Modification of the Baldhill Dam, North Dakota Embankment for Overtopping

Baldhill Dam and Lake Ashtabula were authorized and are operated for low flow augmentation to meet downstream water supply and pollution abatement requirements and for alleviating flooding downstream in the Sheyenne River valley. The reservoir is located about 16.5 river miles upstream of Valley City, which is the principal damage center for flood control. Detailed Risk Assessment evaluations and alternatives analysis confirmed that an 880 foot ungated overflow auxiliary spillway located in the embankment of the dam with a crest elevation of 1271.0 feet is the preferred alternative to correct hydrologic deficiencies on this high hazard dam. An ungated overflow spillway was designed to discharge of 58,000 cfs and will compliment the existing spillway to provide a combined Probable Maximum Flood (PMF) outflow capacity of 125,000 cfs. The maximum PMF surcharge pool is limited to top of dam elevation of 1278.5 feet. A 5-foot parapet wall is provided for freeboard above the crest of the existing embankment. The recommended spillway a 50 foot wide broad crested weir with a 1V:3H downstream chute slope and 1V:2.5H sloped upstream face. A previous ASDSO paper presented at the 7th Annual Conference outlined the project formulation and alternatives study. This paper will review the project formulation and discuss the construction of the project to include the construction staging, problems encountered and their solutions and a review of the completed project. 12 pp., 2 references.