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Floyd Lake Dam has been classified by St Louis District, Corps of Engineers (SLD), as having a high hazard potential; we concur with this classification. The SLD estimated damage zone length extends approximately eight miles downstream of the dam. Within the potential downstream damage zone are 4 dwellings within the first mile and the town of Old Mines, approximately 2 mi downstream. The dam is classified small due to its 50 ac-ft storage capacity. The dam is 21 ft high. The classification for small size dams is based on a height between 25 and 40 ft of a storage capacity between 50 and 1000 ac-ft. The visual inspection and available information indicate the dam is in generally poor condition. The principal deficiencies noted were the inadequate spillway capacity, potential erosion at the toe of the dam by the flow in the discharge channel, and very dense vegetation on the downstream slope of the dam, obscuring the face from a thorough inspection. Seepage and stability analyses comparable to the requirements of the 'Recommended Guidelines for Safety Inspection of Dams' were not available, which is a deficiency that should be rectified. Hydraulic and hydrologic analyses indicate that the dam will be overtopped by the 10 percent probability-of-occurrence event (10-yr flood), and by a flood of greater than 5 percent of the PMF event. The PMF is defined as the flood event that may be expected to occur from the most severe combination of critical meteorological and hydrologic conditions that are reasonably possible in the region.
Sections 1-2. Keyword Index.--Section 3. Personal author index.--Section 4. Corporate author index.-- Section 5. Contract/grant number index, NTIS order/report number index 1-E.--Section 6. NTIS order/report number index F-Z.
Gudaitis Lake Dam is in the small size classification based on its maximum height of 25 ft and on its reservoir storage volume of about 180 ac-ft. The small dam classification includes dams between 25 and 40 ft in height or having storage capacities between 50 and 1000 ac-ft. The inspection and evaluation indicate the dam is in fair condition. This evaluation is primarily based on hydrologic analyses that indicate that a flood greater than 12 percent of the PMF will effectively overtop the dam. The dam will also be effectively overtopped by the one percent probability-of-occurrence (100 year) flood event. A 40 ft wide zone of slow seepage was noted at the toe of the dam, but the seepage water was not transporting any soil particles at the time of our inspection. The downstream face of the dam is heavily vegetated with relatively young trees up to 12 in. in diameter. This vegetation obscured portions of the dam from inspection. The spillway discharge channel is narrow and shallow and is bordered by many large trees. Deep gulley erosion is occurring where this channel traverses steeper slopes. Flood flows in this channel could cause overtopping of the channel and erosion of the channel walls, resulting in possible erosion at the toe of the 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: (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.
The purpose of the inspection was to assess the general condition of the dam with respect to safety. The assessment is based upon an evaluation of the available data, a visual inspection, and an evaluation of the hydrology and hydraulics of the site in order to determine if the dam poses hazards to human life or property. The dam provides impoundment for barite ore tailings. The results of the inspection and evaluation indicated that the spillway does not meet the criteria given in the guidelines for a dam with the size and hazard potential of Bottom Diggins Dam. As an intermediate size dam with a high hazard potential, the Guidelines specify that the discharge capacity and/or storage capacity should be capable of safely handling the Probable Maximum Flood (PMF) without overtopping the crest. The PMF is the flood that may be expected from the most severe combination of critical meteorological and hydrologic conditions that are reasonably possible in the region.
The purpose of this inspection of the Las Brisas Lake Dam was 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. The guidelines used in the assessment indicate that this dam is in the high hazard potential classification. The dam is in the intermediate size classification, since it is greater than 40 ft high but less than 100 ft high. The inspection and evaluation indicate that the combined spillways do not meet the criteria set forth in the guidelines for a dam having the above size and hazard potential. The combined spillways will pass 58 percent of the Probable Maximum Flood without overtopping. The guidelines require that a dam of intermediate size with a high downstream hazard potential pass the PMF. The 100-year frequency flood will not overtop the dam. The 100-year flood is one that has a 1 percent chance of being exceeded in any given year.