In the decades since many of the states’ dam safety programs were first established, significant advances in meteorological, hydrologic, and hydraulic analysis capabilities have occurred and much regulatory experience has been gained in the monitoring, inspection, and safety evaluation of thousands of dams nationwide. In the process, guidelines for dam safety evaluation and design of dams have evolved and new tools developed to assist both regulators and dam owners. Hydrologic and hydraulic design criteria factor prominently in dam safety regulation and have led to re-evaluation of inflow design floods (IDFs), hydraulic performance of spillways and outlet works, and dam failure inundation mapping.
This paper discusses some of the unintended side effects and potential liabilities created by bringing older dams into compliance with updated spillway design criteria imposed by regulation. In almost all cases involving free-overflow (ungated) spillways on older dams, replacement or enlargement is necessary to enable safe passage of prescribed IDFs without overtopping and potential dam failure. While preventing an IDF-induced catastrophic failure, an enlarged spillway can also produce higher spillway discharges in comparison to the original (undersized) spillway for floods less than the IDF, e.g. a 100-year flood. As a result, the original flood easements held by dam owners may be inadequate for a rehabilitated dam with an upgraded spillway. A case example of this phenomenon is illustrated by the Grand Lake St. Marys Dam, which impounds the largest man-made reservoir in Ohio. The dam was originally constructed in between 1837 and 1845, and the original spillway was replaced in 1914 by a spillway with approximate crest length of 100 feet. To bring the dam into compliance with Ohio Dam Safety Laws and Regulations (ORC Title XV as amended), the 1914 spillway was replaced in 1997 by a new spillway with a crest length 500 feet.
