DWR_2801682 - Laserfiche WebLink

17 Considerations Every dam should have an up-to-date Operations and Maintenance (O&M) manual. The O&M manual should cover all facets of the dam and have detailed guidelines and schedules for releases, if applicable. When developing or updating an O&M, a detailed assessment should be made with respect to all the factors listed in the previous section of this article. This must include a clear understanding of flooding impacts downstream as assessed by the local engineering authority with respect to the varied volumes of water released. The methods for discharging water through the outlet works detailed in the O&M manual should take into account the structural capacity of the outlet structures as well as the capacity of the downstream channels, such that they do not overwhelm the system. The O&M manual and EAP should have a comprehensive list of stakeholders who are to be notified prior to routine releases as well as unexpected releases. This not only includes parties downstream, but parties located upstream who may be impacted as well. The notifications may have a tiered arrangement dependent on the magnitude of reservoir drawdown expected. For example, small routine releases may have minimal impacts, there may only be a handful of parties to notify. Conversely, if your dam is experiencing an emergency and requires an immediate dewatering of the reservoir, your notifications may include numerous communities upstream and downstream, local jurisdiction officials (Dam Safety Office, Department of Water Resource Management, Department of Environmental Protection, federal and/or local wildlife management officials, etc.) utilities, and emergency services. Depending on the jurisdiction and the scale of your dam, you may be required to have an Environmental Impact Assessment, which details the effects a major drawdown has on the local wildlife and habitats. Some dams have established protocols to mitigate some of the impacts made by large volume releases. Inspection of the structural integrity of the dam and its outlet works structures should be a routine occurrence for dam operators. However, if a significant release is anticipated with sufficient lead time, an assessment of the condition of the outlet works system should be made. It is common and highly recommended that all of the outlet systems be operated fully, or “exercised” on an annual basis. This can provide notice of damage or operating deficiencies that can be remedied before a real emergency occurs. Repairs should be made if possible before the drawdown; however, if this cannot be achieved prior to the release, there should be a modification to the discharge methodology to account for the deficiencies and monitoring of the system’s performance during operation. For high head dams, a major drawdown puts tremendous stress on the dam system, especially for improperly designed outlet systems or systems with inadequate ventilation (See Volume 1: Issue 2). Following a major reservoir drawdown, the dam structure itself and its appurtenant structures should be inspected for damage that may have occurred during the event prior to refilling the reservoir. Common Pitfalls in Drawdown Design and Operation Listed below are possible outcomes of lack of planning, coordination, or infrastructure that can take place when drawing down a reservoir. Releasing water at too high of a rate, causing upstream slope instability and/or damaging outlet works. Inadequate notifications leading to flooding of downstream areas, disruption of water delivery services, or regulatory enforcement actions and/or penalties. Unintended habitat destruction or wildlife casualties. Major alterations of downstream channel conditions, such as scouring or shoaling. Property damage both upstream and downstream. Inability to drawdown reservoir due to lack of low- level outlet or siphon, sedimentation or debris buildup, or improper design of outlet works inverts. Unvented outlet works during high head releases that lead to cavitation in pipes and outlet works structures.