NCS000402_29_Compass Pointe Stormwater Narrative

Compass Pointe Center
Stormwater Narrative
City of Mebane
By: Aden R. Stoltzfus, PE
Stoltzfus Engineering, Inc.
01/23/2014
A commercial site -development is proposed at the southwest quadrant of the intersection of
Lowes Boulevard and NC Highway 119. The proposed site layout is shown on the construction drawings
prepared by Stoltzfus Engineering, Inc (SEI). This proposed commercial site contains 7.74 acres and is
comprised of one watershed draining to an open channel located along the western property line.
The final grading design will maintain the water within the same watershed area with drainage to an
unnamed tributary to Back Creek.
The commercial site -development project is proposed for five separate commercial lots and
buildings with proposed parking stalls and new sidewalks. The site also constructs a new public road,
Compass Pointe. To manage stormwater draining off the site, one stormwater wet detention pond is
proposed along the western property line. The proposed pond is located on a separate lot to be dedicated
to a jointly held owner's association comprising of the upstream property now consisting of lots 1-5 as
shown on the construction plans. Note that these lots could be re -configured as the outparcels are
developed, however the property will still fall under the established owner's association.
Rooftop runoff from buildings, roadway and parking areas will be piped directly to the wet
detention basin. The wet detention pond is sized to manage the first inch of runoff for water quality
control and reduce peak flows to pre -development levels for the 10-year storm event.
Stormwater discharge from this site drains from the proposed pond barrel into an existing wooded
open channel. The open channel then flows first into an existing pond downstream of the site then to an
unnamed tributary to Back Creek. Based on available maps and aerial information, no structures exist
immediately downstream between the proposed wet detention basin and the existing pond. The existing
channel at the outlet of the pond barrel currently shows no signs of erosion or scouring through the
undeveloped area.
The submitted stormwater calculations were analyzed using Hydra flow hydrograph software.
The SCS method was selected to model the 1 year, 2 year, 10 year and 100 year storm events over a 24
hour period. The rational method was used as a separate calculation to double-check against the SCS
method and to ensure the riser and 100 year emergency spillway were sized adequately. Please refer to
submitted calculations for detailed results.