Geomorphic Impacts of Dam Removal

Rollin H. HotchkissDirector, Albrook Hydraulics Laboratory

Washington State University, Pullman, WArhh@wsu.edu

http://www.matilijadam.org/

DefinedGeomorphic 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 fluctuationV

Modes of Sediment Transport

Suspended Load

Bedload

Yes, It’s NonlinearShear stress designated as S (S is slope)

S V2 (V is velocity)Therefore, V2

Sediment Transport designated as Qs

Qs 1.5 Therefore, Qs V3 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

Finest on bottom

Uniform delta

Eventually, surface is like u.s.

Upstream from the Reservoir

Matilija Dam, So. CaliforniaSediment does not reach the beachDeposited 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

Now

North Platte River, NE; 7 u.s. dams for 50 years

Before Dams

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 BalanceRivers constantly shifting

geomorphology to adjust balanceWhen 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…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

Matilija Dam – helps control rate of downstream transport

http://www.matilijadam.org/

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• Downstream streambed 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 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