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Home My WebLink About25SITE1 - TE Comments 03-21-2025 John U. Faulise, Jr., L.S. David C. McKay, P.E. Gerald J. Stefon, L.S. (1952-2021) Jacob S. Faulise, E.I.T. Boundaries LLC 179 Pachaug River Drive P.O. Box 184 Griswold, CT 06351 T 860.376.2006 | F 860.376.5899 www.boundariesllc.net Attachment: Water Quality Flow and Flow Diversion Guidance from the 2004 Stormwater Quality Manual March 21, 2025 Ms. Meredith Badalucca, CZEO Assistant Planner Town of Montville 310 Norwich-New London Turnpike Uncasville, CT 06382 Re: 25SITE1 – John Dempsey – Homes R Us 2 & 8 Enterprise Lane, Oakdale, CT Site Development Plan Modifications Review Dear Ms. Badalucca, Per your request Boundaries LLC has completed a review of the revised site plans for the proposed commercial building and associated access driveways and loading dock located at 2 & 8 Enterprise Lane (Map 2, Lot 5-B and Map 2, Lot 5-C) prepared by Green Site Design, LLC. The following documents were received on March 21, 2025 as part of the application package: · PVC Direct Revised Site Plan, 2 & 8 Enterprise Lane, Oakdale, CT, Map/Block/Lot: 002-005-00B & 002-005-00C, revised March 21, 2025. The review comments provided on March 7, 2025 have been responded to by the applicant’s engineer except for the following item: · The oil-grit separator is shown in an off-line configuration as requested, however, based on the flow rates reported in the project stormwater management reports and the proposed pipe diameters and invert elevations, flows up to the 10-year storm event will flow through the oil-water separator. Procedures from the 2004 Water Quality Manual for configuring the diversion structure are attached. The pipe sizes or invert elevations should be adjusted to ensure that flows in excess of the water quality flow are bypassed around the treatment structure. Please do not hesitate to contact me with any questions. Sincerely, David C. McKay, P.E. Appendix B Water Quality Flow (WQF) and Flow Diversion Guidance 2004 Connecticut Stormwater Quality Manual B-1B-1 Water Quality Flow Calculation The water quality ﬂow (WQF) is the peak ﬂow rate associated with the water quality design storm. This section describes the recommended procedure for calculating the water quality ﬂow (WQF) for the design of: ❍Grass drainage channels (not water quality swales, which should be designed based on water quality volume - WQV) ❍Pre-manufactured stormwater treatment devices (e.g., hydrodynamic separators, catch basin inserts, and media ﬁlters) ❍Flow diversion structures for off-line stormwater treatment practices The WQF should be calculated using the WQV described in Chapter Seven. This WQV, converted to watershed inches, should be substituted for the runoff depth (Q) in the Natural Resources Conservation Service (formerly Soil Conservation Service), TR-55 Graphical Peak Discharge Method. The procedure is based on the approach described in Claytor and Schueler, 1996. 1. Compute the NRCS Runoff Curve Number (CN) using the following equation, or graphically using Figure 2-1 from TR-55 (USDA, 1986) (reproduced below): CN =1000 [10 + 5P + 10Q–10(Q2 + 1.25QP)1/2] where:CN = Runoff Curve Number P = design precipitation, inches (1” for water quality storm) Q = runoff depth (in watershed inches) =[WQV (acre – feet]x[12(inches/foot)] Drainage Area (acres) Figure 2-1 Solution of Runoff Equation Direct runoff (Q), inches Rainfall (P), inches Curves on this sheet are for the case Ia=0.2S, so that Q =(P - 0.2S)2 P + 0.8S 2004 Connecticut Stormwater Quality ManualB-2 B-2 2. Compute the time of concentration (tc) based on the methods described in Chapter 3 of TR-55. A minimum value of 0.167 hours (10 minutes) should be used. For sheet ﬂow, the ﬂow path should not be longer than 300 feet. 3. Using the computed CN, tc, and drainage area (A) in acres, compute the peak discharge for the water quality storm (i.e., the water quality ﬂow [WQF]), based on the procedures described in Chapter 4 of TR-55. ❍Read initial abstraction (Ia) from Table 4-1 in Chapter 4 of TR-55 (reproduced below); compute Ia /P Table 4-1 Ia values for runoff curve numbers Curve Ianumber (in) 40 . . . . . . . . . . . . . . . . . . . 3.000 41 . . . . . . . . . . . . . . . . . . . 2.878 42 . . . . . . . . . . . . . . . . . . . 2.762 43 . . . . . . . . . . . . . . . . . . . 2.651 44 . . . . . . . . . . . . . . . . . . . 2.545 45 . . . . . . . . . . . . . . . . . . . 2.444 46 . . . . . . . . . . . . . . . . . . . 2.348 47 . . . . . . . . . . . . . . . . . . . 2.255 48 . . . . . . . . . . . . . . . . . . . 2.167 49 . . . . . . . . . . . . . . . . . . . 2.082 50 . . . . . . . . . . . . . . . . . . . 2.000 51 . . . . . . . . . . . . . . . . . . . 1.922 52 . . . . . . . . . . . . . . . . . . . 1.846 53 . . . . . . . . . . . . . . . . . . . 1.774 54 . . . . . . . . . . . . . . . . . . . 1.704 Curve Ianumber (in) 55 . . . . . . . . . . . . . . . . . . . 1.636 56 . . . . . . . . . . . . . . . . . . . 1.571 57 . . . . . . . . . . . . . . . . . . . 1.509 58 . . . . . . . . . . . . . . . . . . . 1.448 59 . . . . . . . . . . . . . . . . . . . 1.390 60 . . . . . . . . . . . . . . . . . . . 1.333 61 . . . . . . . . . . . . . . . . . . . 1.279 62 . . . . . . . . . . . . . . . . . . . 1.226 63 . . . . . . . . . . . . . . . . . . . 1.175 64 . . . . . . . . . . . . . . . . . . . 1.125 65 . . . . . . . . . . . . . . . . . . . 1.077 66 . . . . . . . . . . . . . . . . . . . 1.030 67 . . . . . . . . . . . . . . . . . . . 0.985 68 . . . . . . . . . . . . . . . . . . . 0.941 69 . . . . . . . . . . . . . . . . . . . 0.899 Curve Ianumber (in) 70 . . . . . . . . . . . . . . . . . . . 0.857 71 . . . . . . . . . . . . . . . . . . . 0.817 72 . . . . . . . . . . . . . . . . . . . 0.778 73 . . . . . . . . . . . . . . . . . . . 0.740 74 . . . . . . . . . . . . . . . . . . . 0.703 75 . . . . . . . . . . . . . . . . . . . 0.667 76 . . . . . . . . . . . . . . . . . . . 0.632 77 . . . . . . . . . . . . . . . . . . . 0.597 78 . . . . . . . . . . . . . . . . . . . 0.564 79 . . . . . . . . . . . . . . . . . . . 0.532 80 . . . . . . . . . . . . . . . . . . . 0.500 81 . . . . . . . . . . . . . . . . . . . 0.469 82 . . . . . . . . . . . . . . . . . . . 0.439 83 . . . . . . . . . . . . . . . . . . . 0.410 84 . . . . . . . . . . . . . . . . . . . 0.381 Curve Ianumber (in) 85 . . . . . . . . . . . . . . . . . . . 0.353 86 . . . . . . . . . . . . . . . . . . . 0.326 87 . . . . . . . . . . . . . . . . . . . 0.299 88 . . . . . . . . . . . . . . . . . . . 0.273 89 . . . . . . . . . . . . . . . . . . . 0.247 90 . . . . . . . . . . . . . . . . . . . 0.222 91 . . . . . . . . . . . . . . . . . . . 0.198 92 . . . . . . . . . . . . . . . . . . . 0.174 93 . . . . . . . . . . . . . . . . . . . 0.151 94 . . . . . . . . . . . . . . . . . . . 0.128 95 . . . . . . . . . . . . . . . . . . . 0.105 96 . . . . . . . . . . . . . . . . . . . 0.083 97 . . . . . . . . . . . . . . . . . . . 0.062 98 . . . . . . . . . . . . . . . . . . . 0.041 Exhibit 4-111 Unit peak discharge (qu) for NRCS (SCS) type III rainfall distribution ❍Read the unit peak discharge (qu) from Exhibit 4-III in Chapter 4 of TR-55 (reproduced below) for appropriate tc Unit peak discharge (qu), (csm/in) Time of concentration (Tc), (hours) 2004 Connecticut Stormwater Quality Manual B-3B-3 ❍Substituting the water quality volume (WQV), converted to watershed inches, for runoff depth (Q), compute the water quality ﬂow (WQF) from the following equation: WQF = (qu)(A)(Q) where:WQV = water quality ﬂow (cfs) qu = unit peak discharge (cfs/mi2/inch) A = drainage area (mi2) Q = runoff depth (in watershed inches) = [WQV (acre – feet]x[12(inches/foot)] Drainage Area (acres) Other peak ﬂow calculation methods may be used for determining the WQF, such as those recommended by manufacturers of proprietary treatment systems, provided that the WQF calculated by other methods is equal to or greater than the WQF calculated using the above NRCS Graphical Peak Discharge Method. Flow Diversion Structures Flow diversion structures, also called ﬂow splitters, are designed to deliver ﬂows up to the design water quality ﬂow (WQF) or water quality volume (WQV) to off-line stormwater treatment practices. Flows in excess of the WQF or WQV are diverted around the treatment facility with minimal increase in head at the ﬂow diversion structure to avoid surcharging the treatment facility under higher ﬂow conditions. Flow diversion structures are typically manholes or vaults equipped with weirs, oriﬁces, or pipes to bypass excess runoff. A number of design options exist. Figures B-1 through B-3 show common examples of ﬂow diversion structures for use upstream of storm- water treatment practices. Other equivalent designs that achieve the result of diverting ﬂows in excess of the WQF or WQV around the treatment facility, including bypasses or overﬂows located inside the facility, are also acceptable. The following general procedures are recommended for design of ﬂow diversion structures: ❍Locate the top of the weir or overﬂow structure at the maximum water surface elevation associated with the WQF, or the water surface elevation in the treatment practice when the entire WQV is being held, whichever is higher. ❍Determine the diversion structure dimensions required to divert ﬂows in excess of the WQF using standard equations for a rectangular sharp-crested weir, uniform ﬂow in pipes or channels, or oriﬁce depending on the type of diversion structure. ❍Provide sufﬁcient freeboard in the stormwater treatment practice and ﬂow splitter to accommodate ﬂow over the diversion structure. ❍Limit the maximum head over the ﬂow diversion structure to avoid surcharging the stormwater treatment practice under high ﬂow conditions. Flow to the stormwater treatment practice at the 100-year water surface elevation should not increase the WQF by more than 10 percent. ❍Design diversion structures to withstand the effects of freezing, frost in foundations, erosion, and ﬂotation due to high water conditions. These structures should be designed to minimize clogging potential and to allow for ease of inspection and maintenance. 2004 Connecticut Stormwater Quality ManualB-4 B-4 Figure B-1 Flow Diversion Structure Design Option 1 Source: Adapted from Washington, 2000. Manhole Inﬂow Bafﬂe wall/weir To stormwater treatment practice Steps or ladder access Design WQF or WQVwater surface elevation Reinforced concrete bafﬂe wall or other suitable material 4’ min. or provide separate access to either side of bafﬂe wall 7’ min. AA 6” min. To stormwater treatment practice Manhole cover Inﬂow Bypass pipe To bypass conveyance system Plan View NTS SECTION A-A NTS 2004 Connecticut Stormwater Quality Manual B-5B-5 Figure B-2 Flow Diversion Structure Design Option 2 Source: Adapted from City of Sacramento, 2000. Excess runoff to storm drainage system To stormwater treatment practice Inﬂow Weir height equal to maximum depth of WQV or WQF 2004 Connecticut Stormwater Quality ManualB-6 B-6 Figure B-3 Flow Diversion Structure Design Option 3 References U.S. Department of Agriculture, Natural Resources Conservation Service (formerly Soil Conservation Service), Urban Hydrology for Small Watersheds, Technical Release No. 55, Washington, D.C., June 1986. Claytor, R.A. and T.R. Schueler, Design of Stormwater Filtering Systems, The Center for Watershed Protection, Silver Spring, Maryland, December 1996. Inﬂow Inﬂow Outﬂow Outﬂow To treatment practice Set invert at WQF or WQV water surface elevation Manhole Manhole From treatment practice Flow FlowFlowPlan Section A–A A A