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Northview_DPW Comments on Draft As-Built_20220614CITY OF NORTHAMPTON, MASSACHUSETTS DEPARTMENT OF PUBLIC WORKS 125 Locust Street Northampton, MA 01060 413-587-1570 Fax 413-587-1576 Donna LaScaleia Director MEMORANDUM To: Matthew Taylor, P.E. and Eric Bernardin, P.E., Fuss & O’Neill From: Doug McDonald CC: Carolyn Misch, David Veleta Date: 6/14/2022 Re: Review of Higgins Way As-Built Submissions The Northampton DPW has reviewed the submission of the following information for the Higgins Way/Northview project:  Higgins Way—Northampton, Closed Drainage System Acceptance Letter, dated 5/20/2022, from Matthew Taylor, P.E. and Eric Bernardin, P.E., Fuss & O’Neill.  Higgins Way Project Closeout – As-Built Stormwater Evaluation Memo, dated 5/4/2022, from Matthew Taylor, P.E., Fuss & O’Neill.  Higgins Way – Northampton, Infiltration System Acceptance Letter, dated 4/15/2022, from Matthew Taylor, P.E. and Eric Bernardin, P.E., Fuss & O’Neill.  Draft Village Hill, As-Built, Higgins Way Project Close Out, Plan Sheet AB-01, dated 4/15/2022, Unstamped, Prepared by Fuss & O’Neil.  As-Built HydroCAD Report, 20110084A71_PROP01_5_22, dated 5/20/2022, Prepared by Fuss & O’Neill.  Partially Constructed (2020) – HydroCAD Report, dated 5/20/2022, Prepared by Fuss & O’Neill.  Preconstruction Conditions – HydroCAD Report, dated 5/20/2022, Prepared by Fuss & O’Neill.  Proposed Watershed Area Map, Village Hill Northview Lot, Prepared by Fuss & O’Neill. Draft As-Built Plan The DPW has reviewed the draft As-Built Plan for Higgins Way and cannot approve the plan and accompanying documents without additional information and revisions. The draft As-Built Plan that has been submitted includes a survey of the surface features and layout and inverts of some stormwater structures but additional information is required to confirm the details of the Pg.2/4 constructed stormwater management system. Additionally, discrepancies between the details in the approved plans and drainage calculations and the details in the draft As-Built Plan and as- built drainage calculations will need further analysis, revision and explanation to determine if the system has been built and will function according to the approved plan and in compliance with the Stormwater Management Permit. The Stormwater Management Permit issued by the Northampton DPW on 5/12/2016 for the Village Hill Northview Lot project included the following Standard Permit Condition 3: The owner shall retain the design engineer to document the construction of proposed stormwater infrastructure. The design engineer shall check elevations, inverts, layout and other critical aspects of the system as they are being constructed. At the completion of the work, including construction of stormwater management facilities and landscaping, the applicant shall submit to the DPW as-built plans stamped by a professional engineer of record in the Commonwealth of Massachusetts and a statement from a professional engineer of record in the Commonwealth of Massachusetts certifying that the materials and construction of the stormwater management system was constructed in accordance with the approved plans and will function as designed. All changes to the approved stormwater facilities shall be documented by a professional engineer of record in the Commonwealth of Massachusetts and approved by the DPW prior to submission of as-built plans and the certification statement described above. The preparation of as-built plans requires routine inspections and surveying of stormwater system components while they are being constructed. The DPW requests revisions to the As-Built Plan and additional information to address the following comments: 1. Detention Basin 1 a. The as-built size and volume of Detention Basin 1 has been modified to be smaller than shown in the approved and revised plans. Please confirm that the basin shown in the as-built plan is accurate. The as-built drainage calculations should be revised to be consistent with the as-built basin. b. The Outlet Control Structure (OCS) in Detention Basin 1 has a 2” PVC orifice at elevation 207.98’ as shown in the as-built plans. The as-built drainage calculations should be revised to include this orifice. c. The rim elevation of the OCS for Detention Basin 1 is 209.57’ in the as-built plans. The drainage calculations specifies the rim of the OCS at 210.5’. The as-built drainage calculations should be revised to be consistent with the as-built plans. 2. Detention Basin 2 a. The emergency overflow has been constructed at elevation 194.15’ in the as-built plans. The approved plans and drainage calculations specified the emergency overflow at 195’. The as-built drainage calculations should be revised to be consistent with the as-built plans. b. Detention Basin 2 is constructed with 2 outlet control structures. The grate for each OCS is specified at elevation 195’ in the approved plans and the as-built drainage calculations. The As-Built Plan identifies the rim (grate) at elevations 193.97’ and Pg.3/4 194.01’ for each OCS. The 10” orifice is specified at elevations 192.5’ and 193’ in the approved plan, 193’ for both in the as-built calculations; but the As-Built Plan identifies one 10” orifice at elevation 192.36’ and another 7” orifice at elevation 192.2’. The as-built drainage calculations should be revised to be consistent with the as-built plans. c. The contour elevations for Detention Basin 2 in the As-Built Plan are similar to the contours shown in the approved plan with a bottom elevation of the basin at 192’ but the top of the berm only at 195’ instead of 196’ in the approved plans. If the elevations of the bottom of the basin, the outlet control structure and the emergency overflow are correct in the As-Built Plan, the basin has not been constructed according to plan and the function of the as-built basin should be analyzed in revised drainage calculations to confirm that the peak flows for the system are in compliance. 3. Subsurface Infiltration System #1 a. Details of the system have not been provided in the As-Built Plan. The As-Built Plan must provide critical information confirming that the as-built subsurface infiltration system has been constructed according to design, including the following: bottom elevation of the stone, area and depth of stone, bottom elevation of the chambers, number of chambers installed, make and model of chambers installed, invert elevation of the outlet pipe and size and material of outlet pipe. 4. Subsurface Infiltration System for Individual Houses a. Details of the subsurface infiltration systems for individual houses should be provided in the A-Built Plan as described in #3 above and include details of any piped overflows and connections between systems (see 4c below). b. The Proposed Watershed Area Map and the Higgins Way Project Closeout – As-Built Stormwater Evaluation memo both identify 17 House Infiltration Units. The Proposed Watershed Area Map specified which units were located and which were installed after 9/20 and not located. The As-Built Plan shows only 15 subsurface infiltration systems and is not consistent with the other documents. The documents submitted should all show consistent information. c. A 12” HDPE FES Outlet and rip rap pad is shown on the As-Built Plan behind house #63. This outfall was not included in the approved plans and drainage calculations for the project and is possibly connected to the subsurface infiltration system(s) from individual houses. The details and function of the system connected to this outfall should be described in the As-Built Plan and included in the as-built drainage calculations. 5. Rain Garden a. The As-Built Plan should include details and confirmation that the 3 feet of Soil Filter Mix was installed at the bottom of the rain garden consistent with the specifications in the approved plans. b. The outlet structure in the rain garden was specified as a yard drain with a rim elevation of 211.75 in the approved plans and the as-built drainage calculations. The As-Built Plan identifies the rim at elevation 209.36’. Similar to comments above, the Pg.4/4 as-built drainage calculations should be consistent with the as-built plan and document compliance of the stormwater management system. 6. Pervious Pavers a. Pervious Pavers (Typical) are shown on the As-Built Plan but no details are provided. Details of the constructed pervious paver system including at least manufacturer of pavers and depth and type of stone should be provided in the As-Built Plan. Inspection of the Stormwater Management System The 4/15/2022 and the 5/20/2022 Memos from Fuss & O’Neill included brief statements that “field observations of the subsurface infiltration system” were completed on October 28, 2021 and that “the central subsurface infiltration located in the field grassed area of Higgins Way was found to be functioning as designed and no maintenance is required at this time.” The details of the field observations were not submitted. Based on rainfall records, it appears the inspection of the subsurface infiltration system on 10/28/2021 was conducted during dry weather and therefore no assessment has been made of the infiltration function of the system. During construction of the Northview Project, there were persistent problems with erosion and sediment control that resulted in large amounts of sediment washed to the double catch basin (located in front of 51 Higgins Way) which drains directly to the subsurface infiltration system. A silt sack and other measures were installed to protect the stormwater system but were inconsistently maintained resulting in questions concerning how much sediment may have been discharged to the infiltration system during construction. Sediment discharged to the infiltration system would have deposited in the stone installed below the chambers and may not be observable from the inspection ports but could impact the infiltration function of the basin. Based on this history, a draw down inspection of the subsurface system should be completed during and after a rain event to confirm the infiltration function of the system. The method for a draw down inspection is described in the recorded Stormwater Management Operation, Maintenance and Inspection Agreement. This method can be used as described or a DPW approved comparable evaluation during and after a rain event of at least 1 inch. The details and findings of the inspection of the infiltration system as well as inspection of other components of the stormwater management system should be submitted to the DPW in a full inspection report. A plan to correct any deficiencies and maintenance requirements should be submitted to the DPW as well.