Geomorphic Impacts of Dam Removal http www matilijadam

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

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

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”

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

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

Modes of Sediment Transport Suspended Load Bedload

Yes, It’s Nonlinear Shear stress designated as ta. S t (S is slope) S a V 2 (V is velocity) Therefore, t a V 2 Sediment Transport designated as Qs Qs a t 1. 5 Therefore, Qs a V 3 or higher!

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

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

Within the Reservoir Delta Formation

Delta Formation

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

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

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

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

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

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

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

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

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

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

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

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!

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

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

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

Adjustments after Removal Where on deposit curve are you now?

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

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

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

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

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

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

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

Summary • Know 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