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

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11a!/ Design and Construction Standards Determining the Minimum Volume of the Pond. For accuracy the design must be based on a field topographic survey of the pond site. USGS topographic maps or other sources can be used to determine total drainage area, ground cover, existing land use, etc. Permanent Pool The permanent standing volume must be at least four (4) times the volume of runoff (not the "peak rate of runoff') resulting from a one (1) inch rainfall over the entire watershed. Methods for calculating this volume can be found the in references and other text. This design volume should result in a pond capable of removing 85-90% of suspended solids and 65% of the phosphorus. Purpose of this Volume This volume is designed to achieve an average of two weeks of retention so that sedimentation processes occur for even small particles and to maximize biological uptake of pollutants. The theory behind the design is that "new" polluted runoff into the pond will displace "old" treated water in the pond from previous events. The "new" water will be held and treated until the runoff from the next rainfall event (if the storm is heavy enough to produce any) displaces it. As far as volume is concerned,bigger is better, but a threshold is reached where increasing it has little additional benefit and may disturb more area. Evaluation of the Volume Calculation This calculation is the minimum volume of the permanent pool for water quality purposes. It does not necessarily mean this should be the minimum volume of the pond. The watershed and development being considered may be such that only a small volume is required. However, other factors, such as the ratio of surface area to watershed, fish habitat, stormwater detention, recreation, etc. must also be considered in the design. Therefore,,a larger pond may be needed or desired. Offsite Drainage Area In calculating this volume, if there is offsite drainage area the projected future development of it must be taken into account in calculating the volume and and designing the outlet structures. Reasonable estimates can be taken from the County's land use plan. This is necessary so that the pond's function of capturing pollution and structural integrity is not threatened by future upstream development. Storage Volume Above the Permanent Pool and Control of the Peak Rate of Runoff An additional storage volume between the permanent pool and the top of the riser must be provided to store the runoff resulting from a one (1) inch rainfall over the drainage age. (This DRAFT 5 June 1993