Mitigation List to Encroach BVW Setback 10-02-2020The Engineer Group, LLC
20 Bofat Hill Road
PO Box 262
Chesterfield, MA 01012
Date:10/02/2020
Sarah I. LaValley
Conservation and Preservation Planner
Northampton Office of Planning and Sustainability
City Hall, 210 Main Street, 2nd Floor
Northampton MA, 01060
Re/Project: 36 Hampden St, Conservaition encroachment to Northampton 35' BVW buffer.
Sarah,
We propose the following items which will result in improvements to the wetlands as mitigation for
allowing the referenced project to proceed with a reduction to the 35' setback required by the
Northampton BVW buffer ordinance.
1. There is currently no reduction in TSS (Total Suspended Solids) with the existing catch basin
drain and pipe. The existing pipe drains to the wetland without significant storage, and overflows
directly to the wetland area, taking any silt and debris with the flows into the wetland. We propose
reduction of TSS for storm events to reduce silt flows to the wetland. This is accomplished through
a "deep sump catch basin" and a "filter row" as part of the underground storm mitigation system.
Combined, these two practices achieve 80 % reduction of TSS which will ultimately be discharged
to the wetland. This improvement is required by MA DEP for the first 1" of any storm, or the "first
flush" of stormwater. The system we propose will be built to filter all storms, up to and including
the 100yr rain event, or over a 7", 24 hr rainfall. The proposed system will also be filtering the
water from 32 Hampden, in addition to the water from 36 Hampden.
2. The existing drainage system offers virtually no storm storage capacity to mitigate stormwater
peak flow rates. The DEP regulations require that storms up to the 10 yr storm event be mitigated.
We propose improvement of stormwater collection capacity, detention, and peak flow mitigation for
all storms, up to and including the 100 year storm.
3. Rain barrels at select leaders will allow reuse of rain water for gardens, and offer some
infiltration back to the site.
4. Knotweed grows on nearly the entire slope leading to the wetland area. The root system is coarse,
and offers poor erosion control of the fine soils on the slope. We propose to mow a portion of the
knotweed down to the 35' buffer, along most of the 35' buffer line, and beyond the buffer below the
proposed parking area. We propose to replace the mowed knotweed with grass which will be
maintained behind the existing and new units. Please see "Proposed Conditions" on page 3, titled
"Planting Plan" of the submission drawings. The grass will help stabilize the slope soils where
disturbed, and offer a sediment filter strip for overland flows, which do not enter the stormwater
system above, before they reach the wetland resource. This will further reduce sediment load to the
The Engineer Group, LLC
20 Bofat Hill Road
PO Box 262
Chesterfield, MA 01012
wetland, and provide erosion protection to the steep slope, some of which will be disturbed during
construction. Grass seeding of the mowed slope is proposed to be a mix of 47% Creeping Red
Fescue, 5% Red Top, and 48% Tall Fescue. Hay or straw mulch shall be applied at a rate of
approx100 lbs per 1000 sf.
5. There is no operation plan or formal maintenance plan for the site stormwater system in place
today, or for the property in general. We propose an operation and maintenance plan to keep the
proposed stormwater system in working order, and the site improvements in a condition that will
continue protection of the wetland resource. See N.O.I. documents for operation and maintenance
plans. This will ensure continual operation of improvements made.
6. The developer is proposing a donation of the lower lots to Arcadia. The proposal has been
accepted by Arcadia under the following terms as part of that potential transaction:
a. The developer will cover all costs for the donation (estimated to be approx. $1,000)
b. The developer will make a contribution of $5,000 to Arcadia toward their future
stewardship/control of the knotweed. We understand the $5,000 donation will be used over three
years of treatment for invasive plant management and for ongoing monitoring/maintenance.
Primarily, the control will be the large portion of knotweed that will not be maintained by the
developer.
If you have any questions, please contact us. Thank you.
Sincerely;
John Wallen, P.E.
The Engineer Group, LLC
570-263-7296
johnw@theengineergroup.com
CC:Danielle McKahn, Pioneer Development, LLC