HomeMy WebLinkAboutPreliminary Drainage Report 040623PRELIMINARY DRAINAGE
CALCULATIONS
April 4, 2023
BASIN A 9.06 cfs
BASIN B 0.08 cfs
Pre-Development 25-Year Runoff 9.15 cfs
BASIN A 9.20 cfs
BASIN B 0.08 cfs
Post-Development 25-Year Runoff (Uncontrolled)9.29 cfs
25 -Year
Estimated Detention Volume
Differential Runoff (25-year) → 12.16 - 9.19 = 2.97cfs
2.97cfs X 8 minutes (Tc Post) X 60 sec/min = 1,425.6 cf
Detention Volume Required = 1,430 cf
BASIN NAME
BASIN DESCRIPTION
BASIN AREA 1.84 ACRES
BASIN GROUNDCOVER PERCENTAGE OF
BASIN
25-YEAR RUNOFF
COEFFICIENT WEIGHTED
Drives and Walks 76% 0.91 0.69
Natural Vegetation, Clay Soil,
Average Slopes (2-7%)24% 0.28 0.07
0.76
Longest Flow Path (Sheet)ft
Average Slope of Longest FP
Longest Flow Path (Shal. Conc.)
Average Slope of Longest FP
Time of Concentration*min.
25-Year Storm Rainfall Intensity**in/hr
* Per City of Little Rock Drainage Manual Figure 2-2 using LFP and Slope
** Per City of Little Rock Drainage Manual Figure 2-5 Using TOC
25 - Year Storm Runoff (Q = CIA)cfs
6.5
9.06
Basin consists of most of the site except the part
on the East side of the drive off of Sunnymeade
Drive.
BASIN PRE-A
WEIGHTED RUNOFF COEFFICIENT
175
4.0%
13
244
2.0%
BASIN NAME
BASIN DESCRIPTION
BASIN AREA 0.03 ACRES
BASIN GROUNDCOVER PERCENTAGE OF
BASIN
25-YEAR RUNOFF
COEFFICIENT WEIGHTED
Drives and Walks 32% 0.91 0.29
Clay Soil, Lawns, Average Slopes 2-
7%68% 0.28 0.19
0.48
Longest Flow Path (Sheet)ft
Average Slope of Longest FP
Longest Flow Path (Shal. Conc.)ft.
Average Slope of Longest FP
Time of Concentration*min.
25-Year Storm Rainfall Intensity**in/hr
* Per City of Little Rock Drainage Manual Figure 2-2 using LFP and Slope
** Per City of Little Rock Drainage Manual Figure 2-5 Using TOC
25 - Year Storm Runoff (Q = CIA)cfs
2.0%
5
8.5
0.13
BASIN POST-B
Basin consists of the part on the East side of the
drive off of Sunnymeade Drive.
WEIGHTED RUNOFF COEFFICIENT
24
2.0%
72
BASIN NAME
BASIN DESCRIPTION
BASIN AREA 1.84 ACRES
BASIN GROUNDCOVER PERCENTAGE OF
BASIN
25-YEAR RUNOFF
COEFFICIENT WEIGHTED
Drives and Walks 24% 0.91 0.22
Roofs 53% 0.92 0.49
Clay Soil, Lawns, Average Slopes 2-
7%23% 0.28 0.06
0.77
Longest Flow Path (Sheet)ft
Average Slope of Longest FP
Longest Flow Path (Shal. Conc.)ft.
Average Slope of Longest FP
Time of Concentration*min.
25-Year Storm Rainfall Intensity**in/hr
* Per City of Little Rock Drainage Manual Figure 2-2 using LFP and Slope
** Per City of Little Rock Drainage Manual Figure 2-5 Using TOC
25 - Year Storm Runoff (Q = CIA)cfs
2.0%
5.5
8.5
12.04
BASIN POST-A
Basin consists of most of the site except the part
on the East side of the drive off of Sunnymeade
Drive.
WEIGHTED RUNOFF COEFFICIENT
560
3.0%
250
BASIN NAME
BASIN DESCRIPTION
BASIN AREA 0.03 ACRES
BASIN GROUNDCOVER PERCENTAGE OF
BASIN
25-YEAR RUNOFF
COEFFICIENT WEIGHTED
Drives and Walks 32% 0.91 0.29
Clay Soil, Lawns, Average Slopes 2-
7%68% 0.28 0.19
0.48
Longest Flow Path (Sheet)ft
Average Slope of Longest FP
Longest Flow Path (Shal. Conc.)ft.
Average Slope of Longest FP
Time of Concentration*min.
25-Year Storm Rainfall Intensity**in/hr
* Per City of Little Rock Drainage Manual Figure 2-2 using LFP and Slope
** Per City of Little Rock Drainage Manual Figure 2-5 Using TOC
25 - Year Storm Runoff (Q = CIA)cfs
2.0%
8
8.5
0.13
BASIN POST-B
Basin consists of the part on the East side of the
drive off of Sunnymeade Drive.
WEIGHTED RUNOFF COEFFICIENT
24
2.0%
72