32C-141 (51) The Post-Development Watershed includes 65,450 square feet of impervious surface and
119,287 square feet of permeable soils, which is a net change of+17,072 square feet (0.39 acres)
of impervious area over the Pre-Development Watershed. The combined impervious surface
areas of Drainage Areas 2B and 2C (22,376) square feet require a compensating recharge rate of
I/2 per square foot, which totals 932 cubic feet, which is met by out Detention Pond Design.
We have met this requirement by designing the infiltration bed of the Detention Pond to contain
932 cubic feet of storage, and to recharge up to 222 cubic feet per hour in the infiltration bed
portion of the Pond.
The discharge from the Pond is via a 12" pipe which has energy dissipation rip-rap
positioned at the outlet to prevent erosion, and to disperse the runoff as it heads toward the
abandoned river bed.
The site design allows the untreated runoff from the roof of the new building to be
discharged into the existing 12"pipe at the site, however this additional flow into the pipe is
more than offset by the reduction in overall area and flow that is directed to the existing pipe
system
IV. DEP Stormwater Policy Standards
1. No new stormwater conveyances discharge stormwater into wetlands or waters.
2. The Post-Development Peak Discharge Rates are lower than the Pre-Development Peak
Discharge Rates (see above.) The controls have been designed for the 2, 10, and 100-year
storms. There will be no increased flooding impacts from this design.
3. The annual recharge rates for the post-development watershed will approximate the
existing conditions. We have incorporated an additional recharge mechanism(via
infiltration in the Pond) that is equivalent to 1/2" per acre over the impervious area of the
Site limits (see above). Soil types are identified per NRCS Soils Survey and confirmed by
observation(see above). Infiltration BMP's include an Infiltration Basin in the Detention
Pond.
4. Runoff is treated at a rate of 1/2" per acre of impervious surface of the portion of the
property containing new development. 90% of Total Suspended Solids (TSS) will be
removed by this design. BMP's used include a) Street Sweeping, b) Double In-line Deep
Sump Catch Basins, c) Detention Pond with check Dams and Infiltration Basin. See
attached TSS Removal Worksheet.
5. The Project site does not contain Land Uses with Higher Potential Pollution Loads.
6. The Project site does not contain critical areas with sensitive resources.
7. The proposed activity is not a redevelopment project. Portions of the property have been
previously developed, however the current project does result in a net increase of
impervious area The calculations given reflect a design that improves the existing
conditions in the previously developed area, and meeting the Standards within the area of
new development.
8. Erosion and sediment controls are incorporated into the design plans.
9. An Operation Maintenance plan for construction and post-development is incorporated
into the plans.
Millbank Affordable Housing
Environmental Submission
Page 3 of 3
Connecticut River. The point of entry into the 60" pipe has been taken as the "Site Discharge
Point" for Drainage Calculations for this property.
Soils for this site are all"Hadley" (Ha) silty loam, or very fine sandy, silty loam, in
Hydrologic Group "B" per the SCS Soil Conservation Service Soil Survey of Hampshire County,
and confirmed by observation at the site. The watershed for the site is approximately 4.24 acres
and includes most of the subject property plus land of some abutters, and the westerly end of
Michelman Avenue. The watershed is divided into two sections: Area 1 (divided into five sub-
areas A-E), which includes all parts of the watershed which discharge into drainage structures
which ultimately drain into the 12"pipe, which in turn drains into the abandoned river bed; and
Area 2, which includes all of the rest of the watershed and which ultimately drains directly into
the abandoned river bed. The Pre-development watershed includes 48,378 square feet (1.11
acres) of impervious surfaces and 136,359 square feet (3.13 acres) of permeable soils.
Observed percolation rate at the property(based upon one Pere test) is 35 minutes per
inch, or 1.71 inches per hour. If rainwater runoff percolates into permeable soils all over the
property at this rate, approximately 19,431 cubic feet per hour can infiltrate back into the soil as
groundwater recharge.
Pre-development stormwater runoff at the Site Discharge Point is approximately
(rounded to whole numbers) 2 cfs for the 2-year storm, 4 cfs for the 10-year storm, and 8 cfs for
the 100-year storm, calculated by the SCS TR-55 method.
M. Stormwater Drainage—Post-Development Conditions
Post-Development Watershed is also divided into two areas, Area 1 (reconfigured, but
divided again into sub-areas A-E) discharging into the 12"pipe and Area 2 (now divided into
Subareas A-D) eventually discharging overland into the abandoned river bed.
Post-development stormwater runoff at the Site Discharge Point is approximately
(rounded to whole numbers) 2 cfs for the 2-year storm, 5 cfs for the 10-year storm, and 9 cfs for
the 100-year storm An infiltration/detention pond has been incorporated into the site design
which will slow the discharge rate in the areas that discharge into the pond from rates of 1, 2 and
4 cfs (for 2, 10 and 100 years storms) to a maximum of 1.5 cfs for the 100 years storm, and allow
a discharge rate of up to 2.5 cfs for storms in excess of the 100 year when the detention pond if
filled to over capacity. Thus the net discharge at the Site Discharge Point is 2, 4.5 and 7.5 cfs
for the 2, 10 and 100 year storms, respectively, which are equal to or less than the pre-
development discharge figures.
Detention pond design capacity required is 5421 cubic feet, supplied is 5536 cubic feet, at
a full elevation of 118.5. A "level spreader" is supplied at elevation 119.83 so that in the event of
a major flood that produces flow which exceeds the capacity of the pond, the water will over-top
the southwesterly bank of the pond along a level grade that is about 160 feet long and produce a
shallow sheet flow to minimize erosion.
During normal storms 1558 square feet at the bottom of the detention pond are available
for infiltration of storm water, as the bottom of the pond is 0.6 feet lower than the invert at the
outlet structure. The Detention Pond is equipped with four (4) check dams which serve also as
filters.
Millbank Affordable Housing
Environmental Submission
Page 2 of 3
MILLBANK AFFORDABLE HOUSING
ENVIRONMENTAL SUBMISSION
TO ACCOMPANY A NOTICE OF INTENT
Prepared by
E. B. HOLMBERG &ASSOCIATES
87 UNION STREET
EASTHAMPTON, MA 01027-0945
November 3, 2000
I. Project Summary
The proposed project includes construction of a new housing structure,re-hab of an
existing housing structure, and construction of paved walks, access roadway and parking on a
portion of a 2.9 acre property ("the property") which already contains two housing structures
(including the one to be re-habbed), walks, access roadway and parking. The proposed grading
work will be contained within a 1.23 acre envelope ("the site") at the end of Michelman Avenue.
Portions of the existing pavement will be removed to make way for the new construction.
The property will remain substantially unchanged from the center of the property
southeasterly, while new grades and new pavement will be constructed near the end of
Michelman Avenue. Stormwater routing from the southeasterly portion of the property , which
now flows into a 12" discharge pipe, will not be changed;however the total amount of runoff
routed to the existing 12"pipe will be reduced, since most of Existing Watershed Subarea 1-A
and some of 1-B will be re-routed toward the Detention Pond via new outlet structures.
Re-routed Stormwater runoff will be discharged via two (2) sets of two-in-line deep sump
catch basins (total of four basins) to the opposite ends of the Detention Pond , from which water
will flow toward the center, through two sets of two check dams to a ponding area which will
serve as an infiltration bed, thence to an Outlet Structure which will slow the discharge rate,
thence to an outlet pipe which will discharge at the bottom of the slope in the direction of the
abandoned river bed. Land which is unaffected by the new construction will continue to drain
toward the abandoned river bed as before.
New Site grading will be designed to have minimal sloping wherever possible to
minimize runoff and maximize recharge.
II. Stormwater Drainage—Existing Conditions
The existing conditions at the property are such that all of the easterly portion of the
property, plus land of some abutters and part of Michelman Avenue, currently drain into the
existing parking area of the property, which in turn drains into several catch basins, all of which
finally drain into a single 12" pipe which discharges into the abandoned bed of the Mill River,
which is now a drainage ditch, near the southeasterly comer of the property. Stormwater runoff
the enters a single 60" diameter pipe and flows southeasterly, eventually reaching the
Millbank Affordable Housing
Environmental Submission
Page 1 of 3
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