Dams, sediment, and channel changes and why you should care

Transcription

1 Dams, sediment, and channel changes and why you should care Gordon E. Grant USDA Forest Service PNW Research Station Corvallis, Oregon

2 Dam effects on river regimes FLOW (Q) SEDIMENT (Qs) TEMP CHEMISTRY DAM Channel morphology Aquatic Biology

3 Dam effects on river regimes FLOW (Q) Channel morphology SEDIMENT (Qs) DAM Dams may or may not affect the flow regime Virtually all dams affect the sediment transport regime by trapping sediment Aquatic Biology

4 pre-dam post-dam Discharge record , Colorado River below Glen Canyon Dam

5 25 Peak Discharge, thousands of cubic feet per second Deschutes River near Madras, Drainage area 7,820 mi 2 Deschutes River at Moody Drainage area 10,500 mi 2 Project Construction Dec Feb Water Year Pre- and post- dam discharge record for Deschutes River, Oregon

6 Dams trap sediment Cougar Reservoir drawdown 2002, SF McKenzie River, Oregon

7 If we understand how dams change flow and sediment regimes, can we predict downstream channel response?

8 Principle of River Equilibrium A graded stream responds to a change in conditions in accordance with Le Chatelier's general law: "if a stress is brought to bear on a system in equilibrium, a reaction occurs, displacing the equilibrium in a direction that tends to absorb the effect of the stress." J. Hoover Mackin, The concept of the graded river. GSA Bulletin; v. 59; no. 5; p ; DOI: / (1948)59[463:COTGR]2.0.CO;2

9 Using Lane s Balance to predict downstream changes & channel narrowing Lane s Balance Applied River Morphology, Dave Rosgen, 1996

10 less sediment A. Dam traps sediment, no change to flow

11 coarsening lower slope less sediment incision, narrowing A. Dam traps sediment, no change to flow

12 less sediment?? less water B. Dam traps sediment and reduces flows

13 less water C. Dam reduces flows, sediment influx below dam

14 fining higher slope less water aggradation C. Dam reduces flows, sediment influx below dam

15 Textural coarsening Clackamas River, Oregon

16 Incision / bed degradation Arno River at Empoli

17 Large decrease in eddy sand bars Grand Canyon

18 Post-dam bed degradation Williams & Wolman, 1984

19 ARBITRARY ELEVATION, IN METERS Channel narrowing COTTONWOODS SALTCEDAR & RUSSIAN OLIVES 24 WILLOWS DISTANCE, IN METERS Green River, Utah; Grams and Schmidt, unpublished data

20 Framework for downstream channel response Equilibrium Point

21 How do we bring hydrologic and sediment transport effects together? Δ Sediment Supply Aggradation Textural shifts at confluences Island and bar construction Vegetation enchroachment Channel aggradation Degradation Bed scour Armored channel Bar and island erosion Channel degradation, narrowing Δ Transport Capacity

22 Δ Sediment Supply Rio Grande River? Colorado River (135 km) Yangtze River (673 km) Δ Transport Capacity

23 A summary of sorts Although dams change water and sediment regimes, the nature and direction of those changes depends on the dam itself and how it s operated Trajectories of downstream change are semi-predictable using simple analytical models Changing flow and sediment regimes translate to changes in channel form and aquatic habitat

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25 Sandy River Other examples of channel response

26 Loss of secondary channels (simplification) Colorado River

27 1871 Lodore Canyon, Green River early 1990s Grams and Schmidt, 2002

28 How do we bring hydrologic and sediment transport effects together? Geology Rock properties (erodibility) Deformation and structure Processes Sediment Supply Topography Relief Drainage Density Sediment Transport Regime Climate Precipitation type, intensity and duration Hydrologic Regime Basin Scale Geologically-controlled landforms and events ( history ) Dams Channel and valley floor morphology Channel and reach scale

29 Transport capacity T cr t ( Q Q ) t Q Fraction of time threshold for sediment transport is exceeded (grain size dependent)

30 Change in transport capacity due to dam T * T T post pre Ratio of post-dam to pre-dam fractional transport time

31 Change in sediment supply due to dam S * S S B A Ratio of below dam to above dam sediment supply

32 dam S*<<1 S*<1 but increasing Pre-dam Post-dam

33 Flaming Gorge Dam Downstream Changes in Flow and Sediment Flux, Green River Utah

34 Aggradation Reach 3: initial narrowing (10%) now aggrading Reach 2 narrowing (13%) degrading? Green River (464 km) Reach 1 narrowing (13%) degrading (0.7m) Degradation

35 Pre-dam loads were 35-40% sand (Topping et al., 2000) Glen Canyon Dam Colorado River

36 Elevation (m) Aggradation Distance Downstream (km) 25 Colorado River (135 km) Degradation

37 Elevation above sea level (m) 10/1/57 10/1/61 10/1/65 10/1/69 10/1/73 10/1/77 10/1/81 10/1/85 10/1/89 10/1/93 10/1/97 Deschutes River, Oregon Aggradation Deschutes River pre-pelton (160 km) m incision in 40 years Deschutes River pre-dam (160 km) /3/65 2/26/5812/31/64 2/24/82 2/10/96 Degradation Time

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39 Aggradation Rio Grande River Yangze River (673 km) Degradation

40 Evaluating altered sediment regimes Pre-dam as reference point Reservoir sedimentation rates Magnitude/frequency of sediment transport Ratio of above dam to below dam sediment supply Volume Timing/mechanisms of supply Caliber

41 Q Linking hydrograph changes to geomorphic response Peak discharge Armor breakup (Phase II) Initiation of motion (Phase I) Active bars inundated Secondary channels inundated Time