ORD-2003-142 PD-1-03 State Employees Credit Union Addition Special use permit

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~~2 Hillsborough State Employees Credit Union Stormwater Report Project # 70994252 Project Overview The State Employee's Credit union intends to increase the size of its facility and parking area at the Hillsborough, North Carolina location at the comer of Old Hillsborough -Chapel Hill and Oakdale Roads. The project will involve converting a new piece of land into a parking azea with a small building to service a new drive through teller system Methodology Water Quantity - - Tliis hydraulic study was preformed using HydroCAi3®6: -This-is- a software package hy-Applied- - - Microcomputer Systems, which can be used to model a variety of watersheds, ponds and a v ereo . wasp r~xi i'i'R-5~ii~~ivcfinsgrap~M~dr Please refer to the attached sununary (Appendix A) for the peak flows and water levels and to the attached calculations for details. Wafer 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 parking area that takes up most of the site. A detention pond is located near Oakdale Road, which drains into 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 land 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 yard inlet. The quantity of Stormwater through the existing system will increase slightly with the addition of some runoff from the new site. The pond is capable of handling a large increase in quantity while still detaining 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 Stormwater 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 pazking area. This underground detention system will consist of 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 in lazger storm events. Because the post-development Nitrogen. values will be less than 10 pounds per acre per yeaz, 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 azea will not be directed to the stormwater detention system because of the elevation differences. This water will be directed to the effluent pipe from 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