Sediment Transport in Some Eastern United States Streams

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.

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.

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