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Geomorphic Impacts of Dam Removal

Geomorphic Impacts of Dam Removal. Rollin H. Hotchkiss Director, Albrook Hydraulics Laboratory Washington State University, Pullman, WA rhh@wsu.edu. http://www.matilijadam.org/. Defined.

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Geomorphic Impacts of Dam Removal

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  1. Geomorphic Impacts of Dam Removal Rollin H. Hotchkiss Director, Albrook Hydraulics Laboratory Washington State University, Pullman, WA rhh@wsu.edu http://www.matilijadam.org/

  2. Defined Geomorphic impacts: changes in hydrology and hydraulics which affect stream shape, planform, slope, and sediment transport

  3. Introduction • Dams – geomorphic disruptors! • My purpose – provide a primer on what happens when we tinker with a river by building or removing a dam • Emphasize geomorphic processes and qualitative relationships • What questions should you ask before you “pull the plug”

  4. Road Map • Sediment Transport Processes • Sediment Transport and Reservoirs • Geomorphic Balance • Regime Adjustment • What do you need to Know?

  5. Sediment Transport Process • Sediment weighs more than water! • Forces required to move sediment • “Shear” – tangential dragging force downstream • “Lift” – upward-directed vertical velocity fluctuation V

  6. Modes of Sediment Transport Suspended Load Bedload

  7. Yes, It’s Nonlinear Shear stress designated as t ta S (S is slope) S a V2 (V is velocity) Therefore, ta V2 Sediment Transport designated as Qs Qsat1.5 Therefore, Qsa V3 or higher!

  8. To Further Complicate Matters Armor layer – coarser material on top, finer beneath

  9. Road Map • Sediment Transport Processes • Sediment Transport and Reservoirs • Geomorphic Balance • Regime Adjustment • What do you need to Know?

  10. Within the Reservoir Delta Formation

  11. Delta Formation

  12. Reservoir: a Sorting Machine Due to nature of reservoir, coarser material farthest upstream

  13. Consider a section… Vertical Sorting as Well Don’t just sample on the surface….

  14. Eventually, surface is like u.s. Uniform delta Finest on bottom Combining the Two Processes

  15. Matilija Dam, So. California Sediment does not reach the beach Deposited u.s. from pool Upstream from the Reservoir http://www.matilijadam.org/

  16. Atbara River, Sudan: 12 m deposition u.s. from reservoir pool

  17. Downstream from the Reservoir • Sediment load decreased • Smaller peak discharges • Unfulfilled sediment transport capacity • Excess energy erodes bed and banks • Bed eventually becomes armored

  18. Bank Instability Small dam Lower Elkhorn River, NE Top view: upstream from 5 ft-high dam Lower view: downstream from dam

  19. Downstream Impacts North Platte River, NE; 7 u.s. dams for 50 years Before Dams Now

  20. Secondary Impacts • All things biological adjust • New upstream river type • New downstream river type • How long does that take? • Eventually change reaches equilibrium….

  21. Road Map • Sediment Transport Processes • Sediment Transport and Reservoirs • Geomorphic Balance • Regime Adjustment • What do you need to Know?

  22. Lane, E.W. 1955. The importance of fluvial morphology in hydraulic engineering. ASCE Proceedings, 81, paper 745: 1 – 17. Qs x ds Q x So From Applied River Morphology by Dave Rosgen, Wildland Hydrology, Figure 2-1

  23. Implications of Balance Rivers constantly shifting geomorphology to adjust balance When in approximate balance: “in regime” Dam in or Dam out is out of balance!

  24. Road Map • Sediment Transport Processes • Sediment Transport and Reservoirs • Geomorphic Balance • Regime Adjustment • What do you need to Know?

  25. On the Day of the Ribbon Cutting • These geomorphic shifts begin • River system begins to change • It all takes time…time…time..

  26. Downstream Adjustment Time • For Dam Building OR Removal River 1 and river 2 here

  27. Adjustments after Removal Where on deposit curve are you now?

  28. Depends upon Future Runoff Sequence! • Large flood – lots goes out early • No large flood – wait and wait, and…

  29. Dam Notching http://www.matilijadam.org/ Matilija Dam – helps control rate of downstream transport

  30. Road Map • Sediment Transport Processes • Sediment Transport and Reservoirs • Geomorphic Balance • Regime Adjustment • What do you need to Know?

  31. Hydrologic History • Flooding sequence and history • Change in downstream flood peaks, duration, and timing • Changes in water temperature, turbidity, and quality downstream

  32. Sedimentation History • “Snapshots” of bathymetry over time • Qs coming in and going out • Downstream streambed profiles over time • Location of downstream tributaries

  33. Sediment Particle Size Distribution • Within reservoir • Spatial coverage • Collect cores!!!! • PSD inversion? • Downstream • Armored?

  34. Downstream Habitat • Benthics • Fish compared to pre-dam • Condition of spawning gravels • Riparian vegetation

  35. Summary • Know how hydrology, sediment transport and size have changed • Understand how downstream reaches have adjusted • Need to predict (can we do that?) spatial and temporal scenarios of impact

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