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0195-Letter-DPW-12-21-11December 21, 2011 Doug McDonald Department of Public Works 127 Locust Street Northampton, Ma 01060 RE: Stormwater Permit for Zoe Senior Retirement & Addition to Linda Manor Extended Care Living, 331 & 359 Haydenville Road, Northampton, Massachusetts. Sage Project No. 0195 Dear Mr. McDonald: On behalf of J. Chakalos Investments, LLC . Sage Engineering is submitting the enclosed revised Site Plans and Stormwater Management Report. We have outlined the following responses following your nomenclature to guarantee clear communications. In general, we have revised the design plans and stormwater report to address the comments made and answer the questions posed. -Documentation must be provided of LID measures that were considered for the proposed project as required in the current MA DEP Checklist for Stormwater Report. -Both the addition to Linda Manor and the proposed Zoe Senior Retirement developments have incorporated LID measures as outlined in the MA DEP Checklist. Both projects have designed with no disturbance to any Wetland Resource areas, and have minimized the total area of development by creating a cluster type complex, and limiting development to areas that have been previously developed to the greatest extent possible. Additionally, the projects will provide an overall reduction in stormwater runoff. -The proposed drainage system for Zoe Senior Retirement is shown in the plans connecting to the existing drainage system at Linda Manor. The applicant has provided separate drainage calculations for Zoe Retirement and Linda Manor. These calculations do not show overall how the system will function and what the peak flows will be to the existing basin and outfall at Beaver Brook. The applicant must provide drainage calculations that show pre and post construction peak flows for the entire drainage system for the 1, 2, 10, and 100 year 24 hour design storms. The drainage calculations must correctly model the proposed detention basin discharge to the existing 24 inch drainage pipe at Linda Manor and the stormwater system at Linda Manor to properly analyze how the systems will function together. Additional redesign of the existing drainage system at Linda Manor may be necessary to allow the drainage systems from both projects to function correctly and in accordance to the Stormwater Management Standards. -We have revised the Stormwater Management Report. There is now one report to cover both developments and it analyzes the entire system as whole as requested. The system still meets all ten Stormwater Management Standards. -TSS removal calculations must be provided documenting the treatment trains for all areas of new impervious surface meet. -We’ve completed the Mass DEP TSS Removal worksheet for each treatment train. These are included in the revised stormwater management report. -Soil investigations (by a certified Soil Evaluator or Certified Professional Soil Scientist) must be conducted and documentation submitted including test pits or other tests for areas where construction of infiltration practices will occur. Information must be submitted verifying the soil conditions, elevation of estimated seasonal high groundwater for the proposed infiltration areas, and how the proposed rate of infiltration was calculated. -On site soil investigations were completed at locations of proposed infiltration. The evaluation was completed by Certified Soil Evaluators and Professional Engineers. The locations are shown on the revised plans. The observation logs are included in the revised stormwater management report. The infiltration rate for the Storm-Tech chambers was revised to 1.02 inches per hour based on the results of the field observations and the Rawls Rate from the Massachusetts Stormwater Handbook. Please see revised Stormwater Report for further detail. -The plans must specify where stormwater from the roof of the proposed building addition at Linda Manor flows. -The plans have been revised to show the majority of the roof runoff being captured via downspouts and underground storm piping and discharged into the stormwater management collection system. A small portion of the building will be discharged via downspouts to the grass shoulder and sheet flow into catch basins. -The elevation of the proposed Stormtech Chambers must be specified in the plans and analysis of the structure must be included in drainage calculations discussed above. -The invert elevations for the Stormtech Chambers are shown on detail 12, sheet C-5. The revised stormwater report includes the chambers in the analysis. -The long-term post-construction stormwater maintenance plan must include all components of the proposed stormwater management system. -The operation and maintenance narrative (Section V.) has been revised to include all system components with maintenances requirements as outlined in the Mass DEP Stormwater Handbook. -The applicant must provide documentation of the original design and current functioning of the existing stormwater system at Linda Manor and a record of maintenance of this system. -The owner does not have any copies of the original site plans or maintenance records for Linda Manor, therefore the original design cannot be documented. The drainage information shown on the existing conditions plan is the result of a field survey by this office. -The design of the proposed detention basin must show an emergency spillway and safe flow path of this overflow. -The detention basin outlet structure has been designed with an emergency outlet opening at elevation 507.0. This elevation is above the expected level of the basin for a 100-year storm, but below the top of the detention basin bank elevation. The emergency outlet will discharge into the Linda Manor stormwater system and ultimately to Beaver Brook. An additional emergency overflow culvert has been shown which will discharge to the lawn of Linda Manor and ultimately into Linda Manors detention basin and drainage system. -The design of the proposed detention basin must provide and specify an access route for maintenance. -The proposed detention basin was designed with an 8 foot wide access berm around the north, west, and east sides for maintenance access. The south side of the basin can be accessed from the proposed driveway. A note has been added to the grading plan to identify the access paths. -Appropriate information (TARP or STEP) must be submitted documenting the removal effectiveness of the proposed Stormceptors. The DPW requires that proprietary Stormwater BMP’s such as Stormceptors be designed with an offline configuration during larger storm events (diversion structures) to avoid re-suspension and discharge of sediment. -A TARP report has been included in the revised drainage report. TARP has approved Stormceptors for 75% TSS removal rates. The TSS treatment spreadsheets have been revised to reflect this change. TARP also has approved the installation of Stormceptors inline, as they are designed to allow bypass of larger storm events without re-suspension of previously captured sediment. We recommend leaving the Stormceptor units online as intended by the manufacturer to allow runoff treatment to the greatest extent possible. However, if the DPW prefers, we can design the unit to be offline and allow larger storms to bypass the entire unit. -Snow storage areas must be indicated on the plans. -In general snow storage area is the grasses shoulder around the perimeter of the driveway. Snow storage for the parking lot areas has been added to the site plans. -Easement(s) may be necessary to discharge stormwater from the Zoe Retirement stormwater system to the Linda Manor stormwater system and other adjacent properties. -Both the Linda Manor and the proposed Zoe facility are on land owned by the same entity. There is an existing stormwater easement across land required to reach Beaver Brook. -The proposed rain garden must be designed in conformance with the guidance of the Massachusetts Stormwater Handbook including, but not limited to: sizing of the structure, existing soil and groundwater conditions, proposed plantings, specification of the engineered soil mix, and grading on the Site Grading Plan. -Based on the results of additional soil testing on site, it has been determined that due to high groundwater levels a rain garden is not practical at this site. The rain garden has been removed from the proposed plans. Sincerely, Bryan Balicki, P.E. Project Engineer C.C. J. Chakalos Investments, LLC