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Home My WebLink About1998-04-10 Response Comments SPR MEOW ® A MHE Design Consultants, Inc. 12-8 Manor Parkway > Salem, NH 03079 -TEL(603)893-0720 • FAX (603)893-0733 'A March 23, 1998 I 1' Ms. Kathleen Colwell Planning Director 120 Main Street North Andover, MA 01845 RE: Cofer & Colantonio Engineering Review Channel Building Company Dear Ms. Colwell: Comments received from Cofer& Colantonio in their plan review letter dated March 11, 1998 have been received, The enclosed plans and revised report are intended to specifically address the comments and further questions which have been raised. This letter narrative is formatted in response to Coler& Colantonio's original letter and is intended to direct specifically raised questions, to the appropriate response area and to assist in further clarification and amplification of MHF's response. Information Required: To make the plan more readable, some of the proposed notes have been moved. Zoning Bylaws: 1. The location map on the plan front sheet has been revised to a scale of 1" _ 1,000' as required. 2. The Grading & Drainage Plan has been revised accordingly. ENGINEERS m PLANNERS . SURVEYORS 3. See Below. MHF Design Consultants, Inc. 4. No response necessary. 5. Access drive has been shown. [Refer to the Existing Conditions Plan] 6. Tree line has been shown. [Refer to the Existing Conditions Plan.] 7. No response necessary. Drainage Issues: 8. These plans were sent out overnight delivery on March 12, 1998 to Coler& Colantonio and crossed in the mail. Calls to Coler& Colantonio confirm that they have received these report plan inserts. However, based on conversation with John Chessia the Pre-Development Runoff Worksheet was re-plotted at 1"= 30' which matches the same scale as the Post-Development Runoff Worksheet, so the two plans and drainage areas can be overlaid. In the revised Stormwater Management Report each of the hydrographs have been printed for the 10-Year design Storm (for flows greater than 0.5 efs, except for hydrograph #16 - the out flow of the detention pond which illustrates the 24-hour detention pond flow. As can be seen, the detention pond has a water surface elevation of 242. 31 (9" +1-) 24-hours after the beginning of the storm. [Refer to Stormwater Management Report page 3-47.] This qualifies the detention pond as "long term". The print of these hydrographs illustrate input areas used and the RCN values. An additional worksheet, in Section Two [page 2-5] has been added which modifies the Pre-Development runoff area. This area now includes the drainage runoff area of the detention pond itself (0.13 acres) and all landscaped area between the impervious area and the landscaped area of disturbance (0.45 aces). Previously, the footprint of the impervious area (and landscaped areas within it) was used in the Pre and Post-Development Runoff calculations. These changes, along with the re-graded detention pond have all been incorporated into the revised 2, 10, 50 and 100 Year Hydrographs. 9. Stormwater runoff at each catch basin inlets is provided in the Stormwater Management Report for this site. Proposed grading has been revised beyond the five foot sidewalk for Catch Basin #1. This grading directs the Stormwater runoff for the 50 and 100-year storm to the detention pond. [Refer to page 2-7 and the revised Grading & Utility Plan, sheet 4 of 8.] 10. The detention pond has been revised per this comment and comments numbers 8, 12 and 13. These comments will all be addressed here together. The a f y MWERW RIM mom wuwmx .mn. ..n w detention pond has been revised from a wet bottom design, to a ftliRR�"Consultants, Inc, design. This eliminates the need for the draw down piping and value as discussed in the review letter under Standard Number 4. The proposed outlet of the detention pond is now at elevation 241.50 (the former wet pond bottom elevation). Incorporated into the detention pond re-grading is a four foot wide emercy spillway, in case both the proposed weir outlet (elevation 244.60 ) and the emergency overflow grate (elevation 245.45) should get plugged. The emergency spillway overflow elevation is designed for the 100-Year Storm elevation 245.10), The top of the eight-foot wide berm is at elevation 246,25. [Refer to the Grading & Drainage Plan, sheet 4 of 8 and the Detail Sheet, sheet 7of8] 11. The outlet of the detention pond has been revised. Based on conversations with the town, the conservation commission and the town's review consultant (Coley& Colantonio) the preferred method of discharge is to outlet, to a grass swale outside of the25-footg o disturbance zone_.vers s tying.inpto„the existing catch basin and outlet through the existingestablished outlet. i e. The revised flans show a side discharge from the detention pond outlet control structure (at elevation 240.50). This outlet pipe is one foot below, the 2 112" cored detention pond outlet. Stone for erosion control (8-feet long and 8-feet wide) is proposed at the flared end section (FES #2). [Refer to the Grading & Drainage Plan, sheet 4 of 8 and the Detail Sheet, sheet 7 of 8 (Details Detention Pond Cross Section A-A, Proposed Outlet Structure (MH 0), and Outlet Protection Detail) 12. The berm has been revised to 8-feet wide. [Refer to Grading & Drainage Plan, sheet 4 of 8 and the Detail Sheet, sheet 7 of 8.] 13. A proposed spillway for the 100-Year Storm has been incorporated into the revised detention pond grading. [Refer to Grading & Drainage Plan, sheet 4 of 8 and the Detail Sheet, sheet 7 of 8] 14. A clean out has been proposed on the revised Grading and Drainage Plan. [Refer to Grading & Drainage Plan, sheet 4 of 8.] 15. The treatment grass swale has been revised. [Refer to Grading & Drainage Plan, sheet 4 of 8 and a Detail Worksheet on page 2-22 in the Stormwater --,9 SA Management Report] The proposed treatment Swale has a 4-foot wide flat `-1 bottom with 6.1 side slopes a proposed 4" perforated PVC pipe assists in t draining the 6" sump in the level grass treatment swale. The proposed grass treatment swale is in conformance with the standards put forth in the stormwater Policy Handbook. [Refer to Stormwater Management Report page 2-23] f } ■ 16. The proposed catch basin and manhole layout has been revisedMTA9 gbConsultants, Inc. Grading & Drainage Plan, sheet 4 of 8.] Stormwater Management Policy The original Stormwater Management Report did not include the blank Stormwater Management Form. However the design narrative addressed each point in the Stormwater Management Policy. The blank Stormwater Management Form is now included in the Revised Stormwater Management Report. [Refer to the revised Stormwater Management Report, pages 2-10 to 2-15.] The last page of the design narrative (2-15) has been stamped and certified. A note (#6) has been added to the plans which addresses the parking lot sweeping. A complete 24-hour print out of the detention pond outflow for the 10-Year Design Storm has been incorporated into the Revised Stormwater Management Report. [Refer to Hydrograph #16 in Section 3 of the revised report.] An excerpt of the Design Criterion for Wet Quality Treatment Swales for the Stormwater Policy Handbook is included in Section 2.0 of the revised Stormwater Management Report along with pertinent calculations. [Refer to page 2-23 of the Stormwater Management Report.]] Sin r ly, L rry D. urster, P.E. enior ngineer LDW Enclosure: Revised Stormwater Management Report Revised Plans cc: John Chessia Greig Wiech 56097 CR