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Stormwater Narrative Summary 6.5.201 36 Hampden Street, Northampton Stormwater Narrative Summary Pioneer Development LLC Project Description Pioneer Development LLC is proposing to construct 3 additional dwellings units, situated on a previously developed parcel at 36 Hampden St, Northampton Massachusetts (the Project) which has 2 dwelling units existing on the property at present time. The Project proposes 3 new units, a paved driveway with 5 new parking spaces, removal of a garage structure to create 3 new parking spaces on the remaining garage slab, ancillary landscape improvements. The project is not considered a Land Use with Higher Potential Pollutant Loads (LUHPPL) as defined in 3.10 CMR 10.04 and 314 CMR 9.02. The total disturbance is 0.323 acres which does not require a NPDS Construction General Permit. Site Description The Site is currently an approximately 1.77-acre parcel of land located at the end of Hampden St, Northampton, Massachusetts. It is located near the intersection of Hampden Street and Reed Street, at the south-eastern terminus of Hampden St. The Site borders on the north side of Arcadia Wildlife Sanctuary, and borders an abandoned section of the old Mill River bed. The site owners are proposing to donate the old parcel numbered 14 to the Mass Audubon Society to be added to the Arcadia Wildlife Sanctuary. This will bring the new lot size to 0.63 acres. The area inside the bounds of the Erosion Sediment Controls are proposed to be 0.323 acres. There is a Bordering Vegetated Wetland (BVW) resource and a 100 year flood plain on the south side of the existing site. Work is proposed within the 100' boundary of the BVW and within the 40' Northampton offset of the Flood Plain boundary on the site, as well as a section of the 35' Northampton BVW boundary to expand the paving for a parking space on the south side of the existing garage slab, and to mow some of the Japanese Knot Weed to be replaced with grass for erosion control and maintenance. SCS Soil Survey map shows the soils comprised of 745C, Hinckley-Merrimac-Urban land complex, 3 to 15 percent slopes, which is listed as Class A in the Soils Report, and 52A, Freetown muck, central lowland, 0 to 1 percent slopes, which is listed in the Soils Report as Hydrologic Soil Group (HSG) B/D Soils. Soils analysis in the lowland portion of the site for wetland delineation, confirm the soil type to be consistent with the Freetown Muck with exception that the muck layer below the peat is silty clay, instead of Muck. The HSG Group in the lowland area is HSG D. Probe soil tests of the upland portion of the site show the soil to be consistent with the Hinckley-Merrimac-Urban land complex, sandy gravel, sand, and loamy sand. The HSG of the upland portion of the site, and where all disturbance is proposed is HSG A. Existing hydrology of the upland sand and gravel soil allows water to infiltrate and seep downward until it encounters the underlying clay/silt layer where it is forced outward toward the old Mill River bed. In spring the water seeps along the bottom of the slope, and across the surface to the old Mill River Bed. There is an existing outfall of a Municipal stormwater pipe which measures approx 10'' dia. located on the west side of the property, near the bottom of the slope. It is steel corrugated and collapsed at the end. Above the municipal pipe is an existing outfall of a privately owned storm 4" PVC drain coming from an existing basin located in the existing driveway to the site. There is some minor erosion to the wetland at the outfall of the municipal pipe. To the east of the lot, on the neighboring lot, near the existing garage, is a seep which is creating a visibly wet area as observed throughout the summer. The wet area drains into the wetland and runs toward the old Mill River bed. The BVW generally follows the toe of the slope, until it runs east, where it disappears under debris, then reappears up the slope on the adjacent lot. 2 Proposed Drainage Conditions The Project is a redevelopment of the Site, so as stated in Standard 7 of the Massachusetts Stormwater Standards, it is subject to Standards 2, 3, 4, 5, and 6 to the maximum extent practicable. The Site is subject Standard 2; an overview of planned drainage is provided. Runoff on the site would be split between two drainage areas much as it exists currently. In the northern part of the Site, the grass areas, paved areas, and half the roofs of the existing and proposed buildings will be collected via a deep sump catch basin then piped through a water quality inlet deep sum hooded basin. Water quality will then be provided by a proprietary Cultec Separator Row System within 5 Gally type chambers installed under the new proposed paved driveway. The Separator Row and other chambers will be installed in a bed of stone which will be wrapped with an HDPE membrane to prevent infiltration on the site. The chamber/stone system proposed will drain through an orifice outlet control located in a manhole, then connect to the municipal pipe which will outfall in the wetland in the existing location at the toe of the slope. The City of Northampton DPW has informed us that the collapsed drain will be repaired in the future. The stormwater for the southern part of the Site would be as it exists today. Roof leaders will drain to splash blocks which will overflow disconnected into mulched or grassy areas. The decks in the rear of the buildings will be also disconnected and run off into the grass area behind the building. The system has been designed to mitigate the 2, and 10 year storms for peak runoff rate. Standard 6 Recharge will not be accomplished due to lack of space on the site. When discounting the lot set backs, and setback from foundations listed in the Massachusetts Storm Water Manual, there is no space left to safely locate a recharge system. Environmentally Sensitive and Low Impact Development (LID) Techniques Low Impact Development (LID) techniques and stormwater Best Management Practices (BMPs) implemented into the site design include: The new buildings are designed clustered to minimize impervious area, and maximize pervious. Walkways and some parking areas have been designed to occupy the smallest footprint possible on the site. Rain barrels are proposed for the front of the building. Environmentally Sensitive Elements Environmentally sensitive elements include: A stand of Japanese Knot Weed (invasive species) on the south slope which is proposed to be partially mowed and replaced with grass, then maintained by mowing to provide a filter strip before overland waters enter the wetland. It should be noted that the Hydraulic analysis includes the neighbors property to the north, since it drains primarily to the inlet location existing and proposed. The drainage mitigation for 2 and 10 yr storms encompasses this additional property as well. Donation of the tract of land for conservation will add to the Arcadia Wildlife Sanctuary for public preservation. Stormwater runoff from all impervious surfaces in proposed conditions will receive treatment for stormwater quality prior to discharge to the BVW. Under existing conditions, there is no treatment provided for stormwater runoff. The Site has been designed with a stormwater management system to the maximum extent practicable that has been developed in accordance with the Massachusetts Stormwater Standards.