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HomeMy WebLinkAbout2015-01-06 Stormwater Correspondence DTO SP i d Design Consultants, Inc. Consulting Engineers & Land Surveyors f Matt Office: North Share Office: s 120 MIDDLESEX AVE.STE 20 68 PLEASANT STREET SOMERVILLE,MA 02145 NEWBURYPORT, MA 01950 j Tel: 617.776.3350 Tel: 978.358.7173 October 8, 2014 { Matthew Egge,Town.Planner Jennifer Hughes,Conservation Administrator j Town of North Andover 1 R-E : 4 High Street,West Mill Loop Parldng As-Built Review Comments Dear Mr, Egge: This letter provides responses to the review comments included in the September 19, 2014 review letter-from Lisa Eggleston, P.E., Eggleston Environmental for the West Mill Loop Parking Project. The original comment is included followed by a response in italics. 1. The as-built depths of several of the rain gardens are one to two feet shallower than was shown on the approved plan, so they do not have the capture volume assumed in the design i calculations. Response: The rain gardens were not constructed to the design depths shown oil the approved plans. The as-built conditions have been modelled in HydroCADOO and despite j the depths of the rain gardens heing.shallower than designed, the as-built conditions of the improvements provide a decrease in both the runoff rate ate and volume. See attached as-built a hydrology calculations and revised Table 1. 2. The relative grades between the parking areas and most of the rain gardens are such that runoff is prevented from draining from the parking areas to the rain gardens, as was design E intent, and in some locations runoff from the rain gardens will actually drain onto the parking areas, potentially impacting the long-term viability of the porous pavers. Response: After carefully reviewing the grades of the parking areas with respect to the rain gardens, DC1 believes that three of the five parking areas grade correctly towards the rain gardens. The remaining hvo parking areas are graded in such a way that a portion of the runoff will enter the rain gardens, and the remaining will pond oil the pavers. If fire pavers are maintained properly, the runoff will drain through the pavers to the crushed .stone below, which ii,ill provide pretreatrtient prior•to recharge to groundwater. 3. The design plan called for four of the rain gardens to have overflow drain connecting to the closed drainage system, whereas the As-Built plan shows an overflow drain in only one rain garden. Response: All of the four proposed overflow drains ivere installed according to the plans, The as-built plan has been updated to reflect this. See attached updated as-built plan. 4. The As-Built plan does not indicate whether the peastone and sod filter strips called for on the design plan were installed between the parking areas and the rain gardens to provide pretreatment of runoff flows. Response: The peastone and sod filter strips mere installed per the design plans. The as- built plain has been updated to reflect this. See attached updated as-built playa. P L014 Jlwiect.0014�014 tiigir SIAValer 5t N.Andoseri_Carrespondence)lxaerslIo MEtge I0-8-144-do.. t i i 3 i I I S. As is clearly outlined in the Stormwater Report and in the Site Plan Special Permit,the rain gardens/bioretention basins were an integral part of the drainage design for this project and i were intended to attenuate and treat runoff from the paved driveway, not just the new parking areas, thus providing a net benefit in stormwater management for the site. This design was also the premise for the Conservation Commission's issuance of a negative Determination of Applicability for the project. However under the as-built conditions the ? rain gardens will receive little to no pavement runoff, and the runoff from the driveway tl areas will discharge uncontrolled and untreated through the closed drainage systern. Drainage frorn the new parking areas should be adequately managed by the pervious pavers, provided they are properly maintained and not subject to sedimentation wash-off from the adjacent rain gardens. Response: The drainage improvements to the existing site were intended 10 capture and mitigate the storrnwater runoff front the proposed parking areas only. The existing driveivay's drainage patterns were not changed as a part of this project. The small areas of the existing driveway that will drain into the new parking areas should be easily managed by the permeable pavers, provided(as noted in the comment) they are properly maintained The as-built improvements do provide a net benefit in storntrvater management by providing irnpr oved water quality, increased groundwater recharge and reduced runoff quantity. 6. While l recognize that the site conditions referenced in Mr. Zeren's email may have precluded installation of some of the design features called for on the approved plan, in particular the overflow drain connections, it is not clear to me that they would have prevented the proper grading of the site to drain from the parking areas into the rain garden areas, Some of the inconsistencies between the design and as-built conditions are likely to have been errors in construction and may be correctable by minor re-grading, even if the subsurface profile of the rain gardens has to be reduced. 1 recommend further evaluation of the as-built conditions by the design engineer to determine whether any such corrective actions can be taken to enhance the functionality of the drainage design. Response: After evahrating the as-built conditions with respect to the design plaits, the differences are primarily with the grading of the rain gardens. Although the rain gardens have not been constructed according to the design plaits, after analyzing the as-built hydrology compared to the pre-existing hydrology, the rain gardens and pavers will decrease both the runoffrate and runoff volume frorn the site in the as-built condition. For this reason, DCI believes that no firrther adjustments need to be made to the nervily constructed improvements. 7. l also note that a certification of "substantial compliance" by the design engineer is required by the Special Permit, and that the final As-Built Plan should be stamped and signed by a registered PE or Land Surveyor. Response: Comment noted, DCI believes the as-built improvements substantially comply ivith the design plans. If there is any additional information that you would find helpful, please do not hesitate to contact me. Sincerely, sign Co sultants,Inc. Wayr e K her, P.E., LEED AP bd-+-c Principal P 12014 Pmje tNO€-0 014 theh Si-Wamei S1 10-3-14 dmx 2 1S 2S Propose arking 1 P posed Parking 2 Rain G�den 1 ain Garden 2 O O O Existing Overland Flow Prted Rain Garden 3 Proposed Parking 3&4 A5P Rain on 5S Proposed Parking 5&6 5S Proposed Parking 7 SubCat Reach Potl LICIk. Drainage Diagram for 2014-014 Hydrology-As Built Prepared by Design Consultants, Inc., Printed 1 01812 0 1 4 HydroCAD®9.10 s!n 00884 O 2010 HydroCAD Software Solutions LLC 2014-014 West Mills 2014-014 Hydrology-As Built Type III 24-hr 2-Yr Rainfall=3.20" Prepared by Design Consultants, Inc. Printed 10/8/2014 N droCAD®9.10 sln 00884 02010 H droCAD Software Solutions L.LC Pa e 2 Time span=0.00-48.00 hrs, dt=0.01 hrs, 4801 points Runoff by SCS TR-20 method, UH=SCS Reach routing by Stor-Ind+Trans method - Pond routing by Stor-Ind method SubcatchmentOS: Existing Runoff Area=46,016 sf 36.00% Impervious Runoff Depth=1.61" Tc=5.0 min CN=83 Runoff=2.06 cfs 0.142 of Subcatchment1S: Proposed Parking 1 Runoff Area=1,159 sf 0.00% Impervious Runoff Depth=1,09" Tc=5.0 min CN=75 Runoff=0.03 cfs 0.002 of Subcatchrnent2S: Proposed Parking 2 Runoff Area=1,335 sf 0.00% Impervious Runoff Depth=1.09" Tc=5.0 min CN=75 Runoff=0.04 cfs 0.003 of Subcatchment3S: Proposed Parking 3&4 Runoff Area=1,963 sf 0.00% Impervious Runoff Depth=1.09" Tc=5.0 min CN=75 Runoff=0.06 cfs 0.004 of Subcatchment4S: Proposed Parking 5&6 Runoff Area=997 sf 0.00% Impervious Runoff Depth=1.09" Tc=5.0 min CN=75 Runoff=0.03 cfs 0.002 of Subcatchment5S: Proposed Parking 7 Runoff Area=696 sf 0.00% Impervious Runoff Depth=1.04" Tc=5.0 min CN=74 Runoff=0.02 cfs 0.001 of Subcatchment6S: Overland Flow Runoff Area=40,132 sf 41.27% Impervious Runoff Depth=1.68" Tc=5.0 min CN=84 Runoff=1.88 cfs 0.129 of Reach 1 R: Proposed inflow=1.91 cfs 0.132 of Outflow=1.91 cfs 0.132 of Pond 1P: Rain Garden 1 Peak Elev=0.89' Storage=59 cf Inflow=0.03 cfs 0.002 of Discarded=0.00 cfs 0.002 of Primary=0.00 cfs 0.000 of Outflow=0.00 cfs 0.002 of Pond 2P: Rain Garden 2 Peak Elev=1.47' Storage=77 cf Inflow=0.04 cfs 0.003 of Discarded=0.00 cfs 0,003 of Primary=0.00 cfs 0.000 of Outflow=0.00 cfs 0,003 of Pond 3P: Rain Garden 3 Peak Elev=3.10' Storage=135 cf Inflow=0.06 cfs 0.004 of Discarded=0.00 cfs 0.002 of Primary=0.00 cfs 0.000 of Outflow=0.00 cfs 0.003 of Pond 5P: Rain Garden 5 Peak Elev=1.36' Storage=37 cf Inflow=0.02 cfs 0.001 of Discarded=0.00 cfs 0.001 of Primary=0.00 cfs 0.000 of Outflow=0.00 cfs 0.001 of 2014-014 West Mills 2014-014 Hydrology-As Built Type /// 24-hr2-Yr Rainfall=3.20" Prepared by Design Consultants, Inc. Printed 10/8/2014 HydroCAD89.10 s/n 00884 02010 HydroCAD Software Solutions LLC Page 3 Summary for Subcatchment OS: Existing Runoff = 2.06 cfs @ 12.08 hrs, Volume= 0.142 af, Depth= 1.61" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Type III 24-hr 2-Yr Rainfall=3.20" Areas CN Description 16,564 98 Paved parking, HSG C 29,452 74 >75% Grass cover, Good, HSG C 46,016 83 Weighted Average 29,452 64.00% Pervious Area 16,564 36.00% Impervious Area Tc Length Slope Velocity Capacity Description (rain) (feet) (ft/ft) (ft/sec) (cfs) 5.0 Direct Entry, Direct Entry Summary for Subcatchment 1S: Proposed Parking 1 Runoff = 0.03 cfs @ 12.08 hrs, Volume= 0.002 af, Depth= 1.09" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Type III 24-hr 2-Yr Rainfall=3.20" Area (so CN Descri tion 810 75 Porous Pavers 349 74 >75% Grass cover, Good HSG C 1,159 75 Weighted Average 1,159 100.00% Pervious Area Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs) 5.0 Direct Entry, Dirent Entry Summary for Subcatchment 2S: Proposed Parking 2 Runoff = 0.04 cfs @ 12.08 hrs, Volume= 0.003 af, Depth= 1.09" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Type III 24-hr 2-Yr Rainfall=3.20" Area (sf) CN Description 969 75 Porous Pavers 366 74 >75% Grass cover, Good HSG C 1,335 75 Weighted Average 1,335 100,00% Pervious Area 2014-014 West Mills 2014-014 Hydrology-As Built Type 11124-hr 2-Yr Rainfall=3.20" Prepared by Design Consultants, Inc. Printed 1 0/812 0 1 4 HydroCAD89.10 s/n 00884 02010 HydroCAD Software Solutions LLC Page 4 Tc Length Slope Velocity Capacity Description (min) (feet) Oft) (ft/sec) (cfs) 5.0 Direct Entry, Dirent Entry Summary for Subcatchment 3S: Proposed Parking 3&4 Runoff = 0.06 cfs @ 12.08 hrs, Volume= 0.004 af, Depth= 1.09" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Type 111 24-hr 2-Yr Rainfall=3.20" Areas CN Description 642 75 Porous Pavers 645 75 Porous Pavers 676 74 >75% Grass cover, Good, HSG C 1,963 75 Weighted Average 1,963 100.00% Pervious Area Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs) 5.0 Direct Entry, Dirent Entry Summary for Subcatchment 4S: Proposed Parking 5&6 Runoff = 0.03 cfs @ 12.08 hrs, Volume= 0.002 af, Depth= 1.09" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Type III 24-hr 2-Yr Rainfall=3.20" Area (sf) CN Description 997 75 Porous Pavers 997 100.00% Pervious Area Tc Length Slope Velocity Capacity Description min feet ft/ft ft/sec cfs 5.0 Direct Entry, Dirent Entry Summary for Subcatchment 5S: Proposed Parking 7 Runoff = 0.02 cfs @ 12.08 hrs, Volume= 0.001 af, Depth= 1.04" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Type III 24-hr 2-Yr Rainfall=3.20" 2014-014 West Mills 2014-014 Hydrology-As Built Type 11124-hr 2-Yr Rainfall=3.20" Prepared by Design Consultants, Inc. Printed 10/8/2014 N droCADO 9.10 s/n 00884 ©2010 H droCAD Software Solutions LLC Pa e 5 Area (so CN Description 323 75 Porous Pavers 373 74 >75% Grass cover, Good HSG C 696 74 Weighted Average 696 100,00% Pervious Area Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs) - 5.0 Direct Entry, Dirent Entry Summary for Subcatchment 6S. Overland Flow Runoff _ 1.88 cfs @ 12.08 hrs, Volume= 0.129 af, Depth= 1.68" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Type III 24-hr 2-Yr Rainfall=3.20" Area (so CN Description 16,564 98 Paved parking, HSG C 23 568 74 >75% Grass cover, Good HSG C 40,132 84 Weighted Average 23,568 58.73% Pervious Area 16,564 41.27% Impervious Area Tc Length Slope Velocity Capacity Description min feet ft/ft ft/sec cfs 5.0 Direct Entry, Direct Entry Summary for Reach 1 R: Proposed Inflow Area = 1.062 ac, 35.79% Impervious, Inflow Depth = 1.49" for 2-Yr event Inflow - 1.91 cfs @ 12.08 hrs, Volume= 0.132 of Outflow - 1.91 cfs @ 12.08 hrs, Volume= 0.132 af, Atten= 0%, Lag= 0.0 min Routing by Stor-Ind+Trans method, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Summary for Pond 1 P: Rain Garden 1 Inflow Area = 0.027 ac, 0.00% Impervious, Inflow Depth = 1.09" for 2-Yr event Inflow -- 0.03 cfs @ 12.08 hrs, Volume= 0.002 of Outflow - 0.00 cfs @ 11.61 hrs, Volume= 0.002 af, Atten= 96%, Lag= 0.0 min Discarded = 0.00 cfs @ 11.61 hrs, Volume= 0.002 of Primary = 0.00 cfs @ 0.00 hrs, Volume= 0.000 of Routing by Stor-Ind method, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Peak Elev= 0.89' @ 16.23 hrs Surf.Area= 220 sf Storage= 59 cf Plug-Flow detention time= 457.4 min calculated for 0.002 of(100% of inflow) Center-of-Mass det. time= 457.5 min ( 1,315.3- 857.9 ) 2014-014 West Mills 2014-014 Hydrology-As Built Type 11124-hr2-Yr Rainfall=3.20" Prepared by Design Consultants, Inc. Printed 10/8/2014 HydroCADO 9.10 s!n 00884 0 2010 HydroCAD Software Solutions LLC Page 6 Volume Invert Avail.Stora e Storage Description #1 0.00, 236 cf Custom Stage Data (Prismatic)Listed below (Recalc) Elevation Surf.Area Voids Inc.Store Cum.Store (feet) (sq-ft) (%) (cubic-feet) (cubic-feet) 0.00 220 0.0 0 0 0.01 220 30.0 1 1 1.00 220 30.0 65 66 2.00 220 30.0 66 132 3.00 220 30.0 66 198 3.01 15 100.0 1 199 3.32 220 100.0 36 236 Device Routing Invert Outlet Devices #1 Discarded 0.00' 0.270 inlhr Exfiltration over Surface area #2 Primary 3.15' 5.0' long x 1.0' breadth Broad-Crested Rectangular Weir Head (feet) 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.50 3.00 Coef. (English) 2.69 2.72 2.75 2.85 2.98 3.08 3.20 3.28 3.31 3.30 3.31 3.32 #3 Primary 3.25' 6.0" Horiz. Orifice/Grate C= 0.600 Limited to weir flow at low heads J)iscarded OutFlow Max=0.00 cfs @ 11.61 hrs HW=0.01' (Free Discharge) 1=Exfiltration (Exfiltration Controls 0.00 cfs) Primary OutFlow Max=0.00 cfs @ 0.00 hrs HW=0.00' (Free Discharge) T--3=Orifice/Grate 2=Broad-Crested Rectangular Weir( Controls 0.00 cfs) ( Controls 0.00 cfs) Summary for Pond 2P: Rain Garden 2 Inflow Area = 0.031 ac, 0.00% Impervious, Inflow Depth = 1.09" for 2-Yr event Inflow - 0.04 cfs @ 12.08 hrs, Volume= 0.003 of Outflow = 0.00 cfs @ 11.47 hrs, Volume= 0.003 af, Atten= 97%, Lag= 0.0 min Discarded = 0.00 cfs @ 11.47 hrs, Volume= 0.003 of Primary - 0.00 cfs @ 0.00 hrs, Volume= 0.000 of Routing by Stor-Ind method, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Peak Elev= 1.47' @ 17.87 hrs Surf.Area= 175 sf Storage= 77 cf Plug-Flow detention time= 736.4 min calculated for 0.003 of(100% of inflow) Center-of-Mass det. time= 736.4 min ( 1,594.3 - 857.9 ) Volume Invert Avail.Stora e Storage Description #1 0.00, 224 cf Custom Stage Data (Prismatic)Listed below (Recalc) 2014-014 West Mills 2014-014 Hydrology-As Built Type /// 24-hr 2-Yr Rainfall=3.20" Prepared by Design Consultants, Inc. Printed 10/8/2014 H droCAD®9.10 s/n 00884 ©2010 H droCAD Software Solutions LLC Page 7 Elevation Surf.Area Voids Inc.Store Cum.Store (feet) (sq-ft) (%) (cubic-feet) (cubic-feet) 0.00 175 0.0 0 0 0.01 175 30.0 1 1 1.00 175 30.0 52 53 2.00 175 30.0 53 105 3.00 175 30.0 53 158 3.01 15 100.0 1 158 3.70 175 100.0 66 224 Device Routing Invert Outlet Devices #1 Discarded 0.00' 0.270 in/hr Exfiltration over Surface area #2 Primary 3.50' 5.0' long x 1.0' breadth Broad-Crested Rectangular Weir Head (feet) 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.50 3.00 Coef. (English) 2.69 2.72 2.75 2.85 2.98 3.08 3.20 3.28 3.31 3.30 3.31 3.32 #3 Primary 3.60' 6.0" Horiz. Orifice/Grate C= 0.600 Limited to weir flow at low heads )iscarded OutFlow Max=0.00 cfs @ 11.47 hrs HW=0.01' (Free Discharge) t1=Exfiltrat1on (Exfiltration Controls 0.00 cfs) Primary Outflow Max=0A0 cfs @ 0.00 hrs HW=0.00' (Free Discharge) T__2=Broad-Crested Rectangular Weir( Controls 0.00 cfs) 3=Orifice/Grate ( Controls 0.00 cfs) Summary for Pond 3P: Rain Garden 3 Inflow Area = 0.045 ac, 0.00% Impervious, Inflow Depth = 1.09" for 2-Yr event Inflow - 0.06 cfs @ 12.08 hrs, Volume= 0.004 of Outflow = 0.00 cfs @ 19.18 hrs, Volume= 0.003 af, Atten= 98%, Lag= 425.7 min Discarded = 0.00 cfs @ 11.24 hrs, Volume= 0.002 of Primary - 0.00 cfs @ 19.18 hrs, Volume= 0.000 of Routing by Stor-Ind method, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Peak Elev= 3.10' @ 19.18 hrs Surf.Area= 58 sf Storage= 135 cf Plug-Flow detention time= 1,040.5 min calculated for 0.003 of(62% of inflow) Center-of-Mass det. time= 924.3 min ( 1,782.1 -857.9 ) Volume Invert Avail.Storage Storage Description #1 0.00, 156 cf Custom Stage Data (Prismatic)Listed below (Recalc) 2014-014 West Mills 2014-014 Hydrology-As Built Type III 24-hr2-Yr Rainfall=3.20" Prepared by Design Consultants, Inc. Printed 1 0/812 0 1 4 H droCAD0 9.10 s/n 00884 02010 H droCAD Software Solutions LLC Page 8 Elevation Surf.Area Voids Inc.Store Cum.Store (feet) (sq-ft) (%) (cubic-feet) (cubic-feet) 0.00 145 0.0 0 0 0.01 145 30.0 0 0 1.00 145 30.0 43 44 2.00 145 30.0 44 87 3.00 145 30.0 44 131 3.01 20 100.0 1 131 3.31 145 100.0 25 156 Device Routing Invert Outlet Devices #1 Discarded 0.00' 0.270 inlhr Bxfiltration over Surface area #2 Primary 3.10' 5.0' long x 1.0' breadth Broad-Crested Rectangular Weir Head (feet) 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.50 3.00 Coef. (English) 2.69 2.72 2.75 2.85 2.98 3.08 3.20 3.28 3.31 3.30 3.31 3.32 #3 Primary 3.20' 6.0"Vert. Orifice/Grate C= 0.600 Qiscarded OutFlow Max=0.00 cfs @ 11.24 hrs HW=0.01' (Free Discharge) 1=Exfiltration (Exfiltration Controls 0.00 cfs) Primary OutFlow Max=0.00 cfs @ 19.18 hrs HW=3.10' (Free Discharge) N2=Broad-Crested Rectangular Weir(Weir Controls 0.00 cfs @ 0.08 fps) =0 rif ice/G rate ( Controls 0.00 cfs) Summary for Pond 5P: Rain Garden 5 Inflow Area = 0.016 ac, 0.00% Impervious, Inflow Depth = 1.04" for 2-Yr event Inflow = 0.02 cfs @ 12.08 hrs, Volume= 0.001 of Outflow = 0.00 cfs @ 11.56 hrs, Volume= 0.001 af, Atten= 97%, Lag= 0.0 min Discarded = 0.00 cfs @ 11.56 hrs, Volume= 0.001 of Primary 0.00 cfs @ 0.00 hrs, Volume= 0.000 of Routing by Star-Ind method, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Peak Elev= 1.36' @ 17.73 hrs Surf.Area= 92 sf Storage= 37 cf Plug-Flow detention time= 685.7 min calculated for 0.001 of(100% of inflow) Center-of-Mass det. time= 685.8 min ( 1,546.9- 861.1 ) Volume Invert Avail.Stora e Storage Description #1 0.00, 136 cf Custom Stage Data (Prismatic)Listed below (Recalc) 2014-014 West Mills 2014-014 Hydrology-As Built Type 11124-hr2-Yr Rainfall=3.20" Prepared by Design Consultants, Inc. Printed 10/8/2014 H droCADO 9.10 s/n 00884 O 2010 H droCAD Software Solutions LLC Page 9 Elevation Surf.Area Voids Inc.Store Cum.Store _(feet) - (sq-ft)__ M (cubic-feet) (cubic-feet) 0.00 92 0.0 0 0 0.01 92 30.0 0 0 1.00 92 30.0 27 28 2.00 92 30.0 28 55 3.00 92 30.0 28 83 3.01 15 100.0 1 83 4.00 92 100.0 53 136 Device Routing Invert Outlet Devices #1 Discarded 0.00' 0.270 inlhr Exfiltration over Surface area #2 Primary 3.80' 5.0' long x 1.0' breadth Broad-Crested Rectangular Weir Head (feet) 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.50 3.00 Coef. (English) 2.69 2.72 2.75 2.85 2.98 3.08 3.20 3.28 3.31 3.30 3.31 3.32 #3 Primary 3.90' 6.0" Vert. OrificelGrate C= 0.600 Qscarded OutFlow Max=0.00 cfs @ 11.56 hrs HW=0.01' (Free Discharge) 1=Exfiltration (Exfiitration Controls 0.00 cfs) Primary OutFlow Max=0.00 cfs @ 0.00 hrs HW=0.00' (Free Discharge) t2=B road-C rested Rectangular Weir( Controls 0.00 cfs) 3=Orifice/Grate ( Controls 0.00 cfs) 2014-014 West Mills 2014-014 Hydrology-As Built Type 11124-hr 10-Yr Rainfall=4.80" Prepared by Design Consultants, Inc. Printed 10/8/2014 H droCAD®9.10 sln 00884 ©2010 H droCAD Software Solutions LL.0 Page 10 Time span=0.00-48.00 hrs, dt=0.01 hrs, 4801 points Runoff by SCS TR-20 method, UH=SCS Reach routing by Stor-Ind+Trans method - Pond routing by Stor-Ind method SubcatchmentOS: Existing Runoff Area=46,016 sf 36.00% Impervious Runoff Depth=2.99" Tc=5.0 min CN=83 Runoff=3.83 cfs 0.264 of SubcatchmentlS: Proposed Parking 1 Runoff Area=1,159 sf 0.00% impervious Runoff Depth=2.29" Tc=5.0 min CN=75 Runoff=0.07 cfs 0.005 of Subcatchment2S: Proposed Parking 2 Runoff Area=1,335 sf 0.00% Impervious Runoff Depth=2.29" Tc=5.0 min CN=75 Runoff=0.08 cfs 0.006 of Subcatchment3S: Proposed Parking 3&4 Runoff Area=1,963 sf 0.00% Impervious Runoff Depth=2.29" Tc=5.0 min CN=75 Runoff=0.12 cfs 0.009 of Subcatchment4S: Proposed Parking 5&6 Runoff Area=997 sf 0.00% Impervious Runoff Depth=2.29" Tc=5.0 min CN=75 Runoff=0.06 cfs 0,004 of Subcatchment5S: Proposed Parking 7 Runoff Area=696 sf 0.00% Impervious Runoff Depth=2.21" Tc=5.0 min CN=74 Runoff=0.04 cfs 0.003 of Subcatchment6S: Overland Plow Runoff Area=40,132 sf 41.27% Impervious Runoff Depth=3.09" Tc=5.0 min CN=84 Runoff=3.44 cfs 0.237 of Reach 1 R: Proposed Inflow=3.50 cfs 0.247 of Outflow=3.50 cfs 0.247 of Pond 1P: Rain Garden 1 Peak Elev=2.35' Storage=155 cf Inflow=0.07 cfs 0.005 of Discarded=0.00 cfs 0.004 of Primary=0.00 cfs 0.000 of Outflow=0.00 cfs 0.004 of Pond 2P: Rain Garden 2 Peak Elev=3.50' Storage=194 cf Inflow=0.08 cfs 0.006 of Discarded=0.00 cfs 0.002 of Primary=0.00 cfs 0.000 of Outflow=0.00 cfs 0.002 of Pond 3P: Ram Garden 3 Peak Elev=3.12` Storage=136 cf Inflow=0.12 cfs 0.009 of Discarded=0.00 cfs 0.002 of Primary=0.06 cfs 0.005 of Outflow=0.06 cfs 0.007 of Pond 5P: Rain Garden 5 Peak Elev=3.62' Storage=107 cf Inflow=0.04 cfs 0.003 of Discarded=0.00 cfs 0.001 of Primary=0.00 cfs 0.000 of Outflow=0.00 cfs 0.001 of 2014-014 West Mills 2014-014 Hydrology-As Built Type /// 24-hr 10-Yr Rainfall=4.80" Prepared by Design Consultants, Inc. Printed 10/8/2014 H droCAD®9.10 s/n 00884 ®2010 H droCAD Software Solutions LLC Page 11 Summary for Subcatchment 0S: Existing Runoff - 3.83 cfs @ 12.07 hrs, Volume= 0.264 af, Depth= 2.99" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Type lit 24-hr 10-Yr Rainfall=4.80" Area �sD CN Description 16,664 98 Paved parking, HSG C 29,452 74 >75% Grass cover, Good HSG C 46,016 83 Weighted Average 29,452 64.00% Pervious Area 16,564 36.00% Impervious Area Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs) 5.0 Direct Entry, Direct Entry Summary for Subcatchment 1S: Proposed Parking 1 Runoff - 0.07 cfs @ 12.08 hrs, Volume= 0.005 af, Depth= 2.29" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Type Ill 24-hr 10-Yr Rainfall=4.80" Area (so CN Description 810 75 Porous Pavers 349 74 >75% Grass cover, Good HSG C 1,159 75 Weighted Average 1,159 100.00% Pervious Area Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs) 5.0 Direct Entry, Dirent Entry Summary for Subcatchment 2S: Proposed Parking 2 Runoff -- 0.08 cfs @ 12.08 hrs, Volume= 0.006 af, Depth= 2.29" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Type III 24-hr 10-Yr Rainfall=4.80" Area (so CN Description 969 75 Porous Pavers 366 74 >75% Grass cover, Good HSG C 1,335 75 Weighted Average 1,335 100.00% Pervious Area 2014-014 West Mills 2014-014 Hydrology-As Built Type /// 24-hr 10-Yr Rainfall=4.80" Prepared by Design Consultants, Inc. Printed 10/8/2014 HydroCAD09.10 s/n 00884 02010 HydroCAD Software Solutions LLC Page 12 Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs) 5.0 Direct Entry, Dirent Entry Summary for Subcatchment 3S: Proposed Parking 3&4 Runoff = 0.12 cfs @ 12.08 hrs, Volume= 0.009 af, Depth= 2.29" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Type III 24-hr 10-Yr Rainfall=4.80" Area (sf) CN ❑escription 642 75 Porous Pavers 645 75 Porous Pavers 676 74 >75% Grass cover, Good, HSG C 1,963 75 Weighted Average 1,963 100,00% Pervious Area Tc Length Slope Velocity Capacity Description (min) (feet) (ftlft) (ft/sec) (cfs) 5.0 Direct Entry, Dirent Entry Summary for Subcatchment 4S: Proposed Parking 5&6 Runoff _ 0.06 cfs @ 12.08 hrs, Volume= 0.004 af, Depth= 2.29" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Type ill 24-hr 10-Yr Rainfall=4.80" Area (so CN Description 997 75 Porous Pavers 997 100.00% Pervious Area Tc Length Slope Velocity Capacity Description (min) (feet) (fUft) (ft/sec) (cfs) 5.0 Direct Entry, Dirent Entry Summary for Subcatchment 5S: Proposed Parking 7 Runoff = 0.04 cfs @ 12.08 hrs, Volume= 0.003 af, Depth= 2.21" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Type Ili 24-hr 10-Yr Rainfall=4.80° 2014-014 West Mills 2014-014 Hydrology-As Built Type 11124-hr 10-Yr Rainfall=4.80" Prepared by Design Consultants, Inc. Printed 1 01812 0 1 4 HydroCAD®9.10 sln 00884 0 2010 HydroCAD Software Solutions LLC Page 13 Area (sD CN Description 323 75 Porous Pavers 373 74 >75% Grass cover, Good, HSG C 696 74 Weighted Average 696 100.00% Pervious Area Tc Length Slope Velocity Capacity Description min feet ft/ft ft/sec cfs 5.0 Direct Entry, Dirent Entry Summary for Subcatchment 6S: Overland Flow Runoff = 3.44 cfs @ 12.07 hrs, Volume= 0.237 af, Depth= 3.09" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Type III 24-hr 10-Yr Rainfall=4.80" Area_(so _ CN Description 16,564 98 Paved parking, HSG C 23,568 74 >75% Grass cover, Good, HSG C 40,132 84 Weighted Average 23,568 58.73% Pervious Area 16,564 41.27% Impervious Area Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (fUsec) (cfs) 5.0 Direct Entry, Direct Entry Summary for Reach 1 R: Proposed Inflow Area = 1.062 ac, 35.79% Impervious, Inflow Depth = 2.79" for 10-Yr event Inflow = 3.50 cfs @ 12.07 hrs, Volume= 0.247 of Outflow = 3.50 cfs @ 12.07 hrs, Volume= 0.247 af, Atten= 0%, Lag= 0.0 min Routing by Stor-Ind+Trans method, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Summary for Pond 1 P: Rain Garden 1 Inflow Area = 0.027 ac, 0.00% Impervious, Inflow Depth = 2.29" for 10-Yr event Inflow = 0.07 cfs @ 12.08 hrs, Volume= 0.005 of Outflow 0.00 cfs @ 10.63 hrs, Volume= 0.004 af, Atten= 98%, Lag= 0.0 min Discarded = 0.00 cfs @ 10.63 hrs, Volume= 0.004 of Primary - 0.00 cfs @ 0.00 hrs, Volume= 0.000 of Routing by Stor-Ind method, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Peak Elev= 2.35' @ 19.71 hrs Surf.Area= 220 sf Storage= 155 cf Plug-Flow detention time= 968.1 min calculated for 0.004 of(85% of inflow) Center-of-Mass det. time= 903.3 min ( 1,739.2 - 835.9 ) 2014-014 West Mills 2014-014 Hydrology-As Built Type 11124-hr 10-Yr Rainfall=4.80" Prepared by Design Consultants, Inc. Printed 10/8/2014 HydroCAD®9.10 sin 00884 02010 HydroCAD Software Solutions LLC Paste 14 Volume Invert Avail.Stora e Storage Description #1 0.00, 236 cf Custom Stage Data (Prismatic)Listed below (Recalc) Elevation Surf.Area Voids Inc.Store Cum.Store (feet) (sq-ft) (%) (cubic-feet) (cubic-feet) 0.00 220 0.0 0 0 0.01 220 30.0 1 1 1.00 220 30.0 65 66 2.00 220 30.0 66 132 3.00 220 30.0 66 198 3.01 15 100.0 1 199 3.32 220 100.0 36 236 Device Routing Invert Outlet Devices #1 Discarded 0.00' 0.270 in/hr Exfiltration over Surface area #2 Primary 3.15' 5.0' long x 1.0' breadth Broad-Crested Rectangular Weir Head (feet) 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.50 3.00 Coef. (English) 2.69 2.72 2.75 2.85 2.98 3.08 3.20 3.28 3.31 3.30 3.31 3.32 #3 Primary 3.25' 6.0" Horiz. Orifice/Grate C= 0.600 Limited to weir flow at low heads Qscarded OutFlow Max=0.00 cfs @ 10.63 hrs HW=0.01' (Free Discharge) 1=Exfiltration (Exfiltration Controls 0.00 cfs) Primary OutFlow Max=0.00 cfs @ 0.00 hrs HW=0.00' (Free Discharge) t2=Broad-C rested Rectangular Weir( Controls 0.00 cfs) 3=Orifice/Grate ( Controls 0.00 cfs) Summary for Pond 2P: Rain Garden 2 Inflow Area = 0.031 ac, 0.00% impervious, Inflow Depth = 2.29" for 10-Yr event Inflow = 0.08 cfs @ 12.08 hrs, Volume= 0.006 of Outflow = 0.00 cfs @ 17.26 hrs, Volume= 0.002 af, Atten= 97%, Lag= 311.3 min Discarded = 0.00 cfs @ 10.32 hrs, Volume= 0.002 of Primary 0.00 cfs @ 17.26 hrs, Volume= 0.000 of Routing by Stor-Ind method, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Peak Elev= 3.50' @ 17.26 hrs Surf.Area= 129 sf Storage= 194 cf Plug-Flow detention time= 751.0 min calculated for 0.002 of(40% of inflow) Center-of-Mass det. time= 626.0 min ( 1,461.9 - 835.9 ) Volume Invert Avail.Storage Storage Description #1 0.00, 224 cf Custom Stage Data (Prismatic)Listed below (Recalc) 2014-014 West Mills 2014-014 Hydrology-As Built Type 1/124-hr 10-Yr Rainfall=4.80" Prepared by Design Consultants, Inc. Printed 10/8/2014 H droCADO 9.10 s/n 00884 O 2010 H droCAD Software Solutions LLC Page 15 Elevation Surf.Area Voids Inc.Store Cum.Store (feet) (sg-ft) (%) (cubic-feet) (cubic-feet) 0.00 175 0.0 0 0 0.01 175 30.0 1 1 1.00 175 30.0 52 53 2.00 175 30.0 53 105 3.00 175 30.0 53 158 3.01 15 100.0 1 158 3.70 175 100.0 66 224 Device Routing Invert Outlet Devices #1 Discarded 0.00' 0.270 in/hr Exfiltration over Surface area #2 Primary 3.50' 5.0' long x 1.0' breadth Broad-Crested Rectangular Weir Head (feet) 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.50 3.00 Coef. (English) 2.69 2.72 2.75 2.85 2.98 3.08 3.20 3.28 3.31 3.30 3.31 3.32 #3 Primary 3.60' 6.0" Horiz. Orifice/Grate C= 0.600 Limited to weir flow at low heads �iscarded OutFlow Max=0.00 cfs @ 10.32 hrs HW=0.01' (Free Discharge) 1=Exfiitration (Exfiltration Controls 0.00 cfs) Primary OutFlow Max=0.00 cfs @ 17.26 hrs HW=3.50' (Free Discharge) N=Broad-Crested Rectangular Weir(Weir Controls 0.00 cfs @ 0.08 fps) =Orifice/Grate ( Controls 0.00 cfs) Summary for Pond 3P: Rain Garden 3 Inflow Area = 0.045 ac, 0.00% Impervious, Inflow Depth = 2.29" for 10-Yr event Inflow = 0.12 cfs @ 12.08 hrs, Volume= 0.009 of Outflow 0.06 cfs @ 12.24 hrs, Volume= 0.007 af, Atten= 51%, Lag= 9.6 min Discarded = 0.00 cfs @ 9.88 hrs, Volume= 0.002 of Primary = 0.06 cfs @ 12.24 hrs, Volume= 0.005 of Routing by Stor-Ind method, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Peak Elev= 3.12' @ 12.24 hrs Surf.Area= 67 sf Storage= 136 cf Plug-Flow detention time= 439.5 min calculated for 0.007 of(82% of inflow) Center-of-Mass det. time= 365.3 min ( 1,201.2 - 835.9 ) Volume Invert Avail.Stora a Storage Description #1 0.00, 156 cf Custom Stage Data (Prismatic)Listed below (Recalc) 2014-014 West Mills 2014-014 Hydrology-As Built Type U124-hr 10-Yr Rainfall=4.80" Prepared by Design Consultants, Inc. Printed 10/8/2014 H droCADO 9.10 sln 00884 0 2010 H droCAD Software Solutions LL.0 Page 16 Elevation Surf.Area Voids Inc.Store Cum.Store (feet) (sq-ft) (%) (cubic-feet) (cubic-feet) 0.00 145 0.0 0 0 0.01 145 30.0 0 0 1.00 145 30.0 43 44 2.00 145 30.0 44 87 3.00 145 30.0 44 131 3.01 20 100.0 1 131 3.31 145 100.0 25 156 Device Routing Invert Outlet Devices #1 Discarded 0.00' 0.270 in/hr Exfiltration over Surface area #2 Primary 3.10' 5.0' long x 1.0' breadth Broad-Crested Rectangular Weir Head (feet) 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.50 3.00 Coef. (English) 2.69 2.72 2.75 2.85 2.98 3.08 3.20 3.28 3.31 3.30 3.31 3.32 #3 Primary 3.20' 6.0" Vert. OrificelGrate C= 0.600 Qiscarded OutFlow Max=0.00 cfs @ 9.88 hrs HW=0.01' (Free Discharge) 1=Exfiltration (Exfiltration Controls 0.00 cfs) Primary OutFlow Max=0.05 cfs @ 12.24 hrs HW=3.12' (Free Discharge) t2=Broad-Crested Rectangular Weir(Weir Controls 0.05 cfs @ 0.42 fps) 3=Orifice/G rate ( Controls 0.00 cfs) Summary for Pond 5P: Rain Garden 5 Inflow Area = 0.016 ac, 0.00% Impervious, Inflow Depth = 2.21" for 10-Yr event Inflow 0.04 cfs @ 12.08 hrs, Volume= 0.003 of Outflow = 0.00 cfs @ 10.45 hrs, Volume= 0.001 af, Atten= 99%, L.ag= 0.0 min Discarded = 0.00 cfs @ 10.45 hrs, Volume= 0.001 of Primary - 0.00 cfs @ 0.00 hrs, Volume= 0.000 of Routing by Star-Ind method, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Peak Elev= 3.62' @ 24.05 hrs Surf.Area= 63 sf Storage= 107 cf Plug-Flow detention time= 823.4 min calculated for 0.001 of(34% of inflow) Center-of-Mass det. time= 693.3 min ( 1,531.8-838.5 ) Volume Invert Avail.Stora e Storage Description #1 0.00, 136 cf Custom Stage Data (Prismatic)Listed below (Recalc) 2014-014 West Mills 2014-014 Hydrology-As Built Type I1124-hr 10-Yr Rainfall=4.80" Prepared by Design Consultants, Inc. Printed 10/8/2014 HydroCADO 9.10 sln 00884 ©2010 HydroCAD Software Solutions LLC Page 17 Elevation Surf.Area Voids Inc.Store Cum.Store (feet) (sq-ft) (%) (cubic-feet) (cubic-feet) 0.00 92 0.0 0 0 0.01 92 30.0 0 0 1.00 92 30.0 27 28 2.00 92 30.0 28 55 3.00 92 30.0 28 83 3.01 15 100.0 1 83 4.00 92 100.0 53 136 Device Routing Invert Outlet Devices #1 Discarded 0,00' 0.270 inlhr Exfiltration over Surface area #2 Primary 3.80' 5.0' long x 1.0' breadth Broad-Crested Rectangular Weir Head (feet) 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.50 3.00 Coef. (English) 2.69 2.72 2.75 2.85 2.98 3.08 3.20 3.28 3.31 3.30 3.31 3.32 #3 Primary 3.90' 6.0" Vert. Orifice/Grate C= 0.600 Qiscarded OutFlow Max=0.00 cfs @ 10.45 hrs HW=0.01' (Free Discharge) 1=Exfiltration (Exfiltration Controls 0.00 cfs) Primary OutFlow Max=0.00 cfs @ 0.00 hrs HW=0.00' (Free Discharge) 2=Broad-Crested Rectangular Weir( Controls 0.00 cfs) 13=0 rif ice/G rate ( Controls 0.00 cfs) 2014-014 West Mills 2014-014 Hydrology-As Built Type 11124-hr 25-Yr Rainfall=6.00" Prepared by Design Consultants, Inc. Printed 10/8/2014 Hy droCAD®9.10 s/n 00884 02010 H droCAD Software Solutions LLC Pa e 18 Time span=0.00-48.00 hrs, dt=0.01 hrs, 4801 points Runoff by SCS TR-20 method, UH=SCS Reach routing by Stor-Ind+Trans method - Pond routing by Star-Ind method SubeatchmentOS: Existing Runoff Area=46,016 sf 36.00% Impervious Runoff Depth=4.09" Tc=5.0 min CN=83 Runoff=5.19 cfs 0.360 of SubcatchmentIS: Proposed Parking 1 Runoff Area=1,159 sf 0.00% Impervious Runoff Depth=3.28" Tc=5.0 min CN=75 Runoff=0.11 cfs 0.007 of Subcatchment2S: Proposed Parking 2 Runoff Area=1,335 sf 0.00% Impervious Runoff Depth=3.28" Tc=5.0 min CN=75 Runoff=0.12 cfs 0.008 of Subcatchment3S: Proposed Parking 3&4 Runoff Area=1,963 sf 0.00% Impervious Runoff Depth=3.28" Tc=5.0 min CN=75 Runoff=0.18 cfs 0.012 of Subcatchment4S: Proposed Parking 5&6 Runoff Area=997 sf 0.00% Impervious Runoff Depth=3.28" Tc=5.0 min CN=75 Runoff=0.09 cfs 0.006 of Subcatchment5S: Proposed Parking 7 Runoff Area=696 sf 0.00% Impervious Runoff Depth=3.18" Tc=5.0 min CN=74 Runoff=0.06 cfs 0.004 of Subcatchment6S: Overland Flow Runoff Area=40,132 sf 41.27% Impervious Runoff Depth=4.20" Tc=5.0 min CN=84 Runoff=4.63 cfs 0.322 of Reach 1 R: Proposed Inflow=4.91 cfs 0.342 of Outflow=4.91 cfs 0,342 of Pond 1P: Rain Garden 1 Peak Elev=3.15' Storage=208 cf Inflow=0.11 cfs 0.007 of Discarded=0.00 cfs 0.003 of Primary=0.01 cfs 0.001 of Outflow=0.01 cfs 0.005 of Pond 2P: Rain Garden 2 Peak Elev=3.51' Storage=195 cf Inflow=0.12 cfs 0.008 of Discarded=0.00 cfs 0.002 of Primary=0,01 cfs 0.003 of Outflow=0.01 cfs 0,005 of Pond 3P: Rain Garden 3 Peak Elev=3.16' Storage=139 cf Inflow=0.18 cfs 0.012 of Discarded=0.00 cfs 0.002 of Primary=0.18 cfs 0.009 of Outflow=0.18 cfs 0.011 of Pond 5P: Rain Garden 5 Peak Elev=3.80' Storage=120 cf Inflow=0.06 cfs 0.004 of Discarded=0.00 cfs 0.001 of Primary=0.00 cfs 0,001 of Outflow=0.00 cfs 0.002 of 2014-014 West Mills 2014-014 Hydrology-As Built Type 11i 24-hr 25-Yr Rainfall=6.00" Prepared by Design Consultants, Inc. Printed 10/8/2014 H droCAD®9.10 s/n 00884 02010 H droCAD Software Solutions LLC Page 19 Summary for Subcatchment 0S: Existing Runoff = 5.19 cfs @ 12.07 hrs, Volume= 0.360 af, Depth= 4.09" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Type III 24-hr 25-Yr Rainfall=6.00" Area (so CN Description 16,564 98 Paved parking, HSG C 29,452 74 >75% Grass cover, Good HSG C 46,016 83 Weighted Average 29,452 64.00% Pervious Area 16,564 36.00% Impervious Area Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs) 5.0 Direct Entry, Direct Entry Summary for Subcatchment 1 S: Proposed Parking 1 Runoff = 0.11 cfs @ 12.08 hrs, Volume= 0.007 af, Depth= 3.28" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Type III 24-hr 25-Yr Rainfall=6.00" Areas CN Description 810 75 Porous Pavers 349 74 >75% Grass cover, Good HSG C 1,159 75 Weighted Average 1,159 100.00% Pervious Area Tc Length Slope Velocity Capacity Description min feet ft/ft) (ft/sec) (cfs 5.0 Direct Entry, Dirent Entry Summary for Subcatchment 2S: Proposed Parking 2 Runoff = 0.12 cfs @ 12.08 hrs, Volume= 0,008 af, Depth= 3.28" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Type III 24-hr 25-Yr Rainfall=6.00" _...__..._Area„(sD CN Description 969 75 Porous Pavers 366 74 >75% Grass cover, Good HSG C 1,335 75 Weighted Average 1,335 100.00% Pervious Area 2014-014 West Mills 2014-014 Hydrology-As Built Type I1124-hr 25-Yr Rainfall=6.00" Prepared by Design Consultants, Inc. Printed 1 0/812 0 1 4 HydroCAD®9.10 s/n 00884 O 2010 HydroCAD Software Solutions LLC Pace 20 Tc Length Slope Velocity Capacity Description min feet ft/ft) (ft/sec) (cfs 5.0 Direct Entry, Dirent Entry Summary for Subcatchment 3S: Proposed Parking 3&4 Runoff = 0.18 cfs @ 12.08 hrs, Volume= 0.012 af, Depth= 3.28" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Type ]]I 24-hr 25-Yr Rainfall=6.00" Area (so CN Description 642 75 Porous Pavers 645 75 Porous Pavers 676 74 >75% Grass cover, Good HSG C 1,963 75 Weighted Average 1,963 100,00% Pervious Area Tc Length Slope Velocity Capacity Description min feet ft/ft) (ft/sec) (cfs 5.0 Direct Entry, Dirent Entry Summary for Subcatchment 4S: Proposed Parking 5&6 Runoff - 0.09 cfs @ 12.08 hrs, Volume= 0.006 af, Depth= 3.28" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Type III 24-hr 25-Yr Rainfall=6.00" Area (so CN Description 997 75 Porous Pavers 997 100.00% Pervious Area Tc Length Slope Velocity Capacity Description min feet ft/ft ftlsec cfs 5.0 Direct Entry, Dirent Entry Summary for Subcatchment 5S: Proposed Parking 7 Runoff - 0.06 cfs @ 12.08 hrs, Volume= 0.004 af, Depth= 3.18" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Type III 24-hr 25-Yr Rainfall=6.00" 2014-014 West Mills 2014-014 Hydrology-As Built Type 1// 24-hr 25-Yr Rainfall=6.00° Prepared by Design Consultants, Inc. Printed 10/8/2014 HydroCAD®9.10 sln 00884 ©2010 HydroCAD Software Solutions L.L.0 Page 21 Area (so CN Description 323 75 Porous Pavers 373 74 >75% Grass cover, Good, HSG C 696 74 Weighted Average 696 100.00% Pervious Area Te Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs) 5.0 Direct Entry, Dirent Entry Summary for Subcatchment 6S: Overland Flow Runoff - 4.63 cfs @ 12.07 hrs, Volume= 0.322 af, Depth= 4.20" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Type III 24-hr 25-Yr Rainfall=6.00" Area (sD CN Description 16,564 98 Paved parking, HSG C 23,568 74 >75% Grass cover, Good, HSG C 40,132 84 Weighted Average 23,568 58.73% Pervious Area 16,564 41.27% Impervious Area Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs) 5.0 Direct Entry, Direct Entry Summary for Reach 1 R: Proposed Inflow Area = 1.062 ac, 35.79% Impervious, Inflow Depth = 3.86" for 25-Yr event Inflow 4.91 cfs @ 12.07 hrs, Volume= 0.342 of Outflow -- 4.91 cfs @ 12.07 hrs, Volume= 0.342 af, Atten= 0%, Lag= 0.0 min Routing by Stor-Ind+Trans method, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Summary for Pond 1 P: Rain Garden 1 Inflow Area = 0.027 ac, 0.00% Impervious, Inflow Depth = 3.28" for 25-Yr event Inflow = 0.11 cfs @ 12.08 hrs, Volurne= 0.007 of Outflow - 0.01 cfs @ 14.44 hrs, Volume= 0.005 af, Atten= 94%, L_ag= 141.9 min Discarded = 0.00 cfs @ 9,87 hrs, Volume= 0.003 of Primary = 0.01 cfs @ 14.44 hrs, Volume= 0.001 of Routing by Stor-Ind method, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Peak Elev= 3.15` @ 14.44 hrs Surf.Area= 111 sf Storage= 208 cf Plug-Flow detention time= 871.2 min calculated for 0.005 of(64% of inflow) Center-of-Mass det. time= 766.7 min ( 1,592.2 - 825.5 ) 2014-014 West Mills 2014-014 Hydrology-As Built Type 11124-hr 25-Yr Rainfall=6.00" Prepared by Design Consultants, Inc. Printed 10/8/2014 HydroCADO 9.10 s/n 00884 0 2010 HydroCAD Software Solutions LLC Page 22 Volume Invert Avail.Stora a Storage Description #1 0.00' 236 cf Custom Stage Data (Prismatic)Listed below (Recalc) Elevation Surf.Area Voids Inc.Store Cum.Store (feet) (sq-ft) (%) (cubic-feet) (cubic-feet) 0.00 220 0.0 0 0 0.01 220 30.0 1 1 1.00 220 30.0 65 66 2.00 220 30.0 66 132 3.00 220 30.0 66 198 3.01 15 100.0 1 199 3.32 220 100.0 36 236 Device Routing Invert Outlet Devices #1 Discarded 0.00' 0.270 inlhr Exfiltration over Surface area #2 Primary 3.15' 5.0' long x 1.0' breadth Broad-Crested Rectangular Weir Head (feet) 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.50 3.00 Coef. (English) 2.69 2.72 2.75 2.85 2.98 3.08 3.20 3.28 3.31 3.30 3.31 3.32 #3 Primary 3.25' 6.0" Horiz. Orifice/Grate C= 0.600 Limited to weir flow at low heads piscarded OutFlow Max=0.00 cfs @ 9.87 hrs HW=0.01' (Free Discharge) "-I=Exfiltration (Exfiltration Controls 0.00 cfs) Primary OutFlow Max=0.00 cfs @ 14.44 hrs HW=3.15' (Free Discharge) �2=B road-Crested Rectangular Weir(Weir Controls 0.00 cfs @ 0.18 fps) 3=Orifice/Grate ( Controls 0.00 cfs) Summary for Pond 2P: Rain Garden 2 Inflow Area = 0.031 ac, 0.00% Impervious, Inflow Depth = 3.28" for 25-Yr event Inflow = 0.12 cfs @ 12.08 hrs, Volume= 0.008 of Outflow = 0.01 cfs @ 12.74 hrs, Volume= 0.005 af, Atten= 88%, Lag=40.1 min Discarded = 0.00 cfs @ 9.48 hrs, Volume= 0.002 of Primary = 0.01 cfs @ 12.74 hrs, Volume= 0.003 of Routing by Star-Ind method, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Peak Elev= 3.51' @ 12.74 hrs Surf.Area= 131 sf Storage= 195 cf Plug-Flow detention time= 449.9 min calculated for 0.005 of(58% of inflow) Center-of-Mass det. time= 340.2 min ( 1,165.7- 825.5 ) Volume Invert Avail.Storage Storage Description #1 0.00, 224 cf Custom Stage Data (Prismatic)Listed below (Recalc) 2014-014 West Mills 2014-014 Hydrology-As Built Type III 24-hr25-Yr Rainfall=6.00" Prepared by Design Consultants, Inc. Printed 10i812014 HydroCAD®9.10 sln 00884 ©2010 HydroCAD Software Solutions LLC Page 23_ Elevation Surf.Area Voids Inc.Store Cum.Store (feet) (sq-ft) (%) (cubic-feet) (cubic-feet) 0.00 175 0.0 0 0 0.01 175 30.0 1 1 1.00 175 30.0 52 53 2.00 175 30.0 53 105 3.00 175 30.0 53 158 3.01 15 100.0 1 158 3.70 175 100.0 66 224 Device Routing Invert Outlet Devices #1 Discarded 0.00' 0.270 inlhr Exfiltration over Surface area #2 Primary 3.50' 5.0' long x 1.0' breadth Broad-Crested Rectangular Weir Head (feet) 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.50 3.00 Coef. (English) 2.69 2.72 2.75 2.85 2.98 3.08 3.20 3.28 3.31 3.30 3.31 3.32 #3 Primary 3.60' 6.0" Horiz. Orifice/Grate C= 0.600 Limited to weir flow at low heads Discarded OutFlow Max=0.00 cfs @ 9.48 hrs HW=0.01' (Free Discharge) 1=Exfiltration (Exfiltration Controls 0.00 cfs) Primary OutFlow Max=0.01 cfs @ 12.74 hrs HW=3.51' (Free Discharge) 2=Broad-Crested Rectangular Weir(Weir Controls 0.01 cfs @ 0.25 fps) 13=Orifice/G rate ( Controls 0.00 cfs) Summary for Pond 3P: Rain Garden 3 Inflow Area = 0.045 ac, 0.00% Impervious, Inflow Depth = 3.28" for 25-Yr event Inflow - 0.18 cfs @ 12.08 hrs, Volume= 0.012 of Outflow - 0.18 cfs @ 12.08 hrs, Volume= 0.011 af, Atten= 0%, Lag= 0.3 min Discarded = 0.00 cfs @ 9.03 hrs, Volume= 0.002 of Primary = 0.18 cfs @ 12.08 hrs, Volume= 0.009 of Routing by Stor-Ind method, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Peak Elev= 3,16' @ 12.08 hrs Surf.Area= 80 sf Storage= 139 of Plug-Flow detention time= 299.7 min calculated for 0.011 of(87% of inflow) Center-of-Mass det. time= 242.0 min ( 1,067.5 - 825.5 ) Volume Invert Avail.Stora e Storage Description #1 0.00, 156 cf Custom Stage Data (Prismatic)Listed below (Recalc) 2014 014 West Mills 2014-014 Hydrology-As Built Type 11124-hr 25-Yr Rainfall=6.00" Prepared by Design Consultants, Inc. Printed 10/8/2014 H droCAD®9.10 s/n 00884 02010 H droCAD Software Solutions LLC Pacie 24 Elevation Surf.Area Voids Inc.Store Cum.Store (feet) (sq-ft) (%) (cubic-feet) (cubic-feet) 0.00 145 0.0 0 0 0.01 145 30.0 0 0 1.00 145 30.0 43 44 2.00 145 30.0 44 87 3.00 145 30.0 44 131 3.01 20 100.0 1 131 3.31 145 100.0 25 156 Device Routing Invert Outlet Devices #1 Discarded 0.00' 0.270 inlhr Exfiltration over Surface area #2 Primary 3.10' 5.0' long x 1.0' breadth Broad-Crested Rectangular Weir Head (feet) 0.20 0.40 0.60 0.80 1.00 1,20 1.40 1.60 1.80 2.00 2.50 3.00 Coef. (English) 2.69 2.72 2.75 2.85 2.98 3.08 3.20 3.28 3.31 3.30 3.31 3.32 #3 Primary 3.20' 6.0"Vert, Orifice/Grate C= 0.600 Trded OutFlow Max=0.00 cfs @ 9.03 hrs HW=0.01' (Free Discharge) Exfiltration (Exfiltration Controls 0.00 cfs) Primary OutFlow Max=0.17 cfs @ 12.08 hrs HW=3.16' (Free Discharge) 12=Broad-Crested Rectangular Weir(Weir Controls 0.17 cfs @ 0.63 fps) 3=0 rificeIG rate ( Controls 0.00 cfs) Summary for Pond 5P: Rain Garden 5 Inflow Area = 0.016 ac, 0.00% Impervious, Inflow Depth = 3.18" for 25-Yr event Inflow 0.06 cfs @ 12.08 hrs, Volume= 0.004 of Outflow = 0.00 cfs @ 13.94 hrs, Volume= 0.002 af, Atten= 94%, Lag= 111.9 min Discarded = 0.00 cfs @ 9.64 hrs, Volume= 0.001 of Primary - 0.00 cfs @ 13.94 hrs, Volume= 0.001 of Routing by Star-Ind method, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Peak Elev= 3.80' @ 13.94 hrs Surf.Area= 77 sf Storage= 120 cf Plug-Flow detention time= 624.7 min calculated for 0.002 of(51% of inflow) Center-of-Mass det. time= 509.0 min ( 1,336.8 - 827.9 ) Volume Invert Avail.Stora a Storage Description #1 0.00, 136 cf Custom Stage Data (Prismatic)Listed below (Recalc) 2014-014 West Mills 2014-014 Hydrology-As Built Type 11124-hr 25-Yr Rainfall=6.00" Prepared by Design Consultants, Inc. Printed 1 01812 0 1 4 H droCAD0 9.10 s/n 00884 ©2010 N droCAD Software Solutions LLC Page 25 Elevation Surf.Area Voids Inc.Store Cum.Store feet s -ft % cubic-feet cubic-feet 0.00 92 0.0 0 0 0.01 92 30.0 0 0 1.00 92 30.0 27 28 2.00 92 30.0 28 55 3.00 92 30.0 28 83 3.01 15 100.0 1 83 4.00 92 100.0 53 136 Device Routing_ Invert Outlet Devices #1 Discarded 0.00' 0.270 inlhr Exfiltration over Surface area #2 Primary 3.80' 6.0' long x 1.0' breadth Broad-Crested Rectangular Weir Head (feet) 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.50 3.00 Coef. (English) 2.69 2.72 2.75 2.85 2.98 3.08 3.20 3.28 3.31 3.30 3.31 3.32 #3 Primary 3.90' 6.0"Vert. Orifice/Grate C= 0.600 Qiscarded OutFlow Max=0.00 cfs @ 9.64 hrs HW=0.01' (Free Discharge) 1=Exfiltration (Exfiltration Controls 0.00 cfs) Primary OutFlow Max=0.00 cfs @ 13.94 hrs HW=3.80' (Free Discharge) �2=Broad-C rested Rectangular Weir(Weir Controls 0.00 cfs @ 0.11 fps) 3=Orifice/Grate ( Controls 0.00 cfs) 2014-014 West Mills 2014-014 Hydrology-As Built Type/1/ 24-hr 100-Yr Rainfall=8.60" Prepared by Design Consultants, Inc. Printed 10/8/2014 H droCAD®9.10 sln 00884 ©2010 H droCAD Software Solutions LLC Page 26 Time span=0.00-48.00 hrs, dt=0.01 hrs, 4801 points Runoff by SCS TR-20 method, UH=SCS Reach routing by Stor-Ind+Trans method - Pond routing by Stor-Ind method SubcatchmentOS: Existing Runoff Area=46,016 sf 36.00% Impervious Runoff Depth=6.55" Tc=5.0 min CN=83 Runoff=8.15 cfs 0.577 of SubcatchmentIS: Proposed Parking 1 Runoff Area=1,159 sf 0.00% Impervious Runoff Depth=5.59" Tc=5.0 min CN=75 Runoff=0.18 cfs 0.012 of Subcatchment2S: Proposed Parking 2 Runoff Area=1,335 sf 0.00% Impervious Runoff Depth=5.59" Tc=5.0 min CN=75 Runoff=021 cfs 0.014 of Subcatchment3S: Proposed Parking 3&4 Runoff Area=1,963 sf 0,00% Impervious Runoff Depth=5.59" Tc=5.0 min CN=75 Runoff=0.30 cfs 0.021 of Subeatchment4S: Proposed Parking 5&6 Runoff Area=997 sf 0.00% impervious Runoff Depth=5.59" Tc=5.0 min CN=75 Runoff=0.15 cfs 0.011 of Subcatchment5S: Proposed Parking 7 Runoff Area=696 sf 0,00% Impervious Runoff Depth=5.47" Tc=5.0 min CN=74 Runoff=0.11 cfs 0.007 of Subcatchment6S: Overland Flow Runoff Area=40,132 sf 41.27% Impervious Runoff Depth=6.67" Tc=5.0 min CN=84 Runoff=7.20 cfs 0.512 of Reach 1 R: Proposed lnflow=7.66 cfs 0.559 of Outflow=7.66 cfs 0.559 of Pond 1 P: Rain Garden 1 Peak Elev=3.19' Storage=212 cf Inflow=0.18 cfs 0.012 of Discarded=0.00 cfs 0.003 of Primary=0.09 cfs 0.006 of Outflow=0.09 cfs 0.010 of Pond 2P: Rain Garden 2 Peak Elev=3.56' Storage=201 cf Inflow=0.21 cfs 0,014 of Discarded=0.00 cfs 0.002 of Primary=0.19 cfs 0.009 of Outflow=0.19 cfs 0.011 of Pond 3P: Rain Garden 3 Peak Elev=3.18' Storage=141 cf Inflow=0.30 cfs 0.021 of Discarded=0.00 cfs 0.002 of Primary=0.30 cfs 0.017 of Outflow=0.30 cfs 0.019 of Pond 5P: Rain Garden 5 Peak Elev=3.83' Storage=122 cf Inflow=0.11 cfs 0.007 of Discarded=0.00 cfs 0.001 of Primary=0.06 cfs 0.004 of Outflow=0.06 cfs 0.005 of 2014-014 West Mills 2014-014 Hydrology-As Built Type 11124-hr 100-Yr Rainfall=8.60" Prepared by Design Consultants, Inc. Printed 10/8/2014 HydroCAD®9.10 s/n 00884 0 2010 HydroCAD Software Solutions LLC Page 27 Summary for Subcatchment OS: Existing Runoff - 8.15 cfs @ 12.07 hrs, Volume= 0.577 af, Depth= 6.55" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Type III 24-hr 100-Yr Rainfall=8.60" Area (sf) CN Description_ 16,564 98 Paved parking, HSG C 29,452 74 >75% Grass cover, Good, HSG C 46,016 83 Weighted Average 29,452 64.00% Pervious Area 16,564 36.00% Impervious Area Tc Length Slope Velocity Capacity Description (min) (feet) Oft) (ft/sec) (cfs) 5.0 Direct Entry, Direct Entry Summary for Subcatchment 1S: Proposed Parking 1 Runoff _ 0.18 cfs @ 12.07 hrs, Volume= 0.012 af, Depth= 5.59" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Type III 24-hr 100-Yr Rainfall=8.60" Area (so CN Description 810 75 Porous Pavers 349 74 >75% Grass cover, Good HSG C 1,159 75 Weighted Average 1,159 100.00% Pervious Area Tc Length Slope Velocity Capacity Description min feet ft/ft) (ft/sec) (cfs 5.0 Direct Entry, Dirent Entry Summary for Subcatchment 2S: Proposed Parking 2 Runoff - 0.21 cfs @ 12.07 hrs, Volume= 0.014 af, Depth= 5.59" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Type III 24-hr 100-Yr Rainfall=8.60" Area (so CN Description 969 75 Porous Pavers 366 74 >75% Grass cover, Good, HSG C 1,335 75 Weighted Average 1,335 100.00% Pervious Area 2014-014 West Mills 2014-014 Hydrology-As Built Type /it 24-hr 100-Yr Rainfall=8.60" Prepared by Design Consultants, Inc. Printed 10/8/2014 H droCAD®9.10 s/n 00884 0 2010 H droCAD Software Solutions LLC Pacie 28 Tc Length Slope Velocity Capacity Description min feet fUft ft/sec cfs 5.0 Direct Entry, Dirent Entry Summary for Subcatchment 3S: Proposed Parking 3&4 Runoff - 0.30 cfs @ 12.07 hrs, Volume= 0.021 af, Depth= 5.59" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Type III 24-hr 100-Yr Rainfall=8.60" Areas CN Description 642 75 Porous Pavers 645 75 Porous Pavers 676 74 >75% Grass cover, Good, HSG C 1,963 75 Weighted Average 1,963 100,00% Pervious Area Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs) 5.0 Direct Entry, Dirent Entry Summary for Subcatchment 4S: Proposed Parking 5&6 Runoff = 0.15 cfs @ 12.07 hrs, Volume= 0.011 af, Depth= 5.59" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Type III 24-hr 100-Yr Rainfall=8.60" Area (sf) CN Description 997 75 Porous Pavers 997 100.00% Pervious Area Tc Length Slope Velocity Capacity Description min feet ft/ft) (ft/sec) (cfs 5.0 Direct Entry, Dirent Entry Summary for Subcatchment SS: Proposed Parking 7 Runoff _ 0.11 cfs @ 12.07 hrs, Volume= 0.007 af, Depth= 5.47" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Type Ill 24-hr 100-Yr Rainfall=8.60" 2014-014 West Mills 2014-014 Hydrology-As Built Type 11124-hr 100-Yr Rainfall=8.60" Prepared by Design Consultants, Inc. Printed 10/8/2014 H droCADO 9.10 s/n 00884 ©2010 H droCAD Software Solutions LLC Page 29 Area (sf) CN Description 323 75 Porous Pavers 373 74 >75% Grass cover, Good HSG C 696 74 Weighted Average 696 100.00% Pervious Area Te Length Slope Velocity Capacity Description (min) (feet) (fUft) (ft/sec) (cfs) 5.0 Direct Entry, Dirent Entry Summary for Subcatchment 6S: Overland Flow Runoff = 7.20 cfs @ 12.07 hrs, Volume= 0.512 af, Depth= 6.67" Runoff by SCS TR-20 method, UH=SCS, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Type III 24-hr 100-Yr Rainfall=8.60" Areas CN Description 16,564 98 Paved parking, HSG C 23,568 74 >75% Grass cover, Good, HSG C 40,132 84 Weighted Average 23,568 58.73% Pervious Area 16,564 41.27% Impervious Area Tc Length Slope Velocity Capacity Description min feet ft/ft ftlsec cfs 5.0 Direct Entry, Direct Entry Summary for Reach 1 R: Proposed Inflow Area = 1.062 ac, 35.79% Impervious, Inflow Depth = 6.31° for 100-Yr event Inflow = 7.66 cfs @ 12.07 hrs, Volume= 0.559 of Outflow = 7.66 cfs @ 12.07 hrs, Volume= 0.559 af, Atten= 0%, Lag= 0.0 min Routing by Stor-Ind+Trans method, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Summary for Pond 1 P: Rain Garden 1 Inflow Area = 0.027 ac, 0.00% Impervious, Inflow Depth = 5.59" for 100-Yr event Inflow = 0.18 cfs @ 12.07 hrs, Volume= 0.012 of Outflow - 0.09 cfs @ 12.20 hrs, Volume= 0.010 af, Atten= 48%, Lag= 7.5 min Discarded = 0.00 cfs @ 8.54 hrs, Volume= 0.003 of Primary -- 0.09 cfs @ 12.20 hrs, Volume= 0.006 of Routing by Stor-Ind method, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Peak Elev= 3.19' @ 12.20 hrs Surf.Area= 131 sf Storage= 212 cf Plug-Flow detention time= 458.8 min calculated for 0.010 of(79% of inflow) Center-of-Mass det. time= 380.1 min ( 1,190.4 - 810.3 ) 2014-014 West Mills 2014-014 Hydrology-As Built Type III 24-hr 100-Yr Rainfall=8.60" Prepared by Design Consultants, Inc. Printed 1 01812 0 1 4 HydroCAD09.10 sln 00884 02010 HydroCAD Software Solutions LLC Page 30 Volume Invert Avail.Stora e Storage Description #1 0.00, 236 cf Custom Stage Data (Prismatic)Listed below (Recalc) Elevation Surf.Area Voids Inc.Store Cum.Store (feet) (sq-ft) M (cubic-feet) (cubic-feet) 0.00 220 0.0 0 0 0.01 220 30.0 1 1 1.00 220 30.0 65 66 2.00 220 30.0 66 132 3.00 220 30.0 66 198 3.01 15 100.0 1 199 3.32 220 100.0 36 236 Device Routing Invert Outlet Devices #1 Discarded 0.00' 0.270 inlhr Exfiltration over Surface area #2 Primary 3.15' 5.0' long x 1.0' breadth Broad-Crested Rectangular Weir Head (feet) 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.50 3.00 Coef. (English) 2.69 2.72 2.75 2.85 2.98 3.08 3.20 3.28 3.31 3.30 3.31 3.32 #3 Primary 3.25' 6.0" Horiz. Orifice/Grate C= 0.600 Limited to weir flow at low heads piscarded OutFlow Max=0.00 cfs @ 8.54 hrs HW=0.01' (Free Discharge) '"'--1=Bxfiltration (Exfiltration Controls 0.00 cfs) Primary OutFlow Max=0.09 cfs @ 12.20 hrs HW=3.19' (Free Discharge) �2=Broad-Crested Rectangular Weir(Weir Controls 0.09 cfs @ 0.51 fps) 3=OrificelGrate ( Controls 0.00 cfs) Summary for Pond 2P: Rain Garden 2 Inflow Area = 0,031 ac, 0.00% Impervious, Inflow Depth = 5.59" for 100-Yr event Inflow = 0.21 cfs @ 12.07 hrs, Volume= 0.014 of Outflow = 0.19 cfs @ 12.11 hrs, Volume= 0.011 af, Atten= 9%, Lag= 2.2 min Discarded = 0.00 cfs @ 8.16 hrs, Volume= 0.002 of Primary = 0.19 cfs @ 12.11 hrs, Volume= 0.009 of Routing by Stor-Ind method, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Peak Elev= 3.56' @ 12.11 hrs Surf.Area= 142 sf Storage= 201 cf Plug-Flow detention time= 247.0 rain calculated for 0.011 of(76% of inflow) Center-of-Mass det. time= 161.7 ruin ( 972.0- 810.3 ) Volume Invert Avail.Storage Storage Description #1 0.00, 224 cf Custom Stage Data (Prismatic)Listed below (Recalc) 2014-014 West Mills 2014-014 Hydrology-As Built Type !II 24-hr 100-Yr Rainfall=8.60" Prepared by Design Consultants, Inc. Printed 10/8/2014 H droCAD®9.10 sln 00884 ©2010 H droCAD Software Solutions LLC Page 31 Elevation Surf.Area Voids Inc.Store Cum.Store (feet) (sq-ft) (%) (cubic-feet) (cubic-feet) 0.00 175 0.0 0 0 0.01 175 30.0 1 1 1.00 175 30.0 52 53 2.00 175 30.0 53 105 3.00 175 30.0 53 158 3.01 15 100.0 1 158 3.70 175 100.0 66 224 Device Routing Invert Outlet Devices #1 Discarded 0.00' 0.270 in/hr Exfiltration over Surface area #2 Primary 3.50' 5.0' long x 1.0` breadth Broad-Crested Rectangular Weir Head (feet) 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.50 3.00 Coef. (English) 2.69 2.72 2.75 2.85 2.98 3.08 3.20 3.28 3.31 3.30 3.31 3.32 #3 Primary 3.60' 6.0" Horiz. Orifice/Grate C= 0.600 Limited to weir flow at low heads J?iscarded OutFlow Max=0.00 cfs @ 8.16 hrs HW=0.01' (Free Discharge) 1=Exfiltration (Exfiltration Controls 0.00 cfs) Primary OutFlow Max=0.18 cfs @ 12.11 hrs HW=3.56' (Free Discharge) T--2=Broad-Crested Rectangular Weir(Weir Controls 0.18 cfs @ 0.64 fps) 3=Orifice/Grate ( Controls 0.00 cfs) Summary for Pond 3P. Rain Garden 3 Inflow Area = 0.045 ac, 0.00% Impervious, Inflow Depth = 5.59" for 100-Yr event Inflow - 0.30 cfs @ 12.07 hrs, Volume= 0.021 of Outflow = 0.30 cfs @ 12.08 hrs, Volume= 0.019 af, Atten= 0%, Lag= 0.3 min Discarded = 0.00 cfs @ 7.59 hrs, Volume= 0.002 of Primary - 0.30 cfs @ 12.08 hrs, Volume= 0.017 of Routing by Stor-Ind method, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Peak Elev= 3.18' @ 12.08 hrs Surf.Area= 91 sf Storage= 141 cf Plug-Flow detention time= 178.9 min calculated for 0.019 of(93% of inflow) Center-of-Mass det. time 140.3 min ( 950.6- 810.3 ) Volume Invert Avail.Storage Storage Description #1 0.00, 156 cf Custom Stage Data (Prismatic)Listed below (Recalc) 2014-014 West Mills 2014-014 Hydrology-As Built Type 11124-hr 100-Yr Rainfall=8.60" Prepared by Design Consultants, Inc. Printed 10/8/2014 HydroCADO 9.10 s/n 00884 ©2010 HydroCAD Software Solutions LLC Page 32 Elevation Surf.Area Voids Inc.Store Cum.Store feet s -ft % cubic-feet cubic-feet 0.00 145 0.0 0 0 0.01 145 30.0 0 0 1.00 145 30.0 43 44 2.00 145 30.0 44 87 3.00 145 30.0 44 131 3.01 20 100.0 1 131 3.31 145 100.0 25 156 Device Routing Invert Outlet Devices #1 Discarded 0.00' 0.270 in/hr Exfiltration over Surface area #2 Primary 3.10' 5.0' long x 1.0' breadth Broad-Crested Rectangular Weir Head (feet) 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.50 3.00 Coef. (English) 2.69 2.72 2.75 2.86 2.98 3.08 3.20 3.28 3.31 3.30 3.31 3.32 #3 Primary 3.20' 6.0"Vert. Orifice/Grate C= 0.600 Discarded OutFlow Max=0.00 cfs @ 7.59 hrs HW=0.01' (Free Discharge) "L1=Exfiltration (Exfiltration Controls 0.00 cfs) Primary OutFlow Max=0.30 cfs @ 12.08 hrs HW=3.18' (Free Discharge) t:3=Orif 2=Broad-Crested Rectangular Weir(Weir Controls 0.30 cfs @ 0.76 fps) ice/Grate ( Controls 0.00 cfs) Summary for Pond 5P: Rain Garden 5 Inflow Area = 0.016 ac, 0.00% Impervious, Inflow Depth = 5.47" for 100-Yr event Inflow 0.11 cfs @ 12.07 hrs, Volume= 0.007 of Outflow = 0.06 cfs @ 12.17 hrs, Volume= 0.005 af, Atten= 40%, Lag= 6.0 min Discarded = 0.00 cfs @ 8.31 hrs, Volume= 0.001 of Primary - 0.06 cfs @ 12.17 hrs, Volume= 0.004 of Routing by Stor-Ind method, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs Peak Elev= 3.83' @ 12.17 hrs Surf.Area= 79 sf Storage= 122 cf Plug-Flow detention time= 317.0 min calculated for 0.005 of(72% of inflow) Center-of-Mass det. time= 225.6 min ( 1,038.0 - 812.4 ) Volume Invert Avail.Storage Storage Description _ #1 0.00, 136 cf Custom Stage Data (Prismatic)Listed below (Recalc) 2014-014 West Mills 2014-014 Hydrology-As Built Type 11124-hr 100-Yr Rainfall=8.60" Prepared by Design Consultants, Inc. Printed 1 01812 0 1 4 H droCADO 9.10 s/n 00884 ©2010 H droCAD Software Solutions LLC Pacie 33 Elevation Surf.Area Voids Inc.Store Cum.Store (feet) (sq-ft) (%) (cubic-feet) (cubic-feet) 0.00 92 0.0 0 0 0.01 92 30.0 0 0 1.00 92 30.0 27 28 2.00 92 30.0 28 55 3.00 92 30.0 28 83 3.01 15 100.0 1 83 4.00 92 100.0 53 136 Device Routing Invert Outlet Devices #1 Discarded 0.00' 0.270 inlhr Exfiltration over Surface area #2 Primary 3.80' 5.0' long x 1.0' breadth Broad-Crested Rectangular Weir Head (feet) 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.50 3.00 Coef. (English) 2.69 2.72 2.75 2.85 2.98 3.08 3.20 3.28 3.31 3.30 3.31 3.32 #3 Primary 3.90' 6.0" Vert. OrificelGrate C= 0.600 Iiscarded OutFlow Max=0.00 cfs @ 8.31 hrs HW=0.01' (Free Discharge) 1=Exfiltration (Exfiltration Controls 0.00 cfs) Primary OutFlow Max=0.06 cfs @ 12.17 hrs HW=3.83' (Free Discharge) 1:2=Broad-Crested Rectangular Weir(Weir Controls 0.06 cfs @ 0.45 fps) 3=OrificelGrate ( Controls 0.00 cfs) DCl Project No: 2014-014.00 Table 1 - Existing vs. Proposed vs. As-Built Summary Total Runoff Peak Discharges (cfs)and Volumes (af) Description Existing Conditions Proposed Conditions As-Built Condition Drainage Area 46,016 sf 46,016 sf 46,016 sf Peak Rate Peak Rate Peak Rate Storm Event (cfs) Volume (af) (cfs) Volume(af) (cfs) Volume(af) 2 Yr(3.2") U6 0.142 1.85 0.127 1.91 0.132 10 Yr(4.8") 3.83 0.264 3.38 0.233 3.50 0.247 25 Yr(6.0") 5.19 0.360 4.55 0.317 4.91 1 0.342 100 Yr(8.6") 8.15 0.577 7.08 0.507 7.66 0.559 R\2014 Projects\2014-014 High St-Water St N.Andnver\_"raInage\Ta6Ee 1-Exlsting-Proposed-As 6sil{.x#sx 10/8/2014 vs i t s ! 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Horsley Witten Group Sustainable Environmental Solutions 55 Dorrance Street,Sulte 403•Providence,R102903 , Phone-401-272-1717 • Fax-401-437-8368 •www.horstaywwen.com December 2, 2014 Ms. Lisa Eggleston, PE Eggleston Environmental 32 Old Framingham Road Unit 29 Sudbury, MA 01776 Via email Re: West Mill Redevelopment,4 High Street, North Andover Response to Peer Review Comments Dear Ms. Eggleston: On behalf of RCG, LLC,the Horsley Witten Group, Inc. (HW) is pleased to provide the following response to your engineering peer review letter dated October 30, 2014. We are submitting this response letter to address the comments described below. Revised plans, revised drainage calculations, and a soil evaluation summary by Alliance Environmental are enclosed. Stamped plans and a stamped drainage report will be submitted under separate cover. 1. In general, the proposed plan uses a low impact drainage design approach that maintains existing drainage patterns, mitigates runoff rates,promotes recharge and provides treatment of pavement runoff in a manner that is, with the exceptions noted herein, consistent with the Massachusetts Stormwater Standards and good design practice. RESPONSE: No response required. Z The project as proposed represents a net decrease in impervious area and thus qualifies as a redevelopment project under the Massachusetts Stormwater Standards and the North Andover Wetlands Bylaw.As such it is required to comply with Standards 2, 3 and the pretreatment and structural best management practice(BMP)requirements of Standards 4,5 and 6 only to the maximum extent practicable,provided it is demonstrated that all reasonable efforts have been made to meet the applicable standard fully. Redevelopment projects must fully comply with all other requirements of the Stormwater Management Standards and must improve existing conditions. 1 believe that the applicant needs to demonstrate mare fully why additional groundwater recharge is not feasible on the site, e.g. through the use of pervious pavers in some of the pedestrian walkways. The applicant also needs to demonstrate how the project improves existing conditions with respect to stormwater,particularly since it does represent a significant increase in pavement area and associated traffic volume on the site, with corresponding increases in traffic-related pollutants. RESPONSE:At the time of the submission the site soil evaluations had not been completed, therefore the soil conditions and estimated season high water table(ESHWT) had not been confirmed.Alliance Environmental completed soil evaluations on November 13, 2014,with results summarized in the enclosed report. Due to the poor soils found on site, depth to ESHWT,and the existing building slab to remain in place under much of the proposed parking lots and above the Ms. Lisa Eggleston, PE 2 of 8 West Mill Redevelopment Response to Review Comments existing twin 60-inch culvert,additional recharge on the site will be difficult to achieve. The revised design does allow infiltration at all bioretention systems except BIO 3,which will require an impermeable liner because of its location above the existing concrete slab to remain. All bioretention systems include an underdrain.The revised StormTech detention system includes an impermeable liner due to the limited depth to groundwater.The revised drainage calculations do not take credit for infiltration (conservative). Runoff from new pavement areas within the limit of work will be treated with vegetated low impact design (LID) BMPs, including pre-treatment,for advanced TSS removal to meet Massachusetts Stormwater Standards. Existing pavement areas currently discharging untreated stormwater runoff to Osgood Pond, including the parking/service area behind Building 5 and a significant portion of the loop road near High Street,will also receive advanced water quality filtration with new LID BMPs. 3. All pervious cover types should be modeled as "good"hydrologic condition in the HydroCAD analysis. Particularly for woods, there is no justification for the existing conditions being modeled as `fair"and the post-development conditions as "good". RESPONSE:Although we believe the proposed vegetated areas will be an improvement to the current conditions we have adjusted the cover types accordingly. 4. Subcatchments DA2a and DA2b in the hydrologic model are not shown on the drainage area map or referenced in the water quality calculations. It is not clear where these areas are on the site. RESPONSE:Subcatchments DA2a and DA2b have been added to the Drainage Map for clarification. See comment#5 below. 5. The calculations assume all of the runoff from the north driveway is routed to bioretention basin B104, while the grading plan suggests that much of the runoff will drain toward the smaller ;basin B108. RESPONSE: HW reviewed the grading plan and HydroCAD model. We believe this area is properly modeled in HydroCAD in accordance with the pp d grading plan. Most of the runoff will be directed to BIO 4. Subcatchments QA2b:` DA2a and DA2b have been broken out --- separately in both the Drainage Mapand bioretention sizing calculations to sD help clarify.See post-development Drainage Map enlargement to right. 8r b. It is not clear why DA4 is modeled as having unconnected pavement in the post-development analysis but not in the pre-development analysis. Ms. Lisa Eggleston, PE 3 of 8 West Mill Redevelopment Response to Review Comments RESPONSE: RA4 includes unconnected pavement in the post-development analysis because this subcatchment includes the recently constructed permeable paver parking spaces, most of which are designed to overflow to rain gardens.The drainage report was revised to conservatively add these paver areas as unconnected pavement with a runoff curve number of 98 in the pre-development analysis for consistency.See response to comment #14 regarding incorporation of the recent loop road improvements into the drainage calculations for this phase of the project. 7. The water quality calculations are based on runoff to all of the bioretention systems undergoing pretreatment in sediment forebays, however the plans do not show any forebays or other means of pretreatment for the five "Type 1"bioretention systems on the site. Without adequate pretreatment, the runoff from these five drainage areas would not meet the 80 percent TSS removal requirement of DEP Storm water Standard 4. RESPONSE: HW will be providing sediment forebays for the "Type 1"and "Type 2" bioretention systems.We are currently working on design of a low maintenance forebay system that will align with the client's maintenance schedule. Options being considered for"Type 1"are the installation of a reclaimed granite curb check dam within the bioretention areas in the location of the paved drainage flumes,or the use of a slightly modified version of the Rain Guardian (http://www.rainguardian.biz/). Final design details for all green infrastructure practices will be provided prior to construction. B. Based on the bioretention schedule on Sheet C-9,six of the eight proposed bioretention systems would have less than the minimum 24-inch soil media depth that the DEP Stormwater Handbook states is necessary to provide adequate filtration of runoff flow.Since the landscape plan calls for trees in the bioretention areas, the media depth should be at least 30 inches. RESPONSE:The previous proposed soil depths were based upon assumptions made during the initial design phase regarding the depth to groundwater and the existing pond outlet invert(Outlet 2). Alliance Environmental recently conducted site soil evaluations,as previously noted. Based on the results of the soil evaluations (enclosed), our design has been revised to provide the required soil depth (24 inches minimum, 30 inches for bioretention areas with trees) and the required 2'of separation from groundwater for all proposed bioretention systems. The overall limiting factor for the Lq"" bioretention soil depth and underdrain yrcotvecmu -�. . inverts is based upon the invert at the WMVO lk,45 :3 existing pond outlet(Outlet 2—see figure to MCA, right). We are proposing to lower this 4 ""�'n�-e AxTtlef �3 p p g icr�+�rFit� existing pond outlet invert by 0.9'to 74.35 to ' :�rrran7as. better provide groundwater separation and -- � '• �" '- �a��a,� required media depths at new stormwater BMPs as well as partially address construction precision and tolerances as noted in your comment#21.The surveyed pond water level is 71.7. We are currently in �^ the process of refining the bioretention _ - Ir rt .. 06TTCk Ms. Lisa Eggleston, PE 4 of 8 West Mill Redevelopment Response to Review Comments design and details, including sediment forebays,soil depths,tree plantings, and underdrain configuration. Final design details for all green infrastructure practices will be provided prior to construction. 9. A bioretention soil mix consistent with that called for in the DEP Stormwater Handbook should be specified for both types of bioretention systems. The Type 2 bioretention system detail on Sheet C-9 specifies that the filter media be "approved native backfill; and the Type 1 system detail references a note that i was unable to find on the plan. The specified soil mix should also be consistent with the exfiltration rate assumed in the model. RESPONSE: Based upon our experience as well as the most current data and research,the bioretention soil mix specified has a larger sand content to avoid clogging from finer material found in silt.The bioretention details have been revised for consistency and additional soil mix information added to the plans on Sheet C-9. 10. The design detail for the Type 2 bioretention systems should specify the depth of the concrete forebays and the elevation of the metal spillways that separate the forebays from main part of the bioretention cell. l presume that this is not the some as the spillway elevations listed in the bioretention schedule, since that elevation would preclude any flaw into the basins. The forebay locations should also be identified on the grading and drainage plan, since two of the systems(BIO S and BiO 6)would receive sheet flow from both sides and the flow to BiO 3 would only come from one direction. RESPONSE:The"Type 2" bioretention "spillway"was mislabeled in the detail and has been re- labeled as a check dam.This is not the same as the spillway elevations listed in the bloretention schedule,which refer to the emergency spillway/overflows. Similar to the "Type 1"forebays,the final details and forebay materials are in the process of being refined. Final design details for all the green infrastructure practices will be provided prior to construction.The forebays have been labeled on the plans as recommended. 11. A more detailed planting plan for the proposed bioretention systems is needed. The Conceptual Landscape Plan (Sheet C-12) references typical "bioplantings" and "biotrees" but does not provide any detail as to the nature and density of the proposed plantings. RESPONSE: Per our discussion, a more detailed planting plan will be provided prior to the start of construction. A note has been added to Sheet C-12 stating"Planting list is for permitting purposes only.A final planting list with area appropriate plants with exact plant type, quantities, and sizes will be provided with construction documents." 12. According to the bioretention system sizing calculations, bioretention systems BIO 4 and BiO 7 would fall short of the required water quality volume based on one inch of runoff. i believe, however, that the current North Andover regulations only require a%-inch water quality volume for this site, as do the MA Stormwater Standards. The water quality volume in the basins should therefore be adequate. RESPONSE:Agree. Ms. Lisa Eggleston, PE 5 of 8 West Mill Redevelopment Response to Review Comments 13. The water quality calculations call for the two proposed tree box filters to be 5'x 7'. This is not consistent with the design detail on Sheet C-9, which shows a 2.8'x 2'filter area. RESPONSE:The detail on Sheet C-9 has been revised accordingly. 14. It appears based on the proposed grading that the northern entrance and parking area in DA4A would not drain to the tree pit as is assumed in the water quality calculations, and that the runoff from that new pavement area would not undergo any treatment prior to discharge. RESPONSE: Based on the current design,the inlet to the tree filter box(noted on the plans as CB3.1) is the low point for drainage area DA4a.The runoff for DA4a, including a portion of the existing loop road,will be intercepted by the tree filter prior to discharge to the existing drainage system. HW reviewed the West Mill Loop Parking As-Built Plan dated August 22, 2014,and the response letter to Eggleston Environmental comments dated October 8, 2014, both by Design Consultants, Inc. (DCI). We concur with DCl that if the recently installed loop road pavers are maintained properly the runoff will drain through the pavers to the crushed stone below.Therefore runoff is not expected to drain from the ona� permeable paver parking areas to the proposed tree filter box(CB3.1).To be conservative,we included runoff from the pavers as part of the resubmitted drainage — _ analysis for Phase 11 improvements as unconnected impervious surfaces with a runoff curve number of 98 for both the pre- so ao development and post-development conditions, as noted in the response to oA `'• 82 -szr comment#6. We defer to DC1 as the design F _ engineer for the loop road pavers and rain gardens regarding their revised HydroCAD pre/post analysis,coordination with Eggleston Environmental,and certification of substantial compliance. 15. Additional information is needed on the existing 15,000 gal drainage overflow tank that currently receives drainage from the existing driveway loop in front of the building(Subcatchment DA4)and, under proposed conditions, would also receive drainage from Subcatchments DA4a and DA4b. From my previous discussions with the applicants I had been under the impression that this was some sort of a grit chamber(we'd talked about having it cleaned), and that the discharge to the 60-inch culvert was by gravity. However, the Horsley Witten stormwater report references the discharge from the tank as being pumped via a 6-inch pipe to the 60-inch culvert.Since there is evidently no plan to replace this tank or the pump system its reliability needs to be assessed, as failure of the pump could lead to backups and flooding of on the site.Specifically, is there a separate grit chamber to keep solids out of the pump(and does it get maintained), is the pump always on and activated by a float Ms. Lisa Eggleston, PE 6 of 8 West Mill Redevelopment Response to Review Comments switch vs. by manual control, is there a redundant pump, and is there emergency power to the pump(s)? RESPONSE: RCG has begun the process of inspecting the existing tank and pump system to review operation, condition, power,and maintenance.The existing tank and pumps will be inspected, cleaned, and repaired as necessary to ensure they operate as intended. The loop road drainage system will be re-evaluated to determine if additional repairs or modifications could improve drainage function in this area. 16. The Grading and Drainage Plan (Sheet C-6)does not call for flow to enter the header manifold of the proposed underground recharge chambers URC-1 at its terminus as shown in the system detail on Sheet C-10, therefore the location of the weir in the influent manhole(DMH 1.4)as well as the location of the proposed Isolator Row should be clarified on the plans. RESPONSE: HW revised the recharge chambers(URC-1)to more accurately reflect the design configuration.The isolator row has been labeled along with the weir located in DMH 1.4. 1 Z Based on the Grading and Drainage Plan, outflow from URC-1 would discharge via a 4-inch underdrain to the outlet manhole(DMH 1.5); this should be reflected in the system details on Sheet C-10. it should also be clarified whether there is any connection between the header manifold and DMH 1.5, or DMH 1.5 and the northernmost row of chambers. RESPONSE: HW revised the plans to reflect the updated design. 18. As is indicated in the Stormwater Checklist and noted on the plans, the proposed project will entail the disturbance of more than an acre of land and will therefore require coverage under the EPA Construction General Permit(CGP). Coverage will require preparation and implementation of a Construction Stormwater Pollution Prevention Plan(SWPPP)and filing of an NOi by the site contractor certifying that they have met the eligibility requirements and will comply with the permit's effluent limits and other requirements. The Planning Board and/or Conservation Commission should have the opportunity to review the SWPPP and should be provided with evidence of the filing prior to the start of construction. RESPONSE: No response required.A SWPPP will be provided prior to the start of construction. 19. 1 have the following comments on the Long-Term O&M Plan included with the submittal: ■ The Plan does not appear to be specific to the project site, in that it references a number of BMP components that are inconsistent with or not called for in the plans (sand filter bioretention bed, oil-water/grit separators, bioswales, three types of subsurface recharge systems, open channel systems). ■ The plan should include periodic maintenance of the proposed isolator Row. ■ Snow storage locations consistent with the criteria presented in the O&M Plan should be identified on the plans and the O&M figure. ■ I recommend a simpler, easier-to-read O&M figure identifying the locations of all of the BMPs to be maintained on the site. Ms. Lisa Eggleston, PE 7 of 8 West Mill Redevelopment Response to Review Comments ■ The O&M Plan for the project area should be integrated with the O&M plan for the entire property to ensure that all drainage structures are maintained on a long-term basis. RESPONSE:The Long Term O&M Plan has been revised and will be submitted under separate cover. 20. An illicit discharge statement as is required by DBP Stormwater Standard 10 has not yet been submitted for this project. This should be required as a condition of approval, and it should be based on a thorough physical investigation of the entire storm drainage system on the property to ensure that there are no illicit connections. RESPONSE: Based upon our observations of the existing site drainage system and discussions with the applicant, it is our understanding there are no illicit discharges to the existing drainage system. Per our discussion, HW will conduct a dry-weather inspection at the two existing pipe outfalis to Osgood Pond proposed for reuse as part of this phase of the project.As part of the building renovations the mechanical systems of the building will be evaluated and upgraded by a professional mechanical engineer.This work has not yet begun. If any unknown illicit discharges are uncovered during the building systems evaluation they will be further investigated and removed as part of any proposed stormwater management improvements. No internal building plumbing connections will be allowed to direct non-allowable discharges to the proposed drainage system or wetland resources. 21. Lastly, I wish to point out that the proposed drainage design calls for a great deal of precision-down to the 100ths of feet in elevation-in the construction of the system, including the bioretention basins. Disparities between design and as-built conditions such as occurred with the same applicant's recent construction of the bioretention areas at the front entrance of the site would have significant impact on the functionality of the system, and could lead to flooding issues. If approved, I recommend that the Board(s)require rigorous oversight during the construction of this project by the design engineer, identify a process by which any circumstances requiring adjustment of the design during construction would be brought to the attention of the Board(s)or their designees for their approval, and require as-built plans and an engineer's certification that the drainage design has been constructed and is functioning in strict accordance with the approved design. RESPONSE: No response required. Ms. Lisa Eggleston, PE 8 of 8 West Mill Redevelopment Response to Review Comments If you have any questions or require any additional information, please do not hesitate to contact me directly. Sincerely, HORSLEY WITi'EN GROUP, INC. Jonathan A. Ford, P.E. Senior Project Manager—Community Design cc: Seth Zeren, RCG LLC David Steinbergh, RCG LLC Jennifer Hughes,Town of North Andover Matthew Egge,Town of North Andover Enclosures Horsley Witten Group Sustainable Environmental Solutions 6617orranco Strout•Su/te 403-Providence,R102903 Phone•401-272.1717 • Fax-401-437.8368 .wmvhorsteywitton.coni December 19, 2014 Ms. Lisa Eggleston, PE Eggleston Environmental 32 Old Framingham Road Unit 29 Sudbury, MA 01776 Via email Re: West Mill Redevelopment, 4 High Street, North Andover Response to Peer Review Comments Dear Ms. Eggleston: On behalf of RCG, LLC,the Horsley Witten Group, Inc. (HW) is pleased to provide the following response to your engineering peer review letter dated December 15, 2014.We are submitting this response letter to address the comments described below. Revised materials listed below are enclosed for your review, stamped hard copies will be provided under separate cover: • Revised Grading and Drainage Sheet C-6 • Revised Detail Sheets C-9 and C-10 • Revised Operation and Maintenance Plan 2. Per my discussion with Horsley Witten (HW), the design of the sediment forebays is still being worked on. For the Type 1 bioretention areas the design calls far "Rain Guardians", a proprietary forebay design that I'm not familiar with except what I have found on the manufacturer's website. The product is not included in the MaSTEP database so I requested HW provide performance data or other independent reviews to document their effectiveness. The Rain Guardians should also be labeled on Sheet C-6, and a design detoil far them provided on the plans. RESPONSE:The bioretention Type 1 sediment forebay design has been finalized, as reflected on revised Sheets C-6 and C-9.The Rain Guardian has been deleted from the project design, and a more typical Type 1 forebay design is specified utilizing a granite check dam.Additional detail has been added to Sheet C-6 for clarity. Additional bioretention pretreatment detail including volume calculations are provided in Table-1 an the next page.All bioretention pretreatment systems are sized to provide volume based on 10%of the 0.5-inch storm event,which Is the bioretention sizing criteria for this project as a redevelopment. Due to the nature of the site constraints,several bioretention systems do not fully meet sediment sizing volume based on 10%of the 1-inch storm event. Pretreatment has been incorporated to the maximum extent practicable.See responses to comments#3 and#7 for additional provisions to address sediment forebay sizing and maintenance. Ms. Lisa Eggleston, PE 2 of 6 West Mill Redevelopment Response to Review Comments 2. For the Type 2 bioretention areas the current design calls for 3"deep concrete forebays running the length of the basins, with a metal plate between the forebay and the bioretention cell. 1 question whether o forebay that is only 3 inches deep can provide adequate depth for sediment accumulation, but the issue seems to be how to get it to drain otherwise. HW is still working on how to provide seepage from the forebays into the bioretention cells so that they empty between storms. They mentioned putting weepholes at the bottom of the plate, which 1 thought might be prone to blockage. They also talked about possibly using porous concrete segments in lieu of the metal plate and were going to discuss this option with RCG. It strikes me that this alternative may allow a deeper forebay design. RESPONSE;The Type 2 bioretention forebay detail on Sheet C-9 has been revised to adjust the depth from 3 inches to 6 inches for all Type 2 bioretention systems.A detail specifying filter inserts fitting into the metal spillway plates has been added to Sheet C-9 to provide for drainage from the forebay into the bloretention cell. 3. H W has submitted calculations of the required forebay volume for all of the bioretention cell, but they have not documented the forebay volumes provided by the design. Based on our discussion, it appears that the volumes provided by the design would likely fall short of the calculated required volume but would be easier to maintain on a regular basis.A summary of the actual forebay volume provided for each bioretention cell is needed and, to the extent the numbers do fall short, a written summary of how this would be compensated is also needed. RESPONSE:Table-1 below documents required and provided forebay volumes for each proposed bioretention cell.All bioretention pretreatment systems are sized to provide adequate volume based on 10%of the 0.5-inch storm event,which is the bioretention sizing criteria for this project as a redevelopment. Due to the nature of the redevelopment site constraints, several bioretention systems do not fully meet sizing volume based on 10%of the 1-inch storm event. Pretreatment has been incorporated to the maximum extent practicable.See responses to comment#7 for additional provisions to address sediment forebay maintenance. Table 1 -Bioretention Summary 1-inch Sediment Sediment %Forebay %Forebay WQv Forebay Forebay Provided Provided Pretreatment Req. Required-- Provided (10%of 1-inch (10%of 0.5- System Type (af) 10°o WQv(cf) (cf) WQv) inch WQv) 131O1 Forebay .015 65 84 129% 258% B1O2 Forebay .004 18 11 62% 122% B1O3 Linear channel .005 22 61 276% 555% B1O4 Forebay .012 51 28 54% 110% B1O5 (north) Linear channel .007 30 50 167% 333% B1O5 (south) Linear channel .023 102 53 52% 104% B1O6(north) Linear channel .007 29 41 141% 283% B1O6(south) Linear channel .018 79 40 51% 101% B1O7 Forebay .008 37 31 84% 168% BIOS Forebay .003 13 24 183% 369% Ms. Lisa Eggleston, PE 3 of 6 West Mill Redevelopment Response to Review Comments 4. 1 still have concerns about drainage from the northernmost front entrance to the new parking lot (Comment#14). This area is intended to drain to the proposed tree filter but based on the grading plan and as discussed with HW, it doesn't look like it necessarily will. Per our discussion, the grading should be revised slightly to make sure it drains along the curb line to CB 3.1 and the tree filter. RESPONSE: Proposed grading on Sheet C-6 has been revised to clarify and ensure drainage is to CB 3.1 and the tree filter. 5. The current design calls far the subsurface detention facility to be lined, due to the fact that it does not meet DEP requirements for separation to groundwater. The soil test data indicates that the ESHGW elevation is below the bottom of the basin;therefore it is unlikely that groundwater would impact the storage volume in the system or its capacity to attenuate runoff flows. Consideration should be given to eliminating the impermeable liner and allowing the basin to recharge the runoff to the maximum extent practicable. RESPONSE:We agree with the desire to allow the basin to recharge the runoff to the maximum extent practicable.The impermeable liner was specified because soil evaluation#1 observed the presence of coal ash fill near the location of the proposed underground storage system.At this point we have not eliminated the impermeable liner from the system design.A note has been added to Sheet C-6 requiring further evaluation of soil conditions in this location during construction and elimination of the impermeable liner if possible. 6. As indicated in the HW response to my previous comment#15, the issue of the existing 15,000 gallon tank that receives drainage from the existing loop and some of the new project is still outstanding. Relying on a pumped system to get rid of storm runoff has a certain amount of risk,particularly in the event of a power failure or pump malfunction.An acceptable plan to ensure the reliability and ongoing maintenance of the pump system is needed. RESPONSE:As noted in our previous response, RCG has begun the process of inspecting the existing tank and pump system to review operation, condition, power, and maintenance.This tank and pump system is an existing condition.The existing tank and pumps will be inspected, cleaned,and repaired as necessary to ensure they operate as intended. 7. 1 have the following comments on the revised O&M plan: o The revised O&M Plan calls for inspection of the sediment forebays annually and after major storm events, and cleaning of the forebays when sediment buildup reaches 2 inches. Based on the literature provided for the Rain Guardian chambers and the fact that the total storage volume is likely to be significantly smaller than a traditional sediment forebay, they will need more frequent inspections/cleaning than in called for in the O&M Plan, e,g. after each storm event. The linear forebays on the Type 2 bioretention systems should also be cleaned more frequently, as two inches of sediment accumulation in a forebay that is only three inches deep is likely to result in the sediment being carried over into the bioretention cell Ms. Lisa Eggleston, PE 4 of 6 West Mill Redevelopment Response to Review Comments RESPONSE: The revised O&M Plan calls for inspection of the sediment forebays twice annually (spring and fall)and after major storm events.The Type 2 linear forebay depth has been revised from 3 inches to 6 inches to minimize potential for sediment being carried over into the bioretention cell.Sediment forebays have been incorporated to the maximum extent practicable. Doubling inspection frequency and providing additional sediment forebay volume will address any issues that might have been encountered with pretreatment volumes below the typical standard. o The O&M Plan calls for biannual(once every two years)maintenance of the tree filter pits. Based on the manufacturer's recommendations, this maintenance should be conducted at least twice per year, or semi-annually. RESPONSE:The O&M Plan tree filter pit maintenance requirement has been revised to match the manufacturer's recommendation of twice per year. o Based on the current design, the subsurface Stormtech chambers should not be referred to as a recharge/infiltration system since the system is lined. The O&M Plan should include periodic inspections of the system to ensure that it is draining properly and should also include the maintenance tasks associated with the Stormtech Isolator Row;this information should not be merely appended. RESPONSE:The subsurface Stormtech system has been relabeled as an underground storage chamber(USC)to match the plans.The O&M Plan has been revised to include periodic inspections of the system as well as maintenance tasks associated with the Isolator Row. o The snow storage locations shown on the O&M figure should be referenced in the plan. If that storage is insufficient, offsite alternatives may be needed. RESPONSE:The O&M Plan has been revised to reference the snow storage locations shown on the figure.The figure and plan note that if the identified locations are insufficient,snow will be transported to an alternate location on the property as needed. o The O&M Plan far the project area should be integrated with the O&M Plan for the entire property to ensure that all drainage structures are maintained on a long-term basis. RESPONSE: RCG has contracted with a licensed professional to inspect and maintain West Mill and East Mill stormwater infrastructure. RCG intends to incorporate the requirements of the submitted O&M Plan into their current active maintenance contract to ensure all drainage structures are maintained on a long term basis. An additional comment was posed by Jennifer Hughes via email regarding submittal of a planting plan and pond buffer restoration details: Many of the proposed landscape plantings are not natives, I had also thought there was to be a buffer zone enhancement plan. ~ Ms. Lisa Eggleston, PE 5of6 West Mill Redevelopment Response to Review Comments RESPONSE:As previously noted, a detailed planting plan will be provided prior to the start of construction.A note has been added to Sheet C-12 stating"Planting list is for permitting purposes only.A final planting list with appropriate plants with exact plant type, quantities,and sizes will be provided with construction dncuments." Native species are proposed within bloretention areas and within the Osgood Pond 50 foot no build zone, except for parking islands. Non native species are proposed as ornamentals within parking islands and around the building foundation.These selections were made to utilize hardier plants In these high-traffic locations. Native species can be substituted as part of the final planting list with � construction documents ifdesired. The existing buffer adjacent to Osgood Pond is predominantly mowed grass to the pond edge as shown in Figure 1 below.The revised planting plan to be provided with construction documents will include invasives management and native low-maintenance freshwater coastal habitat plantings to restore buffer areasion the property and within the limit of this phase of work as shown in Figure 2. Figure Existing Osgood Pond edge looking north ot Outlet 2 Ms. Lisa Eggleston, PE 6 of 6 West Mill Redevelopment Response to Review Comments A� °rb^¢r emr RWFFERRESTORATION AREA r`rrrr ;q' Tl _ :�. PILAU IUMIT — OF WORK PCV/°1 bq if b rt a�^ 1 �• Mt+i�/ � W IE4R1N SEAO :rOWMrrou REASE.Mb � ;` R EY 1 .Ma s. iMM ENVIRONMEN TAL / OUR AS lA rl r rrv,�cu.�r y, ..—,.._.. ,��/! w°a ir / /�//J r(or!r�f✓r/r i aaiMrz°�zr /d/ 1Jb/ui/�/rii !r /� 1vn-��sr i�19H, �A, .+. 2AM ` � r 1 v � GxArpT�wvua M1 f b V _.,!,wr0+r a rest Ea M yyyA. vafawy,,. w.nm.www„M...,. . .......... ....m.__ _. ..__....(wp_., ,,. „� 'AC W 1T1f A 5 ■ E7 TP.VP CY ]R NRI'."hYYp t.f+�Y �� RW14O ZONE .LYAA VwY P C k'+WSJ .—� 4r IW+J"MY fYd✓.A nd IY)Mf'1 eq;+W) M CitlAR 6.l Ki RM"ti \ 11'''° � dt., ..,_t h 4 k rzA.M (l�tw G'1+E8 Hw W� �1 TI M�'1.yy °rer Rr. Yew w/r � i Ir mrt mr r°m Figure 2—Proposed Buffer Restoration Areas Shown in Green If you have any questions or require any additional information, please do not hesitate to contact me directly. Sincerely, HORSLEY WITTEN GROUP, INC. Jonathan A. Ford, P.E. Senior Project Manager—Community Design Enclosures cc: Seth Zeren &David Steinbergh, RCG LLC Jennifer Hughes& Matthew Egge,Town of North Andover