Dam Failure Lesson Learned: Downstream flooding can be caused by spillway flows that exceed channel capacity or as a result of reservoir misoperation.

If a dam functions as intended, risks to the dam owner and nearby communities are often assumed to be negligible. Experience suggests this may not always be the case – the immediate reservoir area and downstream floodplains could be subject to flooding from ‘normal’ yet large spillway flows that exceed channel capacity or as a result of spillway misoperation. In many locations around the world, human development downstream of a dam is often a direct result of the flood risk reduction provided by the dam. Here, lessons learned from example case studies are developed to highlight the implications of non-breach release events on downstream communities; Strategies for managing these risks are evaluated. Typically, dam safety risk assessments are focused on the potential for dam failure and uncontrolled release of the reservoir. However, non-breach impacts are more likely to occur. This increases the importance of estimating and communicating the potential for these events to downstream communities. It also supports dam owner planning and decision-making during normal and flood operations. In addition to reducing risks through communication, preparedness, and community resiliency, recognition and mitigation of non-breach risks to downstream populations may reduce potential liability for dam owners.