Phase II Inspection of Lake Manatee Dam

Lake Manatee Dam was constructed during the period 1965-1967. The reservoir is the primary water supply for a quarter of a million people in the Bradenton-Sarasota area of Florida. In early 1978, the Jacksonville District Corps of engineers performed a preliminary (Phase I) inspection of the earth dam and appurtenant structures as part of the National Dam Safety Program. The Corps found that the existing spillway did not have sufficient capacity to prevent overtopping during a large storm and that the downstream slope was potentially unstable, due to inadequate seepage control. The Corps classified the dam as large, with a high hazard potential, and recommended that a more detailed investigation be performed. Design calculations and many construction records for the dam were not available. Therefore, the subsequent Phase II investigation was considerably more extensive than would otherwise have been necessary. This investigation indicated that the embankment has a satisfactory factor of safety against slope failure and the concrete spillway has a satisfactory factor of safety against overturning. However, the drain at the toe of the dam was found to be partially plugged, and construction of a new toe drain was recommended. It was also recommended that an emergency spillway be constructed to supplement the existing spillway.