The Pierce Lake Dam, Missouri Inventory Number 30999, was inspected. This dam is classified as small due to its 37 ft height and storage capacity of 681 ac-ft. A small dam is one which is less than 40 ft in height and impounds less than 1000 ac-ft of storage. The St. Louis District, Corps of Engineers, has classified this dam high hazard; we concur with this classification. The estimated damage zone, as determined by the St. Louis District, Corps of Engineers, extends approximately five miles downstream. The community of Richwoods and several other occupied structures are located within the estimated damage zone. The inspection and evaluation indicate that the dam is in generally poor condition. Specific deficiencies that were noted are insufficient spillway capacity, very steep downstream slope, high erodibility of the embankment materials and lack of maintenance and periodic inspections. Also deemed as a deficiency is the lack of any stability or seepage analyses.

This dam has been classified as unsafe, non-emergency by the St Louis District as a result of the application of the following criteria: (a) Spillway will not pass 50 percent of the Probable Maximum Flood without overtopping the dam; (b) Overtopping of the dam could result in failure of the dam; and (c) Dam failure significantly increases the hazard to loss of life downstream.

Potosi Lake Dam is an earth dam constructed for recreational purposes. The main spillway is a 3-ft dia corrugated metal pipe (CMP), or culvert, which discharges into an unlined earth channel. A 2-ft dia CMP serves as the auxiliary spillway and discharges into the same channel. A siphon (12-in. dia steel pipe) crosses the dam at about the midpoint. It crosses under the road and rests on the downstream slope. It discharges at the dam toe. The inspection and evaluation indicate that the dam is in generally poor condition. Specific deficiencies that were noted are inadequate spillway capacity, the high potential for spillway blockage, the dense growth of large trees and brush on the downstream slope and along the upstream crest, and lack of maintenance and periodic inspections. Hydrologic/Hydraulic studies indicate that the 10 percent probability-of-occurrence event (10-yr flood) will cause overtopping of the dam. These analyses also indicate that the dam will be overtopped for a hydrologic event which produces greater than eight percent of the Probable Maximum Flood.

The St Louis District (SLD), Corps of Engineers, has classified this dam as having a high hazard potential. The dam is classified intermediate size based on its height of about 45 to 50 feet. The downstream face is steep, on the order of 33 to 35 degrees, is constructed of cohesionless sand gravel, and could be severely eroded if overtopped. Scattered large trees up to 12 in. diameter are growing on the downstream face of the dam. The reservoir storage volume is completely filled with fine tailings and storage of water is limited to two small ponds at the upstream end of the impoundment, and shallow, intermittent ponds on the surface of the tailings. Hydraulic/hydrologic analyses indicate the dam and informal spillway will pass the 1 percent probability-of-occurrence event (100-year flood) without overtopping the embankment. These analyses further indicate the dam will be overtopped by a storm which produces greater than 55 percent of the Probable Maximum Flood (PMF). Based on the inspection of the Dam, it is recommended that further study be conducted without undue delay, under the guidance of an engineer experienced in the design and construction of tailings dams.

The purpose of Dresser No. 4 Dam is to impound tailings from a barite separation and beneficiation operation. The dam completely encircles the impoundment except for a 300-foot wide breach at the north (up-stream) end where the structure failed in 1975. The dam cannot retain water in its present state because of this breach in the dam. This dam is classified as large. The U.S. Corps of Engineers has classified it as having a high downstream hazard potential to indicate that failure of this dam could threaten life and property. The damage zone, estimated by the U.S. Corps of Engineers, extends about 6 miles downstream of the dam. Since the embankment cannot retain water, the overtopping potential of the embankment was not analyzed. Instead, studies were made to estimate the amount of flow that could occur in the diversion ditch (located to the west and south of the impoundment) under different flood conditions. It was calculated that the diversion ditch could pass a 100-year flood probably without significant erosion of the ditch or adjacent embankment toe. However, it was estimated that the ditch cannot pass 50 percent of the Probable Maximum Flood without significant erosion of the ditch and adjacent embankment materials.