1. Sediment Transport in Some Eastern United States Streams
James W. Gracie
and
William A. Thomas

2. Sediment Disequilibrium
When 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.
Show enlarging channels prefereably with mid channel bars and/or overgrown point bars

3. 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 Transport
Thus 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.
Show schematic slide of erosion causing accelerated adjustment

4. 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.

5. Difficulties in Bedload Transport Measurement
Eastern streams are storm flow dominated
Short duration of elevated discharges
Only brief periods of steady state flow
Recognizing storm events that allow bedload collection
Brief sampling conditions require many trips over longer time period
Snow melt systems in Rocky Mountain region allow for longer periods of steady state flow.

6. Difficulties in Bedload Transport Measurement
Bedload varies spatially and temporally
Cross channel variation can be great
Bedload 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.

7. Difficulties in Bedload Transport Measurement
Ideal procedure is to collect at 20 equal width segments for two transects
In small streams where width is less than 5 meters 20 equal width segments isn’t practical
Two transects often exceeds the steady state duration of stable flow.

8. Equipment for Bedload Measurement
Helly Smith Bedload Sampler
Hand held and suspended
Cable Way with two reels
one for positioning the sampler across the channel
A-reel for raising and lowering the sampler
Stage rating gage
Current Meter

9. Methods Permanent Monuments were installed at each site
Where bankfull discharges would be too great for wading cableways were installed
Cross 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

10. Helley-Smith Bedload Sampler

11. Cable Way Station
Note 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.

12. Helley-Smith Deployed

13. WHEN TO SAMPLE
Storm 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 conditions
If flow changes during the sampling effort then results are not valid for a given discharge
Need a way to predict conditions for bedload sampling

14. Intellicast Weather Images Available Online

15. Precipitation Intensity

16. Satellite Summary

17. Precipitation Forecast

18. Sampling Locations
PBUSGS
PBDSGH
PBMS GH GS

19. Bedload Stations Station Wbf (m) Dbf (m) Slope (m/m) Qbf (m3/s)
DA (mi2)
Gum Springs
5.64 m
.25
.01009
1.75 or
61.8 cfs
1.0
Good Hope
3.35 m
.26
.0110
0.85 or
29.99 cfs
0.4
Paint Branch u/s Gum Springs
9.38
.47
.0051
4.98 or
165.8 cfs
3.5
Paint Branch d/s Good Hope
9.45 m
.57
.0057
7.24 or
255.5 cfs
5.5
Paint Branch at Rt. 29
14.78
.61
.0074
16.98 or
599.2 cfs 18