Low Hazard, High Hazard, What Hazard??? The Saga of the Lake Mills Dam

摘要

The hazard rating of a dam imparts upon its owner certain responsibilities that can often have significant economic liabilities. Two of the more common dam safety items associated with hazard classification are the required spillway capacity of the dam and the frequency of required dam safety inspections by a professional engineer. Each of these items can translate directly into an expense for the owner: a capital expense for upgrades to the facility in the event the spillway capacity is insufficient to satisfy regulatory requirements for a given hazard rating, and an ongoing expense of periodic inspections, the frequency of which is typically driven by the hazard rating. As the hazard rating is determined by identifying the population at risk of inundation downstream of the dam in the event of a failure, it is important that the assumptions supporting the dam breach analysis and the routing of the ensuing flood wave correctly represent the physical conditions present at the facility and are reasonably applicable to the postulated failure mode sequence. What happens when, in the context of the particular dam safety regulations, the results of the dam breach modeling indicate a high hazard rating, yet the postulated failure mode for the dam is clearly so remote as to be practically non-credible? The engineer and regulator are then faced with the challenge of trying to determine and justify a legitimate hazard rating for the dam. This paper explores the case of the Lake Mills Dam, a dam that hardly merits the moniker of dam, yet is bound by the regulatory requirements dictated by its hazard rating. The authors review the dam breach analysis and the events leading to a high hazard rating, and highlight the owner's approach to reduce the hazard rating through a rehabilitation design that will enhance the community's awareness and appreciation of the dam, and provide improved control of discharges from the impoundment.