sediment transport in some eastern united states streams

Sediment Transport in Some Eastern United States Streams

James W. Gracie and

William A. Thomas

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.

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.

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.

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.

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.

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.

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

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

Helley-Smith Bedload Sampler

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.

Helley-Smith Deployed

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

Intellicast Weather Images Available Online

Precipitation Intensity

Satellite Summary

Precipitation Forecast

Sampling Locations

• PBMS

• GH • GS

• PBDSGH

• PBUSGS

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

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

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

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

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