Agenda - 08-23-1993 - C 2 and C 3 attch

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is the same volume of runoff used to determine the minimum volume of the permanent pool.) The purpose is to capture and store the volume of runoff that contains most of the pollutants. 1 U To capture this runoff the overflow device that controls the water level must be configured with a small orifice that establishes the permanent water level but is small enough to impound this volume and store it. The orifice must be designed so that the runoff from a one inch rainfall is stored for not less than 2 days and not more than 5 days. There are various methods and outlet configurations capable of meeting this requirement. It is beyond the scope of this standard to cover the means and calculations since more than adequate references are readily available. Refer to the references. Sizing the Pond Using the topographic information on the proposed site and the calculations of the required volumes the height of the dam and the extent of the pond can be determined. A trial and error process can be used. However, computer programs that can easily and quickly perform these calculations are recommended that allow minipulation of the variables to discover the best fit for the particular location. The Dam Suggested design procedures for the dam are those from the US Department of Agriculture Soil Conservation Service. A minimum of one foot of freeboard is required between the calculated maximum depth of flow for the design storm through the spillway (where one is used) or the maximum water level (when only a riser is used) and the top of the dam. A maximum 3:1 slope (preferably flatter) of both sides of dam is required to facilitate mowing and maintenance and discourage the growth of undesirable vegetation and to discourage borrowing animals. A qualified person must examine the soil and determine if it is suitable for holding water and constructing the dam. During construction of the dam, supervision must be provided to ensure that good soil is used and properly compacted. The soil used in the dam must be free of rocks larger than 6, inches, stumps, roots, and other debris that could compromise the structure's integrity. If the dam height is 15 feet or more (measured from the highest point in the dam to the toe of the dry slope in the bottom of the downstream channel) and the pond impounds 10 acre/feet or more (measured to the crest of the dam) then the necessary permits under the Dam Safety Act of 1967 must be obtained from the State and copies submitted. The Basin An average depth of 3 to 6 feet is optimal for pollutant removal and treatment. Depths over 8 feet should be avoided to prevent thermal stratification, which tend to become anoxic. However, topography and required volume will also have to be considered. Thermal stratification and thermal impacts can be mitigated by placing the riser in the deepest point and designing a coldwater bottom release in the riser. DRAFT 6 June 1993