This report presents the results of field inspection and evaluation of the Weatherby dam. This dam has been classified as unsafe, non-emergency by the St. Louis District as a result of the application of the following criteria: (1) Spillway will not pass 50 percent of the Probable Maximum Flood; (2) Overtopping could result in dam failure; and (3) Dam failure significantly increases the hazard to loss of life downstream.

Riss Lake Dam was inspected to make an assessment of the general condition of the dam with respect to safety, based upon available data and visual inspection, in order to determine if the dam poses hazards to human life or property. Our inspection and evaluation indicates the spillway does not meet the criteria set forth in the guidelines for a dam having the above size and hazard potential. The spillway will not pass the probable maximum flood without overtopping but will pass 90 percent of the probable maximum flood, which is greater than the estimated 100-year flood. The spillway design flood recommended by the guidelines is 100 percent of the probable maximum flood. The probable maximum flood is defined as the flood discharge which may be expected from the most severe combination of critical meteorologic and hydrologic conditions which are reasonably possible in the region. Deficiencies visually observed by the inspection team were erosion of the upstream face due to wave action, ponded water on the dam crest due to poor drainage of rainwater, trees and brush growing on the embankment, animal burrows, and seepage at several locations below the dam.

The purpose of the inspection was to make an assessment of the general conditions of the dam with respect to safety, based upon available data and visual inspection, in order to determine if the dam poses hazards to human life or property. In accordance with the guidelines and based on visual observation, the dam is classified as having a high potential for damage and loss of life. Failure would threaten life and property. The estimated damage zone extends approximately one (1) mile downstream of the dam. The inspection and evaluation indicates that the spillways meet the criteria set forth in the recommended guidelines for a small dam having a high hazard potential. Considering the small volume of water impounded, one-half of the Probable Maximum Flood is the appropriate spillway design flood. The spillways will pass the 100-year flood (1% probability flood - a flood having a one percent chance of being exceeded in any year) without overtopping the dam. The spillways will pass 65% of the Probable Maximum Flood without overtopping the dam. The Probable Maximum Flood (PMF) is defined as the flood that may be expected from the most severe combination of critical meteorologic and hydrologic conditions that are reasonably possible in the region.