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1 Session: For more information:

2 San Juan, Puerto Rico World Hydropower Congress Sustainable Sediment Management and Hydropower Reservoirs 21 May 2015 Dr. Gregory Morris PhD, PE Needle valve erosion after 24 hrs. operating with sand

3 Main Concepts 1. Sustainable sediment management is a current best management practice rather than a future problem to be solved by others 2. Transition to sustainable long-term operation is feasible and should be planned for: Operational changes Structural modifications Reduction in benefits may occur 3. Different management strategies are applicable to different types of sediment 4. A regular program of monitoring is essential to understand the problem and find optimal solutions

4 Long-term operation is achieved by replacing the Useful Life model with the Sustainable Use model Useful Life Model Continuously Trap Sediment End-of-Life Scenario Sustainable Use Model Sediment Balance Inflow = Outflow Sustainable Use is recommended as a Best Practice Model for hydropower

5 What is sustainable sediment management? Sustainable sediment management in reservoirs seeks to: (1) minimize the rate of volume loss and (2) balance sediment inflow and discharge while maximizing storage volume or other benefits. Benefits are not directly related to storage volume Not all storage volumes have equal value. For hydropower, the marginal value of storage value depends on both hydrology & market condtions. For example, peaking volume is much more valuable than seasonal storage.

6 Sustainable operation does not happen automatically. It results from foresight and appropriate actions

7 Sustained operation is not always the preferred strategy (Removal of Glines Canyon hydropower dam, Elwha River, USA) Dam has been removed

8 Main Concepts 1. Sustainable sediment management is a current best management practice rather than a future problem to be solved by others 2. Transition to sustainable long-term operation is feasible and should be planned for: Operational changes (may reduce seasonal storage volume) Structural modifications (intake, low level outlets) Reduction in benefits may occur 3. Different management strategies are applicable to different types of sediment 4. A regular program of monitoring is essential to understand the problem and find optimal solutions

9 Classification of Sediment Management Strategies for Reservoirs 1-Reduce Sediment Inflow from Upstream 2-Route Sediments (maintain transport, minimize deposition) 3- Focus or Remove Sediment Deposits Reduce Sediment Production Sediment Trapping Above Reservoir Sediment Bypass Sediment Pass-Through Mechanical Excavation Hydraulic Scour Soil Erosion Control, Revegetation Forests Rangeland Farms Construction sites and Developed Areas Channel Erosion Control Large Dams Dispersed structures Gully Stabilization Stream Channel Stabilization- Restoration Flood Bypass Tunnel High Level (no drawdown) Low Level (drawdown) Offstream Reservoir Reservoir Drawdown, Sluicing Flood Event Seasonal Vent Turbid Density Currents Dry Excavation Hydraulic Dredge (slurry pump) Hydrosuction (gravity) Dredge Air Lift Dredge Dredging Mechanical-lift (bucket, clamshell, dragline, backhoe) Agitation Dredge Pressure Scouring Empty Flushing Modify Operating Rule (focus or redistribute sediment) Sediment Discharge Options Below dam Off Channel 4-Adaptive Strategies (sediments not manipulated) Reallocate Storage, Increase Operational Efficiency Modify Intakes, Turbine Coatings, etc. Raise Dam to Increase Volume Water Loss Control & Conservation Construct Replacement Project Decommission Infrastructure

10 1- Reduce Sediment Yield Erosion control Costly, may involve many thousands of land users, slow and uncertain implementation and downstream response Upstream Sediment Trapping Small check dams have little capacity and are subject to eventual failure, releasing stored sediment, but can be effective if installed in large numbers. Large debris dams are costly. Upstream dams built for hydro, irrigation or other benefits are typically the fastest and surest way to reduce watershed inputs, but these projects may not get built as planned. Upstream Sediment Release Sedimented upstream reservoirs can change their operation to start releasing sediment.

11 China s watershed reforestation success is exemplary. However, watershed management may not protect against extreme events & debris flows (Intake to Jagra 30 MW RoR plant, Pakistan Administered Kashmir) May, 2013

12 Extreme rainfall produced extensive landslides and debris flows Antioquia, Colombia

13 Calderas Powerhouse Boulders INSIDE the powerhouse

14 Impact of Large Events on Sediment Yield Storage loss at Kulekhani reservoir, Nepal. (Shrestha 2012)

15 2 - Sediment Routing: (Minimize Sediment Deposition) Sediment Bypass (offstream storage, bypass tunnel) Highly effective, method of choice where feasible Dramatically reduces but does not eliminate sedimentation Reservoir Drawdown (sluicing) Reservior level is reduced during floods to maximize flow velocity through the reservoir and minimize sedimentation Some sediment may be scoured (as in flushing), but this is not the primary objective Release Turbidity Currents While most reservoirs have turbidity currents, not all of them transport a substantial amount of sediment to the dam Requires low-level release Minimum impact on operations

16 Passing Sediment Sanmenxia Dam, Yellow River, China Flow First major reservoir successfully managed for control of sediment Bottom Outlets

17 Use SBT to bypass headpond pool, which is used as a sand trap to avoid constructing a desanding basin

18 Sediment Bypass Tunnel operational since 1922 (Pfaffensprung, Switzerland. Amsteg power plant)

19 Turbidity currents are important in many reservoirs and may be used to release sediment through turbines

20 Turbid density current running through Dos Bocas reservoir, Puerto Rico Turbid water exiting turbines below dam Dam Clear surface water Sediment-laden water entering reservoir Dos Bocas was built in 1942

21 3 - Sediment Removal Pressure Flushing Dredging Costly. Not normally economically feasible on a large scale, unless cheap electricity from hydropower is available If not discharged below the dam, problem of finding a disposal site for a never-ending supply of sediment. Emptying and Flushing Empty the reservoir and allow the river to scour sediment Downstream release of muddy water Large downstream environmental impacts if not properly managed Only a limited volume can be recovered

22 Pressure Flushing Semi-circular scour hole formed in front of an intake at a reservoir in Brazil.

23 Dredging since 1962 at hydropower reservoir Sauyerman dragline Hidraulic cutterhead dredge (74 MW, 83 m 3 /s, 72 m head) Bajo Anchicayá, Colombia

24 Power Intake Bottom Outlet At this site the original designer did not think about sedimentation, and did not put the power intake in a location where it would be cleared of sediment by flushing. Now reservoir Gregory is emptied L. Morrisfor cleaning by hand & machine.

25 Flushing Channel and Intake Location By placing the intake on the left side of the dam sediment control at the intake would be easier BOTTOM OUTLET INTAKE Intake Channel FLUSHING CHANNEL maintained mechanically

26 Flushing will only scour out a channel about as wide as the original aluvial river channel Because the discharge rate and transport capacity of the flushing release is limited by the low-level outlet capacity, it will typically not be possible to flush out all of the coarse sediment that enters the reservoir. Flushing channel helps sustain turbidity current movement to the dam. Submerged floodplains keep accumulating sediment. These deposits not removed by flushing Full Reservoir Level Turbid density currents deposit fine sediment in the flushing channel, which can be released in the next flushing event. Flushing Channel Pre-dam channel width

27 Main Concepts 1. Sustainable sediment management is a current best management practice rather than a future problem to be solved by others 2. Transition to sustainable long-term operation is feasible and should be planned for: Operational changes Structural modifications Reduction in benefits may occur 3. Different management strategies are applicable to different types of sediment 4. A regular program of monitoring is essential to understand the problem and find optimal solutions

28 Generalized Sediment Deposition Patterns Within a Reservoir

29 Venting Turbidity Current Through Turbines It is possible to pass fine sediment through turbines with little damage. However, coarse delta sediments cannot be allowed to enter turbines. To avoid using deep intakes or outlet works, use siphon to aspirate turbid water from greater depths.

30 Main Concepts 1. Sustainable sediment management is a current best management practice rather than a future problem to be solved by others 2. Transition to sustainable long-term operation is feasible and should be planned for: Operational changes Structural modifications Reduction in benefits may occur 3. Different management strategies are applicable to different types of sediment 4. A regular program of monitoring is essential to understand the problem and find optimal solutions

31 Problems due to change in bathymetric study methodology The presence of error is obvious when reservoir storage increases over over time. When volume decreases we don t see the error, and assume bathymetry is correct. Volume'(Mm 3 )' 450" 400" 350" 300" 250" 200" 150" 100" 50" Valle"de"Bravo" El"Bosque" 0" 1940" 1950" 1960" 1970" 1980" 1990" 2000" 2010" 2020" Year'

32 The rate of storage loss is not necessarily constant. Without successive surveys these changes in sedimentation rate will not be observed Data from reservoirs in Kansas, Oklahoma, y main stem Missouri River (red) PROBABLE CAUSES: Sediment consolidation over time. Reduction in sediment yield over time. LESS PROBABLE: Change in survey methodology. Reduction in trap efficiency. AVERAGE RATE OF ANNUAL STORAGE LOSS (% / year) 1.00% 0.10% Canton Wister 0.01% SURVEY YEAR

33 Sediment sampling to monitor the advance of sand toward the intake. (Vibracore Sampling of Sediment Cores to 60 m depth, La Esmeralda, Colombia) Winch Extruded sediment core Core tube Vibrating head Floats to keep core vertical despite boat Movement.

34 PDF of 748 page book Reservoir Sedimentation Handbook McGraw-Hill Book Co., New York Free download at:

35 Final thoughts: Although there are many sediment management options, there is not yet a feasible solution for preserving large storage volumes.