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• Clean out and conduct a camera inspection of the drain pipes under the dam; add <br />inspection of these pipes to the regular O &M schedule. Following the drain inspection, <br />evaluate the need for modifications to the drainage outfall area at the dam toe to address <br />uncontrolled seepage exits on the dam face near the outfall pipes. <br />• Evaluate and design appropriate channel erosion protection of the recently (2013) <br />repaired gully erosion in the right downstream abutment groin. Seeding of the area can <br />proceed as soon as conditions allow. Channel erosion protection placement will require <br />engineered design of appropriately sized riprap or other protection to suitably route and <br />channel the flow. <br />Recommendations for Additional Studies to Upgrade Facility Performance for Rare Extreme <br />Flood Events: <br />An Inflow Design Flood (IDF) has not been identified for this facility. If the objective is that the <br />facility is to perform within tolerable risk guidelines under a PMF or other approved IDF that is <br />substantially larger than a 100 -year event, additional engineering studies will be needed. These <br />studies should proceed with the objectives being to identify and recommend appropriate <br />structural upgrades to the facility that will reduce risk of severe erosion at the Drop Structure and <br />South Diversion Ditch during a PMF (or other approved IDF), which could potentially result in <br />an unacceptable release of flood water or tailing material stored in the impoundment. The <br />following additional studies are recommended: <br />• Evaluate and design modifications to prevent uncontrolled overflow at the Drop Structure <br />during extreme storm / runoff events. It is anticipated that an engineered Auxiliary <br />Overflow Structure would be needed. The Auxiliary Overflow Structure would operate in <br />tandem with the existing Drop Structure. Design alternatives for the Auxiliary Overflow <br />Structure should include options for directing peak flood flows into portions of the <br />impoundment area using internal berms to separate floodwaters from the existing <br />free -water storage pool. This would provide flood attenuation using storage capacity <br />within reclaimed portions of the impoundment. Temporary flood detention in reclaimed <br />portions of the impoundment area would allow optimizing the size of the Auxiliary <br />Overflow Structure and eventual direct discharge of flood waters which would not have <br />intermingled with the free -water pool or tailing. <br />• Evaluate and, if determined appropriate and necessary, design modifications to control <br />overflow from the South Diversion Ditch into the impoundment area during extreme <br />flood events to control erosion and scour of the ditch and berm. If overflow erosion <br />protection is determined to be required, additional engineering analysis will be needed to <br />optimize the location(s), size and type of armored overflows along various reaches of the <br />ditch upstream from the Drop Structure. <br />ES -3 <br />