measurement of hydrodynamic conditions, sediment transport and

Measurement of hydrodynamic conditions,

sediment transport and bed morphology in a

tidal inlet

J.J. Williams, P.S. Bell & P.O. Thome

Centre for Coastal and Marine Sciences, Proudman OceanographicLaboratory, Bidston Observatory, Bidston Hill, Prenton CH64 7RA, UK

Abstract

In many tidal inlets strong tidal currents and/or large waves generally prevent thedeployment of instrumentation using benthic frames or boats and consequentlymeasurements of hydrodynamic and sediment parameters necessary forcalibrating numerical models remain scarce. Recently a solution to this problemhas been found using a multi-sensor instrument deployed from a small 'jack-up'barge. Observations demonstrating successfully the acquisition of a range ofhydrodynamic and sedimentological data are presented.

1 Introduction

As part of the International Project INDIA (Inlet Dynamics Initiative: Algarve),fieldwork has been undertaken in the Barra Nova tidal inlet located in southernPortugal (Figure 1), during January to March, 1999. The inlet is approximately150 m wide with an average depth of 4 m. The spring tidal range of 4 m givesrise to depth-mean currents 0(2 m/s). DSQ for the inlet sediments is 1.2 mm.

The three principal objectives of the field study were: a) to measure in detailhydrodynamic and sediment parameters pertaining to sediment transportprocesses; b) to quantify the sediment budget and migration rate of the channel;and c) to provide data sets required to calibrate, validate and verify a suite ofgeneric numerical models of the site. In order to help develop strategies for

Environmental Coastal Regions III, C.A. Brebbia, G.R. Rodriguez & E. Perez Martell (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-827-9

262 Environmental Coastal Regions HI

future coastal zone management, observed and predicted results have beencombined with data from other inlet studies to examine generically processes inand adjacent to tidal inlets.

The acquisition of data at this site presented a number of difficulties: a) the depthmean current speed exceeds 2 m/s during spring tides; b) the sites is exposed towaves exceeding 3.0 m during storms; and c) the channel bed, comprising deep,highly mobile coarse sand is subject to very rapid morphological changes. Inaddition, an environmental assessment indicated that during storm conditions,the inlet had a potential to migrate along the coast several tens of metres over aperiod of one or two days, O'Connor et al., [1].

Figure 1. (a) Location of the field site; and (b) aerial view of the Barra Nova

Ria Formosa Barrier Islands1. Ancao 5, Tavira2. Barretta 6, Cabanas3. Cuiatra 7. Cacela4. Armotia

ATLANTICOCEAN

Ria Formosa

Study area *̂"**_,. 500m —. q ^Km


Environmental Coastal Regions HI 263

The present paper describes the use of a small jack-up barge to deploy a multi-sensor instrument at locations in the Barra Nova. Selected data pertaining toflow turbulence, suspended sediments and bed morphology at locations in theinlet are then presented and discussed. Since this paper is aimed primarily atconveying information about the new deployment technique, the intention here isnot to present or to discuss in detail results from the present series ofexperiments. In the sections below only selected data illustrating PIPcapabilities are presented.

2 Measurements2.1 The 'jack-up' bargeThe Seacore Skate II jack-up barge used in the Barra Nova study (Figure 2)consisted of four pontoon sections that were linked together to form a workingplatform of size 14.65 m x 9.62 m. A drilling tower fixed to the deck facilitatedthe lowering and raising of the equipment described below into the waterbeneath the barge through the moon pool in the deck. Generators provided

Figure 2. The jack-up barge used in the Barra Nova study

Drilling tower and winchScience

/ laboratory



electrical and hydraulic power and cabins were installed to house a workshop, acanteen and scientific equipment. Hydraulic legs of length 20 m were positionedat each corner of the platform and could be raised or lowered independently ofthe others. Once raised above the water surface, the platform was ably towithstand the strong tidal currents and wave action present in the Barra Nova.

2.2 The CCMS-POL Instrument Package (PIP)In a number of past studies, the present authors and others have used datagathered by the autonomous, multi-sensor instrument STABLE (Humphery &Moores, [2]) to investigate and model wave - current - sediment interactions(e.g. Williams et al., [3]). Recently, these technologies have been refined andbrought together as the POL Instrument Package (PIP). The PIP, shown inFigure 3, consisted of: a) an adjustable deployment frame; and b) a suite ofsensors and sampling devices.

Figure 3. The POL Instrument Package (PIP)


Environmental Coastal Regions III 265

The PIP deployment frame comprised two parallel precision ground stainlesssteel shafts fixed vertically using ridged steel plates with a movable instrumentcarriage fabricated using lengths of scaffold pole and elbow joints connected tothe shafts. A hydraulic ram was used to raise and lower the carriage therebyallowing the instrumentation to be positioned at any desired height above the bedin the range 0 m < z < 1.0 m. Specially designed seals and wipers were used toensure sand did not enter the points of contact between the carriage and theshafts. The upper plate of the PIP was attached to a 15 m length of standard drillpipe sections (diameter 35 cm) and then connected to the winch on the bargedrilling tower by mean of a steel cable. In this way, the PIP could be lowered tothe bed of the inlet through the moon pool and raised onto the deck whenrequired. A spike approximately 1 m long was fixed beneath the lower plate tolocate the PIP rigidly into the bed. During deployment, as the locating spikepenetrated the bed under the combined weight of the PIP and drill pipe sections,contact between the lower surface of this plate and the sea bed was confirmedusing a sensor. Above the PIP, the drill pipe was held in position by thecombined forces of the tidal flow against the PIP and support pipe and ahydraulic deck clamp around the supporting drill pipe as it emerged from themoon pool.

A suite of sensors to measure hydrodynamic, sedimentological andmorphological parameters were fixed to the instrument carriage. These consistedof the following instruments to measure hydrodynamic conditions: two pairs ofValeport Series 800 electromagnetic current meters, ECM's, and a Digiquartzpressure sensor, sampling at 8 Hz; a Sontek acoustic Doppler Velocimeter,ADV, sampling at 25 Hz; a Nortek ADV, sampling at 25 Hz; and the CCMS-POL 3D Acoustic Coherent Doppler Velocity Profiler, 3DACDV, sampling witha vertical resolution of 4.6 cm up to approximately 80 cm above the sea bed at 16Hz. A triple frequency (1.0 MHz, 2.0 MHz and 4.0 MHz) acoustic backscattersystem, ABS, sampling at 4 Hz was deployed to measure instantaneous verticalsuspended sediment concentration profiles (C-Profiles). Pump-samplingequipment on the PIP was used to measure time-averaged and horizontallyspatially averaged C-Profiles and to obtain samples of suspended sediment forthe provision of information on suspended sediment grain size, shape anddensity. For any given experiment, the collection of pump-samples from allsampling locations above the bed took approximately 15 minutes to complete.

Beneath the PIP acoustic images of the bed morphology in a circular area ofradius approximately 2.5 m were obtained using a sector-scanning sonar device,SSS. In addition, a scanning acoustic ripple profiler, ARP was used to measurethe morphology of the bed along a single line of length approximately 3 mcentred beneath the PIP and running along the x axis. Cables from theinstruments were run into a clean laboratory cabin where data were logged usinga suit of PC's. Pump-sample tubes were connected to the pumping and samplingsystem on the deck of the barge. All samples were inspected in situ and thenbagged for more detailed analysis in the laboratory.


266 Environmental Coastal Regions III

3 Results and discussion

Data presented and discussed below were obtained during the flood tide on 28February 1999 between approximately lOhOO GMT and 13h30 GMT (hereafterreferred to as Day 59). During this time, depth-mean tidal flows exceeded 2 m/sand resulted in the transport of appreciable quantities of sediment as bedload fora period of approximately three hours.

3.1 Hydrodynamics

The 3DACDV was deployed in the field for the first time during theseexperiments. Intercomparison between (/, and V flow components measured overa period of 100 s at z ~ 4 cm by the ADV and by 3DACDV are shown in Figure4a. Agreement between U and V time-series is good (/?< = 0.94) demonstratingthe accuracy of the 3DACDV. Differences between U or V time series areconsidered real and attributable primarily to the physical separation between thetwo measurement locations. Figure 4b shows time-averaged vertical currentprofiles from the 3DACDV. The profiles exhibit a linear increase in currentspeed, S, with logeU) up to z = 40 cm. In contrast, profiles shown using opensquares and diamonds have two distinct gradients: the first shallower gradient inthe range 2 cm < z < 10 cm; and a second steeper curve in the range 10 cm < z <50 cm. In all cases, the gradient of the profile decreases at z > 50 cm owing tocomplex near-field effects discussed by Lohrmann et. al [4]. Vertical profiles ofthe time-averaged bed shear stress derived from turbulent kinetic energy, TKE,profiles are shown in Figure 4c. The profiles are consistent with theoreticalexpectations for conditions at the field site.

Figure 4a Comparison between the 3DACDV and the Sontec ADV

100

80

60

40

4O302O10

; o-10-20-30-4O

U

SonTec ADV, *r«4 cm3D ACDV, z 4 cm



Figure 4b & c Results from the 3DACDV

] Q average current speed profiles i .0 —. TKE profile

o.iN

0.01-̂

&0.5-

A 0 O

0. 0.5Current speed (m/s)

3.2 Bedforms and bedload

I1.0

0. -

Da

0.05

K̂g (m V)

0.10

The ARP measured the migration of ripples beneath the PIP for a period ofapproximately 4.5 hours on Day 59 (Figure 5). To assist assessment of scale inFigure 5, the position of the PIP framework at an arbitrary point in time is alsoshown schematically in this figure. Figure 5 shows an initial reversal in rippleasymmetry shortly after the start of the run in response to a change in flowdirection. Thereafter, rates of ripple migration increase to reach a maximum rate0(5 m per hour) during the peak flow. The presence of such ripples in the coarseinlet sediments is not predicted by theory.

Also evident in Figure 5 is scour associated with flow around the PIP supportpile and the apparent deposition of sediment downstream of the frameworkattributable to interactions between flow and the PIP. Despite the interruption tosediment transport processes downstream of the PIP, bedload transport is shownto proceed essentially unhindered at all positions beneath the PIP. Clearly therates of ripple migration shown in Figure 5 account for the transport ofappreciable quantities of bed sediments.

If it is assumed that all grains in transport roll over the stoss slope and down thelee face of ripples before coming to rest, it is possible to calculate the volumetricbedload transport rate, g/,, expressed in units of rrf/s, by considering the ripplemigration speed, S/?̂ , so that g/, = kmp.hr.SRip. Here &#//, is an empirical constantrelated to ripple shape and generally assumed to be 0.32, Soulsby, [5]. Here S/?/,,is estimated using cross-correlation analyses between successive ARP profilesmeasured from the PIP so that 5^ = X̂ /dt, where X^ is the spatial lag


268 Environmental Coastal Regions III

Figure 5 Ripple migration beneath the PIP measured by the ARP

r- PIP support pilescour depression / , PIP frame

corresponding to the maximum cross-correlation coefficient for successive ARPprofiles and dt is the time interval between ARP profiles (63 seconds).

Using regression analysis, a simple expression to predict rate of bedloadtransport at the present study site has been derived using the present observationsof ripple migration rates. The most statistically significant correlation was foundbetween 0/, and (/giving the simple expression Q/, = 1.756/* x 10"̂ nf/s, whereU is the depth-mean current speed. Q\, values predicted by this simple expressionagree favourably with predictions from Van Rijn [6]. Agreement betweenobserved and predicted (2/> values indicates that the PIP has little influence uponthe processes driving bedload transport in the inlet.

3.3 Suspended sediments

Given the magnitude of the tidal flow through the inlet, one might expect thesuspended load to be correspondingly high. However, in Figure 6a, showingtypical examples C-profiles derived from laboratory analysis of pump samplesobtained at approximately lOhOO, llhOO, 12hOO and 13h30 on Day 59,suspended sediment concentrations are shown to be only 0(0.1 g/1). A measuredC-Profile has been compared with C-Profiles predicted by the well-known



Power-Law profile and a semi-empirical expression derived by Williams et. al[3], W&R, in Figure 6b. Profiles / (Power-Law) and 2 (W&R) are calculatedusing a reference concentration given by Zyserman & Freds0e [7] and profile 3(W&R) is fitted to the data using a single concentration measurement.

Figure 6 (a) Measured time-averaged vertical suspended sediment profiles; and(b) modelled C-Profiles.

(a)1.0 -n

0.5 -

N

0.1

D

D

# •D

OlOhOO•llhOO•12hOOai3h30

00oo

j 1 1 ! 1 I ! 1 I !

0.001 0.01

$>

i i i i 1 1 1 1

0.1

A Measured at 1 IhOO,28/2/99 —7

0.10

0.01-

o.oo i-JI I T I 1

0.00001 0.0001 0.001 0.01 0.1 1.0

All terms in the relevant equations used to derive the profiles shown in Figure 6have been derived from PIP measurements including bed shear stress, in situsettling velocity and ripple dimensions. Clearly, the power-law is not valid inthe present situation and reference concentrations given by the Zyserman &Freds0e expression are too low. The fitting if the Williams et al. expression to asingle measured concentration value gives the best predicted results.



4 Conclusions

Using a small jack-up barge, a new technique for the deployment of instrumentsin a tidal inlet has been developed successfully. Data pertaining tohydrodynamic conditions and sediment transport at locations in an inlet havebeen obtained successfully using state-of-the-art instrumentation and have beenused to study processes and to calibrate and validate numerical models. Forreasons of practicality, safety and cost, future deployments of the presentinstrument package from a similar jack-up barge in water depths up to 20 m isrecommended.

Acknowledgements

The work was supported jointly by the European Communities by the MAST 3project 'Inlet Dynamics Initiative: Algarve' (INDIA), contract MAS3-CT97-0106,and by the UK NERC. The authors would like to acknowledge the Director of theRia Formosa Natural Park and Port Authority at Faro for granting specialpermissions to work in the Barra Nova.

References

[1] O'Connor B. A., Williams J. J., Dias J. M. A, Collins M. B., Davidson M.A., Arens S. M., Howa H., Sarmento A. J., Seabra-Santos F., Aubrey D.,Salles P., Smith J. S., Heron M., Pires H. O., Silva A., Bell P. S. AND PanS. (1998) Tidal Inlet Monitoring/Modelling Project (INDIA). ProceedingsOceanology International, Singapore, 14pp.

[2] Humphery J.D. & Moores S.P. (1994) STABLE II - An improved benthiclander for the study of turbulent wave-current-bed interactions andassociated sediment transport. Electronic Engineering in Oceanography,IEE Conference Publication No. 394, 170-174.

[3] Williams J. J., Rose C. P., Thome P. D., Humphery J. D., Hardcastle P. J.,O'Connor B. A, Moores S. P., Cooke J. A. & Wilson D. J. (1999) FieldObservations and Predictions of Bed Shear Stresses and Vertical SuspendedSediment Concentration Profiles in Wave-Current Conditions. ContinentalShelf Research, 19(4), 507-536.

[4] Lohrmann A., Hacket B. & Roed L. P. (1990) High resolutionmeasurements of turbulence, velocity and stress using a pulse to pulsecoherent source. Journal of Atmospheric and Ocean Technology, 7, 19-37.

[5] Soulsby R. L. (1997) Dynamics of marine sands: a manual for practicalapplications. Thomas Telford Publications, 249pp.

[6] Van Rijn L. C. (1989) Handbook of Sediment Transport by Currents andWaves. Delft Hydraulics Report H461.

[7] Zyserman J. A. & Freds0e J. (1994) Data analysis of bed concentration ofsediment. Journal of Hydraulic Engineering, 120(9), 1021-1042.


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