10822 EVERGREEN WAY 2016-11-07 - Laserfiche WebLink

calculations shows that there is about 0.0.51 cfs increase for the maximum 100 year storm. The pond configuration is trapezoidal and the north and south dimensions of 41 and 61 are averaged to 51' by 30' across, norht to south. As a result a square shape is assumed for the trapezoid at 2.25' of live water depth and 1' of freeboard. The actual calculations show that only 2.07' of depth is needed. It should be noted that the time for concentration for the development site uses the minimum time since the developed project site will have a higher more impervious area. With the minimum time the hydrograph for the developed conditions is maximum at 0.95 cfs and will provide the greatest possible ftow difference for the pond to detain. B. Conveyance Analysis of Onsite Drainage System Using the conveyance storm of 100 years the project site generates 0.95 cfs flow going into the pond(see Appendix B, page 1) The new 108th St storm drain takes away any upstream flows coming down from the north onto the site. As a result the maximum 100 year storm will be just from the site and which will test the ditch capacity just upstream of the de4ention pond. See the DHA plan for the ditch invert elevations used in the co�veyance analysis. Total basin = 1.9 acres and use Cn = 89 for grav�led parking lots after development. The ditch is set up per section D- D for earth as a network of one ditch and the input hydrograph of the 100yr-24hr storm of 0.94cfs is stored in a hydrograph unit #1. It is routed through the nefinrork of one ditch and a hydraulic gradefine is set up as HGL1. The nefinrork is then checked for hydraulic gradeline. The result is that the one foot ditch is filled to 0.37' deep at the downstream end and even here the 100 year storm will not overFlow the ditch or the 0.5 freeboard. The ditch will contain the maximum storm without overflowing. �