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.

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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