HomeMy WebLinkAboutPreliminary Drainage Letter 050924JOE WHITE & ASSOCIATES, INC.
CIVIL ENGINEERING - CONSULTING SERVICES - LAND SURVEYING
May 9, 2024
Mr. Chris Smith, P.E.
City of Little Rock
Planning & Development
723 W. Markham St.
Little Rocic, AR 72201
RE: Parker Audi, Lot 1, Parl< West Commons
Preliminary Drainage Analysis
Mr. Smith,
This letter is to accompany the submitted Short Form PCD for the subject project and provides a
summary of the preliminary drainage analysis. The project will consist of the construction of a parking
lot. The existing site is currently undeveloped but contains storm drainage infrastructure from a past
project.
The rational method was used to determine the existing peak runoff for the watersheds. For the offsite
undeveloped areas, a rational coefficient of 0.4 was used. A rational coefficient of 0.55 was used for on -
site drainage. Time of concentrations were calculated using the TR-55 method. A time of concentration
of 5 min. was used when the TR-55 method calculated a time of less than 5 minutes.
The existing site drains south to north and has two outlet points into the existing drainage
infrastructure on West Markham St. 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)
A
#1
2063
12
0.4
8.31
#2
0.61
11
0.55
2.73
Total to Release Point
10.79
B
#3
0.14
5
0.85
1.17
#4
0.96
5
0.55
5.19
Total to Release Point B
6.36
Total Site Discharge
17.15
25 RAHLING CIRCLE, SUITE A-2 LITTLE ROCK, ARKANSAS 72223 Pk -ZONE: (501) 214-91d1
JOE WHITE & ASSOCIATES, INC.
J, CIVIL ENGINEERING - CONSULTING SERVICES -LAND SURVEYING
The drainage patterns will remain the same in the proposed plan, We plan to use the existing drainage
infrastructure on site to convey and detain the drainage to the existing outfalls. The mitigation of the
additional post -developed stormwater runoff will be included in the design per the City of Little Rock
Drainage Manual.
Sincerely,
Jacob White, PE
25 RAHL[NG CIRCLE, SUITE A-2 LITTLE ROCK, ARKANSAS 72223 PHONE: (501) 214-9141