SW3191105_Stormwater Narrative - Millbridge Phase 7E_12/3/2019

STORMWATER NARRATIVE
Millbridge Subdivision
Phase 7E
Waxhaw, NC
November 18, 2019
W. K. Dickson & Co., Inc.
1213 W. Morehead Street, Suite 300
Charlotte, North Carolina 28208
[phone]
NC License No. F-0374

GENERAL
Millbridge Subdivision is an existing single-family subdivision located in Waxhaw, North
Carolina. Phase 7EF is a proposed phase of the greater Millbridge subdivision located on the
southwest side of the subdivision and is a total of 45.14 acres. Phase 7E consists of a total of 75
proposed single-family homes with a lot width of 52'. A Future Phase 7F will consist of a total of
62 proposed single-family homes with lots of mixed widths varying 52', 60' & 70' homes. The
subdivision currently has entrances off Waxhaw-Marvin Road and Kensington Drive.
Phase 7E is also being submitted to Town of Waxhaw for Preliminary Plat & Erosion Control and
Union County Public Works for utility permitting. The estimated start of construction is Fall of
2020.
PROPOSED IMPROVEMENTS
The project will collect stormwater with valley gutter and storm drainage inlets. The stormwater
will be conveyed in concrete storm pipes. Per the Town of Waxhaw's stormwater detention
requirements and NCDEQ Water Quality requirements, the stormwater will be conveyed from
the storm drainage system to one of two wetpond facilities. The stormwater, once treated for
the first T' of rainfall and detained per the Town's requirements will be discharged from the
wetponds to a rip rap apron and vegetative filter strip.
The site has three drainage points of interest. Point of interest #1 (POI#1) is located on the
Northwest side of Phase 7E where the Existing Pond Drains into South Carolina. Point of Interest
#2 (POI#2) is located at the Southwest corner of the site where the site drains into a culvert
under the CSX Railroad. Point of Interest #3 (POI#3) is located at the Southwest corner of the
site on the opposite side of the CSX Railroad from POI #2. The table below represents the pre -
development and post development discharge. The wetponds provide stormwater quality for
the first inch of rainfall (2-5 Day Drawdown) and detention for the peak stormwater runoff for
the 2 &10-year storm events to meet the pre -development rates. Also provided is a table that
shows the available freeboard in the two wetponds.

2 YEAR
10 YEAR
PRE (CFS)
POST (CFS)
DELTA
PRE (CFS)
POST (CFS)
DELTA
P01#1
4.02
3.26
-0.84
26.49
13.13
-13.36
POI#2
0.43
0.26
-0.17
3.68
1.58
-2.10
POI #3
0.77
0.76
-0.01
6.77
6.71
-0.06
Free Board Check
Peak Elv. (50YR)
Top of Berm
Freeboad (ft)
BMP 7E-1
570.47
573.00
2.53
BMP 7E-2
562.91
565.00
2.09
BMP 7E-3
572.86
575.00
2.14