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Sediment Transport in Some Eastern United States Streams
James W. Gracie and
William A. Thomas
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Sediment DisequilibriumWhen watersheds are developed, hydrologic
regime can change significantly Increase in impervious surfaces can cause
increases in peak, frequency and cumulative duration of elevated flows.
This in turn causes channel enlargement as the channel tries to accommodate the increased discharges.
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Sediment Disequilibrium(continued) When this enlargement process occurs at a
rate greater than the channel can transport the supplied sediment the process goes into disequilibrium.
Sediment Supply > Sediment TransportThus excess sediment creates depositional
features which cause even more erosion and greater supply of excess sediment which in turn creates even more depositional features and on and on.
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Sediment Disequilibrium(continued)Direct measurement of bedload sediment
transport has been a difficult proposition especially in small streams in the eastern United States.
Sediment transport models can be in disagreement by more than an order of magnitude.
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Difficulties in Bedload Transport MeasurementEastern streams are storm flow dominated
Short duration of elevated dischargesOnly brief periods of steady state flowRecognizing storm events that allow bedload
collection Brief sampling conditions require many trips
over longer time periodSnow melt systems in Rocky Mountain
region allow for longer periods of steady state flow.
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Difficulties in Bedload Transport MeasurementBedload varies spatially and temporally
Cross channel variation can be greatBedload varies in time (can move in
intermittent slugs)The key to representative transport
measurements is to sample across the width at equal distances and to conduct sampling long enough to average out the changes in rates.
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Difficulties in Bedload Transport MeasurementIdeal procedure is to collect at 20 equal
width segments for two transectsIn small streams where width is less than 5
meters 20 equal width segments isn’t practicalTwo transects often exceeds the steady state
duration of stable flow.
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Equipment for Bedload MeasurementHelly Smith Bedload Sampler
Hand held and suspendedCable Way with two reels
one for positioning the sampler across the channel
A-reel for raising and lowering the samplerStage rating gageCurrent Meter
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MethodsPermanent Monuments were installed at
each siteWhere bankfull discharges would be too
great for wading cableways were installedCross sections were measured
Width, average depth, cross section including flood prone area, water surface slope, pebble counts
Stage reading rods were installed to allow visual observation of stage
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Helley-Smith Bedload Sampler
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Cable Way StationNote two sets of cables, one for positioning the sampler across the channel and another for raising and lowering it.Used for Helley-Smith and Current Velocity Meter.
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Helley-Smith Deployed
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WHEN TO SAMPLEStorm events must generate enough
discharge to move bedload. (Usually 1/3 of bankfull depth as a rule of thumb)
Event must last long enough to create steady flow conditionsIf flow changes during the sampling effort then
results are not valid for a given dischargeNeed a way to predict conditions for bedload
sampling
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Intellicast Weather Images Available Online
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Precipitation Intensity
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Satellite Summary
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Precipitation Forecast
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Sampling Locations
• PBMS
• GH • GS
• PBDSGH
• PBUSGS
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Bedload StationsStation Wbf (m) Dbf
(m)Slope(m/m)
Qbf (m3/s) DA (mi2)
Gum Springs 5.64 m .25 .01009 1.75 or61.8 cfs
1.0
Good Hope 3.35 m .26 .0110 0.85 or29.99 cfs
0.4
Paint Branch u/s Gum Springs
9.38 .47 .0051 4.98 or165.8 cfs
3.5
Paint Branch d/s Good Hope
9.45 m .57 .0057 7.24 or255.5 cfs
5.5
Paint Branch at Rt. 29
14.78 .61 .0074 16.98 or599.2 cfs
18
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1 10 100 10000.1
1
10
100
R² = 0.923442932285763
Good Hope Tributary
Series1Exponential (Series1)
Discharge, cfs
Bed
load
Tra
nsp
ort
Rat
e, t
on
s/d
ay
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1 10 1000.01
0.1
1
10
100
R² = 0.827384075881545
Gum Springs TributaryBedload Rating Curve
Series1Exponential (Series1)
Discharge, cfs
Bed
load
Tra
nsp
ort
Rat
e, t
on
s/d
ay
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10 100 10000.01
0.1
1
10
R² = 0.837144698756919
Paint Branch Upstream of Gum SpringsBedload Rating Curve
Series1 Linear (Series1)
Discharge, cfs
Be
dlo
ad
Tra
ns
po
rt, t
on
s/d
ay
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10 100 1000 100001
10
100
R² = 0.620720691258752
Bedload Transport CurvePaint
Branch at U.S. Rt. 29
Discharge. cfs
Bed
load
Tra
nsp
ort
Rat
e, t
on
s/d
ay