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16 <br />significant reduction in the lake level can expose <br />saturated soils and cause slope and foundation <br />failures. <br />Photo 3. Residential Boat Dock Left High and Dry by Reservoir Drawdown <br />A fairly obvious impact of reservoir drawdown is the <br />loss of the water resource. Drawdowns can leave <br />irrigation intakes “high and dry” and lot of <br />irrigators…irritated. <br />Photo 4. Irrigation Well Left High and Dry by Reservoir Drawdown <br />Increased amounts of debris may be produced from <br />the newly exposed rim, which can begin to clog <br />spillways and trashracks. For large volume releases, <br />the increased energy in the reservoir can transport <br />previously stable sediments in upper reaches of the <br />reservoir. In select cases, significant reservoir <br />drawdown could expose culturally or historically <br />sensitive areas that were previously submerged. <br />Photo 5. Reservoir Debris at Intake during a Period of Reservoir <br />Drawdown. Photo by Wyoming State Engineer’s Office <br />Damage to Dam and Appurtenant Structures – For <br />earthen embankment dams, rapidly reducing the <br />reservoir level by a significant magnitude can develop <br />excess pore pressures in the upstream slope of the <br />dam, leading to a slope failure in the upstream portion <br />of the embankment. This loading condition is called <br />rapid drawdown. While some maximum drawdowns <br />may be specific to local regulations, many dam experts <br />advocate a maximum 1 foot of reservoir drawdown per <br />day as a rule of thumb. While large volumes of water <br />are discharged through the outlet works system, high <br />energy flows have the potential to damage <br />components of the outlet works system. Particularly <br />for unvented outlet works, cavitation can cause <br />significant damage to the outlet works components <br />and the dam itself. A detailed discussion about venting <br />conduits was presented in Volume 1: Issue 2 of <br />Western Dam Engineering: Technical Notes. <br />Photo 6. Damage Incurred to Outlet Conduit during Operation