HomeMy WebLinkAboutENG 3 - Stormwater Memo and Report 2021-05-24memo
Page 1
TO: Karl Johnson
FROM: Celine Saucier, PE
DATE: May 24, 2021
JOB NO.: 4192.025
RE: Supplemental Storm Information
CC:
Urgent For Review Please Comment Please Reply For Your Use
This memo presents information for Engineering Comment #3 for the Boulder Creek Apartments
Site Plan Review:
“It is unclear how runoff from drainage area 2 will be retained and treated. A drainage
swale is shown grading offsite. The drainage report must identify how this drainage will
reach the intended destination of retention pond #2. The report must clearly demonstrate
runoff is being treated and post development runoff is kept to predevelopment
conditions.”
Per the attached stormwater report SD1.0 Exhibit, Drainage Area 2 (DA2) encompasses the
eastern portion of the proposed Phase 1 development as well as the eastern portion of the
future development. As a part of the Stormwater Management Design Report by C&H, June
2015, the existing Retention Pond #7 in the northeast corner of the Boulder Creek Subdivision
was designed to hold the runoff from DA2 using a C value of 0.5. However, since the C value of
the proposed site is 0.7, Retention Pond #2 will be constructed to retain the stormwater volume
difference between C=0.7 and C=0.5.
The on-site Retention Pond #2 will have an overflow into a swale that will drains into the existing
Retention Pond #7. Since the ultimate destination of this stormwater runoff is to two retention
ponds which do not discharge into any adjacent waterways, retention ponds #2 and #7 act as a
method of treatment, removing sediments and pollutants from the stormwater runoff.
Retention Pond #2 in the northeast corner of Lot 4, will be constructed with Phase 1 to retain the
portion of DA2 that will be constructed with this project. The portion of DA2 within Phase 1 will
flow to Retention Pond 2 using a 6” deep swale at 1.3% (See Grading Sheet C601 and Detail
7/C050).
Supplemental Storm Information
Page 2
The 25-year storm flow that the temporary swale will be required to convey for the DA2 flows on
proposed property is as follows:
A = 0.8 acres (34822.77SF)
C- 0.7
Overland Tc = 5min
Intensity at Tc = 3.8 in/hr
Post-development Peak Runoff: 2.13cfs
The swale will be able to contain the 25-year storm flow from the portion of DA2 within Phase 1
as the capacity of the swale is 4.48cfs. This calculation is shown in the attached channel report.
Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc.Tuesday, Jun 1 2021
6in Deep Swale Capacity
Trapezoidal
Bottom Width (ft) = 1.00
Side Slopes (z:1) = 4.00, 4.00
Total Depth (ft) = 0.50
Invert Elev (ft) = 100.00
Slope (%) = 1.30
N-Value = 0.025
Calculations
Compute by: Q vs Depth
No. Increments = 5
Highlighted
Depth (ft) = 0.50
Q (cfs) = 4.481
Area (sqft) = 1.50
Velocity (ft/s) = 2.99
Wetted Perim (ft) = 5.12
Crit Depth, Yc (ft) = 0.50
Top Width (ft) = 5.00
EGL (ft) = 0.64
0 1 2 3 4 5 6 7
Elev (ft)Depth (ft)Section
99.75 -0.25
100.00 0.00
100.25 0.25
100.50 0.50
100.75 0.75
101.00 1.00
Reach (ft)