HomeMy WebLinkAboutPreliminary Plat Drainage Letter 051324JOE WHITE & ASSOCIATES, INC.
CIVIL ENGINEERING - CONSULTING SERVICES - LAND SURVEYING
May 10, 2024
Mr. Chris Smith, P.E.
City of Little Rock
Planning & Development
723 W. Markham St.
Little Rock, AR 72201
RE: Mountain Brool<Subdivision, Preliminary Plat
Preliminary Drainage Analysis
Mr. Smith,
This letter is to accompany the submitted Preliminary Plat for the subject project and provides a
summary of the preliminary drainage analysis. The project will consist of 162 residential lots located off
of Valley Ranch Drive and will include paving, drainage, sidewalk, water, sewer, and additional utility
extensions. The subdivision will be constructed in phases. The existing site is currently undeveloped with
some steep grades and heavily wooded.
The rational method was used to determine the existing peak runoff for the watersheds. Considering the
existing cover and steep grade, an existing rational coefficient of 0.50 was used for the site. Time of
concentrations were calculated using the TR-55 method. The existing site drains two ways with both
outlets ultimately draining into Little Maumelle River. The north 25.93 acres drains to the northeast
corner of the property. Runoff from the south 11.32 acres drains to the south into existing developed
areas. The preliminary analysis includes four individual drainage basins separated into on -site and off -
site areas. These drainage basins are summarized below in Table 1 and are shown on the Preliminary
Drainage Analysis drawing.
Table 1: Existing Drainage Areas
Release Point
Drainage Basin
Area (Acres)
Tc (min.)
C
Q100 (cfs)
North
#1(offsite)
46.22
19
0.5
153.56
#2
25.93
13
0.5
99.66
Total to North
207.22
South
#3
2.02
13
0.5
7.76
#4
3.63
14
0.5
13.59
#5
4.5
16
0.5
16.03
#6
1,17
14
0.5
4.38
Total to South
39.16
Total Site Discharge
246.38
25 RAHLING CIRCLE, SUITE A-2 LITTLE ROCK, ARKANSAS 72223 PHONE: (501) 214-914]
JOE WHITE & ASSOCIATES, INC.
I, CIVIL ENGINEERING - CONSULTING SERVICES - LAND SURVEYING
Drainage Basins #1-#6 will be captured by the proposed storm drainage apart from the rear yards of Lots
101-129. Run-off to the neighboring developments will be reduced by capturing a large portion of the
existing drainage areas and conveying this drainage to the proposed detention facility. The proposed
storm drainage infrastructure will be designed to convey the post -developed run-offs to a detention
facility located in the northeast corner of the project area. The detention basin will be sized to mitigate
the additional post -developed stormwater runoff per the City of Little Rock Drainage Manual.
Sincerely,
V/0Ci ��ob White, PE
25 RAHLING CIRCLE, SUITE A-2 LITTLE ROCK, ARKANSAS 72223 PHONE: (501) 214-9141