9630 EVERGREEN WAY 2018-01-02 MF Import

IV. DEVELOPED DRAINAGE The developed project will include the new detention pond and biofiltration Swale. The areas used to calculate the flows were the some as those areas used in the predeveloped drainage calculations. The detention pond was sized to meet the performance criteria established by the City for streambank erosion protection. This requires that the developed 2 year peak flow be restricted to a maximum of 50% of the predeveloped 2 year peak flow. The 10 year and 100 year peak flows must not exceed the predeveloped peak flows for the respective storm events. There is also a sizing factor required to be added to the design pond volume to determine the actual size. The proposed developments will have an impervious surface of approx. 85% of the total area. The pond volume sizing fac; ,)r was calculated to be 145%. The pond also will include a 1 ft. freeboard. Similar to the existing condition, the SBUH hydrograph method was used to determine the peak flows for the various storm events. A level pool analysis was performed to evaluate the pond performance during the various required scenarios. The computer data sheets for the developed i flows follow this page as well as the works sheets and the time of concentration calculations. A brief summary of the developed drainage characteristics are: time of concentration: 3.81 minutes 2 year peak flow: 1.55 CFS 10 year peak flow: 2.40 CFS 25 year peak flow: 2.82 CFS 100 year peak flow: 3.62 CFS total area: 5.57 acres pervious area: 0.80 acres curve number: 86 impervious area: 4.77 acres curve number: 98 8