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

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The upper ends of the basin must be configured so that inflow minimizes turbulence to promote sedimentation and prevent resuspension of accumulated sediment and pollutants. 14 1 Cutoff Trench Care must be taken when excavating the cut off trench not to expose underlying rock fractures that may cause leaks and effect the pond's ability to maintain the desired water level as well as its appearance if multiples uses are planned. Soils While infiltration into the soil is desirable for pollutant removal, it should not be to the extent that it threatens to drain the pond or lower the water level below the designed permanent volume. (Some drawdown during the summer and draughts is almost inevitable in ponds with small watersheds such as is expected with detention ponds.) Soil at the site should be suitable for dam construction. A soils investigation by a qualified professional is necessary as part of the plans. Other Permits Wetlands If the pond site involves a wetland, the necessary permits must be obtained. This should be determined as part of the site investigation. Floodplains If the pond will be constructed in a designated floodplain the approvals and permits necessary to place fill will be required. This will usually not be the case since ponds designed and built under this standard will not be located on streams large enough to have a designated floodplain. However, maps should be checked to make sure that a floodplain from a larger stream does not extend up a tributary where the dam is proposed. Erosion Control If more than 20,000 square feet will be disturbed, erosion control plan approval and grading permit will be required. If streets and other improvements are to be made the pond can be included in the erosion control plan for all the work. Others Depending upon the location, size of project, area disturbed, and other factors additional permits may be necessary, such as an environmental assessment or evaluation, natural areas study, etc. Shape of the Basin A long, narrow basin maximizes the length of flow between points of inflow and the outlet structure. Baffles can be used to increase the flow length where a long basin is not possible. A baffle near the inlet(s) is necessary to concentrate sediment so that it can be removed periodically. The shape and dimensions should be such that, for reasons of economy, the pond above the baffle can be reached by a rubber-tired backhoe to remove and load sediment into a dump truck for removal to an onsite disposal area. Depth of Basin To maximize pollutant capture and treatment at least 25% of the pond should be less than two DRAFT 7 June 1993