20170694 Ver 1_SW Impact Analysis_20170605

HIDDEN VALLEY
STORMWATER IMPACT ANALYSIS
MARCH 23, 2017
PREPARED FOR:
Pulte Group
1225 Crescent Green Drive, Suite 250
Cary, NC 27518
Randy King 919.816.1146
PREPARED BY:
Timmons Group, NC License No. C-1652
Jim Chandler, P.E.
5410 Trinity Road, Suite 102
Raleigh, North Carolina 27607
919.866.4507 phone [phone] fax
jim.chandler(u�timmons.com
www.timmons.com
TIMMONS GROUP
YOUR VISION ACHIEVED THROUGH OURS.

Hidden Valley
Fuquay Varina, NC
ect Number: 37913
Project Summary
Introduction
The Hidden Valley Subdivision is a 214 + acre tract of land located between Hwy. 55 and Purfoy Road about
3.5 miles south of Main Street in Fuquay-Varina, NC. The subdivision consists of 505 proposed lots, common
areas, recreation areas, and natural areas. The existing site conditions vary between a former golf course to
wooded areas. On Site storm drainage is collected through a series of structures and pipes that outlet, to
storm water control measures before outlets to natural areas, streams, wetlands etc. This report analyzes the
entire storm drainage system for using the 10 year storm event to size pipes and the 4" year storm event for
gutter spread calculations and structure placement.
Stormwater Methodology
Y For each individual storm drainage inlet, a drainage area was measured as well as
assigning impervious surface percentage. From this impervious percentage a rational
'C' value was calculated. Impervious surfaces were given a `C' value of 0.95. This data
was used for pipe sizing. The average "C" value for the site is 0.65
Y RCP is specified for all storm pipe on site and off site with a Manning's N value of 0.013
Y The various inlet types are shown on the plan detail sheets. Flared end sections are
used at all culvert inlet and discharge points.
Y Spread calculations have been performed using the 4" storm event..
Y Spread calculations have been performed using a 5 minute ToC. Maximum
allowable spread is 8 feet of a lane width.
Y The storm water network has been designed to pass the 10 -year event.
Y Structures are spaced/placed to prevent flow crossing an intersection at intersecting
streets.
Y Pipes are sized to handle the 10 -YR storm keeping the HGL within crown of pipe.
Y The pipes were sized using "Hydraflow Storm Sewers. This program accepts the input
data from each inlet, as well as physical characteristics of the storm system to be
designed, and calculates flow rates and pipe sizes through the system. For rainfall data,
the Town of Fuquay Published IDF curve was utilized. Each inlet was modeled to capture
100% of the drainage area flow. The minimum storm drainage pipe size used for this
project is 15 inches.
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YOUR VISION ACHILILD THROUGH OURS.

Hidden Valley
Fuquay Varina, NC
ect Number: 37913
Stormwater Treatment and Detention:
Six wet detention basins have been proposed and designed to treat the requirements for 85% TSS removal,
drawdown of the first inch of rainfall between 2-5 days and reduction of the peak flow for the 1 "- 24 hour storm
event. In addition, we have also routed the 100 -year event to ensure safe passage through the site. As the
site drains to two common point, treatment is being accomplished at both these points.
Detention:
This project is required to reduce the post -development peak flow rates for the 1 -year, 24 hour storm event
such that it does not exceed the pre -development peak flow rates. Detention within the proposed BMPs will be
used to achieve the required results. The results of the preliminary routing are provided below.
Analysis Point 1
re -Development
Tc Drainage Area Q1 Q100
Min Ac cfs cfs
5.0 61.90 66.93 426.16
Post -
Development
Tc Drainage Area Q1 Q100
Min Ac cfs cfs
5.0 61.90 24.02 415.66
Analysis Point 2
re -Development
Tc Drainage Area Q1 Q100
Min Ac cfs cfs
5.0 20.40 20.38 136.83
Post -
Development
Tc
Drainage Area
Q1
Q100
Min
Ac
cfs
cfs
5.0
20.40
15.00
121.34
v PondPack software has been used to calculate the peak flow rates using SCS Method. A report
of all the calculations for the site has been included for review.
Y The Natural Resources Conservation Service (NRCS) web based soils report has been used to
identify that the onsite soil types consist primarily of Hydrologic Soil Group type "B".
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TIMMONS GROUP
1- -11- ACHIL iHH . OURS.

Hidden Valley
Fuquay Varina, NC
ect Number: 37913
Rainfall Data
Town of Fuquay-Varina OF Chart
Table 110.01 -1-D-F Table*
Duration
1 -year
2 -year
10 -year
25 -year
50 -year
100 -year
(minutes)
(inches/hour)"
(inches/hour)
(inches/hour)
(inches/hour)
(inches/hour)
(inches/hour)
5
5.10
5.82
7.44
8.23
8.77
9.28
10
4.20
4.65
5.95
6.56
6.98
7.37
15
3.50
3.90
5.02
5.55
5.90
6.21
30
2.38
2.69
3.64
4.11
4.44
4.76
60
1.50
1.69
2.37
2.73
3.01
3.27
120
0.89
1.00
1.43
1.67
1.85
2.03
180
0.65
0.70
1.02
1.20
1.35
1.50
360
0.38
0.42
0.61
0.73
0.82
0.91
720
0.23
0.25
0.36
0.43
0.49
0.55
1440
0.13
0.15
0.21
0.25
0.29
0.32
24-hour
3.00
3.51
5.13
6.11
6.89
7.69
rainfall
- Based on NOAA - National Weather Service Atlas 14 Volume 2 Version 2.1
** - The 1 -year storm is a misnomer, since this would result in a 100% occurrence in any given year, but it
is so named herein for ease of reference.
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