Agenda Packets - 2021/07/06 - Laserfiche WebLink

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The attached drawing shows the calculated unit hydrograph for the area tributary to Oak Lake Park pond. C. Storage at Oak Lake Park Pond The following calculations are based on the originally approved final contours for the ponding area with the following changes. a. No excavation work was assumed south of the inplar_e ditch near the Scottland Green Apartments. This includes the "floating bog" area and east of that area to near County Road I. b. -To the west of the Scottland Green Apartments, only the area shown to be excavated to elevation 889 was removed. No grading was assumed between the deep part of the pond and the apartment property line. A revised grading plan for the site is enclosed with this report. Available storage volumes at the site are as follows: Contour Area, acres Al + A L/2 Volume Sum Volume 893 11.0 0.0 25.3 0.5 12.7 894 14.3 12.7 30.8 0.5 15.4 895 16.5 28.1 37.3 0.35 13.0 895.7 20.8 41.1 This calculation does not allow for any settlement in areas that will not be excavated to the east and south of the major pond excavation areas. However, such settlement will occur when the general water levels are lowered. D. Rating of the Outlet Structure Plans for the outlet structure show it as having a 12 inch diameter opening at the pond outlet elevation of 893.0. When water rises to elevation 894.9, flow will begin over a weir having a width of 4'9". This has been narrowed from previous designs to reduce the 100 year pond discharge. At a water level of 895.7, the total flow through the structure will be approximately 13.2 cfs. The following assumptions were made. For orifice flow through the 12" diameter hole, we assumed that tailwater on the orifice would be below elevation 893.5 for the interval from normal pool (893.0) to the weir overflow elevation (894.9). Above that level, tailwater would be higher on the orifice rising to 894.9 when the pond level would be at elevation 895.7.