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memo_NCC_response07212015CITY OF NORTHAMPTON, MASSACHUSETTS DEPARTMENT OF PUBLIC WORKS 125 Locust Street Northampton, MA 01060 413-587-1570 Fax 413-587-1576 Edward S. Huntley, P.E. Director Pg.1/6, C:\Users\wfeiden\Downloads\memo_NCC_response07212015.doc.docx Memorandum To: Northampton Conservation Commission From: James R. Laurila, P.E., City Engineer Nicole Sanford, Senior Wetland Scientist Date: July 21, 2015 Re: City of Northampton Department of Public Works Hinckley Street Reconstruction Project - Notice of Intent DEP File Number: 246-0683 Response to Public Discussion Project and NOI Hearing Summary On May 14, 2015, the Northampton Department of Public Works (DPW) presented the Notice of Intent (NOI) for the Hinckley Street Reconstruction Project (Project) to the Northampton Conservation Commission (NCC). The project proposes to fully reconstruct Hinckley Street including water, sewer and drain utility replacements. Additionally, stormwater from upper Hinckley, upper Warner Street and Maplewood Terrace will be rerouted down Hinckley Street to a newly constructed outfall to the Mill River on the southeast side of Riverside Drive. Other than the proposed outfall, the roadway reconstruction project is outside of jurisdictional areas protected by the Massachusetts Wetlands Protection Act (MWPA). As detailed in the NOI and confirmed by Sarah LaValley, Northampton Conservation, Preservation and Land Use Planner, in an email to DPW staff on May 15, 2015; the project meets all applicable performance standards for working within certain resource areas (Riverfront Area, Bordering Vegetated Wetland, and Intermittent Bank). Further, the Department of Environmental Protection (DEP) issued a file number for the project on April 14, 2015 with no comments related to the MWPA. At the public hearing and the site visit on June 2, 2015, neighbors and abutters to the project site voiced comments pertaining to:  alternative outfall options  the need for the proposed access road to the outfall;  the width of the proposed access road; Pg.2/6  concerns about precedence being set by allowing a permanent access road for a drainage structure;  stormwater alternatives including Stormceptors versus deep sumps and water quality;  Low Impact Development (LID) options;  alternate finished surfaces for the access road; and  replanting possibilities. We also provide a summary of the anticipated reduction in drainage flow to the undersized Elm Street Brook drainage system. One of the benefits that would result from construction of the new outfall. The following are responses to these comments. Response to Comments Outfall alternative options: Alternative drainage and outfall options were considered. The first option was to retain the existing drainage configuration and rebuild the existing outfall. The existing 10-inch outfall discharges near the top of the steep slope to the west of the proposed outfall. It has deteriorated, is undersized for current flows and has resulted in erosion on the slope. Rebuilding this outfall would require a similar degree of disturbance and access in order to install a new pipe that would provide a gradual transition down the slope and mitigate erosion, yet doing so would not provide the additional benefit of larger drainage system improvements that are being proposed. For the proposed system improvements, in order to minimize site disturbance, building an access road along the proposed pipe alignment was also considered. Due to the steepness of the slope, an access road with a 20% grade would have been required, which is too steep for maintenance equipment to safely navigate. Finally, an access road approach from the west was considered, but the grades were even less favorable from this direction than the proposed approach, and both property line and wetland locations made this design idea infeasible. Also, maintaining the existing drainage flows to Elm Street Brook was considered to not be preferred because of the repeated flooding issues there. The need for the proposed access road to the outfall and the width of the proposed access road: Construction of a 14 foot wide access road is necessary for both the construction of the 36 inch drain line with associated drain manholes and outfall in order for construction equipment to safely access the site; and for ongoing maintenance, repairs and replacements of this section of the drainage system that may occur in the future. Given that cost will be incurred to build the road needed to construct the outfall we believe it’s a prudent fiscal decision to keep the road for future use rather than paying for the road to be removed at the end of the construction job. There are other functional benefits to keeping the road. Permanent access will prevent having to re- disturb the site every time the stormwater structure is in need of maintenance or repair. Future repairs may require heavy equipment and as with construction, the width of the road is necessary to allow safe access to the site. The following photos detail areas in Northampton both with limited access and areas with existing access; and show the type of equipment that may be needed for future repairs or replacements of this proposed drainage system.   Pg.3/6 Photo 1 shows an existing culvert and drainage outfall located off of Hatfield Street. The culvert was filled to the top with sediment and the drainage line from the existing catch basin in the street had failed. The embankment off of Hatfield Street is steep and there was no established access to this culvert making routine maintenance and repairs extremely difficult. Establishing access for this repair required tree cutting, clearing and the installation of a temporary access. Photo 2 illustrates the type of equipment necessary to clean out the culvert and repair the drain line. Often times the excavations necessary to repair utility lines and structures are deep making the “do-it by hand” approach impossible especially during emergency situations like this repair. When the drain line failed the embankment eroded exposing a high pressured gas main. Pg.4/6 Photo 3 shows an existing paved access road off of the Norwottuck Rail Trail (behind Stop and Shop). This existing sediment basin located within the Barrett Street Marsh requires annual maintenance and the paved access way is necessary for the heavy equipment to access the basin for cleaning and/or repair. Photo 4 shows the equipment necessary to clean out the accumulated sediment from the basin. The proposed outfall off of Riverside Drive includes a gravel access road because heavy equipment may be needed, like the equipment shown in this picture. If the system requires repair, a gravel access road with the appropriate width to safely allow necessary equipment to access the site is practical and important for this site. Setting precedent by allowing an access road to a proposed drainage structure: Concern has been raised that a precedent will be set by allowing an access road to this proposed outfall and that all future repairs of existing outfalls or installations of new outfalls will have access roads. The need for an access road is site specific. There are instances where access to culverts and outfalls within the City are easy and do not require access roads; however, similarly to the photos of infrastructure off of Hatfield Street, there are and will likely be sites that have Pg.5/6 steep topography or are located further away from a main road where roadway access is necessary for maintenance and repairs. Water quality concerns and questions regarding the use of Stormceptors versus deep sump catch basins: A question was asked regarding the water quality of the stormwater entering the Mill River from the proposed outfall and whether Stormceptors were evaluated instead of deep sump catch basins. The proposed use of deep sump catch basins will result in an improvement in water quality over existing conditions. A water quality unit such as a Stormceptor would provide greater pollutant removal than deep sump catch basins alone. For maintenance and functional purposes the unit would need to be located at the end of the system where the drainage system crosses Riverside Drive. A large sanitary sewer main in Riverside Drive, as well as other utilities, make this location infeasible to install a water quality structure. The use of a Stormceptor would be a considerable added expense to the project on an element that is not required for project permitting. A new Stormceptor unit would cost on the order of $20,000 to install. Low Impact Development (LID) options LID Options were considered for this project. The outfall was designed with a long natural flow path between the outfall and the Mill River. The narrow footprint of road right of way in this existing neighborhood limited the consideration of additional LID options for this project. Vegetated swales along Hinckley Street and permeable pavement for the new sidewalk were considered. After investigating soil and groundwater conditions and considering the topography and possible locations of the structures, it was determined that both these options were not feasible. The soils were found to be tight and not well drained, groundwater was high and the area required to install swales was not available. Alternate finishes for the road: The access road will be gravel and will not be paved. Gravel is the minimum material required to build a lasting and stable road. The embankment associated with Riverside Drive is steep and a wetland is located near the toe of the embankment. The road was designed to limit the area of disturbance and not permanently impact the wetland and, as a result, the proposed embankments associated with the access road will be steep. Grades that are steeper than 3 horizontal to 1 vertical require rip rap for adequate stabilization purposes. Replanting possibilities: The project meets the performance standards of the MWPA and there are no replanting requirements per the regulations. While there are no proposed plantings or seeding of the rip rap areas, over time vegetation will become established on the slopes. The vegetation that grows in the rip rap on the access road embankment will not be removed. Additional trees could be planted onsite, but as the entire site will not be cleared of vegetation, these trees would be planted underneath the understory of the existing tree canopy and would need to be located within the proposed easement unless the property owner granted permission to plant trees outside of the easement. The limit of work area shown on the project plans is also the limit of the easement. Much of the easement contains the proposed stormwater structures, access road and Pg.6/6 an existing wetland, and the area to plant trees is limited to a small area located southwest of the proposed outfall within the existing bike jump area. Reduction in Flow to Elm Street Brook Since the last Conservation Commission hearing we worked with CDM Smith to calculate how much water is being removed from the Elm Street Brook to the new outfall. We verbally discussed the benefits of reducing drainage to the undersized Elm Street Brook drainage system at the last hearing. The modeling completed by CDM Smith revealed the following flows into Elm Street Brook and what the proposed flow and percent reduction would be if the new outfall is constructed: Storm Event Existing Discharge (cfs) Proposed Discharge(cfs) Percent Reduction Flow Reduction (cfs) Flow Reduction (gpm) 2- year 123.6 117.1 5.3 6.5 2,917 10-year 261 244.4 6.4 16.6 7,450 25-year 376.8 350.9 6.9 25.9 11,625 50-year 485.6 462.6 4.7 22.8 10,233 100-year 664.3 636.7 4.1 27.6 12,388 The construction of the new outfall will provide important relief to the undersized Elm Street Brook Drainage system, as this table illustrates. This current project is a cost-effective means to make drainage improvements that will benefit many properties in this area of the City.