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WATERRESOURCESESEARCH,OL. 29,NO. 8, PAGES 925-2939,UGUST1993

FlumeSimulation f Recirculatinglow and Sedimentation

JOHN C. SCHMIDT

Department f GeographyndEarthResources, atershedcience nit, Utah StateUniversity, ogan

DAVID M. RUBIN

U.S. GeologicalSurvey,Menlo Park, California

HIROSHI IKEDA

EnvironmentalResearchCenter, Universityof Tsukuba, baraki, Japan

Experimentswere conductedn a 4-m-wide lume o simulate ecirculating low and sedimentation

in a lateral eddywithin a channel xpansion. he percentage f main stem sediment hat was captured

by the eddy decreasedrom 37% (when he eddywas empty) o 24% (when sand illed approximately

32% of the eddyvolume).The reattachment ar within he eddygrew n an upstream irection,and he

finest size sedimentwas deposited n the lee of the obstruction; oth observationsare consistentwith

field observations. easurementsf reattachmentengthduringsediment ransport 0.5-1.0 kg/s)at

constantischarge0.60m3/s) howhat eattachmentength ependsotonlyoncharacteristicsf

the expandinget, but alsoon the topography f the channelbed downstream; eattachmentength

decreased henpart of the channel xpansion as illedby an aggradingmidchannel ar. Comparison

of these resultswith measurementsn the ColoradoRiver in Grand Canyon suggestshat downstream

channel rregularities lay a largerole in controllinghe lengthof eddies n natural rivers.

INTRODUCTION

Recirculatingcurrents, or eddies, develop n rivers wher-

ever the orientation of downstream flow and the channel

banks are sufficiently divergent, such as at sharp meander

bends [Leeder and Bridges, 1975] and downstream rom

channel confluences Best, 1986]. These currents also de-

velopwhereconstrictedlowenters wider each,such s n

the lee of debris fans or bedrock that partly obstructs

downstream low. Recirculating lows are weaker han adja-

centdownstream low, whichcauses edimento accumulate

in recirculationzones; sedimentstorage n recirculation

zones may comprisea high proportionof total sediment

storagen somebedrock anyons. arsdepositedt high

dischargen recirculatingurrentsandsubsequentlymer-

gent)may become ubstrateor riparian egetationr be

usedas campsites.n canyonsffected y upstreamams,

recirculatingurrent arsmaybeerodedSchmidtndGraf,

1990],and future restorationmanagementtrategies ay

include igh-discharge,egulatedeleasesntendedo recon-

struct these bars.

Flow separationnd associatedecirculationre long-

studiedhenomenan the aboratoryEaton nd ohnston,

1981;Simpson, 989]and coastal ettingsSignell nd

Geyer, 991;Wolanskit al., 1984], uthave eceivedess

attentionn rivers. hegeneralydraulicnd edimentologic

characteristics f recirculation ones and associated ars

have beendescribedn GrandCanyon Rubinet al., 1990;

Schmidt, 990;Schmidt nd Graf, 1990], ut few field

observations ave been madeof the rate and style of bar

developmentnd ydrauliconditionsnriver ddies.here

Copyright993 y theAmericaneophysicalnion.

Papernumber93WR00770.

0043-1397/93/93WR-00770505.00

have also been few physical model experimentswhich

measured ecirculating low conditionsduring bar aggrada-

tion. This paper summarizes he results of experiments

intended to simulate flow and sedimentation in a bedrock

canyon;hese xperiments ereundertakenn the 4-m-wide

flume at the Environmental Research Center, University of

Tsukuba. We describe (1) flow patterns, velocities, and

changesn reattachmentength n a large ecirculation one,

(2) evolutionof bar topographyand bed forms, (3) areal

sortingof sediments,and (4) efficiencyof this zone in

capturing ediment. he experimentalesults re related o

field studies.

LITERATURE REVIEW

We have observed arge recirculationzones and associ-

ated sandbars along the Colorado River in Grand Canyon,

Arizona,andCataractCanyon,Utah; along he SnakeRiver

in Hells Canyon, daho and Oregon;along he River of No

Return reach of the Salmon River, Idaho; along the Green

River in Lodore and Desolation Canyons, Utah; and on

smaller streams in the western United States and New

England.Accounts f river runners ndicate hat suchzones

occur n bedrockcanyons hroughout he world, and recir-

culating urrentsedimentationasbeendescribedn paleo-

flood studies hroughout he western United States Baker,

1984]. n the ColoradoRiver in Grand Canyon, ecirculation

zones re argeand persistent,ypicallyoccupying ne third

to one half of the channel width downstream from constrict-

ing debris ans. In the upstream 95 km of Grand Canyon,

about400 argeeddiesexist at mostdischargesSchmidtand

Graf, 1990].

Experiments how that wakes with simple ecirculating

flow develop n the lee of cylindersat Reynoldsnumbers

greater han about 4 [Chang, 1966].At Reynoldsnumbers

2925

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2926 SCHMIDTET AL.: FLUME SIMULATIONOF FLOW AND SEDIMENTATION

Shear ayer Time-Averaged

. ' Dividing

.Hi-,._,,/1Streamline

...

Recirculation x "'

Separation Zone Reattachment

Point Zone

Fig. 1. Plan view of flow features at a backward facing step

adapted from Driver et al. [ 1985]. H is step height.

greater than 40, flow within the wake becomes unsteady, and

vortices develop along the shear zone between the main flow

and the wake. Two flow zones develop: a near wake of

closed recirculation and a far wake within which shed

vortices move downstream. In open-channel low, the near

wake is referred to as the separation bubble or recirculation

zone.

The simplest reattaching flow occurs at the backward

facing step at the boundary of an otherwise semi-infinite or

uniform flow field (Figure 1). The point of flow separation,

which Simpson [1989] has shown to be a zone of varying

detachment conditions, is generally considered fixed in

space where the step corner is sharp, although Eaton and

Johnston [1981] have documented unsteadinessof the sepa-

ration point even at this geometry. Near the separation

point, the detached boundary layer forms a free shear ayer

which is initially thin and parallel to the orientation of the

constricted flow. Further downstream, the separated shear

layer curves sharply toward the wall. Simplified drawingsof

experimental conditions typically show a time-averaged

mean streamline impinging the wall at the time-averaged

reattachment point, which is located within a zone through

which the instantaneous eattachment point fluctuates. Re-

attachment point fluctuation has been related to the migra-

tion of large turbulent structures along the shear layer, and

the length of the reattachment zone is of the order of 4 step

heights. Simpson [1989] also reported that the maximum

backflow velocity is typically greater than 20% of the free

stream velocity. Experiments and numerical modeling have

demonstrated that reattachment length is highly sensitive o

the longitudinal pressure gradient and to bottom friction

[Eaton and Johnston, 1981; Yeh et al., 1988].

Schmidt and Graf [1990] measured great variability in

reattachment length in Grand Canyon, and found that recir-

culation zones lengthen with increasing discharge until dis-

charge becomes so great that the constricting debris fan is

overtopped and the surface expression of the recirculation

zone appears to become washed out. Flow in recirculation

zones is organized into a simple one-cell primary eddy

rotating such that upstream flow occurs along the fiver bank.

At higher discharges, primary eddies lengthen downstream

and secondary eddies develop upstream from the primary

eddy on flooded parts of the constricting debris fan. Schmidt

[ 1990] showed hat there is a maximum ength to which each

recirculation zone can extend, controlled at the upstream

end by the constriction tself and at the downstream nd by

irregularities n channel geometry. Schmidt [ 1990] ound that

narrow and deep constrictions create longer recirculation

zones than do wider, shallower constrictions.

Baker [1984] distinguished nd described our major bar

typeshatoccurn narrow, eepbedrock tream hannels,

including he expansionbars and eddy bars that are the

subjectof this paper. The categoryof eddy bars, formed

beneath ecirculating urrents, was subdividedby Schmidt

[1990], asedon observationsn GrandCanyon Figure ).

Reattachmentbars form in the reattachment zone and be-

neath the primary eddy. Separation bars mantle the con-

stricting ebris an andare formed t higherdischargesy

secondary eddies that submerge parts of these fans

[Schmidt, 1990]. Sorting occurs within recirculation zones

[Pageand Nanson, 1982]. n GrandCanyon, hosesepara-

tion bars formed by high annual peak discharges n 1983-

1985 were composed of finer sediment than were reattach-

ment bars formed by the same flows, presumablybecause

sorting rocessesaused he coarsersediment o be depos-

ited in the area where sediment first entered recirculation

zones, the reattachment zone [Schmidt, 1990]. Bed load

transport directions beneath the primary eddy, inferred rom

Fig. 2. Photograph, ookingdownstream,of the ColoradoRiver

in Grand Canyon about 1 km upstream from Blacktail Canyon.

Three reattachment arson fiver right exist at this discharge; ne s

at the bottom, one in the middle, and one near the top of the

photograph. ach reattachment ar occurs n associationwith a flow

separationnducingdebris an. A small rapid in the centerof the

photograph asbeen ormedby a debris an on river right;a small

patchof sandon top of this debris an is a separation ar. Note that

reattachmentarsare highestn elevation n theirdownstreamnd

shoreward side, and that each reattachment bar has a small remnant

channelseparating he low-elevationpart of the bar from the talus

bank. This channel s maintained by backflow at higher discharges.

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SCHMIDTTAL.:FLUMEIMULATIONFFLOWND EDIMENTATION 2927

TABLE 1. ExperimentalConditions

Experiment

EmptyFlume BedAggradation

Flow ate,m3/s 0.15-0.61 0.60

Total run time, hours 22 30

Number of runs 13 4

Sediment eed rate, k/s 0 0.5-1

Sediment feed characteristics:

mean, mm '" 1.3

standarddeviation, phi units --- 0.96

Number of runs ndicatesnumberof times lumewas drainedand

bed observations were made.

bed form orientations nd sedimentary tructures,mimic

surfacelowpatterns nd ndicateotarymotion Rubin t

al., 1990].The only largearea of structuresndicating

downstreamransport s in the downstream art of the

reattachment one. Typical sedimentary tructuresound

within eattachmentndseparationarsare oresets epos-

itedby thebar as t migrates nshore nd oresets eposited

by ripples nddunesmigratingn a rotarypattern.Symmet-

rical straight-crested ipplesare common n the reattachment

zone Rubinet al., 1990].Separationar developmentas

beendescribed y Schmidt nd Graf[1990,Figure13],who

documented pstreammigrationof a separation ar within a

secondary ddy duringa regulated lood n GrandCanyon.

Field studiesof topographic hanges nd sedimentologic

characteristicsof selectedbars in Grand Canyon are the

basisof conceptualmodelsof bar buildingwithin recirculat-

ing currents proposedby Rubin et al. [1990] and Schmidt

[1990]. These modelspredict hat reattachment ars build n

an upstream direction from the reattachment one and that

sediment deposited at high discharge s reworked during

upstream retreat of the reattachment zone during flood

recession.Separationbars are formed at highdischarges nd

are reworked during flood recession o a lesserextent than

are reattachment bars.

METHODS

These experimentswere conductedn the 160-m-long,

4-m-wide,2-m-deep lume at the University of Tsukuba;

details f theTsukuba acilityare provided y keda [1983].

Water surfaceslopewas controlledby a tailgateat the

downstreamnd of the flume. Bed topography nd bed

formswereexamined ndmeasured uring xperimentsy

raising he tailgate, pending he flow, and then slowly

draining the flume. After measurementswere made, flow

wasreintroduced nd experimentsesumed.Flow velocities

were measuredwith a two-dimensional lectromagnetic ur-

rent meter and from time-lapse photography of surface

floats. Electromagnetic urrent meter measurements t 16

sitesweremade0.1 m beneath he water surface very0.75

s for a duration f 5 min. Becauselowdepth ncreasedn the

downstream irection, he positionof the measuring oint

relative o the bed differed ongitudinally. esultant orizon-

tal flow vectorsand rose diagrams f the relative amount

(duration imesvelocity) of flow in differentdirectionswere

computed.The vertical componentof the flow field was not

determined,althoughNelson [1991] has shown that the

verticalcomponents an mportant art of the flow ieldnear

the shear layer. Locations in the flume are referenced to

distancedownstream rom the headgatewhere water and

sedimentwere supplied; or example,station120 is located

120m downstreamrom the headgate.All references o left

or right side of the flume are made as if the viewer were

facing downstream.

We report the resultsof two experimentsn this paper

(Table 1). In both, a semicircular bstruction constructed

with sandbagsand coveredby large plasticsheets)con-

stricted low width to about1.5 m (Figure3). This obstruc-

tionwas ocated etweenstations 0 and95. For purposes f

comparison ith laboratory tudies f backward acingstep

flow, the step heightduringour experimentss taken as the

distancerom the right wall to the obstruction pex, 2.5 m.

In order to simulate the increased elevation of the bed that

occursat constrictionsn Grand Canyon [Kieffer, 1988], a

0.1-mcinder lockstepwasplaced t the constriction uring

deceleratinget shearayer cross-overfmn ownstreamlow

90 95 / 100 105 / 110 115 119

-.., .,-,-,.. ..... :.: ..,r;'. 4:-:...., -N-" ,,r-_ ,. -. / --X.-

...... , ....

,> /"-.'.....-.,'.-.',.'..,.'..'.-...':....:.....x,_ " -: - ,,, .' r ....... '".t.5..--,2:. .--'-

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2928 SCHMIDT T AL FLUMESIMULATION F FLOWAND SEDIMENTATION

1.6

1.4

1

o.

0.6

0.4

8O

total range of location f mean

energy reattachmentoint

ater surface

;"T;,'bar-crestleVatsiOn

..... after hr

empty flume experiment;

Initialconditionor bar-buildingxperiment

90 100 110 120 30 140

DISTANCE,N METERS,FROMHEADGATE

5O

1.6

1,4

0.6

0.4

i i i i i i

water-surface elevation after

Indicated hours of run time

,

I

22 5 30

flumeloor ' bar rest

ind lactatd

. i i i i i rnime

90

100 110 120 130 140

DISTANCE,N METERS,FROMHEADGATE

150

Fig. 4. Longitudinalprofile of water surface levationand max-

imum bed elevation during bar-building experiment for different

durations. (a) Water surface elevation and total energy grade line

after 14 hours and bed conditionsat start of experimentand after 11

hours. Total energy grade line calculated rom v a, using 1) from

text. (b) Water surface elevation after 20 and 26 hours and bed

elevation after 22.5 and 30 hours.

the secondexperiment and for some of the runs of the first

experiment Figure 4). Downstream rom the obstruction,

zone of recirculating current extended to within about 20 m

from the tailgate. Bed slopewas 0.01 for the first experiment

and was variable due to bed aggradationduring the second

experiment. Water surface slope was always less than the

bed slope, and water depth increased greatly downstream.

This physical model is an analogueof some of the condi-

tions in large rivers that flow within bedrock canyons. In

these canyons, the longitudinal prof-fie consists of steps

(debris fans) and pools (channel expansions) [Leopold,

1964]. Debris fans function as natural tailgates, often acting

as local controls upstream to the base of the preceding step

[Kieffer, 1985]. Recirculation in bedrock gorges occurs

within the channel and is not restricted to overbank settings

investigated by Tamai et al. [1986].

The size of the experimental channel expansion was

greater than the mean size of expansions n Grand Canyon.

In the experiments, the flow width constriction ratio (down-

stream flow width divided by constricted flow width) of 0.38

was narrower han the mean flow width constriction atio n

Grand Canyon which Kieffer [1985] ound to be 0.5, but the

experimental angewas within the rangeof all measuredield

sites. The flow-width expansion atio was 2.7 in all runs

becausehe flumewallswere neveraltered;Schmidt 1990]

found that the mean flow-width expansion ratio in Grand

Canyon is 2.9. The flow area expansion ratio (downstream

flow area at station 130 divided by constricted flow area at

station 92) during the experiments varied between 3.4 and

14.2 and greatly exceeded the flow width expansion atio

(Table 2). Because low depth increased ongitudinally, he

flow area expansion ratio was even greater at points down-

stream rom station 130. Some expansionsn Grand Canyon

are as large as those in the experiments because depthsof

individual scourholesare as much as 9 times the depth of the

constricted hannel.Theseexperiments re therefore epre-

sentative of some of the field conditions observed in Grand

Canyon; however, these experimentsmay not be represen-

tative of sites where the downstream increase in flow area is

small. In the latter case, deceleration of streamwise flow due

to increasing low area is smaller, and the relative impor-

tance to deceleration of lateral diffusion of momentum

across the shear zone is greater.

In the first experiment, changes in recirculating flow

characteristics were measured for different clear water dis-

charges and tailgate elevations. During this experiment,

sediment formed a linear bar with maximum relief of 0.18 m

in the center of the flume downstream from station 120. In

the secondexperiment, a heterogeneoussand mixture was

addedo a 0.60m3/s lowat a ratebetween .5 and1.0kg/s,

and the rate and style of bar aggradation was measured5

times during 30 hours.

Water surface elevations were measured from a movable

platform above the flume. Mean downstreamvelocity was

computedusing he following equation:

va = Q/Aa (1)

where va is the mean downstream elocity, Q is discharge,

andA a is the cross-sectionalreaof downstreamlowbased

on estimates of the location of the time-averaged mean

separation urfacehat dividesdownstream ndrecirculating

flow. Estimatesof va are only a general approximation

because he straight ine defining he mean dividing stream-

line can only be an approximate epresentation f a highly

unsteady hree-dimensional urface Figure 3). Froudenum-

berswerecalculated asedon va anddepthsmeasured.5m

from the eft wall. Hydraulic umps occurred t the constric-

tion where the Froude number of constricted flow was as

small s0.8 because a is less hanmaximum elocityn the

constriction.

The location of the reattachment point during each run

was determined n three ways. First, 1-s-exposure hoto-

graphsof surface loatswere used o identify the instanta-

neous eattachmentointat the time when he photograph

was taken. Second,digitized ongitudinal racesof the water

surface .5 m from the fight wall were used o identify he

reattachment oint by determining he most downstream

extentof adverseupstream) atersurface lope t the ime

of the water surface measurements. The time of these

measurementsasgenerally ithin15min rom he imeof

the photography. hird, visual observations f the time-

averagedocationof the reattachment ointwere made.

Bed opographyasdeterminedy repeatedlyurveying

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SCHMIDT TAL.: FLUMESIMULATiONFFLOWANDSEDIMENTATION 2929

z N

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2930 SCHMIDT ET AL.' FLUME SIMULATION OF FLOW AND SEDIMENTATION

105 109 113

117

Scale of resultant flow vectors

0.25 m/s

Fig. 5. Rose diagrams showing resultant flow vectors and amount of flow (speed times duration) toward each

indicated direction during run 7.

cross sections located at 2-m intervals between stations 89

and 125 and at 4-m intervals elsewhere. Measurements at

each cross section were taken at 0.2-m intervals and are

accurate to within 0.1 cm. Volume calculations were made

using surface and volume modeling software, and mass

transport rates were calculated assuming that the specific

gravity of the sediment was 2.65 and the porosity of the

sediment deposits was 35%. Samples of bed sediment were

collected when the flume was drained and were later sieved.

Sediment transport rates past the recirculation zone could

not be determined from the sediment feed rate because the

deposition rate upstream from the constriction was variable.

We determined the transport rate through the constriction by

measuring the total volume of sediment aggraded in the

flume downstream from the constriction during each run.

The resulting transport rates are accurate because no sedi-

ment washed out of the flume; in fact, little aggradation

occurred in the most downstream 20 m of the flume. Aggra-

dation rates in different parts of the flume were used to

determine the efficiency of the eddy in capturing sediment

transported by the main downstream current. These rates

were calculated by determining the difference in sediment

volume between successive measurements for five areas,

each of constant dimension. This procedure eliminated bias

in estimating he dimensionsof the recirculation zone during

each measurement period but may slightly overestimate the

proportion of sediment deposited n the eddy during the final

period because the recirculation zone decreased in size.

RESULTS

Flow Patterns and Velocity

The flow field downstream from the constriction can be

subdivided into (1) the decelerating et, (2) the shear layer

where vortices increase in diameter in the downstream

direction, (3) nearly stagnant flow in the lee of the obstruc-

tion and extending 1-1.5 channel widths downstream, and

(4) strong backflow in an area immediately downstream from

the stagnant flow area (Figure 3). Vortices eventually in-

creased n diameter such that they extended across he entire

flume. The recirculation zone typically was truncated where

the high-velocity core of downstream current crossed to the

right side of the flume. The location of the reattachment

point fluctuated over distances as great as 5 times the step

height, similar to fluctuation ranges reported by Simpson

[1989] (Table 2).

Although the flow field can be described by average flow

conditions, instantaneous vectors were widely variable, es-

pecially in the shear ayer. Figure 5 showsrose diagramsof

instantaneous velocity at selected locations using the two-

dimensional current meter. In the upstream part of the

expanding et, downstream velocities were so much greater

than the rotary flow of the vortices that the instantaneous

flow directions of the combined flow remained within 45

degreesof directly downstream. In the center of the recir-

culation zone, mean velocities were weaker and flow direc-

tions were more variable. Farther downstream where the jet

slowed and combined with the vortices, instantaneous vec-

tors varied over 360 degrees. Instantaneousvelocies in the

reattachment zone were bidirectional, and upstream-

downstream low reversals occurred. Along the wall, flow

directionwasrestricted,but speeds aried greatly. Backflow

along the wall was typically about half of v,/, but instanta-

neous speeds ranged from a minimum of nearly zero to a

maximum that approached the mean speed of the down-

streamcurrent. Upstreamvelocity was between 0.00 and

0.10 m/s in the lee of the constriction. Backflow speeds

greatly exceededvalues reported by Simpson [1989] and

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SCHMIDT TAL.: FLUME IMULATIONFFLOWANDSEDIMENTATION 2931

3.5

, 3

n- 1.5

z 1

- 0.5

' ' ' 1 ' ' I ' ' ' I ' ' '' I ' ' '' I ' ' ' I ' ' '

short recirculation zone

or onghin 13

recirculationzone 2

with oorly- O

developedirculation

(13.4-13.))

2 well-developedirculation

2 16

10.2

10.2-13

1 0 (9.4-9.8)

9-10.6 13 8.2-11

O (7-10.6) rm

O (10.2) O 8.6 7-1 1

(7.4-9.4) poorly-developedirculation

, , , I , , , I , , I , , , I , , , I , , , I , , ,

4 6 8 10 12 14

FLOW-AREA EXPANSION RATIO

Fig. 6. Recirculationzone lengthat differentexperimental onditions.Numbers are time-averaged eattachment

length divided by step height. Numbers n parentheses re the range of instantaneous eattachment ength where

time-averaged aluesare not available.Squares re runswith 0.1 m stepat constriction.Solid squaresare bar-building

experiment.

exceeded he values (0.2-0.4 times vd) measured y Schmidt

[1990] at three sites in Grand Canyon.

Reattachtnent Point Migration and Fluctuation

During Clear Water Experiment

Over the range of conditions of the runs of the first

experiment, reattachment engthswere bimodal:either very

short or were much longer; intermediate conditionsdid not

exist (Table 2). The very short reattachment lengths (less

than 2 step heights) were associatedwith shooting flow

through he constriction,a hydraulic ump locatedabout 1

channel width downstream from the constriction, a Froude

number of constricted flow exceeding 2, and a flow area

expansion atio less han 8 (Figure 6). Very slightchanges n

these conditions resulted in narrow recirculation zones with

reattachment engths 13 times the step height (Figure 7). At

Froude numbers less than 1.5, reattachment lengths were

between 9 and 16.6 times step height. At these conditions, a

recirculation zone similar to that shown in Figure 3 devel-

oped; this condition best represented ield conditions ob-

served in Grand Canyon. At Froude numbers typically less

than 0.5, poorly developed recirculation existed and time-

Contraction Froude number > 2

area expansion atio< 8

Contraction Froude number > 2.2

area expansion ratio > 5

Contraction Froude number 0.8 - 1.6

Contraction Froude number < 0.5'

Fig 7. Schematiciagramf low atternsndecirculationoneengthtdifferentlow onditionsatchuredine

is ocation f hydraulicumpanddashedine s average hear urface.

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2932 SCHMIDT ET AL.: FLUME SIMULATION OF FLOW AND SEDIMENTATION

100

0hrs

110 120 130

linearidge

mid-channel bar

flatbed ripples dunes

11 hrs

175 hrs

.-"" "*'- 22.5 rs

.% %,,. . 'y ....

...

... ..... ,::_____ 30hrs

RP

Fig. 8. Bed topography and distribution of bed forms during

bar-buildingexperiment. Contour interval is 10 mm. Dark patterned

areas were covered with dunes migrating in the directions ndicated

by the arrows. Intermediate pattern areas are rippled areas, and

lightest patterned area are fiat bed areas. Zone over which reattach-

ment point oscillated during experiments also shown. Cumulative

run time indicated at right.

averaged reattachment length was between 7 and 11 times

step height; vortices migrated slowly downstream, and re-

circulation was poorly developed. We never observed in-

stantaneous reattachment lengths between 2 and 6.2 times

step height, and only rarely were reattachment lengths ess

than 8.2 times step height. Variation in the instantaneous

location of the reattachment point during runs where con-

stricted flow Froude numbers were less than 2 was nearly of

the same order as variation in time-averaged location among

all these runs. The large variation in instantaneous eattach-

ment length in the first experiment (Figure 6, open circles

and squares) masked any relation between reattachment

length and the flow area expansion ratio [Eaton and

Johnston, 1981].

Topography and Bed Forms During

Bar-Building Experiment

During the secondexperiment, sedimentwas deposited 1)

in the pool upstream from the constriction, (2) in and near

the recirculation zone, and (3) downstream from the recir-

culation zone. Downstream from the constriction, sediment

was first deposited beneath the shear zone as a linear ridge,

near the tight wall near station 120, and as a midchannel

expansion bar near station 130 (Figure 8).

The highest vertical aggradation rates in the entire flume

occurred at the midchannel expansion bar downstream from

the recirculation zone (Figure 9). These rates were as high as

5 cm/h and occurred over a larger area than the area of

recirculation zone aggradation. With time, the midchannel

bar became attached to the left wall and may have steered

downstream flow from the left to the right wall. Vertical

aggradationateswithin he recirculationonewerealways

less than 2 cm/h. The highest rates occurred in the down-

stream part of the recirculation zone, and these rates were

between 2 and 3 times as great as rates that occurred at the

linear ridge. Deposition within the recirculation zone did not

occur at the reattachment zone, but instead always occurred

well upstream.

Although the linear tidge was a persistent feature, its

volume increased at a slower rate than that of downstream

parts of the recirculationzone. The location of the ridge

changedwith time as parts of the ridge were scoured while

other parts aggraded.

Bed forms hat developed n the ridgeand platformduring

the bar-building experiment are also mapped on Figure 8.

Within the recirculation zone, upstream-migrating dunes

existednear the reattachmentpoint, and upstream-migrating

ripples developedelsewhere. With time, dunes migrated nto

areas previously covered by ripples, so that a vertical

sequence exposed in the bar would have passed upward

from tipples to dunes. Areas of highest recirculation zone

aggradation were areas covered by dunes. Symmetrical

tipples (Figure 10) orientedwith crests ransverse o the long

axis of the flume existed near the center of the eddy.

Evidently, lateral shifts in the location of the eddy center

subject this region to flow that reverses in direction.

Sediment Sorting and Capture Rates

Aggraded sediments within the eddy were finer than the

sediment feed. Within the eddy, the coarsest sediments were

deposited near the reattachment point and the finest sedi-

ments collected in the lee of the obstruction near the

separationpoint (Figure 11). Sorting within the recirculation

zone was similar to that reported for flood deposits in Grand

Canyon [Schmidt, 1990] and for other recirculation zone

deposits [Page and Nanson, 1982].

The rate of deposition within the recirculation zone was

roughly proportional to the transport rate through the con-

striction (Figure 12). The relation in Figure 12 (which is not

temporally ordered) could be expected to depend in part on

the extent to which the eddy was filled at the time of each

measurement. Based on stratigraphic evidence in bars in

Grand Canyon [Rubin et al., 1990], we suspected that the

A 1,00

110 1,20 1,30

"' ...........:...--

::.-. ,:.........:.......:..::

RP

========================

Fig. 9. Aggradation atesduring runs, determinedby subtract-

ing previousbed topographyrom resultantbed topography.Con-

tour interval 10 mm/h, with supplemental 5 mm/h dashed contours.

(a) Zero o elevenhours; b) 11-17.5hours; c) 17.5-22.5hours;d)

22.5-30 hours.

8/10/2019 Schmidt 1993

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SCHMIDTTAL.: LUMEIMULATIONF LOWND EDIMENTATION

2933

Fig. 10.

Photographf symmetricalippleshatdevelopedeneathhe center f theeddy.

capture ateswould be greatestwhen the eddywas empty

and would decreaseas the eddy became illed. To evaluate

the effect of such illing on the capture ate, the data were

plotted in temporal order in Figure 13, which shows hat the

cumulativepercentageof sediment apturedby the eddy

decreased hrough time. The percentageof main stem sedi-

ment that was captured by the eddy between observations

(Table 3, column I0) decreased rom 37% (when the eddy

was empty) to 24% (when sand filled approximately32% of

the eddy volume). We hypothesize hat f the experimenthad

continued long enough, the mean capture rate would have

approached zero as the eddy filled to such an extent that

circulating currents and sediment nput were restricted or

eliminated.

Changes n Reattachrnent Length and Do)t,nstrearn

Changes n Mean Velocity During

Bar-Building Experiment

During the bar-buildingexperiment, reattachment ength

decreasedand velocity increased as deposition occurred.

During the initial part of this experiment, the reattachment

length was 13 to 16.6 times step height' after 17.5 hours the

reattachment ength decreased o 10.2 to 13.8 times step

height, and at the end of the experiment the reattachment

length was 9 to 10.6 times step height (Figure 8). The

location of the reattachment zone was compared to the

longitudinal pattern of deceleration of streamwise flow.

Mean downstreamvelocity in the channel expansion was

lOO

a 80

::) 20

o

o

o.{

(separationoint) / // leaIra... t point

,'7 -

/

/ / / sedimenteed

o.1 1 lO

SIZE IN MILLIMETERS

Fig. l 1. Size distribution f composite amples f sediment ol-

lectedduringdifferent unsat indicatedocations.

i 0.25

_ 0.2

Z 0.15

E 0.1

uj 0.05

7.5 HRS

y - 0.0192 0.226 / C

r 2 - 0 933 / '

/ 5 HRS

11 HRS

O

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

MAIN CHANNEL TRANSPORTRATE, N KILOGRAMSPER SECOND

Fig. 12. Eddy deposition ate and main channel ransport ate

for bar-building xperiment.Measurementperiod, n cumulative un

time, is shown for each value.

8/10/2019 Schmidt 1993

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2934 SCHMIDT T AL.: FLUMESIMULATION FFLOWANDSEDIMENTATION

volume of

recirculation zone

y=0.08240.340x0.00236x 30HRS

r2 = 0.999 .-- --- -"

11 HRS -e '-' "- 22.5 HRS

--- 17.5 HRS

-, i , , .. I .... I .... I , , , , I , , ,

5 10 15 20 25 30

CUMULATIVE VOLUME OF SEDIMENT TRANSPORTED

THROUGH CONSTRICTION, N CUBIC METERS

Fig. 3. Cumulative volume of recirculation one filled with

sediment in relation to cumulative sediment transported hrough

constrictionduring bar-buildingexperiment,both divided by the

volumeof theemptyoriginal ecirculationone.The totalvolume f

theoriginalecirculationonewas 5.2m3 28m ength 2 mwidth

x 0.45 m average water depth between stations97 and 125).

Cumulative run time for each measurement is shown.

calculatedrom (1) by calculating d from measurementsf

bed topography nd water surfaceelevationand estimating

the location of the dividing streamlinebetween streamwise

and recirculating low. The longitudinal atternof very high

velocity at the constrictionand steadydecreasedownstream

shown n Figure 14 exists becauseconservation f mass

dictates that flow accelerate through the constrictionand

decelerate as flow cross-sectional area increases down-

stream; after 4 hours cumulative run time, v, decreased

continuously downstream. Velocity increased with time

between stations 120 and 130 because bed aggradation

decreased d; following he initial measurement eriod,

decreased to a minimum but increased further downstream

between stations 120 and 130. The magnitudeand location of

the cross section of minimum streamwise velocity increased

and migratedupstream.Reattachmentoccurredwhere mean

downstream elocity was about0.3 m/s (4 hours),0.4 m/s (14

hours), 0.5 m/s (20 hours), and 0.55 m/s (26 hours), respec-

tively. The location of the reattachmentzone was between

the cross section of minimum downstream velocity and the

cross section where accelerated flow reached its maximum

value at the two intermediate measurements eriods (14 and

20 hours),and was ocatedupstream rom the locationof the

minimum value in the last (26 hours) measurementperiod.

FIELD ESTIMATES OF AGGRADATION

Available estimates of deposition rates in recirculation

zones in Grand Canyon provide minimum values of the

percentageof sediment rapped n eddies hat are at leastone

order of magnitude ess than those measured n theseexper-

iments. Schmidt [1987] estimated an aggradation rate at a

reattachmentbar along the Colorado River 1.5 km down-

stream from the Paria River (see Schmidt and Graf[ 990] for

locations). At this site, topographic surveys and recovered

scour chains showed that 0.18 m of brown silty sand ag-

graded between October 1985and January 986. During this

period, only one discharge vent occurred n the Paria River

(October 0, 1985,peakdischarge1 m3/s), he onlyup-

stream tributary source of sediment. Schmidt [ 987] con-

cluded, based on textural characteristics, hat all deposition

had occurred during this one Paria flood event. He useda

locally developedstage-to-dischargeelation at the reattach-

ment bar to estimate the duration of inundation of the bar

andestimatedhemass f aggraded edimentromsurveying

and trenching.Basedon these measurementsnd assump-

tions, Schmidt [ 987] estimatedan aggradation ate of 4.2-

7.5 kg/s (verticalaggradationate of 0.6-0.9 cm/h) during

4.5-8.5 hours of inundation.

Schmidt [ 1987] estimated a capture rate by the recirculat-

ing current by comparinghis estimateof the mass of sedi-

ment delivered by the Paria with the estimate of the massof

sediment deposited n the recirculation zone. The massof

delivered sediment was estimated by using the average

sediment ransport elation for the Paria River gage Randle

and Pemberton, 1987], the 25% value suggestedby Randle

and Pemberton [1987] for the proportion of transported

sediment within the sand size fraction (0.0625-2 mm), and

hourly water discharge alues. Basedon the estimated ate

of sediment delivery to the Colorado River, the maximum

suspended edimentconcentrationof the Colorado River

immediately downstream rom the Paria was 55,000 rag/L,

and concentrations exceeded 10,000 mg/L for the entire

periodof bar aggradation. he total deliveredsand oad o

the iverwasbetween .0 x 10? and10.0x 107kg.Themass

of sedimentggradedt the eattachmentar,1.2x 105 g,

was at least 0.1 and 0.2% of the total estimated sand load

deliveredby the Paria River flood, and at least 0.03 to 0.06%

of the total suspendedoad. The averageaggradationate or

the entire bar of 4.2-7.5 kg/s was not less than two to three

orders of magnitude ess than the estimatedsedimentdis-

charge ate of the ColoradoRiver. Theseestimates re only

minimumvalues of eddy capture rate because (1) the pro-

portionof the delivered ediment eposited n the channel

bedupstreamrom the measurementite s unknownand 2)

the actual delivered sediment load and its size distribution is

unknown.

Schmidt nd Graf[1990] estimated n aggradationate of

0.3 kg/s (vertical aggradation ate of 0.04 cm/h) duringa

33-dayperiodof inundation f a separation ar at Eighteen

Mile Wash in Grand Canyon. Using the sand transport

relation or the ColoradoRiver at Lees Ferry, located 30 km

upstreamrom this site [Randleand Pemberton,1987], he

average aterdischargeuring heperiodof inundation,nd

the local aggradation ate, this site accumulated t least

0.04% of the total main stem transported sand.

DISCUSSION

Bar Construction and Bed Forms

There s general imilaritybetween he topographicorm

of reattachmentbars in Grand Canyon and the bar that

developed uring he bar-buildingxperiment.n bothcases,

thehighest artof thebarexistsn thedownstreamartof

the recirculation one, althoughn GrandCanyon, here s

greatvariability n the degree o which hesebars ill the

upstream artsof recirculationones.Field evidences

consistent ith the progressionf bar constructionuring

theexperiments.edimentransportirectionsnterpreted

from the orientation f bed formsand the progressionf

topographichangen the lume howshat hebarbuilt n

8/10/2019 Schmidt 1993

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SCHMIDTTAL.'FLUME IMULATIONFFLOW ND EDIMENTATION 2935

O

an upstream irection.Upstreammigrationof the reattach-

ment zone also caused scour of the farthest downstream

areasdeposited ithin the nitially onger ecirculation one.

There is inconsistency etween ield and flume in aggra-

dationpatterns n the reattachment one, however. In the

field, we have observed the results of reattachment zone

aggradationn the formof thicksequencesf climbingipple

structureswith migration directions consistentwith reat-

tachmentzone flow [Rubin et al., 1990]. In the flume,

deposition id not occur n the reattachment one adjacent

to the wall; in fact, this zone was an area of local scour.

This discrepancymay be related to the experimental

channelgeometry.First, in these experimentshere were

only smalldifferences etween he streamwise nd upstream

velocity in the vicinity of the reattachment one becauseof

the large magnitudeof streamwisedecelerationupstream

from the reattachment one (Figure 14). In Grand Canyon,

thesedifferences re muchgreater Schmidt,1990, Table 2

and Figure 1], such that more sediment-ladenwater is

delivered to reattachment zones in the field. Second, there

may be differencesbetween flume and field in the relative

position of main channel scour hole and the reattachment

zone leading to differences n the location of shoaling of

streamwiselow. Third, the area of low velocity surrounding

the instantaneouseattachmentpoint was much smaller n

the flume than is observed in the field. This is because the

rough slopingbank of the river slows the flow more than

does the smooth vertical wall of the flume. In the field, the

area of low velocity is largerand the average elocity of the

reattachment one may be lower than in the flume. Scour n

downstreamparts of the reattachmentzone in the flume

could occur f high velocity downstream low occasionally

exists in the reattachment zone when the instantaneous

reattachment oint is at the upstreamend of the zone.

Characteristics f RecirculatingFlow

The courseof floating objectsreflects he sum of all the

instantaneouslow directionsover the object's path. Obser-

vations of these path lines are consistentwith bed form

migrationdirections nterpreted rom analysisof sedimen-

tary structures Rubinet al., 1990].However, both field and

experimental esults ndicate hat there is great variation in

the nstantaneouselocity field, especially ear the centerof

the recirculationzone and near the reattachmentpoint.

These instantaneousvariations are great enough to even

causesymmetrical ipples to develop beneath areas where

there is net onshore flow.

Some flow pattern characteristics f these experiments

were influenced y the strongstreamwisedeepening nd are

consistent ith experiments oncerningets debouchingnto

still water. A high rate of deceleration eads to relatively

rapid decreasen the velocity head (Figure 4), increase n

water surface elevation downstream, and existence of an

adverse pressure gradient. First, current meter measure-

ments showed that instantaneous backflow velocities were

about 0.5 times v d, values much greater than typically

observed n the field. The high ratio of backflow to down-

streamvelocity may simplybe due to fact that deceleration

of vd was so great. Second,unusualcrossover f streamwise

flow from eft to right wall occurred n some uns, suchas is

depicted earstation119 n Figure3. Vorticesadvected rom

upstream evelopedwherevelocity and cross-stream hear

8/10/2019 Schmidt 1993

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2936 SCHMIDT ET AL.: FLUME SIMULATION OF FLOW AND SEDIMENTATION

2.5

1.5 , .

I1mO RANGENOCATIONFEATTACHMNT

i Wm'"'XX POINTFTERNDICATED

O3 01 X 14

-- SED "-.

."-.

0.54., ..:

TIMErea-fd

vvn;3',...,'nr-a. bar deposition

0 ' ,, I, , , , I ,

80 90 100 110 120 130

DISTANCE, IN METERS

140

Fig. 14. Mean section velocity during different runs of bar-building experiment. Range of location of reattachment

point also shown.Values of v, determined y estimating rea of downstream low.

was high, and thesevorticesmay have been strongenough o

produce upstream flow along the left wall. Third, reattach-

ment lengthswere very long for the experimental low width

expansiongeometry.

Controls on Reattachment Length

Our laboratory experiments and field observationshave

documentedboth shorter and longer recirculationzones han

would be predictedusingpublished elationsbetweenstep

height and reattachment length. Relative to the relations of

Abbott and Kline [1960] for clear water flow at a single

backward step (Figure 15, line c), the reattachment engths

in our empty flume experimentsare too long. Reattachment

lengths n the bar-buildingexperimentsare shorter and tend

to be similar to those of published elations; field reattach-

ment lengths ypically are too short (Figure 15). As will be

discussedbelow, these deviations result from topography

that influences he pressuregradient. n flows with obstacles

downstreamsuchas the bar that was depositedn our flume

experiments or cobble bars in Grand Canyon) the flow

accelerates, esulting n a diminishedadversepressuregra-

dient and a shorter reattachment ength. In contrast, where

downstream flow is a deep pool (as in our empty flume

experimentand in very deep pools n rivers), flow deceler-

ates, resulting n an increasedadversepressuregradientand

longer reattachment ength.

Seven runs were conductedat different discharges ut at

the same tailgate elevation (Table 2, thirteenth column) and

thereforesimulate hanginglow conditionsn an expansion

whose water surface elevation is set by a downstream

control. The experimentalresults of these runs (1, 6, 10,

14--17) are consistent with field data that show that time-

averagedreattachment ength increaseswith discharge.Al-

though instantaneous fluctuations in the location of the

reattachment point varied greatly in these runs, the time-

averaged reattachment length was greatest at highest dis-

charge (Figure 16). Lengthening at higher discharge was

associatedwith higher Froude numbers of constricted lows

and more adverse pressure gradients. In these runs, Froude

number was higher at higher discharges (Table 2, ninth

column) because water depth through the constriction did

not change Table 2, eleventhcolumn). The magnitudeof the

adverse pressure gradient was higher at higher discharges

because the elevation of the water surface downstream

(Table 2, twelfth column) increased with increasing dis-

charge.

At constant discharge and nearly constant flow conditions

in the constrictionduring he bar-buildingexperiment Table

2, ninth column), reattachment length decreased as the

magnitudeof streamwisedecelerationdecreased Figure 14).

The decrease in the rate of deceleration was due to the

decrease in flow area expansion ratio (Figure 6, solid

squares;Figure 16, solid squares); there was no change n

the magnitudeof the adverse water slope (Table 2, runs

15-17, eleventh to twelfth columns).

Reattachment length shortening was also related to the

fact that streamwise elocity v d) near the initially very long

reattachment zone nearly doubled due to midchannel bar

deposition.At the beginningof the bar-buildingexperiment,

streamwise elocity was about 0.3 m/s opposite he reattach-

ment zone (Figure 14, station 130, 4 hours). At the time of

the last measurement, ctwas more than0.5 m/s at this same

site. As the channel filled with sediment and velocity neces-

sarily ncreased, reas hat had formerlybeen of low stream-

wise velocity or stagnantbecame areas of higher streamwise

velocity. The midchannel ar essentially illed areas hat had

been part of the reattachment zone.

These results suggest hat there shouldbe wide variation

in reattachment lengths in natural rivers, due in part to

presenceor absenceof downstreambars and narrowingof

downstream walls which would also accelerate flow. Figure

15 showsGrand Canyon eattachmentengthdata, reattach-

ment lengthsduring the bar-buildingexperiment, and other

experimental ataploted n relation o stepheight.Data from

specific ield sitesare shownas separate ines extending ver

the range of different measured reattachment engths and

8/10/2019 Schmidt 1993

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SCHMIDTTAL.:FLUME IMULATIONFFLOW ND EDIMENTATION 2937

3::

I--

Z

Z

0

30

z 2O

0

o

z

o10

I

I

i

I

I'-'

I

I

I

-=

I

I

71 --

A B

rangeorirstxperiment largeid-channel

-

j, ,EXPLA-']

ar-building._.,,,t'"" iTM

_ oxporimort /'' -

c .... ? , L

.;.,;

* o

- /- \

':' 11/'"'eb;isfanhortens

o , I

0 1 2 3 4 5 6 7 8

SCALED-STEPEIGHTV-Wc)AN

Fig. 15. Changen recirculationone ength n relation o changen channel xpansion eometry howinghat ield

siteshave shorterscaled ecirculation ones han in flumeexperiments. ines A, B, and C are for flume studiesof

double-backwardteps maximum bserved alues),double-backwardteps minimumobserved alues),and single-

backward steps, respectively, rom Abbott and Kline [1962]. Solidvertical ines above abscissa alue of 1.7 are range

in reattachment lengths or first experiment longer ine) and range in recirculation zone length for bar-building

experiment (shorter ine) after indicated un time. Inset diagramshowshow field measurementswere made. Number

of each ield site refers o Schmidt 1990,Table 1]. Scaled-step eightchangesn field with changing ischarge ecause

we is held constantat value for lowest observeddischarge.

geometric conditions. Reattachment lengths were deter-

minedby mapping ecirculation one low patterns t differ-

ent dischargesonto 1:5000 scale air photos and then esti-

mating the unconstrained reattachment ength [Schmidt,

2O

.4.9

.3-4.9

3.9

3.4

... I

0.1 0.2 0.3 0.4 0.5 0.6 0.7

DISCHARGE,NCUBIC ETERSERSECOND

Fig. 16. Time-averagedeattachmentengthor hesame own-

stream ateheight0.800m above narbitraryoint, eeTable ,

thirteenth olumn). rrorbars ndicateotal angen instantaneous

reattachmentength,n stepheights. umbersdjacento circles

are low-areaxpansionatiooreachun.Open irclesre orclear

water, mptylume xperiment,ndsolid quaresre or bar-

buildingxperiment.rrowndicatesheprogressf thesecond

experiment.

1990, Table 1]. The parameter unconstrained eattachment

lengthwas proposed o permit comparisonof reattachment

length in channel expansions of different curvature and

shape. These data show that unconstrained eattachment

length increases as step height increases. There is great

variability n the rate of lengthening t each ield site, but all

reattachment lengths are shorter than those reported by

Abbott and Kline [1962] for double backward facing steps

(Figure 15, A and B) and are typically shorter han for single

backward facing steps (Figure 15, C).

What field conditions might explain the shorter reattach-

ment engths n Grand Canyon? n Grand Canyon, midchan-

nel cobble and gravel bars are common downstream from

many constrictions, specially apids with the steepestgra-

dients [Webb et al., 1988]. These bars are analogous o the

expansion ars describedby Baker [1984]and analogouso

the midchannelbar that developed n the secondexperiment.

In other field cases, a downstream debris fan or talus cone

narrows the channel and shortens he length of the channel

expansion.Of the field sites plotted on Figure 15, the

channel expansionsat sites 5, 10, and 11 are strongly

influenced y a midchannelbar (site 10, Granite Rapids)or a

debris fan (site 5, Eighteen Mile Wash, and site 11, One

Hundred Twentytwo Mile Creek). These three sites plot the

furthest from the experimental values of Abbott and Kline

[1962],suggestinghat downstream hannelrregularities o

constrain hese reattachment lengths.

Midchannel bars not only shorten the length of recircula-

8/10/2019 Schmidt 1993

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2938 SCHMIDT T AL.: FLUMESIMULATION F FLOWAND SEDIMENTATION

tion zonesat high discharges y necessitatingcceleration

over them, but at low dischargeseattachmentengthmay be

shortenedbecausebars are emergent.Schmidt [1990] m-

plied that recirculationzone length is controlledby the

chance intersection of the shear zone and irregularly ori-

ented downstream hannelbanks. Theseexperiments how

that acceleration ausedby narrowing f the channel, hoal-

ing over midchannel ars, and associated ecreasen flow

area expansion atio also nfluenceshe point where low

reattaches. In rivers where midchannel bars are larger and

more numerous, reattachment lengths should be even

shorter than those measured n Grand Canyon. The results

also ndicate hat successfulmodelingof reattachmentength

in riverswill dependon predictionof flow conditionsn the

constriction, dentificationof the downstream hannelcon-

trol, prediction f the water surface levation pstreamrom

that control nto the expansionn question, nd prediction f

main channel bed behavior during the modeled flows.

Implications or Restorationof Eddy Bars

in Regulated Rivers

These experiments show that deposition rates within

recirculation zones increase as main channel transport in-

creases due to increased sediment concentration at constant

discharge.Aggradation rates within the recirculation zone

were highly correlated with main current sediment ransport

rates and indicate, within the range of these experimental

conditions, that about 20% of main channel transport was

depositedwithin the recirculation one. We only conducted

our bar-building experiment at one constant low, and we

have no data to suggestwhether the percent of sediment

trapped n an eddy increasesat higher main channel rans-

port rates causedby (1) further increases n sedimentcon-

centration at constant discharge or (2) increased water

discharge and increased sediment concentration, such as

indicated by the calculationsof Andrews [1991].

Although we found a direct correlation between cumula-

tive transport hrough he constriction nddegreeof fillingof

the recirculation zone, our experiment only continueduntil

the recirculation zone was filled to about 40% of its original

volume. These data suggest hat this relation decreases n

slope;sucha relation shouldexist becauseas the recircula-

tion zone fills it can be expected that circulationwithin the

eddy and delivery of sediment into the eddy would both

decrease. This idea is consistent with stratigraphic se-

quences observed in Grand Canyon reattachment bars. In

these sequences, rain size decreases s reattachment ones

fill and sedimentary structureschange rom those indicating

vigorouscirculation (dunes and erosional lutes filled with

coarse sand deposited rom turbulent suspension)o those

indicating weak circulation (ripples and mud drapes). An

important unresolvedquestionconcernshow the proportion

of capturedsedimentvaries as a function of the strengthof

recirculating low, which is not only a function of the extent

to which the recirculation zone is filled but is also a function

of the spatial structure of the decelerating et. Flume and

field measurementsof eddy capture rates likely differ by at

least one order of magnitude; however, available field mea-

surements are subject to substantial error.

While these data show that the rate of eddy deposition

dependson the rate of main channel ransport, he extent o

which a recirculation zone fills depends on the duration of

the sediment-transportingvent. In the planningof any

regulated flood intended to reconstruct eroded reattachment

and separationbars, one must be sure that there is sufficient

sedimentsupply available to maintain high main stem rans-

port rates for durations sufficient o result in net deposition.

Field evidence shows that some recirculating current bars

are scouredduring ise of a regulated lood [Schmidt,1990;

Schmidt and Graf, 1990]. These observations, f representa-

tive, imply that reattachmentbars are either a primary

sourceof sedimentor the main stemor a relativelyquickly

mobilized source. The bars can not begin to rebuild until

sediment concentration in the main stem exceeds that in the

eddy. To achieve net aggradation, any flow designed o

reconstruct bars must first rebuild bars to the level that

existedprior to initial scour, then continue o add sediment.

The role of sediment availability in the design of bar

reconstruction loods places great importance on accurate

accounting of sediment sources prior to design of such

floods. If it turns out that most sediment n bedrock gorges s

stored in recirculation zones, rather than in main channel

pools, than it may be impossible o sustainnecessary edi-

ment transport rates for sufficient durations without aug-

mentingnaturally availablesupplies.Otherwise, net erosion

of recirculation zones in upstream sediment-deficient

reaches will result. Planning of bar reconstruction floods

must be based on predicting eddy depositionrates, predict-

ing the duration of high main channel sediment transport,

and accounting or the wide variation in channel and recir-

culation zone geometry that exists in natural rivers.

CONCLUSIONS

During a 30-hour experimentwhen a mixed size distribu-

tion of sediment was added to the flow at a rate of 0.5-1.0

kg/s, a bar formed within the zone of recirculating current

downstream from a channel constriction. The general topo-

graphic orm of this bar was that of a linear ridge widening

downstream nto a broad platform. Within the recirculation

zone, the highestdeposition ates occurred near the down-

stream end, but these rates were always less than rates on a

midchannelbar that formed ust downstream rom the reat-

tachment zone. These results are consistent with field inves-

tigationsof the sedimentology f reattachment ars n Grand

Canyonwhichshow hat deposition ccurs n the vicinityof

the reattachment zone and fills the primary eddy in an

upstream direction.

Bed forms on the surfaceof the ridge and platform and

repetitive topographicsurveys show that the entire bar

aggradedn an onshore nd upstreamdirectionduring he

experiment.Sort'rag f sedimentwithin the recirculation

zone s sufficiento form a deposit hat is generally iner han

the size distributionn transportby the main channel; he

finestsizesare depositedn the lee of the debris an. This

areal distribution is similar to that described in Grand

Canyon or separation nd reattachmentbars, and the pro-

gression f bar depositions consistent ith the modelof

GrandCanyonbar deposition evelopedrom interpretation

of sedimentarytructures nd sequentialield topographic

surveys.The percentageof main stem sediment hat was

capturedby the eddy during the bar-buildingexperiment

decreasedrom 37% (when he eddywas empty) o 24%

(whensand illedapproximately2%of the eddyvolume).

The reattachmentointoccurswhereexpanding ortices

8/10/2019 Schmidt 1993

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SCHMIDTTAL.:FLUME IMULATIONFFLOW ND EDIMENTATION 2939

impinge n the flumewall, and his point luctuated ver a

range f about5 times he stepheight.Some artsof the

recirculation one had instantaneouselocitieshat varied

overan entire360-degreeange.Resulting ed orms hat

developedear hecenter f theeddy esembledscillatory

ripples. scillatoryippleshat n the ieldare ormed y

reattachmentoint luctuationid not develop uring ur

experiments.n fact, deposition id not occurnear the wall

in the reattachment zone.

Recirculationengths eresimilarwithinbroad anges f

hydraulic onditions.Whereconstrictedlow was highly

supercritical, he length of the recirculationzone was about

two step heights, but where Froude numberswere less han

about2, the zone of recirculatingcurrentwas of similarsize

over a range of area expansion atios. To someextent,

reattachment engths were longer, and recirculation low

patternsbetter developed, at intermediateFroude numbers

between 0.8 and 1.6. As aggradationof a midchannelbar

proceededand the bar migrated upstream,reattachment

length decreased and the recirculation zone shortened.

These processeswere linked because low acceleratedover

the midchannelbar, which preventedstagnation long he

wall in areas previously of adverse water slope. As the

midchannelbar retreated upstream, accelerationoccurred

where decelerationhad previouslybeen the case.

Sedimentation can occur in bedrock gorgeswhere flow

separationand flow stagnation xist [Baker, 1984].The areal

extent and magnitude of' deposition depend on the areal

extent of the stagnated low, the areal extent of the zone over

which reattachment point oscillation occurs, he flow struc-

ture of the remainder of the recirculation zone, the rate of

main channel sediment transport, and the duration of the

transportingevent. In regulated ivers where degradation f

reattachment and separationbars has occurred, he task in

planningbar reconstruction loods s to predict the location

of stagnationpoints, the size of the zone over which these

stagnationpoints oscillate, the nature and stability of the

general ecirculating low field, the rate of sedimentdelivery

from main channel to recirculation zone, and the duration

during which that delivery from main channel o recircula-

tion zone will occur.

Acknowledgments.Field work was supported y the GlenCan-

yon EnvironmentalStudiesprogramof the U.S. Bureauof Recla-

mation. Travel funds were providedby the MiddleburyCollege

FacultyDevelopment und J.C.S.)and heU.S. Geological urvey

G. K. GilbertFellowshipD.M.R.). T. O. Manley,Departmentf

Geology,MiddleburyCollege, onductedhe surface ndvolume

modeling alculations. ssistance uring he experiments aspro-

videdby Y. Kodama ndH. Iijima. We thank . G. Bennett, . M.

Nelson,andT. L. Vallier or careful eviewof themanuscriptnd

suggestions or improvements.

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J. C. Schmidt, Department of Geography and Earth Resources,

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(Receivedctober8, 1992;

revised March 1, 1993;

accepted March 16, 1993.)