Agenda - 03-04-2004-9a - Laserfiche WebLink

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FiiIlsborough State Employees Credit Union Stormwater Report //~ Project # 70994252 ~ Y~ Project Overview The State Employee's Gredit union intends to increase the size of its facility and parking area at the Hillsborough, North Carolina location at the corner of Old Hillsborough -Chapel HiII and Oakdale Roads. The project will involve converting a new piece of land into a pazking azea with a small building to service a new drive through teller system. Methodology Water Quantity . - Tliis hydraulio study was preformed using HydroC-Ai)®6.-This-is a software package by-Applied - -. _... _ _-. Microcomputer ~ Systems, which can be used to model a variety of watersheds, ponds and a erea Please refer to the attached summary (Appendix A) for the peak flows and water levels and to the attached calculations for details. - Water Quality For nitrogen runoff- analysis, the two lots were combined and treated as one large area. The nitrogen was calculated using method 1 from the Orange County Stormwater ordinance for lands - within the Neuse River Basin. Site Description The existing credit union is on 2.26 acres, consisting of a building and pazking area that takes up most of the site. A detention pond is located near Oakdale Road, which drains iota a culvert beneath Oakdale Road. The new property is to the south of the existing building, consisting of a few small buildings, driveways, fencing and a house. The new ]and will have a small building for new drive through tellers, and additional parking. Proposed Stormwater Management The existing pond and stormwater system will remain in place, with the addition of only one yazd inlet. ___~_~____-__..__-_____The_quantity of sto_ rmwator through the existing system will increase slightly with the addition of sonic -~ - - runoff from the new site. 'T'he pond- rs capable of handling-a-large-increase-in- quantitx while.~till.-do~3ni~_._. - ,-_:._.., ... - the runoff from the site. The current pipe system can accommodate the slight increase in flow, but would - not be able to -handle the runoff from the whole new property. Therefore a separate starmwater detention - . system for the new development was designed_ The new property will decrease runoff quantity to the predevelopment values through an underground detention system beneath the proposed parking area. •This underground detention system will consist o€ 400' of 36" HDPE piping. The outflow of the detention system will be controlled through the cap at the end of the pipes. A 5" pipe will restrict the outflow in the two year storm to the pre-construction levels, and the cap will be open at the top for free outflow is larger storm events. - Because the post-development Nitrogen values will be less than 10 pounds per acre per year, the owners plan to make a payment toward the wetland mitigation fund and. not implement any nutrient reduction practices. The nitrogen calculations can be seen in Appendix B. A small area will not be duected to the stormwater detention system because of the elevation differences. This water will be directed to the effluent pipe fmm the detention system, and the' water will be directed to - a rip-rap dissipater pad. The value of this flow has been taken into consideration in water quantity analysis. 08/08/03 Page 1