SEDIMENTATION STUDIES FOR DAM REMOVAL USING HEC-6T

1. SEDIMENTATION STUDIES FOR DAM REMOVAL USING HEC-6T

2. Introduction • HEC-6T is • a general purpose, Computational- Sedimentation Computer Program • an enhancement and an extension of the Corps of Engineers programHEC-6 • A proprietary computer program owned by MBH Software, Inc

3. Introduction It performs a continuous simulation of the movement of the water-sediment mixture • the water surface profile over time • the bed surface profile over time • erosion and deposition subject to sorting and armoring of the bed mixture, and • the concentration and delivery of sediment particles, by size class, for non-equilibrium sedimentation processes

4. ASCE Manual 54CHAPTER I.—Nature of Sedimentation Problems “A. Introduction 1. General.—Sedimentation embodies the processes of erosion, entrainment, transportation, deposition, and the compaction of sediment. These are natural processes that have been active throughout geological times and have shaped the present landscape of our world. …”

5. 2 points … • When evaluating the applicability of a computer program, state the questions you want the program to answer. • By coupling computational features in HEC-6T with river morphology principles we can answer many questions that are pertinent to dam removal issues.

6. To illustrate point No. 1 … The following examples show typical questions that one can ask HEC-6T. Removal of Washington Water Power Dam Breach of the N1 Structure at Mt. Saint Helens Pumping Spirit Lake at Mt. Saint Helens

7. Fig 1. Removal of Washington Water Power Dam

8. Fig 2. Erosion of Reservoir Deposit at Dam

9. Fig 3. Deposition Downstream of Dam, River Mile 3.48

10. Removal of Washington Water Power Dam • Questions to ask HEC-6T • How much of the reservoir deposit will erode? • Will the downstream channel transport the eroded sediment? • How much time will pass before the river has returned to a state of equilibrium?

11. Point No. 1 (cont) … The following examples illustrate the questions that one can ask HEC-6T. Removal of Washington Water Power Dam Breach of the N1 Structure

12. Breach of the N1 Structure • was applied to first dam removal study in 1977

13. Fig 4. Breach of N1 Structure (View from Right Bank Side to Left Bank Side, Downstream of Structure)

14. Flow through Breach, N1 Structure N. Fork Toutle River - Mt. St. Helens, 1981

15. Point No. 1 (cont) … The following examples illustrate the questions that one can ask HEC-6T. Removal of Washington Water Power Dam Breach of the N1 Structure Pumping Spirit Lake

16. Pumping Spirit Lake • was applied to first dam removal study in 1977

17. Fig 5. Channel from Pumping Spirit Lake (Formed by a discharge of 182 cfs flowing for 1 Year)

18. Pumping Spirit Lake Where C = bank resistance coefficient Q = channel forming water discharge W = top width of channel

19. What questions did we ask HEC-6 about Mt. Saint Helens watersheds? • How much sediment, by particle size, will be eroded from that network of streams as a function of time? • How much of that volume will be transported to the Columbia River? • What will the range of concentrations be over the next 50 years?

20. What questions did we ask HEC-6 about Mt. Saint Helens watersheds? • How much of the volume in transport could be stored in a sediment retention basin? • How much time would be available to construct the basin before sediment deposits blocked the bypass channel.

21. Point No. 2 … Special Features in HEC-6T

22. Physical properties of interest in channel evolution studies are

23. Features that allow one to- determine channel width • The conceptual, Channel Evolution Model of Schumm, Harvey and Watson. • Shows 5 stages of development • Stage II is predominantly erosional • Stage IV is highly depositional

24. Figure 18-1. Stage II Bed Erosion h < hc h = bank height NOTE:hc= critical bank height

25. Figure 18-1. Stage V Flood Plain Evolution NOTE:hc= critical bank height

26. - determining channel width • Pumping Spirit Lake convinced me: • It is not necessary to model the details of channel evolution from one stage to the next. • The hydraulic geometry equation for channel width gives a reasonable top width even in highly disturbed watersheds. • We need only to convert that top width to a bottom width and assign it to the original cross section.

27. Features in HEC-6T- coding bottom width • By restricting erosion to an assigned bottom width, erosion and deposition computations will establish the new bottom slope and the new bank height. Normal Pool Final Cross Section Elevation 1 14 Erosion Limits 13 8 12 Initial Cross Section 6 2 3 Station

28. Features in HEC-6T for channel evolution computations

29. Features in HEC-6T-fail the banks • If the computed height becomes greater than a stable bank height, HEC-6T will fail the banks • The bank sediment will slump into the channel where it will be removed by the flow as transport capacity becomes available. • All computations are made by particle size.

30. Features in HEC-6T-dam removed in stages • The dam can be removed in stages by placing cross sections in the simulation hydrograph at the times when notches will be constructed. • If tributaries are present, they can be coded into a stream network. • HEC-6T will increase transport of the sands and gravels due to the high concentration of fines.

31. Sedimentation Studies For Dam Removal Using HEC-6T Summary and Conclusions

32. Summary and Conclusions HEC-6T is a general purpose computer program that calculates changes in cross sections resulting from the 5 basic sedimentation processes.

33. Summary and Conclusions • 5 Basic Sedimentation Processes • Erosion • Entrainment • Transportation • Deposition • Compaction of Deposited Sediments

34. Summary and Conclusions • It can be applied to study channel development and sediment delivery resulting from the removal of a dam by coupling it with • the Channel Evolution Model, and • river morphology principles.

35. Typical Questions one might ask HEC-6T.(Examples, not an exhaustive list) 1. How deep will the bed erode? 2. How deep will the sediment deposit downstream? 3. How wide will the top width become due to bank failure from channel invert erosion? 4. What will the average boundary shear stress be? 5. What will the gradation of the surface layer be and how will it change over time?

36. Typical Questions (cont) … 6. How will the concentration of sediment in the water column vary over time? 7. How many tons of sediment will be delivered downstream? 8. How much sediment will be transported out of the reservoir? 9. What will the average bed profile become, upstream and downstream of the dam, and how will it change over time?

37. Summary and Conclusions • Observations at Mt. Saint Helens convinced me that river morphology principles can be used to predict channel width even in highly disturbed watersheds.

38. Summary and Conclusions • The channel width can be converted into a bottom width and coded into the initial cross section using the “erosion limits” feature in HEC-6T. Normal Pool Final Cross Section Elevation 1 14 Erosion Limits 13 8 12 Initial Cross Section 6 2 3 Station

39. Summary and Conclusions • The dam can be removed in stages by placing cross sections in the simulation hydrograph at the times when notches will be constructed. • If tributaries are present, they can be coded into a stream network. • HEC-6T will erode silt and clay, and the transport of the sands and gravels will be increased if the concentration of fines becomes significant.

40. Summary and Conclusions • My presentation today shows features in HEC-6T that are not in HEC-6.