HomeMy WebLinkAboutLots 7 and 8, Block 1 Gramercy Sub - Storm Drainage Report (2)Prepared For:
Dave Evans Construction
and City of Meridian
I=
Engineers_ Surveyors. Planners
Lots 7 and 8, Block 1 Gramercy
Subdivision, Meridian, Idaho
Storm Drainage Report
Kerby Kirkham, P.E.
Project Engineer
KM Engineering, LLP
9233 W. State Street
Boise, Idaho 83714
208.639.6939
kkirkham@kmengllp.com
May 22, 2013
Project No: 13-008
TABLE OF CONTENTS
Introduction and Summary
Project Description ...........
Basins ....I ........................
Drainage Basin Figure ...
Calculation .............I.
.....Appendix A
.....Appendix B
Introduction and Summary:
The purpose of this report is to show that the storm drainage facilities are designed to
meet City of Meridian standards. All drainage calculations were derived using the
standard of one inch of stormwater over the impervious area. This methodology is
typically more conservative than the Rational Method, which is the standard method for
small catchments. Based on our calculations we verified that for the 100 year storm the
infiltration swale and seepage bed are adequately sized.
Project Description:
This project is located on the South side of Overland Road and consists of two platted
subdivision lots in the Gramercy Subdivision in Meridian, Idaho. The proposed
improvements include two new structures, utility connections, hardscape, drainage
facilities, and landscaping.
All storm water for the proposed site sheet flows across the concrete asphalt where it is
collected by concrete gutter and directed to either: a sand and grease trap and disposed
of through an underground infiltration facility or a surface infiltration swale located on
the west side of the project site.
Basins:
There are two drainage basins:
A — 35,600 sf (Eastern and Central Portions of Site)
B — 5,720 sf (Westernmost Portion of Site)
Seepage Beds:
Based on our calculations all seepage beds are adequately sized to ensure no ponding
occurs on the surface and that storm water will infiltrate within the seepage beds in a 24
hour period.
The time necessary for infiltration of storm water into the ground has been calculated to
be less than nine hours. An infiltration rate of 8"/hr was used for these calculations
based upon the limited soils investigation for the site for the soils the seepage bed is
designed to be excavated into.
Infiltration Swale:
The swale is sized based upon the design storm using an infiltration rate of 8 per hour
(per the limited soils report). The design storm has been calculated to infiltrate into the
ground within 4 hours. This is based upon the excavation of the swale to the native
free -draining soil stratum, which is indicated on the plan -set detail sheet.
The storm drainage, seepage bed and .infiltration calculations are included in the
calculations section of this report.
Appendix A
Drainage Basin Figure
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Appendix B
Calculations
Project No.: 13-008
Project Description: Gramercy Lots 7 and 8
Tributary Areas
Sq Ft
Swale Tributary Area
Sq Ft
SW -1 0 A
SW -2 5,720 B
SW -3 0 C
Date: 5/15/2013
Tributary Areas
STORM DRAINAGE CALCULATIONS
Project Description: 13-008
Project No.: Gramercy Lots 7 and 8
Date: 5/15/2013
Swale B: SW -1
STORM RUNOFF CALCULATIONS FOR BASIN "B"
Runoff Event Frequency: 100 year
METHOD: 1" STORM OVER IMPERVIOUS AREA
Design storm event for the 100 -year storm is for 1" of storm
water over the entire tributary area
Area 5720.0 ft`
Intensity 1.0 in
VOLUME TO BE RETAINED = 476.7 ft3
SWALE #: SW -1
Runoff Volume (per other calculations):
476,7
ft3
Sediment Storage:
15.0
%
Volume Required (VR):
548.2
ft3
Depth:
1.5
ft
High Water Level Area:
597.0
ftz
Bottom of Swale Area:
28.0
ftz
Average End Area:
312.5
ftz
Average Depth:
1.5
ft
Swale Volume Provided:
468,8
ft3
INFILTRATION:
90% Infiltration is 24 hours
Volume to be infiltrated: 476.7 ft3
Infiltration Rate 8 in/hr
Infiltration Area 180 ft2
Volume Retained in Filter Sand: q0 ft3
First Hour Infiltration 120 ft3
Volume Provided (VP): 678,8 ft3
Time to Infiltrate: 4.0 hr
km
i
OK, VP>VR
OK, < 24 hours
Project No.: 13-008
Project Description: Gramercy Lots 7 and 8
Trubary Areas
A
B
C
D
E
F
G
H
J
K
L
M
N
Sq Ft
35,600
0
n
U
0
0
0
0
0
0
0
0
0
0
Date: 5/15/2013
Tributary Areas
Seepage Bed/ Swale Tributary Area
Sq Ft
SB -1
35.600 A
SB -2
0
SB -3
0
SB -4
0
SB -5
0
SB -6
6 -
SB -7
0
SB -8
0
STORM DRAINAGE CALCULATIONS
Project No.: 13-008
Project Description: Gramercy Lots 7 and 8
STORM RUNOFF CALCULATIONS FOR BASIN "A"
Runoff Event Frequency:
Date: 5/15/2013
Seepage Bed: SB -1
100 year
METHOD: 1"STORM OVER IMPERVIOUS AREA
Per City of Meridian BMP standards: design storm event for the 100 -year storm is for 1" of storm
water overs 13329
5361
Tirburaty Area 35600.0 ft?
Intensity 1.0 in
Volume to Retained= 2966.7 ft3
SEEPAGE BED 1 VOLUME:
Depth: 5.0 ft
Width: 25.0 ft
Length: 46.0 ft
Voids: 40.0 %
Volume Provided by Bed:
2300.0
ft3i
INFILTRATION:
100% Infiltration is 48 hours
Volume to be infiltrated:
2966.7
ft3
Infiltration Rate
8
in/hr
Infiltration Area
1150
ftz
Time to infiltrate:
3.9
hr OK, < 48 hours
TOTAL VOLUME PROVIDED:
Volume from Seepage Bed
2300.0
ft'
Volume from Infiltration (1 -hour)
766.7
ft'
Total Volume Provided (VP)
'+'r:=77
3066.7 ft OK, VP > VR
Gramercy Lots 7 and 8
Rational Method Storm Drainage, 13-008
Basin A,C - Max Flow / Sand and Grease Trap
A =
0.82 Acres Total Area
Ai =
C =
0.82 Acres Area Impermeable (C=0.95)
0.90 Weighted Run -OK Coefficient
L =
S =
100 feet Maximum Overland Travel Length
2.00 % Average Slope
to =
10 13329 Time of Concentration
5361 (Use Minimum of 10 Minutes)
100 Year Storm
I =
1 in/hr Storm Intensity
4=
0.74 9808 Max Flow
V =
0.18 ft/S Velocity Trough S 8 G Trap
5/15/2013