of the
profile reflects high-energy rîver flows adjusting to the change in direction of the
tidal current faster near the surface than near the bed. Shear velocities
calculated from these profiles do not represent total shear stress in the flow.
During the flood tide, velocity profiles are poorly developed and also do not
provide a reliable rneasure of shear velocity. It is therefore recommended that
any estirnates of U. calculated during accelerating or decelerating fiows be done
with a great deal of caution. Where flow approaches low tide, the upper
segments of kinked velocity profiles provide a reasonable estimate of total shear
velocrty .
(3) Dune height increased on the falling tide and decreased on the fising tide,
indicatirtg that the response time required for the adjustment of dune height to
mean velocity is considerably less than one tidal cycle. Dune length did not
respond over one tidal cycle. Scour of the bed in the trough region between
dunes seems ta produce the increase in dune height. The decrease in m a n
velocity on the rising tide results in infilling of the trough.
(4) Dune steepness follows a similar pattern to dune height and is greatest at
low tide when flow velocity is highest. Intermittent flow separation is likely to
occur at tow tide but becorne increasingly infrequent during the ebb or flood when
near-bed velocities are low and may not occur at al! near high tide.
5.5 Recomrnendations for Future Research
Although this study determined that response of dune height to flow
velocity does occur over a single tidal cycle, more research into the nature of this
effect needs to be undertaken. Dune height may exhibit a hysteresis response
over the course of the tidal cycle but additional data needs to be assembled
before this conclusion can be reliably confined. Flow memory occurs
throughout the tidat cycle, but further rneasurements frorn an anchored launch
need to be made in order to test this hypothesis further. Additionally, ADP
measurement is inaccurate across much of the trough area of dunes. As this
region is generally regarded as the origin for macroturbulent events a more
reliable instrument with which to quant@ fiow behavior over the dune lee side
would allow for a clearer picture of dunemow interaction.
Allen, J.R.L. 1983. RNerb 1s: progress and problem ent Fluvial Systems. International Association of Sedimentologists Special Publication, 6: 19-33. (Ed. J.D. Collison and J. Lewin). Blackwdl Scientific PuMications, Oxford.
Allen, J . R L 1976. Conceptuai rnodds for dune tirne lag: general ideas, diffiwities, and earfy m u s . Seûimentary Gedogy. 15: 1 -53.
Allen, J.R.L. and P.F. Friend. 1976a. Relaxation time of dunes in decderating flows. Journal of the Geological Society of London, 132: 17-26.
Allen, J.R.L. and P.F. sprïng-neap cycles, Seâimentdogy, 23:
Friend. 1976b. Changes in intertidal dunes during two Lifeboat Station Bank, Weils-next-the-sea, Norfok (England). 329-346.
Alliota, S. and G.M.E. Perillo. 1987. A sand wave fie(d in the entranœ to Bahia Blanca estuary, Argentina. Marine Geology, 76: 1-14.
Ashiey, G.M. 1990. Classification of largescale wbaqueous bedforms: a new look at an old problem. Journal of Seâirnentaty PetmIogy, W: 160-1 72.
Bennett, S.J. and JIL- Best. 1995. Mean flow and turbulenœ structure over fixed, two-dimensional dunes: implications for sediment transport and bedform stability.
Best, J.L., R.A. Kostaschuk and P.V. Villard. In press. Quanütaüve visualization of flow fields associated with alluvial sand dunes: resub from the labofatory and field using uhsonic and acoustic Doppler anemometry.
Black, K.P. and T.R. Healy. 1988. The sediment oireshold over tidalty induced megaripples. Marine Gedogy, 69: 21 9234.
Bohacs, K.M. 1 981. Hume Studies of the Kinematics and Dynamics of Large- =ale Bedfoms. Cambridge Massachusetts Instituts of Technology, Department of Planetary and Earth Sciences.
Bokuniewia. H.J., RB. Gordon and K.A. Kastens. 1977. Form and migration of sand waves in a large estuary, Long Island Sound. Marine Geology, 24: 185- 199.
Booîhroyd, 3.C. and D. K. Hubbard. 1975. Genesis of bedforms in mesotidal estuaries- In. Esfuanne Reseamh, Vol- II (ed- L-E- Cronin), Academic Press, New York, 21 7-296.
Bowden, K.F. 1978. Physical problems of the benthic boundary layer. Geophysical Surveys, 3: 255-296.
Carey, W.C. and M.D. Keller. 1957. Systematic changes in the beds of alluvial riven. Proceedings of the Amencan Society of Civil Engineers, 83: (paper 1331 ).
Carling, P.A., E. Golz, HG. Orr and A. Radecki-Pawlik. 2000. The morphodynamics of fluvial sand dunes in the River Rhine, near Mainz, Gerrnany. 1. Sedimentology and morphology. Sedimentology, 47: 1 227-252.
Dalrymple, R.W. 1984. Morphology and internai structure of sandwaves in the Bay of Fundy. Sedimentology, 31 : 365-382.
Dalrymple. R.W., R.J. Knight and J.J. Lambiae. 1978. Bedforms and their hydraulic stability relationships in a tidal environment, Bay of Fundy, Canada. Nature, 275: 1 00-1 04.
Dalrymple, R.W. and R.N. Rhodes. 1995. Estuarine dunes and Bars. Geomorphology and Sedimentology of Estuanes. Developments in Sedimentology 53 (Ed . G.M. E. Perillo), p. 1 747. Elsevier Science.
Dingman, S.L. 1984. Fluvial Hydrology. W.H. Freeman and Company, New York.
Dyer, K.R. 1989. Sediment processes in estuaries: future research requirements. Journal of Geophysical Research, 94C: 1432744339-
Dyer, K R . 1 986. Coastal and Estuarine Sediment Oynamics. John Wiley & Sons, Toronto.
Dyer, K.R. 1973. Estuanes a Physical introduction. Wiley and Sons, London.
Einstein, H.A. 1950. The bedload function for sediment transportation in open channel flows. Soil Conservation Sewice, U S . Dept. Agnc. Tech. Bull., 1026: 78-1 04.
Engle, P. and Y.L. Lau. 1980. Friction factor for two dimensional dune roughness. Journal of Hydraulic Research, 1 9: 2 1 3-225.
Engle, P. and Y.L. Lau. 1981. Bed load discharge coefficient. Journal of the Hydraulics Division, American Society of Civil Engineers, HY 1 1 : 1 445- 1 453.
Englund, F. and J. Fredsoe. 1982. Sediment ripples and dunes. Annual Reviews, Fluid Mechanics, 14: 1 3-37.
Fenster, M.S. and D.M. FitzGerald. 1996. Morphodyarnics, stratigraphy, and sediment transport patterns of the Kennebec River estuary, Maine, USA. Sedimentary Geology, 1 07: 99-1 20.
Fredsoe, J. 1979. Unsteady flow in straight alluvial streams: modification of individual dunes. Fluid Mechanics, 91 :3 487-51 2.
Gabel, S. 1993. Geometry and kinematics of dunes during steady and unsteady Rows in the Calamus River, Nebraska, USA. Sedimentology, 40: 237-269.
Harbor, D.J. 1998. Dynarnics of bedforms in the lower Mississippi River. Joumal of Sedimentary Research, 685 750-762.
Hodgins, D.O., T.R. Osborn and MC. Quick. 1977. Numerical model of stratified flow. Amerka Society of Civil Engineers, Journal of the Waterways, Ports and Coastal Division, WW 1 : 25-42.
Jackson, R.G. 1976. Sedimentological and fiuid-dynamic implications of the turbulent bursting phenornenon in geophysical Rows. Joumal of Fluid Mechanics, 77: 531 -560.
Julien, P.Y. and G.J. Klaassen. 1995. Sand-âune geornetry of large rivers during floods. Journal of Hydraulic Engineerfng, 1 2 1 :9 657-663.
Komar, P.D. 1998. Beach Processes and Sedimenfation. Prentice Hall, New Jersey.
Kostaschuk, R.A. and LA. Atwood. 1990. River Discharge and tidal controls on salt-wedge position and implication for channel shoaling. Canadian Joumal of Civil Engineering, 17: 452459.
Kostaschuk, R., M. Church and J. Lutemauer. 1 989. Bedforms, bedmaterial, and bedload transport in a saltwedge estuary: Fraser River. British Columbia. Canadian Journal of Earth Science, 26: 1440-1452.
Kostaschuk, R A . and M.A. Church. 1993. Macro turbulence generated by dunes: Fraser River, Canada. Sedimentary Geology, 85: 25-37.
Kostaschuk, R.A. and S.A. Illersich. 1995. Dune geornetry and sediment transport: Fraser River, British Columbia. In River Geomorphology (Ed. E.J. Hicken), John Wiley 8 Sons, Sussex, England, p. 19-36.
Kostaschuk, R. and J. Lutemauer. 1987. Large-scale sedimentary processes in a trained, high energy, sand rich, salt-wedge estuary: Fraser River, Canada. In: Curren t Research, Part A; Geological Survey of Canada, Paper 87- IA, 727-734.
Kostaschuk, R., J. Luternauer and M. Church. 1998. Sedimentary process in the estuary. In Geology and Natural Hazards of the Fraser River Delta, British Columbia (Ed. J . Clague, J Lutemauer and D. Mosher) Geological Survey of Canada, Buletin 525, p 4 7-56.
Kosbschuk, R.A. and G.M. Macdonald. 1988. Multitrack surveying of large bedfarms. Geo-Marine Letters, 8: 57-62.
Kostaschuk, R. and P.V. Villard. 1996a. Flow and sediment transport over large subaqueous dunes: Fraser River, Canada. Sedimentology, 43: 849-863.
Kostaschuk, R. and P.V. Villard. 1996b. Turbulent sand suspension events: Fraser River, Canada. In: Coherent Flow Structure in Open Channels, (Ed. P.J. Ashworth, S. J. Bennett, J.L. Best and S. J. McLelland) John Wiley and Sons, Sussex, England, p. 305-320.
Kostaschuk, R. and P.V. Villard. 1999. Turbulent sand suspension over dunes. in: Fluvial Sedimentology VI, Special Publication of the International Association of Sedimentologists (Ed. N. D. Smith and J . Rogers), 28: 3-1 3
Kostaschuk, R., P.V. Villard and J. Best. In prep. Measuring velocity over large subaqueous dunes using an acoustic Doppler profiler.
Leeder, M.R. 1983. On the dynamics of sediment suspension by residual Reynolds stresses - confirmation of Bagnold's theory. Sedimentology. 30: 485- 491.
Lueck, R.G. and Y. Lu. 1998. The logarithrnic layer in a tidal channel. Continental shelf research, 1 7: 1 4 1 785- 1 80 1 .
Lyons, M.G. 1997. The dynamics of suspended sediment transport in the Ribble estuary. Water, Air and Soi1 Pollution, 99: 141 -1 48.
McKenna, G.T., J.L. Luternauer and R.A. Kostaschuk. 4992. Large-scale mass- wasting events on the Fraser River detta front near Sand Heads, British Columbia. Canadian Geotechnical Journal, 29: 1 51 -1 56
McLean, DG. and B. Tassone. 1991. A sediment budget of the lower Fraser River. Proceedings of the S" Federal lnteragency Sedimentation Conference, Las Vegas, Nevada, 240.
Mclean, S.R. 1990. The stability of ripples and dunes. Earth Science Reviews, 29: 431-144.
McLean, SR., J.M. Nelson and S.R. Wolfe. 1994. Turbulence structure over two- dimensional bed forms: implications for sediment transport. Journal of Geophysical Research, 99: C6 1 2729: 1 2747
McLean. SR and J.D. Smith. 1986. A mode1 for flow over two-dimensional bedforms. Journal for Hydraulic Engineering, 1 12:4 300-31 7.
Mctean, SR., S R . Wolfe and J.M. Nelson. 1999a. Spatially averaged flow aver a wavy boundary revisited. Journal of Geophysical Research, 104C7 15743- 15753.
McLean, SR., S.R. Wolfe and J.M. Nelson. 1999b. Predicting shear stress and sed iment transport over bedforms. Journal of Hydraulic Engineering, 1 25:? 725- 736.
Milliman, J.O. 1980. Sedimentation on the Fraser River and its estuary, British Columbia. Estuarine and Coastal Marine Science, 1 0: 609-633.
Nasner, H. 1974. Prediction of the height of tidal dunes in estuanes. In: Proceedings of the Fourteenth Coastal Engineefïng Conference. American Society of Civil Engineers, New York, 1036-1050.
Nasner, H. 1978. Tirne-lag of dunes for unsteady flow conditions. In: Proceedings of the Sixteenth Coastal Engineemg Conference. American Society of Civil Engineers, New York, 1801 -1 81 7.
Nelson, J.M., S.R. Mctean and S.R. Wolfe. 1993. Mean flow and turbulence fields over two-dimensional bed forms. Water Resources Research, 29: 1 2 3935-3953.
Ocean Data Equipment Corporation. 2000. Bathy 1500 Survey Echo Sounder, Installation, Operation, Maintenance, Rernote Display, Walpole, Massachusetts.
Onslow, R.J., N.H. Thomas and R.J. Whitehouse. 1993. Vorticity and sandwaves: the dynamics of ripples and dunes. Turbulence: Perspectives on Flow and Sediment Transport (Ed. N.J. Clifford, J.L. French and J. Hardisty), John Wiley and Sons Ltd., Toronto, 278-293.
Orton, G.J. and H.G. Reading. 1993. Variability of deltaic processes in ternis of sediment supply, with parbicular ernphasis on grain size. Sedimentology, 40: 475-51 2.
Pethick, J. 1984. An Introduction to Coastal Geomophology. Edward Arnold. Australia.
Raudkivi, A.J. 1963. Study of sedirnent rippte formation. Joumal of the Hydraulics Division, American Society of Civil Engineers, 89: HY6 1 5-33.
Raudkivi, A. J. and H.H. Witte. 1990- Oevelopment of bed features. Journal of Hydraulic Engineemg, 1 ?6:9 1 063-1 079.
Reid, 1, and L. Frostick. 1994. Fluvial sediment transport and deposition. Sediment Transport and Depositional Processes (Ed. K. Pye), p. 89-t45. Blackwell Scientific Publications, Oxford.
Rubin, D.M. and H. Ikeda. 1990. Flume experiments on the alignment of transverse, oblique, and longitudinal dunes in directionally varying fiows. Sedimenfology, 37: 637-684.
Rubin, D.M. and D.S. McCulloch. 1980. Single and superimposed bedforms: a Synthesis of San Francisco Bay and flume observations. Sedimentary Geology, 26: 637-684.
Shaw, G. and D. Wheeier. 1 985. Statistical Techniques in Geographical Analysis. John Wiley â Sons Inc., Toronto.
Sleath, J.F.A. 1984. Sea Bed Mechanics. John Wiley 8 Sons Inc., Toronto.
Smith, J. D. and SR. McLean. 1977. Spatially averages flow over a wavy surface. Journal of Geophysical Research, 82: 1 735-1 746.
SonTek. 1999. From SonTek web page (www.SonTek.com)
Soulsby, R. 1983. the bottom boundary layer of shelf seas. In: Physical Oceanography of Coastal Shelf Seas (Ed. B. Johns), Elsevier Oceanography Series, Elsevier, Amsterdam, 35: 189-266.
Soulsby, R. 1997. Dynamics of Mame Sands, a Manual for Practical Applications. Thomas Telford Publications, London.
Soulsby, R.L. and K.R. Dyer. 1981. The fom of the near-bed velocity profite in a tidatly accelerating flow. Journal of Geophysical Research. 86:C9 8067-8074.
Southard, J.B. and L.A. Boguchwal. 1990. Bed configurations in steady unidirectional water flows, part 2, synthesis of flume data. Journal of Sedimentary Petrology, 60:5 658-679.
Terwindt, J. H.L. and M. J. Brouwer. 1986. The behavior of intertidal sand waves during neap-spring tide cycles and the relevance of paleoftow reconstructions. Sedimentology, 33: 1 -31.
Thomson. R.E. 1981. Oceanography of Vle British Columbia Coast. Canadian Special Publication of Fisheries and Aquatic Sciences 56, Deparbnent of Fisheries and Oceans, Ottawa.
Van Den Berg, J.H. 1987. Bedfom migration and bed-load transport in some rivers and tidal environments. Sedimenfology, 34: 681 -698.
Van Rijn, L.C. 1 994. Plinciples of Sediment Transport in Rivers, Estuaries and Coastal Seas. Aqua Publications, Amsterdam
Villard, P. and R. Kostaschuk. 1998. The relation between shear veiocity and suspended concentration over dunes: Fraser Estuary, Canada. Marine Geology, 148: 71-81.
Wewetzer, S.F., R.W. Duck and J.M. Anderson. 1999. Acoustic Doppler current profiler measurements in coastal and estuanne environments; examples f o m the Tay Estuary. Scotfand. Geomorphology, 29: 21 -30.
Zyserrnan. J.A. and J. Fredsoe. 1994. Data analysis of bed concentration of suspended sediment. Journal of Hydraulic Engineering, 120: 1021 -1 042.
Appendix A - Flow statistics over dunes with significant fogarithmic velocity profiles
Transect Dune Mean U Mean U No. of Slope Total Surface observa- (m)
(cms") (orns-') tions (n) 95.96 107.63 42 31 -95 96.66 105.39 43 31 -49 99.79 108.56 42 27.74
intersest1 8 / ( s 1 u- cm) (cms")
lntersect 8 zo u, (cm) (cms-')
No. of Slope lntersect r' observa- (m) tions (n)
34 21 .13 135.35 0.359 34 14.19 189.78 0.377 24 18.17 156.67 0.443 29 23.68 116.34 0.315 22 55.03 -107.36 0.494 32 9.15 218.55 0.223 34 27.61 25.39 0.553
Transect Dune Mean U Mean U Total Surface
(cms-') (cms") T l 2 d3 241.16 266.71 T l 2 d4 248.97 276.19 T l 2 d5 245.27 265.50 T l 2 d6 235.62 263.79 T l 2 d7 227.52 252.75 T l 2 d8 241.19 273.14 T l 3 d2 179.23 198.57 T l 3 d3 198.41 226.99 T l 3 d4 210.71 236.39
(cm) (cms-l) 'o I 14*
I I Tl(1) 1 d l 1 NIA 1 N/A 1 II Tl(I) 1 d2 1 N I A 1
T l (1) d3 NIA NIA
T l (1) d4 NIA NIA , P II Tl(]) 1 d5 1 NIA 1 NIA 1 10 1 14.35 1 -17.68 10.4651 3.43 1 5.74 1
II Tl(1) ] d6 1 NIA 1 NIA 1 10 1 10.66 1 -5 69 In 6701 i -71 1 d - î K 1
NIA NIA NIA
Transect
T m T2(1)
T m T2(1) T3(1) T3(1)
T3(1) T3(1) T3(1) T3(1)
1 T3U)
1C.
(cms")
6.33 8.21 6 -87 9.16 6.97 7.06 1 0.04 2.37
8.76 6.0 1 9.20 8.04
Dune
d4 d5 d6
d8 d9 d l d2 d3 d4
d5 d6 d7
1
Mean U Total
(crns") N/A N/A N/A N/A NIA NIA NIA N/A N/A N/A N/A N/A
. --
Slope (m)
15.83 20.51 17.19 22.91 17.43 17.65 25.1 1 5.93
21.89 15.02 23.00 20.1 1
Mean U Surface (cms-')
NIA N/A NIA N/A NIA NIA NIA NIA N/A N/A NIA N/A 1
Intersect
14.20 -10.22 -3.77 -19.81 25.16 -4.98
35.38 54.36 -5.75
25.38 -14.83 -15.25
No. of observa- tions (n)
9
6
5 8 10 10 9 7
14 12 14 10
-
r'
0.381 0.685 0.906 0.719 0.653 0.466 0.548 0.540 0.530 0.394 0.573 0.559
Appendix B - Dune Measurements
Transect t 1 t 1 t 1 t l t l t 1
i 11 1 d8 I 1.45 1 37.64 I 0.0385 1 7 1
Dune d2 d3 d4 d5
Height (m) 1.21 0-97 0.95 0.89
Length (m) 35.57 28.1 8 22.89 78.81
d6 d7
23.04 27.50
1-10 0.72
HIL 0.0339 0.0342 0.041 5 0.01 13
Lee dope angle 5 10 8 10
0.0475 0.0262
12 7
Transect t6
Dune d7
L
t l O t l O t l O t l1 tl1 t l l t l l t l1 tl1
1.80 1.55 2.70 1.65 1.50 1.50 2.75 2.75 1.48
d6 d7 d8 d 2 d3 d4 d 5 d6 d7
Height (m) 1 -41
L
31.80 27.20 27.00 65.50 30.20 16.1 O 33.70 19.60 12.40
Length (m) 16.84
0.0566 0.0570 0.1 O00 0.0252 0.0497 0.0932 0.081 6
HIL 0.0834
14 2 3 15 7 6 7
Lee dope angle 11
0.1403 0.1 190
24 49
Transect t13 t13 t13 t13 t13 t l 5 t15 t15 t15 t15
Dune d4 d5 d6 d7 d8 62 63 d4 d5 d6
Height (m) 1.15 1 -38 1.53 2.00 1.60 2.08 1.85 1.35 1.71 1 -46
Lee dope angle 5 9 14 5 9 11
-
20 11 21 9
Length (m) 45.00 34.00 28.90 31 -30 33.80 35.79 35.59 26.98 37.20 33.47
HIL 0.0256 0,0404 0.0528 0.0639 0.0473 0.0581 0.0520 0.0499 0.0458 0.0436