Risk Based Design and Evaluation of Spillway Structures: An Overseas Perspective

Criteria and guidelines for the design and evaluation of spillway structures vary from country to country as well as from state to state. In some countries, recent current guidelines allow for the design and evaluation of flood security measures for dams to be based on the risk associated with a catastrophic dam failure caused by an extreme flooding event. This is somewhat different from the traditional standards-based approach, which is generally utilized in the United States. For example, current State of Colorado dam safety criteria require that the flood security of existing dams is based on the hazard classification and the size of the structure. Design rainfall values for an existing large, high hazard dam typically range from the 0.5 Probable Maximum Precipitation (PMP) event to the full PMP event. However, an Incremental Damage Assessment (IDA) may be performed in order to determine the incremental damages associated with a potential dam failure during different magnitude flooding events. If incremental damages are small, a lower level of protection may be warranted.