Overtopping Flow Spillway for Baldhill Dam, Sheyenne River, North Dakota

An existing gated spillway structure for Baldhill Dam and Reservoir (Lake Ashtabula, North Dakota) is not capable of passing floods up to the probable maximum flood (PMF) without overtopping the earth embankment. A risk assessment completed in 1986 demonstrated that Baldhill Dam does not have adequate spillway capacity and is considered a potential safety hazard under current engineering standards. During the PMF, the dam could be overtopped by more than 4 ft if the dam does not fail. Overtopping to this extent would probably cause failure unless adequate modifications are provided. A 1:45-scale physical model was used to evaluate alternative spillways for increasing the discharge capacity for Baldhill Dam. An ungated auxiliary overflow spillway was designed, tested, and recommended during 1989 to augment the existing spillway capacity and serve as an emergency spillway. Planning is in progress for prototype construction during the 1990's.