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DPW Stormwater Review Response Letter 10-31-19 \\vhb\gbl\proj\Springfield\42540.00\docs\Permits\Stormwater Permit\2019-10-31 DPW Comments\DPW Stormwater Review Response Letter 10-31-19.docx October 31, 2019 Ref: 42540.00 City of Northampton Department of Public Works Doug McDonald 125 Locust Street Northampton, MA 01060 Re: Hawley Street Residential Development Dear Mr. McDonald, On behalf of our client, O’Connell Development Group, VHB, Inc. submits for your review and use, responses to comments received as part of the review process for the above referenced project. VHB, Inc. received the comments via email dated October 4, 2019 regarding your review of the Hawley Street Residential Development project. VHB’s responses below are bold italic text following the original comment. General: 1. No Low Impact Development (LID) designs have been included in the proposed stormwater management system. The previous stormwater design for this site included rain gardens, an infiltration system, green roof and permeable pavers. The consideration of LID measures is required by Mass stormwater standards and by Northampton permits. The permit application requires the submission of a MA DEP Stormwater Report Checklist which outlines the LID options. A completed MA DEP Checklist was not submitted. Please submit a complete MA DEP Stormwater Report Checklist and any other information and revisions regarding stormwater LID design proposed or describe technical reason that LID measures have not been considered or designed? Response: The stormwater design has been updated based on the comments received as part of this submittal. This new design incorporates LID techniques such as underground infiltration trenches where the geotechnical report allows based on seasonal high groundwater. There are two different infiltration systems on site. One is located at the middle of the site on the south side and the other is located under a grass swale at the far east side of the site. The stormwater system at the front of the site will continue to be a storage system only with no infiltration due to the seasonal high groundwater elevation. A MA DEP stormwater checklist has been completed and is included in the stormwater report. Hawley Street Residential Development Ref: 42540.00 October 31, 2019 Page 2 \\vhb\gbl\proj\Springfield\42540.00\docs\Permits\Stormwater Permit\2019-10-31 DPW Comments\DPW Stormwater Review Response Letter 10-31-19.docx 2. No soil information has been submitted and is required to be submitted. The documents submitted state that soil conditions on the property are "poor" and "groundwater elevations were high" based on previous soil investigation that were not submitted. No infiltration has been proposed. The test pits that were submitted in 2016 for the site showed B soils (fine sandy loam and fine loamy sand). Groundwater in one test pit was at 66 inches and not encountered in another at 104 inches. According to this information, infiltration is feasible. Since the impervious area is proposed to be increased, a recharge system should be included in the design to provide sufficient recharge for this increase. Response: A geotechnical report has been completed which includes on-site soils analysis and in-situ hydraulic conductivity rates. The stormwater design has been updated based on this analysis and infiltration systems have been provided. 3. The proposed stormwater system specifies a 24" HDPE pipe connecting to the City's 15 inch drain line in Hawley Street. This is not acceptable. The DPW will allow a 12 inch RCP connection to the City's system in Hawley Street. The plans and drainage calculations should be revised to show this. Response: This was an oversight during the original design. Based on comments provided by engineering, the pipe connection to the City’s system cannot be larger than the existing pipe in the road; therefore, we have reduced the proposed overflow pipe to a 15” RCP. Based on the HydroCAD model, a 12” overflow pipe will exceed its capacity somewhere between the 10- and 25-year storm event. Based on the design of the system and the existing invert in the street the slope of the pipe cannot be increased. A 15” pipe will be able to handle up to and including the 100-year storm event with the pipe only being between half and 3/4 capacity. 4. Subsurface detention system #1 close to Hawley Street has been specified with 265 feet of 24 inch pipe in the drainage calculations. The plans show only approximately 205 feet of 24 inch pipe. Either the plans or the drainage calculations should be revised to be consistent. Response: The stormwater design has been updated based on the comments received as part of this submittal. Some stormwater has been redirected to different systems. All systems have been updated with new total lengths of pipe. 5. It appears that there are 4 existing yard drains in the grass area north of the church building. The plans show the elimination of 3 yard drains and one yard drain is proposed to be utilized to collect an increased flow from this area. A new catch basin or basins that connects to the proposed stormwater system is preferred for this area. If the old yard drain is to be utilized, Hawley Street Residential Development Ref: 42540.00 October 31, 2019 Page 3 \\vhb\gbl\proj\Springfield\42540.00\docs\Permits\Stormwater Permit\2019-10-31 DPW Comments\DPW Stormwater Review Response Letter 10-31-19.docx additional information must be submitted detailing where this drain flows to, confirm it is not connected to sewer and if the drain line (4 inch) has adequate capacity. Response: A demolition plan has been created for the project. A call-out has been provided on this plan that indicates the contractor to verify destinations of each of the 5 yard drains and provide the information to the engineer prior to removal. Contractor also to verify if any other pipes enter the yard drain system. The plans now show proposed grading at the area north of the church with two new area drains to collect water from the new development slope and convey it to the proposed manhole in Hawley Street. The existing yard drains north of the church collected a larger area under existing conditions than what will be required under proposed conditions. The new development buildings now occupy a large portion of this area and the stormwater will be collected from the roof and conveyed to the underground system which creates a smaller area being collected by the new area drains. 6. The design of subsurface detention #3 does not seem correct. The drainage calculations have elevations and outlet configurations that do not allow much storage in this system. Please check the design and calculations and correct as necessary. Response: The stormwater design has been updated based on the comments received as part of this submittal. Some stormwater has been redirected to different systems. All systems have been updated with new design calculations. Please let me know if you have any questions or require further information. Sincerely, Vanasse Hangen Brustlin, Inc. Jeffrey A. Galarneau, PE Project Engineer jgalarneau@vhb.com Cc: Andrew Crystal, O’Connell Development Charles Roberts, Kuhn Riddle Architects