Geomorphic Impacts of Dam

coriandercultureMechanics

Feb 22, 2014 (3 years and 5 months ago)

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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

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
t




t

a

S (S is slope)



S
a

V
2

(V is velocity)



Therefore,
t

a

V
2

Sediment Transport designated as Q
s



Q
s

a

t

1.5




Therefore, Q
s

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


Finest
on
bottom

Uniform delta

Eventually, surface is like u.s.

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

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.

Q
s

x d
s

Q x S
o

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…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


Q
s

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