HomeMy WebLinkAbout2012-06-05 Planning Board Supplemental Materials (72)DEP Stormwater Management Standards
The project is an eight unit residential condominium project not tributary to a critical area so is
subject to the Massachusetts Stormwater Standards to the maximum extent practicable. In
addition, the project will remove existing pavement and existing, wood frame garages resulting
in a net decrease in impervious surface areas once the project is complete. As such, the project is
also a redevelopment of an existing developed site. The new work will meet the local
requirement of maintaining a 25-foot no disturbance zone from the bordering wetland of Sutton
Pond.
Standard #1 No New Untreated Discharges
Runoff from the proposed paved driveways discharges to a deep sump, hooded catch basin then a
precast concrete infiltration system which then discharges to a depression and level spreader to
dissipate outlet velocity.
Standard #2 Peak Rate Attenuation
Runoff from the project discharges in a general easterly direction towards Sutton Pond. As noted
above, the work will result in a net decrease in impervious surfaces, peak rates of runoff to
Sutton Pond from the parcel and tributary off parcel areas have been calculated.
Peak rates of runoff for the 1,2, 10, and 100-year storm events are as follows:
1-year storm 2-year storm 10-year storm 100-year storm
Pre =1.4 cfs pre =1.9 cfs pre =3.4 cfs pre=6.1 cfs
Post =1.2 cfs post =1.7 cfs post =3.1 cfs post=5.7 cfs
(was 1.1 cfs)(was 1.6 cfs)(was = 3.0 cfs)(was 5.6 cfs)
As can be seen, the peak rates of runoff, post construction, are less than the pre construction rates due to
the decrease in impervious areas and the conversion of the bare soil/weedy surface cover to lawn.
Standard #3 Recharge
The soils at the site according to the USDA Web Soil Survey consist of Canton fine sandy loam.
Canton soils are HSG B soil.The target depth to recharge is 0.35 inches.
Infiltration/Recharge Calculations
Total, existing impervious area =13,015 sf
Total proposed, new impervious area =10,545 sf + 650 sf = 11,195 sf
Net new impervious area =-1,820 sf
There is a decrease in impervious area resulting in no net loss of recharge post construction.
However, the proposed precast concrete galley system will provide recharge as detailed below.
Precast concrete galley infiltration system:
Volume required =11,195 sf x 0.35”/ 12” per foot =326.5± cf (2445 gallons)
Provide 2 -8 x 14 precast concrete galleys 4’-8” high
Volume provided below invert = 2 x 7 ft x 13 ft x 2.36 ft = 429.5 cf
429.5 cf >326.5 cf, ok
Note: rear half of the roof (2,148 sf) will discharge to the grass area behind the structure.
Time to drain (static method)
Total volume = vol. =430cf
Bottom area = A =7 ft x 13 ft x 2=182 sf
k (HSG B soil =0.52 in/hr (0.0433 ft/hr)
t = vol/ (k * A), t =430/ (0.0433*182) =54.5 hrs < 72 hrs, ok
Standard #4 Water Quality
Water Quality Calculations
The infiltration system will provide the 80% TSS removal with pretreatment provided by a deep
sump hooded catch basin.
Calculate the water quality volume:
Tributary area (new)= 11,195-(2,148 (roof)) =9,047 sf
Required WQV:
Use 0.5” depth for WQV calculation.
WQV = 0.5/12 ft x 9,047 = 377± cf
Provided WQV = 429.5 cf > 377 cf, ok
INSTRUCTIONS:Version 1, Automated: Mar. 4, 2008
1. In BMP Column, click on Blue Cell to Activate Drop Down Menu
2. Select BMP from Drop Down Menu
3. After BMP is selected, TSS Removal and other Columns are automatically completed.
Location:
B C D E F
TSS Removal Starting TSS Amount Remaining
BMP1 Rate1 Load*Removed (C*D)Load (D-E)
Deep Sump and Hooded
Catch Basin 0.25 1.00 0.25 0.75
Subsurface Infiltration
Structure 0.80 0.75 0.60 0.15
0.00 0.15 0.00 0.15
0.00 0.15 0.00 0.15
0.00 0.15 0.00 0.15
Total TSS Removal =85%
Separate Form Needs to
be Completed for Each
Outlet or BMP Train
Project:Pondview
Prepared By:jsf *Equals remaining load from previous BMP (E)
Date:4/17/2012 which enters the BMP
TS
S
R
e
m
o
v
a
l
Ca
l
c
u
l
a
t
i
o
n
W
o
r
k
s
h
e
e
t
26 Main St North Andover, MA
Standard #5 Land Uses With Higher Potential Pollutant Loads (LUHPPLs)
The site is not a land use with higher potential pollutant loads.
Standard #6 Critical Areas
The project will not create any discharges to Critical Areas.
Standard #7 Redevelopment Project
There will be a reduction in impervious areas by the proposed construction in an area previously
developed and qualifies as a redevelopment project.
Standard #8 Construction Period Pollution Prevention and Erosion and
Sedimentation Control
A Construction Period Pollution Prevention and Erosion and Sedimentation Control plan has
been prepared and will be implemented to protect the adjacent properties from sedimentation
during construction. The project is not covered by a NPDES Construction General Permit due to
the small scale of the project.
Standard #9 Operation and Maintenance Plan
An Operation and Maintenance Plan (O&M Plan) has been developed which lists the
condominium association as the responsible party. The O&M Plan details the schedule for
routine maintenance and inspection.
Standard #10 Prohibition of Illicit Discharges
All illicit discharges at the site will be prohibited. An Illicit Discharge Compliance Statement
will be submitted prior to the start of any land disturbance activities.