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Stormwater Recharge As Built Memo 3-9-21
T 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: 3/10/2021 T Reynolds Engineering (TRE) inspected the above facility in January 2021 and 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 156,816 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 = 156,816sf x 0.25" x 1'/12" = 3,267 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 41,414 sf of roof area is currently routed into dry wells on site. During a 1-Inch storm event (1.22” over 24hrs), 3,471 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 3,936 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 cc: Pat Melnick Beaver Brook Estates‐‐Summary of As‐Built Recharge Structures Assessors Parcel ID Property Address Unit # Building Recharge Structure on 2020 As‐ Built Plan Type of Rechage Structure Constructed Volume (cf) of Recharge Structure Impervious Area (sf) Directed to Recharge Structure 06 ‐064‐002 58 CHESTNUT AV EXT 6 Yes yes 1 Drywell Recharge Structure 143 1608 Has grated riser. 06 ‐064‐003 60 CHESTNUT AV EXT 7 Yes Yes 1 Drywell Recharge Structure 0 0 Has grated riser. No Downspouts tied in. 06 ‐064‐004 62 CHESTNUT AV EXT 8 Yes Yes 1 Drywell Recharge Structure 143 1780.75 Has grated riser. 85% tied in. 06 ‐064‐005 64 CHESTNUT AV EXT 9 Yes Yes 1 Drywell Recharge Structure 143 484.8 Has grated riser. 30% tied in. 06 ‐064‐006 66 CHESTNUT AV EXT 10 No No 0 0 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 Has grated riser. 06 ‐064‐009 72 CHESTNUT AV EXT 13 Yes Yes 1 Drywell Recharge Structure 143 2839 No inspection riser. 06 ‐064‐010 74 CHESTNUT AV EXT 14 Yes Yes 2 Drywell Recharge Structure 286 1895 No inspection riser. 06 ‐064‐011 2 BEAVER BROOK LOOP 15 Yes Yes 1 Drywell Recharge Structure 143 2563 Has manhole riser. 06 ‐064‐012 4 BEAVER BROOK LOOP 16 Yes No 0 0 06 ‐064‐013 6 BEAVER BROOK LOOP 17 Yes Yes 2 Drywell Recharge Structure 286 805 No inspection riser. 06 ‐064‐014 10 BEAVER BROOK LOOP 18 Yes Yes 1 Drywell Recharge Structure 143 1818 No inspection riser. 06 ‐064‐015 12 BEAVER BROOK LOOP 19 Yes Yes Stone Rechage Structure 0 933 No inspection riser. 60% tied in. Stone recharge volume is unknown. 06 ‐064‐016 14 BEAVER BROOK LOOP 20 Yes Yes 1 Drywell Recharge Structure 143 1788 No inspection riser. 06 ‐064‐017 65 CHESTNUT AV EXT 21 Yes Yes 1 Drywell Recharge Structure 143 2497 Has grated riser 06 ‐064‐018 63 CHESTNUT AV EXT 22 Yes Yes Stone Rechage Structure 0 2388 No inspection riser. Stone recharge volume is unknown. 06 ‐064‐019 61 CHESTNUT AV EXT 23 Yes Yes Stone Rechage Structure 0 2595 No inspection riser. Stone recharge volume is unknown. 06 ‐064‐020 67 CHESTNUT AV EXT 24 Yes Yes 2 Drywell Recharge Structure 286 1804 No inspection riser. 06 ‐064‐021 69 CHESTNUT AV EXT 25 Yes Yes 1 Drywell Recharge Structure 143 1898 No inspection riser. 06 ‐064‐022 71 CHESTNUT AV EXT 26 Yes No 0 3560 06 ‐064‐023 7 BEAVER BROOK LOOP 27 Yes Yes 1 Drywell Recharge Structure 143 2292 No inspection riser. 06 ‐064‐024 9 BEAVER BROOK LOOP 28 Yes Yes 1 Drywell Recharge Structure 143 1778 Not connected, No inspection cover. 06 ‐064‐025 11 BEAVER BROOK LOOP 29 Yes Yes 1 Drywell Recharge Structure 143 1780 No inspection riser. 11A‐003‐001 48 CHESTNUT AVE EXT 1 Yes Yes 143 358.75 Grated Riser. 25% possible overland flow to structure. 11A‐003‐002 50 CHESTNUT AVE EXT 2 Yes Yes 1 Drywell Recharge Structure 143 478.375 Grated riser. 50% overland flow to structure. 11A‐003‐003 52 CHESTNUT AVE EXT 3 Yes No 0 956.75 11A‐003‐004 54 CHESTNUT AVE EXT 4 Yes No 0 956.75 11A‐003‐005 56 CHESTNUT AVE EXT 5 Yes Yes 1 Drywell Recharge Structure 143 478.375 Grated riser. 50% possible overland flow to structure. Loop Road Infiltration Basin 527 1715 Loop Road Infiltration Basin 2976 3824 Constructed Recharge Volume (cf) 6,649 Original Required Recharge Volume (cf) 2,178 Revised Recharge Volume (cf)1 3,267 Total Impervious Area to Recharge Structures (sf) 46,953 Total Impervious Area (sf) 157,687 Percent of Impervious Area to Recharge2 30% 1 Revised required recharge volume includes Beaver Brook Loop and additional as‐built impervious totalling 3.62 acres 2 The Massachusetts Stormwater Handbook requires that at least 65% of impervious area be directed to recharge structures 3S Southerly Roadway 4S Northerly Roadway 5S Roof Area 6P Drywell 7P Small Basin2 8P Small Basin1 Routing Diagram for Dry Well Recharge Prepared by T Reynolds Engineering, Printed 3/10/2021 HydroCAD® 10.00-25 s/n 04155 © 2019 HydroCAD Software Solutions LLC Subcat Reach Pond Link Dry Well Recharge Printed 3/10/2021Prepared 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) 41,414 98 Roofs, HSG B (5S) 46,953 98 TOTAL AREA Type III 24-hr 1-Inch Event Rainfall=1.22"Dry Well Recharge Printed 3/10/2021Prepared 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=41,414 sf 100.00% Impervious Runoff Depth=1.01"Subcatchment 5S: Roof Area Tc=6.0 min CN=0/98 Runoff=1.04 cfs 3,469 cf Peak Elev=96.25' Storage=1,722 cf Inflow=1.04 cfs 3,469 cfPond 6P: Drywell Outflow=0.06 cfs 3,471 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 = 46,953 sf Runoff Volume = 3,933 cf Average Runoff Depth = 1.01" 0.00% Pervious = 0 sf 100.00% Impervious = 46,953 sf Type III 24-hr 1-Inch Event Rainfall=1.22"Dry Well Recharge Printed 3/10/2021Prepared 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 = 1.04 cfs @ 12.09 hrs, Volume= 3,469 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 41,414 98 Roofs, HSG B 41,414 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 Printed 3/10/2021Prepared by T Reynolds Engineering Page 5HydroCAD® 10.00-25 s/n 04155 © 2019 HydroCAD Software Solutions LLC Summary for Pond 6P: Drywell Inflow Area = 41,414 sf,100.00% Impervious, Inflow Depth = 1.01" for 1-Inch Event event Inflow = 1.04 cfs @ 12.09 hrs, Volume= 3,469 cf Outflow = 0.06 cfs @ 14.11 hrs, Volume= 3,471 cf, Atten= 95%, Lag= 121.4 min Discarded = 0.06 cfs @ 14.11 hrs, Volume= 3,471 cf Routing by Dyn-Stor-Ind method, Time Span= 2.00-72.00 hrs, dt= 0.05 hrs Peak Elev= 96.25' @ 14.11 hrs Surf.Area= 1,504 sf Storage= 1,722 cf Plug-Flow detention time= (not calculated: outflow precedes inflow) Center-of-Mass det. time= 305.1 min ( 1,086.6 - 781.5 ) Volume Invert Avail.Storage Storage Description #1 95.00' 1,389 cf 5.33'D x 2.83'H Vertical Cone/Cylinder x 22 Inside #2 #2 94.00' 1,749 cf 9.33'D x 3.83'H Vertical Cone/Cylinder x 22 5,761 cf Overall - 1,389 cf Embedded = 4,372 cf x 40.0% Voids 3,138 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=0.06 cfs @ 14.11 hrs HW=96.25' (Free Discharge) 1=Exfiltration ( Controls 0.06 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 Printed 3/10/2021Prepared 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)