1. Prevention and Cure

2. Contents Introduction to Reservoirs Preventing Siltation Cure Cost Benefit Analysis Conclusion

3. Introduction - Reservoirs Facts and Figures Definition: A lake or large pool, natural or man-made, used to store water for public and industrial use, for irrigation, etc. Also more generally: a large expanse of water held back by a dam. 1 Constructed for 3 main purposes 1. Irrigation 2. Storage/Flood Control 3. Hydro-power 75% of water consumption is used during irrigation 2 Irrigated land produces 33% of the worlds food 1% of reservoir storage is lost due to siltation every year 3 Lake Kariba in Zimbabwe and Zamiba

4. Preventing Siltation Prevention through Design Catchment Characteristics Upstream Trapping Bypassing High Sediment Loads Minimising Deposits Sluicing Density Current Venting

5. Drainage Basin Area Catchment Characteristics Slope Gradient Length Runoff Velocity Catchment Orientation Southern Facing Slopes in the Northern Hemisphere Drier Soil -> Higher erosion Large Catchment Lower Slope Lower Sediment Yield Smaller % of erodible rock More opportunity for sediment to be deposited

6. Human Impact Drainage Density Catchment Characteristics Deforestation Urbanisation Increases Erodibility Land Use Cultivation of Clay Soils Decrease Erosion Rates High Density Higher erosion rates Agriculture Cultivation of Sandy Soils Increase Erosion Rates Increasing Drainage Density

7. Catchment Characteristics Vegetation Root Systems Reduces surface runoff and velocity Dissipates Rainfall Energy Prevents Rain reaching the ground Reduces “Splash” Erosion Holds soil in place Maintains the soils capacity to absorb water Increase Infiltration

8. Upstream Trapping Intercept sediments as they travel downstream Vegetation Screen Check Dam Decrease speed of water flow Reduce erosion Allow sediment and pollutants to settle

9. Bypassing High Sediment Loads High flow rates increases sediment concentration These waters should be bypassed by channels and/or tunnels Water collected will have low/medium flow rates, where sediment concentration is lower

10. Minimising Sedimentation Sluicing Passing of sediments during flooding Requires reduction in water level Preserves the seasonal sedimentation loads downstream Practical alternative to flushing Density Current Venting No reduction in water level required Only used in extreme circumstances Density Current Venting (Morris and Fan, 1998)

11. Cure the Removal of Accumulated Sediment Flushing Siphoning Dredging and Digging

12. Not suitable for all reservoir shapes Shape may prohibit suitable scouring channels River discharge may be incapable of moving sediment in large enough quantities Flushing Requires a significant change in the operation of the reservoir Water level needs to be dropped dramatically

13. Water Level Drop

14. Inadequate Drawdown

15. Final Reservoir Condition due to Inadequate Drawdown

16. May cause environmental issues downstream: (1) Higher nutrient & metal content Abrades fish gills Reduces light penetration Switzerland stipulates sediment concentration levels: 10 mg/l at the reservoirs in crystalline areas 70 mg/l at the reservoirs in non-crystalline areas Flushing Cont. Drawdown reduces the capacity to generate power Reduction in annual water yield

17. A Success Mangahao Reservoir New Zealand Constructed 1924 Lost 59% of storage in 34 years 1969 flushing removed 75% of sediment

18. Siphoning Involves special pumps to redeposit sediment downstream There are two approaches to siphoning sediment: 1. The pipeline goes over the dam crest into a downstream basin 2. The pipeline travels through the dam’s body bottom outlet

19. Dredging and Digging Most expensive of removal techniques Process requires: Extraction Transportation Disposal/Use Mechanical Digging – carried out by specially equipped excavators in shallow waters Dredging – fixed or mobile platforms

20. Cost Analysis 300-400 new dams would need to be constructed a year to keep current storage levels Costing between $9 and $13 billion 1 Not accounting social, environmental or maintenance costs Dredging costs approximately 3 $8m -3 Flushing costs 10-30% of the cost to construct a new similar sized reservoir 2 Performed once a 40-60% reduction of storage capacity is seen

21. Cost Cont. ProcessCost Construction of new dam$44 Million Cost of Flushing average dam$13.2 Million Cost to Dredge Mangahao at $8m -3 $70 Million

22. Conclusion Try to prevent through design 100% prevention is unrealistic so implement sluicing and density current venting if possible If removal is necessary flushing is the most cost effective method Prevention is better than cure!!