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Stormwater Recharge Memo 12-6-21T REYNOLDS ENGINEERING | CIVIL ENGINEERS | PLANNING, DESIGN, AND PERMITTING SERVICES 152 MAPLEWOOD TERRACE, FLORENCE, MA 01062 | PHONE: 413-387-8078 | FAX: 413-727-3477 terry@treynoldsengineering.com | www.treynoldsengineering.com To: Northampton Department of Public Works From: Terrence Reynolds P.E. Subject: Beaver Brook Estates, Stormwater Compliance Date: revised 12/6/2021 T Reynolds Engineering (TRE) inspected the above facility in January 2021and October 2021 and then evaluated the installed stormwater recharge BMPs on site for compliance with Northampton Stormwater Permit at the request of the Pat Melnik. The intent of the evaluation is to determine if the as-built conditions of the overall development meet the required recharge volume as specified in the 1997 Stormwater Policy Handbook. The overall impervious area of the site is estimated to be 159,898 sf. The required recharge volume is the target depth factor for a given soil ("B" soil) time the impervious area, Rv = F x impervious area. Rv = 159,898sf x 0.25" x 1'/12" = 3,331 cf. A summary of impervious areas and a Hydrocad analysis was done for impervious areas determined to be routed to infiltration BMPs (see the attached Summary of As-built Recharge Structures and Hydrocad analysis). Based on the inspection of the installed dry wells and associated roof areas routed into them, it was determined that 28,641 sf of roof area is currently routed into dry wells on site. During a 1-Inch storm event (1.22” over 24hrs), 2,400 cubic feet of stormwater from the site will be infiltrated in the dry wells. An additional 5,539 sf of road area is directed to two infiltration basins located above the main detention basin. During a 1-Inch storm event (1.22” over 24hrs), an additional 465 cubic feet of stormwater from the site will be infiltrated in the two basins for a total of 2,865 cf. The instantaneous infiltration volume within the dry wells and small infiltration basins is ~6,649 cubic feet (volume below the lowest outlet invert). Based on the evaluation of the installed infiltration BMPs, it appears that the project meets the required recharge volume as calculated from the 1997 Stormwater Policy Handbook. Terrence Reynolds Beaver Brook Estates‐‐Summary of As‐Built Recharge Structures December 6, 2021 Assessors Parcel ID Property Address Unit # Building Recharge Structure on 2021 As‐ Built Plan Type of Rechage Structure Constructed Volume (cf) of Recharge Structure Impervious Area (sf) Directed to Recharge Structure Notes Roof Downspouts Connected Into Ground and to Drywell DPW Comment based on 10/6/21 inspection 06 ‐064‐002 58 CHESTNUT AV EXT 6 Yes No 1 Drywell Recharge Structure 143 1608 Grate appears to have been recently buried.Yes OK‐needs to be located and inspected in next 5 years 06 ‐064‐003 60 CHESTNUT AV EXT 7 Yes Yes 1 Drywell Recharge Structure 0 0 Has grated riser. No Downspouts tied in.No Drywell not connected, no credit 06 ‐064‐004 62 CHESTNUT AV EXT 8 Yes Yes 1 Drywell Recharge Structure 143 1780.75 Has grated riser. 85% tied in.Partially OK 06 ‐064‐005 64 CHESTNUT AV EXT 9 Yes Yes 1 Drywell Recharge Structure 143 484.8 Has grated riser. 30% tied in.Partially Ok 06 ‐064‐006 66 CHESTNUT AV EXT 10 Yes Yes 2 Drywell Recharge Structure 286 1200 No risers Yes 06 ‐064‐007 68 CHESTNUT AV EXT 11 Yes No 0 0 06 ‐064‐008 70 CHESTNUT AV EXT 12 Yes Yes 1 Drywell Recharge Structure 143 1078 Front‐has grated riser, Back‐ no riser House‐yes, Outbuilding‐No OK‐needs to be located and inspected in next 5 years 06 ‐064‐009 72 CHESTNUT AV EXT 13 Yes Yes 1 Drywell Recharge Structure 143 2839 No inspection riser.Yes OK‐needs to be located and inspected in next 5 years 06 ‐064‐010 74 CHESTNUT AV EXT 14 Yes Yes 2 Drywell Recharge Structure 286 1895 No inspection riser.Yes OK‐needs to be located and inspected in next 5 years 06 ‐064‐011 2 BEAVER BROOK LOOP 15 Yes Yes 1 Drywell Recharge Structure 143 2563 Has manhole riser.Yes Ok 06 ‐064‐012 4 BEAVER BROOK LOOP 16 Yes No 0 0 No 06 ‐064‐013 6 BEAVER BROOK LOOP 17 Yes Yes 1 Drywell Recharge Structure 0 0 No inspection riser.Only 1 Needs to be located 06 ‐064‐014 10 BEAVER BROOK LOOP 18 Yes Yes 1 Drywell Recharge Structure 143 1818 No inspection riser.Yes OK‐needs to be located and inspected in next 5 years 06 ‐064‐015 12 BEAVER BROOK LOOP 19 Yes Yes Stone Rechage Structure 0 0 No inspection riser. 60% tied in. Stone recharge volume unknown.No No credit unless more information submitted 06 ‐064‐016 14 BEAVER BROOK LOOP 20 Yes Yes 1 Drywell Recharge Structure 143 1788 No inspection riser.Yes OK 06 ‐064‐017 65 CHESTNUT AV EXT 21 Yes Yes 1 Drywell Recharge Structure 143 2497 Has grated riser.Yes Ok 06 ‐064‐018 63 CHESTNUT AV EXT 22 Yes Yes Stone Rechage Structure 0 0 No inspection riser. Stone recharge volume is unknown.Yes No credit unless more information submitted 06 ‐064‐019 61 CHESTNUT AV EXT 23 Yes Yes Stone Rechage Structure 0 0 No inspection riser. Stone recharge volume is unknown.Yes No credit unless more information submitted 06 ‐064‐020 67 CHESTNUT AV EXT 24 Yes No 2 Drywell Recharge Structure 286 1804 No inspection riser.Yes OK‐needs to be located and inspected in next 5 years 06 ‐064‐021 69 CHESTNUT AV EXT 25 Yes No 1 Drywell Recharge Structure 143 1898 No inspection riser.Yes OK‐needs to be located and inspected in next 5 years 06 ‐064‐022 71 CHESTNUT AV EXT 26 Yes No 0 0 06 ‐064‐023 7 BEAVER BROOK LOOP 27 Yes Yes 1 Drywell Recharge Structure 143 2292 No inspection riser.Yes OK‐needs to be located and inspected in next 5 years 06 ‐064‐024 9 BEAVER BROOK LOOP 28 Yes Yes 1 Drywell Recharge Structure 0 0 0 No No Credit, not connected 06 ‐064‐025 11 BEAVER BROOK LOOP 29 Yes Yes 1 Drywell Recharge Structure 143 1780 No inspection riser.Yes OK‐needs to be located and inspected in next 5 years 11A‐003‐001 48 CHESTNUT AVE EXT 1 Yes Yes 1 Drywell Recharge Structure 143 358.75 Grated riser. 25% overland flow to structure.No OK 11A‐003‐002 50 CHESTNUT AVE EXT 2 Yes Yes 1 Drywell Recharge Structure 143 478.375 Grated riser. 50% overland flow to structure.No OK 11A‐003‐003 52 CHESTNUT AVE EXT 3 Yes No 0 0 11A‐003‐004 54 CHESTNUT AVE EXT 4 Yes No 0 0 11A‐003‐005 56 CHESTNUT AVE EXT 5 Yes Yes 1 Drywell Recharge Structure 143 478.375 Grated riser. 50% overland flow to structure.No Needs berm to collect flow Loop Road Small Infiltration Basin 527 1715 No Add basin number. Can pretreatment be added? Must add to stormwater O&M plan to get credit. Add label to as‐built plan. Loop Road Large Infiltration Basin 2976 3824 No Add basin number. Can pretreatment be added? Must add to stormwater O&M plan to get credit. Add label to as‐built plan. Constructed Recharge Volume (cf) 6,506 Volume meets recharge requirement Original Required Recharge Volume (cf) 2,178 Revised ReguiredRecharge Volume (cf)1 3,331 Total Impervious Area to Recharge Structures (sf) 34,180 Total Impervious Area (sf) 159,898 Percent of Impervious Area to Recharge 21% This is a very low percentage of impervious directed to recharge and would not meet current standards. 3S Southerly Roadway 4S Northerly Roadway 5S Roof Area 6P Drywell 7P Small Basin2 8P Small Basin1 Routing Diagram for Dry Well Recharge 12-2-21Prepared by T Reynolds Engineering, Printed 12/3/2021 HydroCAD® 10.00-25 s/n 04155 © 2019 HydroCAD Software Solutions LLC Subcat Reach Pond Link Dry Well Recharge 12-2-21 Printed 12/3/2021Prepared by T Reynolds Engineering Page 2HydroCAD® 10.00-25 s/n 04155 © 2019 HydroCAD Software Solutions LLC Area Listing (selected nodes) Area (sq-ft) CN Description (subcatchment-numbers) 5,539 98 Paved roads w/curbs & sewers, HSG B (3S, 4S) 28,641 98 Roofs, HSG B (5S) 34,180 98 TOTAL AREA Type III 24-hr 1-Inch Event Rainfall=1.22"Dry Well Recharge 12-2-21 Printed 12/3/2021Prepared by T Reynolds Engineering Page 3HydroCAD® 10.00-25 s/n 04155 © 2019 HydroCAD Software Solutions LLC Time span=2.00-72.00 hrs, dt=0.05 hrs, 1401 points Runoff by SCS TR-20 method, UH=SCS, Split Pervious/Imperv. Reach routing by Dyn-Stor-Ind method - Pond routing by Dyn-Stor-Ind method Runoff Area=3,824 sf 100.00% Impervious Runoff Depth=1.01"Subcatchment 3S: Southerly Roadway Tc=6.0 min CN=0/98 Runoff=0.10 cfs 320 cf Runoff Area=1,715 sf 100.00% Impervious Runoff Depth=1.01"Subcatchment 4S: Northerly Roadway Tc=6.0 min CN=0/98 Runoff=0.04 cfs 144 cf Runoff Area=28,641 sf 100.00% Impervious Runoff Depth=1.01"Subcatchment 5S: Roof Area Tc=6.0 min CN=0/98 Runoff=0.72 cfs 2,399 cf Peak Elev=95.58' Storage=1,068 cf Inflow=0.72 cfs 2,399 cfPond 6P: Drywell Outflow=0.05 cfs 2,400 cf Peak Elev=418.11' Storage=37 cf Inflow=0.04 cfs 144 cfPond 7P: Small Basin2 Outflow=0.01 cfs 144 cf Peak Elev=417.15' Storage=92 cf Inflow=0.10 cfs 320 cfPond 8P: Small Basin1 Outflow=0.02 cfs 321 cf Total Runoff Area = 34,180 sf Runoff Volume = 2,863 cf Average Runoff Depth = 1.01" 0.00% Pervious = 0 sf 100.00% Impervious = 34,180 sf Type III 24-hr 1-Inch Event Rainfall=1.22"Dry Well Recharge 12-2-21 Printed 12/3/2021Prepared by T Reynolds Engineering Page 4HydroCAD® 10.00-25 s/n 04155 © 2019 HydroCAD Software Solutions LLC Summary for Subcatchment 3S: Southerly Roadway Runoff = 0.10 cfs @ 12.09 hrs, Volume= 320 cf, Depth= 1.01" Runoff by SCS TR-20 method, UH=SCS, Split Pervious/Imperv., Time Span= 2.00-72.00 hrs, dt= 0.05 hrs Type III 24-hr 1-Inch Event Rainfall=1.22" Area (sf) CN Description 3,824 98 Paved roads w/curbs & sewers, HSG B 3,824 98 100.00% Impervious Area Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs) 6.0 Direct Entry, Summary for Subcatchment 4S: Northerly Roadway Runoff = 0.04 cfs @ 12.09 hrs, Volume= 144 cf, Depth= 1.01" Runoff by SCS TR-20 method, UH=SCS, Split Pervious/Imperv., Time Span= 2.00-72.00 hrs, dt= 0.05 hrs Type III 24-hr 1-Inch Event Rainfall=1.22" Area (sf) CN Description 1,715 98 Paved roads w/curbs & sewers, HSG B 1,715 98 100.00% Impervious Area Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs) 6.0 Direct Entry, Summary for Subcatchment 5S: Roof Area Runoff = 0.72 cfs @ 12.09 hrs, Volume= 2,399 cf, Depth= 1.01" Runoff by SCS TR-20 method, UH=SCS, Split Pervious/Imperv., Time Span= 2.00-72.00 hrs, dt= 0.05 hrs Type III 24-hr 1-Inch Event Rainfall=1.22" Area (sf) CN Description 28,641 98 Roofs, HSG B 28,641 98 100.00% Impervious Area Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs) 6.0 Direct Entry, Type III 24-hr 1-Inch Event Rainfall=1.22"Dry Well Recharge 12-2-21 Printed 12/3/2021Prepared by T Reynolds Engineering Page 5HydroCAD® 10.00-25 s/n 04155 © 2019 HydroCAD Software Solutions LLC Summary for Pond 6P: Drywell Inflow Area = 28,641 sf,100.00% Impervious, Inflow Depth = 1.01" for 1-Inch Event event Inflow = 0.72 cfs @ 12.09 hrs, Volume= 2,399 cf Outflow = 0.05 cfs @ 13.62 hrs, Volume= 2,400 cf, Atten= 93%, Lag= 91.7 min Discarded = 0.05 cfs @ 13.62 hrs, Volume= 2,400 cf Routing by Dyn-Stor-Ind method, Time Span= 2.00-72.00 hrs, dt= 0.05 hrs Peak Elev= 95.58' @ 13.62 hrs Surf.Area= 1,436 sf Storage= 1,068 cf Plug-Flow detention time= (not calculated: outflow precedes inflow) Center-of-Mass det. time= 206.6 min ( 988.1 - 781.5 ) Volume Invert Avail.Storage Storage Description #1 95.00' 1,326 cf 5.33'D x 2.83'H Vertical Cone/Cylinder x 21 Inside #2 #2 94.00' 1,669 cf 9.33'D x 3.83'H Vertical Cone/Cylinder x 21 5,499 cf Overall - 1,326 cf Embedded = 4,173 cf x 40.0% Voids 2,995 cf Total Available Storage Device Routing Invert Outlet Devices #1 Discarded 94.00'1.020 in/hr Exfiltration over Surface area Conductivity to Groundwater Elevation = 90.00' Discarded OutFlow Max=0.05 cfs @ 13.62 hrs HW=95.58' (Free Discharge) 1=Exfiltration ( Controls 0.05 cfs) Summary for Pond 7P: Small Basin2 Inflow Area = 1,715 sf,100.00% Impervious, Inflow Depth = 1.01" for 1-Inch Event event Inflow = 0.04 cfs @ 12.09 hrs, Volume= 144 cf Outflow = 0.01 cfs @ 12.52 hrs, Volume= 144 cf, Atten= 80%, Lag= 26.0 min Discarded = 0.01 cfs @ 12.52 hrs, Volume= 144 cf Routing by Dyn-Stor-Ind method, Time Span= 2.00-72.00 hrs, dt= 0.05 hrs Peak Elev= 418.11' @ 12.52 hrs Surf.Area= 355 sf Storage= 37 cf Plug-Flow detention time= (not calculated: outflow precedes inflow) Center-of-Mass det. time= 25.2 min ( 806.7 - 781.5 ) Volume Invert Avail.Storage Storage Description #1 418.00' 527 cf Custom Stage Data (Prismatic) Listed below (Recalc) Elevation Surf.Area Inc.Store Cum.Store (feet) (sq-ft) (cubic-feet) (cubic-feet) 418.00 306 0 0 419.00 748 527 527 Device Routing Invert Outlet Devices #1 Discarded 418.00'1.020 in/hr Exfiltration over Surface area Conductivity to Groundwater Elevation = 410.00' Type III 24-hr 1-Inch Event Rainfall=1.22"Dry Well Recharge 12-2-21 Printed 12/3/2021Prepared by T Reynolds Engineering Page 6HydroCAD® 10.00-25 s/n 04155 © 2019 HydroCAD Software Solutions LLC Discarded OutFlow Max=0.01 cfs @ 12.52 hrs HW=418.11' (Free Discharge) 1=Exfiltration ( Controls 0.01 cfs) Summary for Pond 8P: Small Basin1 Inflow Area = 3,824 sf,100.00% Impervious, Inflow Depth = 1.01" for 1-Inch Event event Inflow = 0.10 cfs @ 12.09 hrs, Volume= 320 cf Outflow = 0.02 cfs @ 12.55 hrs, Volume= 321 cf, Atten= 83%, Lag= 27.8 min Discarded = 0.02 cfs @ 12.55 hrs, Volume= 321 cf Routing by Dyn-Stor-Ind method, Time Span= 2.00-72.00 hrs, dt= 0.05 hrs Peak Elev= 417.15' @ 12.55 hrs Surf.Area= 693 sf Storage= 92 cf Plug-Flow detention time= (not calculated: outflow precedes inflow) Center-of-Mass det. time= 35.0 min ( 816.5 - 781.5 ) Volume Invert Avail.Storage Storage Description #1 417.00' 2,976 cf Custom Stage Data (Prismatic) Listed below (Recalc) Elevation Surf.Area Inc.Store Cum.Store (feet) (sq-ft) (cubic-feet) (cubic-feet) 417.00 538 0 0 418.00 1,581 1,060 1,060 419.00 2,252 1,917 2,976 Device Routing Invert Outlet Devices #1 Discarded 417.00'1.020 in/hr Exfiltration over Surface area Conductivity to Groundwater Elevation = 410.00' Discarded OutFlow Max=0.02 cfs @ 12.55 hrs HW=417.15' (Free Discharge) 1=Exfiltration ( Controls 0.02 cfs)