Incremental Damage Analysis - The Perverse Perspective and The Lessons from Success and Failure

An incremental damage analysis (IDA) is a procedure that can be used to possibly justify a smaller spillway capacity than would normally be required by regulating agencies given the dam size and hazard classification. The IDA is based on the idea that dam owners should not be held responsible for flood damage that would occur for the design storm were the dam not in place, and it tries to account for the additional damage that would occur from the released storage volume caused by a dam failure. An IDA compares two floods in an attempt to account for that additional damage: the base flood and a dam breach flood. The base flood analysis looks at the percentage of the PMP flood that will just cause a dam failure by overtopping, and then routes that flood through the downstream areas as if the dam were not in place. It represents the level of damage that would occur naturally for the given storm if the dam had never been built. The dam breach flood analysis is the same rainfall event as the base flood but considers the effects of the dam and includes the release of the storage volume that would result from the dam failure. The incremental zone is the difference between the flood stage elevations of the two events. Many state regulations specify that the spillway capacity that passes the base flood would be acceptable where it can be shown that the dam failure would not cause additional loss-of-life and significant damage in the incremental zone downstream. The 100-year storm capacity is usually the minimum allowable spillway size for jurisdictional dams. Boyle Engineering Corporation has performed many IDAs in Colorado over the past several years, and this paper will present the lessons learned from both the successes and failures. The techniques that have been developed for Colorado will be briefly discussed, and the situations or circumstances that often lead to success or failure will be presented. 9 pp., 1 table, 13 references.