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HomeMy WebLinkAboutCorrespondence - 250 CLARK STREET 3/26/2015 Project No. NAND-0020 Prepared for: KEYW Corporation(Flight Landata, Inc.) 250 Clark Street North Andover, MA 01854 978-682-7767(Ph) 978-682-6735 (Fax) Comparative Drain a g e Analysis Flight Landata 250 Clark Street North Andover, MA 01845 February 7, 2013 Revised March 26, 2013 C�(' WILLIAMS SPARAGES ENGINFERS PLANNE0.S SNRVEYORS T S 191 S.Main Street,Suite 103 Middleton,MA 01949 Office Tel:978-539-8088 Fax:978-767-8579 www.wsengineers.com Comparative Drainage Analysis Flight Landata 250 Clark Street North Andover, Massachusetts February 7,2013 Revised: March 26,2013 Purpose: The purpose of this analysis is to compare the existing and proposed watershed conditions by analyzing the surface runoff rates to the limit of the watershed analysis shown on the accompanying watershed maps. The results of which are presented on the summary table that follows. Introduction• The project is located at 250 Clark Street found within the Lawrence Municipal Airport complex. The land is owned by the City of Lawrence and is managed by an organization known as the Lawrence Airport Commission. The property is accessed from Clark Street via Holt Road which connects then with Osgood Street (Route 125). The premises currently is occupied by an existing 9,525 s.f. commercial building, pavement&walkways equaling approximately 30,350 s.f.,lawn,woods,wetlands and other landscaped areas. The proposal is to construct a new 9,680 s.f.building to be used for Hangar&Office Space, a new septic system,an additional 19 paved parking spaces to accommodate the addition,walkways,and two (2) Biofilters for stormwater-treatment from-the new paved areas.-In addition,a 15-7Lx4`Wx2`D stone trench will be constructed to receive stormwater runoff from the south half of the proposed roof area with a 2 year storm capacity before overflowing across the proposed lawn area. It should be noted that there are no proposed disturbances to the existing wetland resource areas located adjacent to the project. Existing Condition Soils Analysis: In order to model the runoff parameters for both the existing and proposed watershed parameters, the parent soils on site were mapped using the Web Soil Survey (WSS) made available on the United States Department of Agriculture (USDA) National Resources Conservation Service (NRCS)website. The WSS provides vital soil data and information such as Hydrologic Soil Group (HSG)which is then input into a mathematical model to generate runoff curve numbers. - --- - --— ---- — — ®F �'r�s PETER M. •, BLAISDELL,JR. , CIVIL ' No.41613 A0 Comparative Drainage Analysis 250 Clark Street, North Andover, MA February, 7 2013 Revised March 26,2013 The user inputs the soil cover type as well as the hydrologic soil group to generate a weighted curve number (CN) and also uses the topography of the land to generate a time of concentration (Tc) from which the stormwater runoff rate as well as volume may be calculated for a certain watershed for comparison. The soils present on site are Urban Land,Paxton fine sandy loam and Ridgebury fine sandy loam. The Paxton and Ridgebury soils both have hydrologic soil groups (HSG) of"C; however,the Urban Land does not have on assigned to it by the NRCS. However,for the comparative purposes of this analysis we have assumed that its HSG is also "C. Stormwater Modeling MethodoloM. The mathematical model used in this analysis was provided using the HydroCAD 9.10 Version developed by HydroCAD Software Solutions LLC. HydroCAD is a program that is used to model the hydrology and hydraulics of stormwater runoff and is based largely on programs and techniques developed by the Soil Conservation Service (SCS)now known as the NRCS,specifically TR-20 and TR-55 as well as other hydraulic calculation methods. HydroCAD allows the user,for a given rainfall event, to generate runoff hydrographs for single or multiple watersheds and is used to determine if a given drainage system is adequate under the desired conditions as well as to predict flooding or other impacts at specified locations such as erosion. In this analysis a 24-hour Type III distribution for using the Rainfall Data found on page 19 of the North Andover Stormwater Management and Erosion Control Regulations was used. Four (4) design storm events were analyzed and the results presented in the tables that follow for the one (1) year,two (2)year,ten(10) year and one-hundred (100) year storm events for comparison. Existing Condition Watershed: We have revised the limit of watershed analysis which was previously the limit of existing disturbance and lease lines to be the flows tributary to the north,west and southeast wetland resource areas at the request of the peer reviewing_engineer,Egglston_Environmental in order to_ fully assess any impacts of the project on these areas. Using the methods described in the stormwater modeling explanation above,runoff curve numbers and times of concentration were generated for each watershed for the existing condition to be used for comparison with the proposed condition described below. A schematic of the mathematical model as well as the results of the calculations for the 1 year,2 year, 10 year and 100 year Type 111, 24-hour storm events are included in this report. 2 Comparative Drainage Analysis 250 Clark Street, North Andover, MA February,7 2013 Revised March 26,2013 Proposed Condition Watershed: In order to mitigate the peak rates of runoff for the 4 design storms we are proposing to construct two (2) Biofilters that will treat the stormwater runoff,provide some limited infiltration of the runoff into the ground. The Biofilters were chosen as a result of our discussions with the peer reviewing engineer as the best solution to this site given the high groundwater elevations that were observed during the conducting of soil testpits. Using the methods described above,runoff curve numbers and times of concentration were generated for each watershed for the existing condition to be used for comparison with the proposed condition described below. A schematic of the mathematical model as well as the results of the calculations for the 1 year,2 year, 10 year and 100 year,Type III,24-hour storm events are included in this report. Conclusion: As can be seen by examining the following summary table and the performance summary of the biofilters,the design will be effective for mitigating the peak flow rates of runoff from the limit of the watershed analysis for the 1,2,10&100 year storm events. Further,an overall reduction in the peak rate of runoff is expected due to the detention of stormwater runoff and groundwater recharge opportunity available within the Biofilters. We have broken the proposed condition watershed analysis results into 2 categories below. The first column illustrates the peak flow rates and generated runoff volume assuming that there is no exfiltration through the bottom of the Biofilters. The second column illustrates the results assuming that there is some limited exfiltration. As you can see the results are very similar as the rate of exfiltration is very small,i.e. 0.27 inches per hour for loamy sands. According to the mathematical model there will be a slight increase in the peak rate of runoff and volume of runoff to the wetland resource area to the North for the 1,2 and 100 year storm,i.e. 0.1 cfs. However, as explained earlier, the wetland resource area to the north is joined with the wetland resource area to the west approximately 150' off of the existing edge of pavement and we do not anticipate any adverse impact from this de minimis increase. 3 Comparative Drainage Analysis 250 Clark Street, North Andover, MA February,7 2013 Revised March 26,2013 Total Peak Rate/Volume of Runoff Comparison Table Description 24 hour Type III Existing Proposed Proposed Storm Event Condition Condition 1 Condition 2 (year) (Assume No (Assume Limited Infiltration in the Infiltration in the Biofilters) Biofilters) Peak Runoff Peak Runoff Peak Runoff Rate of Volume Rate of Volume Rate of Volume Runoff Runoff Runoff (Q)c s. Acre- Q cfs. Acre-ft Q cfs. Acre-ft Flow towards the 1 (2.5 inch) 1.2 0.092 1.0 0.118 0.9 0.088 WEST 2 (3.2 inch) 1.8 0.136 1.7 0.174 1.4 0.140 10 (4.8 inch) 3.1 0.243 3.0 0.308 3.0 0.268 100 8.6 inch) 6.4 0.512 6.0 0.636 6.0 0.590 Flow towards the 1 (2.5 inch) 0.7 0.046 0.8 0.049 0.8 0.049 NORTH 2(3.2 inch) 0.9 0.062 1.0 0.065 1.0 0.065 10 (4.8 inch) 1.5 0.100 1.5 0.102 1.5 0.102 100 8.6 inch 2.7 0.189 2.8 0.188 2.8 0.188 Flow towards the 1 (2.5 inch) 0.7 0.051 0.5 0.043 0.5 0.043 SOUTHEAST 2 (3.2 inch) 1.1 0.083 1.0 0.073 1.0 0.073 10 (4.8 inch) 2.4 0.168 2.0 0.148 2.0 0.148 100 8.6 inch) 5.6 0.398 4.2 0.341 4.2 0.341 Total Flow 1 (2.5 inch) 2.2 0.189 1.8 0.210 1.8 0.176 leaving the limit 2 (3.2 inch) 3.4 0.281 3.2 0.312 2.8 0.274 of watershed 10 (4.8 inch) 6.3 0.510 6.3 0.558 6.3 0.513 analysis 100 (8.6 inch) 13.3 1.099 12.6 1.165 12.5 1.114 Link 119L 4 Comparative Drainage Analysis 250 Clark Street, North Andover, MA February, 7 2013 Revised March 26,2013 Proposed Biofilter 1 Performance Table HydroCAD 117P Top of Berm=133.95 ft 12' wide stone spillway @ 133.45 Bottom of berm=133.2 ft 24 hour Type III Peak Rate of Peak Rate of Runoff Out Peak Water Level Storm Event Runoff In (Q) cfs. (W.L.)ft (YR) (Qin) cfs. 12' wide 12' wide No *With spillway Spillway Exfiltration Exfiltration (No (*With Exfiltration) Exfiltration 1 0.9 0.75 0.57 133.54 133.52 2 1.3 1.11 1.02 133.56 133.56 10 2.0 1.79 1.76 133.61 133.60 100 3.6 3.41 3.38 133.69 133.69 *The design infiltration rate is of 0.27 inches per hour for NRCS Hydrologic Soil Group "C" is as shown on Table 2-1 Infiltration Rates for Various Soil Textures found in the Hydrology Handbook for Conservation Commissioners,March 2002. It is understood that there will be limited infiltration through the bottoms of the biofilters at certain times during the year when the groundwater is at a lower elevation than was observed on February 8,2013 by Williams&Sparages. Therefore,we have included an additional calculation which takes into account the limited exfiltration that would be expected based on the soil properties in order to be conservative. However,as can be seen above,the limited amount of exfiltration that we are accounting for has a negligible affect on the performance of the biofilter. 5 Comparative Drainage Analysis 250 Clark Street, North Andover, MA February, 7 2013 Revised March 26,2013 Proposed Biofilter 2 Performance Table H.ydroCAD 120P Top of Trench=135.0 ft Bottom of berm=133.0 ft 4" PVC Invert's In=134.0 24 hour Type III Peak Rate of Peak Rate of Runoff Out Peak Water Level Storm Event Runoff In (Q) cfs. (W.L.)ft (YR) (Qin) cfs. 12' wide 12' wide No *With spillway Spillway Exfiltration Exfiltration (No (*With Exfiltration) Exfiltration) 1 0.24 0.21 0.19 133.31 133.31 2 0.4 0.35 0.35 133.33 133.33 10 0.7 0.67 0.66 133.38 133.38 100 1.5 1.4 1.4 133.46 133.46 *The design infiltration rate is of 0.27 inches per hour for NRCS Hydrologic Soil Group"C" is as shown on Table 2-1 Infiltration Rates for Various Soil Textures found in the Hydrology Handbook for Conservation Commissioners,March 2002. It is understood that there will be limited infiltration through the bottoms of the biofilters at certain times during the year when the groundwater is at a lower elevation than was observed on February 8,2013 by Williams&Sparages. Therefore,we have included an additional calculation which takes into account the limited exfiltration that would be expected based on the soil properties in order to be conservative. However, as can be seen above,the limited amount of exfiltration that we are .E. accounting for.haswa-negligible`affect on.the-performances of-the-biofi-lter.- _ ____ _._.._ ___._ _ 6 Comparative Drainage Analysis 250 Clark Street, North Andover, MA February, 7 2013 Revised March 26,2013 Proposed Stone Trench Performance Table HydroCAD 126P Top of Berm=133.75 ft Bottom of berm=133.0 ft 4" PVC Invert's In from roof drains=134.00 24 hour Type III Peak Rate of Peak Rate of Runoff Out Peak Water Level Storm Event Runoff In (Q) cfs. (W.L.) ft (YR) (Qin) cfs. Top of stone/Lawn Edge acting as a 157' L Spillway 1 0.3 0.04 135.00 2 0.5 0.22 135.01 10 0.8 0.82 135.02 100 1.5 1.5 135.02 *The design infiltration rate is of 0.27 inches per hour for NRCS Hydrologic Soil Group"C" is as shown on Table 2-1 Infiltration Rates for Various Soil Textures found in the Hydrology Handbook for Conservation Commissioners,March 2002. The bottom of the stone trench will be a minimum of 2 feet above the Estimated Seasonal High Groundwater Table and therefore we have assumed that there will be limited infiltration consistently throughout the year. 7 � J (n J `Q c 2 rc� in OT (A N N �m d N 'D N O Q •� i O O 2 O o _m _U N B (0 L ca N /L r Q a 11O1— �O .� 0 ; o U '..,. ---, co "0 U ca E cu 0 cu � .0) >+ cu cu Z U) > co s i t6 N � t6 N 0 O— O � H Existing Condition Watershed Analysis- 122-130 South Main Street , Middleton MA Revised March 26, 2013 existi ng_r2 Prepared by Microsoft Printed 4/2/2013 HydroCAD®9.10 s/n 06611 @2010 HydroCAD Software Solutions LLC Page 1 Area Listing (all nodes) Area CN Description (sq-ft) (subcatchment-numbers) 6,374 70 Woods, Good, HSG C (116S, 125S) 40,506 74 >75% Grass cover, Good, HSG C (116S, 122S, 125S) 30,350 98 Unconnected pavement, HSG C (116S, 122S, 125S) 9,525 98 Unconnected roofs, HSG C (116S, 122S, 125S) 86,755 TOTAL AREA Existing Condition Watershed Analysis - 122-130 South Main Street , Middleton MA Revised March 26, 2013 existing_r2 Prepared by Microsoft Printed 4/2/2013 HydroCAD®9.10 s/n 06611 ©2010 HydroCAD Software Solutions LLC Page 2 Soil Listing (all nodes) Area Soil Subcatchment (sq-ft) Group Numbers 0 HSG A 0 HSG B 86,755 HSG C 116S, 122S, 125S 0 HSG D 0 Other 86,755 TOTAL AREA Existing Condition Watershed Analysis- 122-130 South Main Street , Middleton MA Revised March 26, 2013 existing_r2 Type 111 24-hr 1 yr Rainfall=2.50" Prepared by Microsoft Printed 4/2/2013 HydroCAD®9.10 s/n 06611 @2010 HydroCAD Software Solutions LLC Page 3 Time span=0.00-40.00 hrs, dt=0.05 hrs, 801 points x 3 Runoff by SCS TR-20 method, UH=SCS Reach routing by Dyn-Stor-Ind method - Pond routing by Dyn-Stor-Ind method Subcatchment 116S: WEST Runoff Area=38,726 sf 50.55% Impervious Runoff Depth=1.24" Flow Length=210' Tc=7.9 min CN=86 Runoff=1.20 cfs 4,013 cf Subcatchment 122S: NORTH Runoff Area=12,331 sf 87.65% Impervious Runoff Depth=1.96" Flow Length=75' Tc=0.8 min CN=95 Runoff=0.70 cfs 2,017 cf Subcatchment 125S: SOUTHEAST Runoff Area=35,698 sf 26.59% Impervious Runoff Depth=0.74" Flow Length=100' Tc=4.8 min UI Adjusted CN=77 Runoff=0.67 cfs 2,202 cf Link 119L: Total Surficial Flow leaving limit of watershed analysis ultimately Inflow=2.24 cfs 8,233 cf Primary=2.24 cfs 8,233 cf Total Runoff Area =86,755 sf Runoff Volume=8,233 cf Average Runoff Depth = 1.14" 54.04% Pervious=46,880 sf 45.96% Impervious=39,875 sf Existing Condition Watershed Analysis - 122-130 South Main Street , Middleton MA Revised March 26, 2013 existing_r2 Type ///24-hr 2 yr Rainfall=3.20" Prepared by Microsoft Printed 4/2/2013 HydroCAD®9.10 s/n 06611 ©2010 HydroCAD Software Solutions LLC Page 1 Time span=0.00-40.00 hrs, dt=0.05 hrs, 801 points x 3 Runoff by SCS TR-20 method, UH=SCS Reach routing by Dyn-Stor-Ind method - Pond routing by Dyn-Stor-Ind method Subcatchment 116S: WEST Runoff Area=38,726 sf 50.55% Impervious Runoff Depth=1.84" Flow Length=210' Tc=7.9 min CN=86 Runoff=1.77 cfs 5,922 cf Subcatchment 122S: NORTH Runoff Area=12,331 sf 87.65% Impervious Runoff Depth=2.64" Flow Length=75' Tc=0.8 min CN=95 Runoff=0.93 cfs 2,718 cf Subcatchment 125S: SOUTHEAST Runoff Area=35,698 sf 26.59% Impervious Runoff Depth=1.21" Flow Length=100' Tc=4.8 min UI Adjusted CN=77 Runoff=1.14 cfs 3,605 cf Link 119L: Total Surficial Flow leaving limit of watershed analysis ultimately Inflow=3.40 cfs 12,245 cf Primary=3.40 cfs 12,245 cf Total Runoff Area=86,755 sf Runoff Volume = 12,245 cf Average Runoff Depth= 1.69" 54.04%Pervious=46,880 sf 45.96% Impervious=39,875 sf Existing Condition Watershed Analysis - 122-130 South Main Street , Middleton MA Revised March 26, 2013 existing_r2 Type ///24-hr 10 yr Rainfall=4.80" Prepared by Microsoft Printed 4/2/2013 HydroCAD®9.10 s/n 06611 @2010 HydroCAD Software Solutions LLC Page 2 Time span=0.00-40.00 hrs, dt=0.05 hrs, 801 points x 3 Runoff by SCS TR-20 method, UH=SCS Reach routing by Dyn-Stor-Ind method - Pond routing by Dyn-Stor-Ind method Subcatchment 116S: WEST Runoff Area=38,726 sf 50.55% Impervious Runoff Depth=3.28" Flow Length=210' Tc=7.9 min CN=86 Runoff=3.13 cfs 10,588 cf Subcatchment 122S: NORTH Runoff Area=12,331 sf 87.65% Impervious Runoff Depth=4.22" Flow Length=75' Tc=0.8 min CN=95 Runoff=1.45 cfs 4,338 cf Subcatchment 125S: SOUTHEAST Runoff Area=35,698 sf 26.59% Impervious Runoff Depth=2.46" Flow Length=100' Tc=4.8 min Ul Adjusted CN=77 Runoff=2.39 cfs 7,308 cf Link 119L: Total Surficial Flow leaving limit of watershed analysis ultimately Inflow=6.25 cfs 22,233 cf Primary=6.25 cfs 22,233 cf Total Runoff Area=86,755 sf Runoff Volume =22,233 cf Average Runoff Depth =3.08° 54.04%Pervious=46,880 sf 45.96% Impervious=39,875 sf Existing Condition Watershed Analysis - 122-130 South Main Street , Middleton MA Revised March 26, 2013 existing_r2 Type 111 24-hr 100 yr Rainfall=8.60" Prepared by Microsoft Printed 4/2/2013 HydroCADO 9.10 s/n 06611 ©2010 HydroCAD Software Solutions LLC Page 3 Time span=0.00-40.00 hrs, dt=0.05 hrs, 801 points x 3 Runoff by SCS TR-20 method, UH=SCS Reach routing by Dyn-Stor-Ind method - Pond routing by Dyn-Stor-Ind method Subcatchment 116S: WEST Runoff Area=38,726 sf 50.55% Impervious Runoff Depth=6.91" Flow Length=210' Tc=7.9 min CN=86 Runoff=6.37 cfs 22,313 cf Subcatchment 122S: NORTH Runoff Area=12,331 sf 87.65% Impervious Runoff Depth=8.00" Flow Length=75' Tc=0.8 min CN=95 Runoff=2.66 cfs 8,220 cf Subcatchment 125S: SOUTHEAST Runoff Area=35,698 sf 26.59% Impervious Runoff Depth=5.83" Flow Length=100' Tc=4.8 min UI Adjusted CN=77 Runoff=5.62 cfs 17,336 cf Link 119L: Total Surficial Flow leaving limit of watershed analysis Inflow=13.30 cfs 47,869 cf Primary=13.30 cfs 47,869 cf Total Runoff Area= 86,755 sf Runoff Volume =47,869 cf Average Runoff Depth =6.62" 54.04% Pervious =46,880 sf 45.96% Impervious=39,875 sf U U J W OD J H r d O O co (q L � ID ® �N N d C,4 w 0 C) cn m Q� o 0 4- 2 E po O N o to a L r O O + m ^-0^`` N c i O •L Q N LL G No 0 g a o c O • U co L U > N C4 77,W H y 3 L LL � i H U) LU LU LL 0 �� 00 CL Proposed Condition Watershed Analysis - 122-130 South Main Street , Middleton MA Revised March 26, 2013 proposed—r2 Prepared by Microsoft Printed 4/2/2013 HydroCADO 9.10 s/n 06611 ©2010 HydroCAD Software Solutions LLC Page 1 Area Listing(all nodes) Area CN Description (sq-ft) (subcatchment-numbers) 1,668 70 Woods, Good, HSG C (122S) 29,095 74 >75% Grass cover, Good, HSG C (116S, 120S, 121 S, 122S, 123S, 127S) 32,550 98 Unconnected pavement, HSG C (116S, 120S, 121 S, 122S, 123S) 14,557 98 Unconnected roofs, HSG C (116S, 120S, 122S, 123S) 8,585 98 Water Surface, HSG C (116S, 121 S, 127S) 86,455 TOTAL AREA Proposed Condition Watershed Analysis - 122-130 South Main Street , Middleton MA Revised March 26, 2013 proposed—r2 Prepared by Microsoft Printed 4/2/2013 HydroCAD®9.10 s/n 06611 @2010 HydroCAD Software Solutions LLC Page 2 Soil Listing (all nodes) Area Soil Subcatchment (sq-ft) Group Numbers 0 HSG A 0 HSG B 86,455 HSG C 116S, 120S, 121 S, 122S, 123S, 127S 0 HSG D 0 Other 86,455 TOTAL AREA Proposed Condition Watershed Analysis - 122-130 South Main Street , Middleton MA Revised March 26, 2013 proposed_r2 _ __Type 111 24-hr I yr Rainfall=2.50" Prepared by Microsoft Printed 4/2/2013 HydroCAD®9.10 s/n 06611 ©2010 HydroCAD Software Solutions LLC Page 3 Time span=0.00-40.00 hrs, dt=0.01 hrs, 4001 points x 3 Runoff by SCS TR-20 method, UH=SCS Reach routing by Dyn-Stor-Ind method - Pond routing by Dyn-Stor-Ind method Subcatchment 116S: PR-131 Runoff Area=24,968 sf 84.92% Impervious Runoff Depth=1.87" Flow Length=230' Tc=15.0 min CN=94 Runoff=0.93 cfs 3,890 cf Subcatchment 120S: PR-SE Runoff Area=13,448 sf 33.45% Impervious Runoff Depth=1.00" Flow Length=70' Tc=9.2 min CN=82 Runoff=0.32 cfs 1,118 cf Subcatchment 121S: PR-B2 Runoff Area=7,937 sf 37.13% Impervious Runoff Depth=1.06" Flow Length=100' Tc=3.5 min CN=83 Runoff=0.24 cfs 698 cf Subcatchment 122S: PR-WEST Runoff Area=21,303 sf 52.57% Impervious Runoff Depth=1.24" Flow Length=80' Tc=0.8 min CN=86 Runoff=0.86 cfs 2,207 cf Subcatchment 123S: PR-NORTH Runoff Area=11,931 sf 93.86% Impervious Runoff Depth=2.16" Flow Length=70' Tc=0.8 min CN=97 Runoff=0.77 cfs 2,151 cf Subcatchment 127S: PR-TRENCH Runoff Area=6,868 sf 67.68% Impervious Runoff Depth=1.53" Tc=1.0 min CN=90 Runoff=0.34 cfs 876 cf Pond 117P: BIOFILTER 1 Peak Elev=133.54' Storage=1,337 cf Inflow=0.93 cfs 3,890 cf Outflow=0.75 cfs 2,923 cf Pond 120P: BIOFILTER 2 Peak Elev=133.31' Storage=154 cf Inflow=0.24 cfs 698 cf Outflow=0.21 cfs 576 cf Pond 126P: Stone Trench Peak Elev=135.00' Storage=496 cf Inflow=0.34 cfs 876 cf Discarded=0.00 cfs 441 cf Primary=0.04 cfs 165 cf Outflow=0.04 cfs 607 cf Link 119L: Total Surficial Flow leaving limit of watershed analysis Inflow=1.81 cfs 9,142 cf Primary=1.81 cfs 9,142 cf Link 124L: Flow to the west Inflow=1.02 cfs 5,131 cf Primary=1.02 cfs 5,131 cf Link 125L: Flow to wetlands to southeast Inflow=0.52 cfs 1,860 cf Primary=0.52 cfs 1,860 cf Total Runoff Area= 86,455 sf Runoff Volume= 10,941 cf Average Runoff Depth = 1.52" 35.58% Pervious=30,763 sf 64.42% Impervious= 55,692 sf Proposed Condition Watershed Analysis - 122-130 South Main Street , Middleton MA Revised March 26, 2013 proposed-r2 Type 111 24-hr 2 yr Rainfall=3.20" Prepared by Microsoft Printed 4/2/2013 HydroCAD®9.10 s/n 06611 @2010 HydroCAD Software Solutions LLC Page 1 Time span=0.00-40.00 hrs, dt=0.01 hrs, 4001 points x 3 Runoff by SCS TR-20 method, UH=SCS Reach routing by Dyn-Stor-Ind method - Pond routing by Dyn-Stor-Ind method Subcatchment 116S: PR-131 Runoff Area=24,968 sf 84.92% Impervious Runoff Depth=2.54" Flow Length=230' Tc=15.0 min CN=94 Runoff=1.25 cfs 5,293 cf Subcatchment 120S: PR-SE Runoff Area=13,448 sf 33.45% Impervious Runoff Depth=1.54" Flow Length=70' Tc=9.2 min CN=82 Runoff=0.50 cfs 1,724 cf Subcatchment 121S: PR-132 Runoff Area=7,937 sf 37.13% Impervious Runoff Depth=1.61" Flow Length=100' Tc=3.5 min CN=83 Runoff=0.38 cfs 1,064 cf Subcatchment 122S: PR-WEST Runoff Area=21,303 sf 52.57% Impervious Runoff Depth=1.84" Flow Length=80' Tc=0.8 min CN=86 Runoff=1.27 cfs 3,258 cf Subcatchment 123S: PR-NORTH Runoff Area=11,931 sf 93.86% Impervious Runoff Depth=2.86" Flow Length=70' Tc=0.8 min CN=97 Runoff=1.01 cfs 2,840 cf Subcatchment 127S: PR-TRENCH Runoff Area=6,868 sf 67.68% Impervious Runoff Depth=2.17" Tc=1.0 min CN=90 Runoff=0.47 cfs 1,241 cf Pond 117P: BIOFILTER 1 Peak Elev=133.56' Storage=1,447 cf Inflow=1.25 cfs 5,293 cf Outflow=1.11 cfs 4,326 cf Pond 120P: BIOFILTER 2 Peak Elev=133.33' Storage=168 cf Inflow=0.38 cfs 1,064 cf Outflow=0.35 cfs 942 cf Pond 126P: Stone Trench Peak Elev=135.01' Storage=496 cf Inflow=0.47 cfs 1,241 cf Discarded=0.00 cfs 455 cf Primary=0.22 cfs 513 cf Outflow=0.22 cfs 968 cf Link 119L: Total Surficial Flow leaving limit of watershed analysis Inflow=3.16 cfs 13,604 cf Primary=3.16 cfs 13,604 cf Link 124L: Flow to the west Inflow=1.66 cfs 7,584 cf Primary=1.66 cfs 7,584 cf Link 125L: Flow to wetlands to southeast Inflow=1.04 cfs 3,180 cf Primary=1.04 cfs 3,180 cf Total Runoff Area=86,455 sf Runoff Volume= 15,420 cf Average Runoff Depth =2.14" 35.58%Pervious =30,763 sf 64.42%Impervious= 55,692 sf Proposed Condition Watershed Analysis - 122-130 South Main Street , Middleton MA Revised March 26, 2013 proposed r2 Type ///24-hr 10 yr Rainfall=4.80" Prepared by Microsoft Printed 4/2/2013 HydroCAD®9.10 s/n 06611 @2010 HydroCAD Software Solutions LLC Page 2 Time span=0.00-40.00 hrs, dt=0.01 hrs, 4001 points x 3 Runoff by SCS TR-20 method, UH=SCS Reach routing by Dyn-Stor-Ind method - Pond routing by Dyn-Stor-Ind method Subcatchment 116S: PR-131 Runoff Area=24,968 sf 84.92% Impervious Runoff Depth=4.11" Flow Length=230' Tc=15.0 min CN=94 Runoff=1.96 cfs 8,553 cf Subcatchment 120S: PR-SE Runoff Area=13,448 sf 33.45% Impervious Runoff Depth=2.90" Flow Length=70' Tc=9.2 min CN=82 Runoff=0.94 cfs 3,251 cf Subcatchment 121S: PR-132 Runoff Area=7,937 sf 37.13% Impervious Runoff Depth=2.99" Flow Length=100' Tc=3.5 min CN=83 Runoff=0.70 cfs 1,980 cf Subcatchment 122S: PR-WEST Runoff Area=21,303 sf 52.57% Impervious Runoff Depth=3.28" Flow Length=80' Tc=0.8 min CN=86 Runoff=2.24 cfs 5,824 cf Subcatchment 123S: PR-NORTH Runoff Area=11,931 sf 93.86% Impervious Runoff Depth=4.45" Flow Length=70' Tc=0.8 min CN=97 Runoff=1.53 cfs 4,422 cf Subcatchment 127S: PR-TRENCH Runoff Area=6,868 sf 67.68% Impervious Runoff Depth=3.68" Tc=1.0 min CN=90 Runoff=0.78 cfs 2,108 cf Pond 117P: BIOFILTER 1 Peak Elev=133.61' Storage=1,631 cf lnflow=1.96 cfs 8,553 cf Outflow=1.79 cfs 7,587 cf Pond 120P: BIOFILTER 2 Peak Elev=133.38' Storage=194 cf Inflow=0.70 cfs 1,980 cf Outflow=0.67 cfs 1,858 cf Pond 126P: Stone Trench Peak Elev=135.02' Storage=496 cf Inflow=0.78 cfs 2,108 cf Discarded=0.00 cfs 479 cf Primary=0.82 cfs 1,356 cf Outflow=0.83 cfs 1,835 cf Link 119L: Total Surficial Flow leaving limit of watershed analysis Inflow=6.31 cfs 24,298 cf Primary=6.31 cfs 24,298 cf Link 124L: Flow to the west Inflow=2.98 cfs 13,411 cf Primary=2.98 cfs 13,411 cf Link 125L: Flow to wetlands to southeast Inflow=2.04 cfs 6,465 cf Primary=2.04 cfs 6,465 cf Total Runoff Area=86,455 sf Runoff Volume=26,139 cf Average Runoff Depth =3.63" 35.58% Pervious =30,763 sf 64.42%Impervious= 55,692 sf Proposed Condition Watershed Analysis - 122-130 South Main Street , Middleton MA Revised March 26, 2013 proposed r2 Type ///24-hr 100 yr Rainfall=8.60" Prepared by Microsoft Printed 4/2/2013 HydroCAD®9.10 s/n 06611 @2010 HydroCAD Software Solutions LLC Paae 3 Time span=0.00-40.00 hrs, dt=0.01 hrs, 4001 points x 3 Runoff by SCS TR-20 method, UH=SCS Reach routing by Dyn-Stor-Ind method - Pond routing by Dyn-Stor-Ind method Subcatchment 116S: PR-131 Runoff Area=24,968 sf 84.92% Impervious Runoff Depth=7.88" Flow Length=230' Tc=15.0 min CN=94 Runoff=3.64 cfs 16,393 cf Subcatchment 120S: PR-SE Runoff Area=13,448 sf 33.45% Impervious Runoff Depth=6.43" Flow Length=70' Tc=9.2 min CN=82 Runoff=2.03 cfs 7,207 cf Subcatchment 121S: PR-132 Runoff Area=7,937 sf 37.13% Impervious Runoff Depth=6.55" Flow Length=100' Tc=3.5 min CN=83 Runoff=1.48 cfs 4,334 cf Subcatchment 122S: PR-WEST Runoff Area=21,303 sf 52.57% Impervious Runoff Depth=6.91" Flow Length=80' Tc=0.8 min CN=86 Runoff=4.55 cfs 12,274 cf Subcatchment 123S: PR-NORTH Runoff Area=1 1,931 sf 93.86% Impervious Runoff Depth=8.24" Flow Length=70' Tc=0.8 min CN=97 Runoff=2.77 cfs 8,192 cf Subcatchment 127S: PR-TRENCH Runoff Area=6,868 sf 67.68% Impervious Runoff Depth=7.40" Tc=1.0 min CN=90 Runoff=1.52 cfs 4,233 cf Pond 117P: BIOFILTER 1 Peak Elev=133.69' Storage=1,988 cf Inflow=3.64 cfs 16,393 cf Outflow=3.41 cfs 15,427 cf Pond 120P: BIOFILTER 2 Peak Elev=133.46' Storage=246 cf Inflow=1.48 cfs 4,334 cf Outflow=1.44 cfs 4,212 cf Pond 126P: Stone Trench Peak Elev=135.02' Storage=496 cf Inflow=1.52 cfs 4,233 cf Discarded=0.00 cfs 512 cf Primary=1.53 cfs 3,448 cf Outflow=1.53 cfs 3,960 cf Link 119L: Total Surficial Flow leaving limit of watershed analysis Inflow=12.55 cfs 50,761 cf Primary=12.55 cfs 50,761 cf Link 124L: Flow to the west Inflow=6.03 cfs 27,701 cf Primary=6.03 cfs 27,701 cf Link 125L: Flow to wetlands to southeast Inflow=4.22 cfs 14,867 cf Primary=4.22 cfs 14,867 cf Total Runoff Area=86,455 sf Runoff Volume= 52,634 cf Average Runoff Depth =7.31" 35.58% Pervious=30,763 sf 64.42% Impervious=55,692 sf r- F7 Q IL _ p M Cl) r =_ C) N U N a) a 9D w M N -0 p 1— C3 N Q IN 1�4 zi ix CL U) m 0 O = 1- 00 O L O ON a N E2 > r L -QCD r o a (o '� O 3 N c N o LL Q O s Q z 3 N '17 EL o U c3! t 0)w `- cn > m _ m m H 3 N 3 rte-" o LL J H of U) w a. o m �+ mi rshed Analysis (With Exfiltration in Biofilters) - 122-130 South Main Street , Middleton MA Revised March 26, 2013 proposed_r2(exfiltration) Prepared by Microsoft Printed 4/2/2013 HvdroCAD®9.10 s/n 06611 ©2010 HvdroCAD Software Solutions LLC Page 1 Area Listing (all nodes) Area CN Description (sq-ft) (subcatchment-numbers) 1,668 70 Woods, Good, HSG C (122S) 29,095 74 >75% Grass cover, Good, HSG C (116S, 120S, 121 S, 122S, 123S, 127S) 32,550 98 Unconnected pavement, HSG C (116S, 120S, 121 S, 122S, 123S) 14,557 98 Unconnected roofs, HSG C (116S, 120S, 122S, 123S) 8,585 98 Water Surface, HSG C (116S, 121 S, 127S) 86,455 TOTAL AREA rshed Analysis (With Exfiltration in Biofilters) - 122-130 South Main Street , Middleton MA Revised March 26, 2013 proposed_r2(exfiltration) Prepared by Microsoft Printed 4/2/2013 HydroCAD®9.10 s/n 06611 @2010 HydroCAD Software Solutions LLC Page 2 Soil Listing(all nodes) Area Soil Subcatchment (sq-ft) Group Numbers 0 HSG A 0 HSG B 86,455 HSG C 116S, 120S, 121 S, 122S, 123S, 127S 0 HSG D 0 Other 86,455 TOTAL AREA rshed Analysis (With Exfiltration in Biofilters) - 122-130 South Main Street , Middleton MA Revised March 26, 2013 proposed_r2(exfiltration) Type ///24-hr 9 yr Rainfall=2.50" Prepared by Microsoft Printed 4/2/2013 HydroCADO 9.10 s/n 06611 ©2010 HydroCAD Software Solutions LLC Page 3 Time span=0.00-40.00 hrs, dt=0.01 hrs, 4001 points x 3 Runoff by SCS TR-20 method, UH=SCS Reach routing by Dyn-Stor-Ind method - Pond routing by Dyn-Stor-Ind method Subcatchment 116S: PR-131 Runoff Area=24,968 sf 84.92% Impervious Runoff Depth=1.87" Flow Length=230' Tc=15.0 min CN=94 Runoff=0.93 cfs 3,890 cf Subcatchment 120S: PR-SE Runoff Area=13,448 sf 33.45% Impervious Runoff Depth=1.00" Flow Length=70' Tc=9.2 min CN=82 Runoff=0.32 cfs 1,118 cf Subcatchment 121S: PR-132 Runoff Area=7,937 sf 37.13% Impervious Runoff Depth=1.06" Flow Length=100' Tc=3.5 min CN=83 Runoff=0.24 cfs 698 cf Subcatchment 122S: PR-WEST Runoff Area=21,303 sf 52.57% Impervious Runoff Depth=1.24" Flow Length=80' Tc=0.8 min CN=86 Runoff=0.86 cfs 2,207 cf Subcatchment 123S: PR-NORTH Runoff Area=11,931 sf 93.86% Impervious Runoff Depth=2.16" Flow Length=70' Tc=0.8 min CN=97 Runoff=0.77 cfs 2,151 cf Subcatchment 127S: PR-TRENCH Runoff Area=6,868 sf 67.68% Impervious Runoff Depth=1.53" Tc=1.0 min CN=90 Runoff=0.34 cfs 876 cf Pond 117P: BIOFILTER 1 Peak Elev=133.52' Storage=1,274 cf Inflow=0.93 cfs 3,890 cf Discarded=0.03 cfs 2,270 cf Primary=0.57 cfs 1,620 cf Outflow=0.60 cfs 3,890 cf Pond 120P: BIOFILTER 2 Peak Elev=133.31' Storage=152 cf Inflow=0.24 cfs 698 cf Discarded=0.00 cfs 285 cf Primary=0.19 cfs 413 cf Outflow=0.19 cfs 698 cf Pond 126P: Stone Trench Peak Elev=135.00' Storage=496 cf Inflow=0.34 cfs 876 cf Discarded=0.00 cfs 441 cf Primary=0.04 cfs 165 cf Outflow=0.04 cfs 607 cf Link 119L: Total Surficial Flow leaving limit of watershed analysis Inflow=1.79 cfs 7,676 cf Primary=1.79 cfs 7,676 cf Link 124L: Flow to the west Inflow=0.86 cfs 3,828 of Primary=0.86 cfs 3,828 cf Link 125L: Flow to wetlands to southeast Inflow=0.50 cfs 1,697 cf Primary=0.50 cfs 1,697 cf Total Runoff Area=86,455 sf Runoff Volume= 10,941 cf Average Runoff Depth = 1.52" 35.58% Pervious =30,763 sf 64.42% Impervious= 55,692 sf shed Analysis (With Exfiltration in Biofilters)- 122-130 South Main Street , Middleton MA Revised March 26, 2013 proposed_r2(exfiltration) Type 111 24-hr 2 yr Rainfall=3.20" Prepared by Microsoft Printed 4/2/2013 HydroCAD®9.10 s/n 06611 @2010 HydroCAD Software Solutions LLC Page 1 Time span=0.00-40.00 hrs, dt=0.01 hrs,4001 points x 3 Runoff by SCS TR-20 method, UH=SCS Reach routing by Dyn-Stor-Ind method - Pond routing by Dyn-Stor-Ind method Subcatchment 116S: PR-131 Runoff Area=24,968 sf 84.92% Impervious Runoff Depth=2.54" Flow Length=230' Tc=15.0 min CN=94 Runoff=1.25 cfs 5,293 cf Subcatchment 120S: PR-SE Runoff Area=13,448 sf 33.45% Impervious Runoff Depth=1.54" Flow Length=70' Tc=9.2 min CN=82 Runoff=0.50 cfs 1,724 cf Subcatchment 121S: PR-132 Runoff Area=7,937 sf 37.13% Impervious Runoff Depth=1.61" Flow Length=100' Tc=3.5 min CN=83 Runoff=0.38 cfs 1,064 cf Subcatchment 122S: PR-WEST Runoff Area=21,303 sf 52.57% Impervious Runoff Depth=1.84" Flow Length=80' Tc=0.8 min CN=86 Runoff=1.27 cfs 3,258 cf Subcatchment 123S: PR-NORTH Runoff Area=11,931 sf 93.86% Impervious Runoff Depth=2.86" Flow Length=70' Tc=0.8 min CN=97 Runoff=1.01 cfs 2,840 cf Subcatchment 127S: PR-TRENCH Runoff Area=6,868 sf 67.68% Impervious Runoff Depth=2.17" Tc=1.0 min CN=90 Runoff=0.47 cfs 1,241 cf Pond 117P: BIOFILTER 1 Peak Elev=133.56' Storage=1,421 cf Inflow=1.25 cfs 5,293 cf Discarded=0.03 cfs 2,464 cf Primary=1.02 cfs 2,828 cf Outflow=1.05 cfs 5,293 cf Pond 120P: BIOFILTER 2 Peak Elev=133.33' Storage=167 cf Inflow=0.38 cfs 1,064 cf Discarded=0.00 cfs 297 cf Primary=0.35 cfs 768 cf Outflow=0.35 cfs 1,064 cf Pond 126P: Stone Trench Peak Elev=135.01' Storage=496 cf Inflow=0.47 cfs 1,241 cf Discarded=0.00 cfs 455 cf Primary=0.22 cfs 513 cf Outflow=0.22 cfs 968 cf Link 119L: Total Surficial Flow leaving limit of watershed analysis Inflow=2.75 cfs 11,931 cf Primary=2.75 cfs 11,931 cf Link 124L: Flow to the west Inflow=1.41 cfs 6,086 cf Primary=1.41 cfs 6,086 cf Link 125L: Flow to wetlands to southeast Inflow=1.03 cfs 3,005 cf Primary=1.03 cfs 3,005 cf Total Runoff Area=86,455 sf Runoff Volume= 15,420 cf Average Runoff Depth =2.14" 35.58% Pervious=30,763 sf 64.42% Impervious = 55,692 sf rshed Analysis (With Exfiltration in Biofilters) - 122-130 South Main Street , Middleton MA Revised March 26, 2013 proposed_r2(exfiltration) Type ///24-hr 10 yr Rainfall=4.80" Prepared by Microsoft Printed 4/2/2013 HydroCAD®9.10 s/n 06611 @2010 HydroCAD Software Solutions LLC Page 2 Time span=0.00-40.00 hrs, dt=0.01 hrs, 4001 points x 3 Runoff by SCS TR-20 method, UH=SCS Reach routing by Dyn-Stor-Ind method - Pond routing by Dyn-Stor-Ind method Subcatchment 116S: PR-1131 Runoff Area=24,968 sf 84.92% Impervious Runoff Depth=4.11" Flowtength=230' Tc=15.0 min CN=94 Runoff=1.96 cfs 8,553 cf Subcatchment 120S: PR-SE Runoff Area=13,448 sf 33.45% Impervious Runoff Depth=2.90" Flow Length=70' Tc=9.2 min CN=82 Runoff=0.94 cfs 3,251 cf Subcatchment 121S: PR-132 Runoff Area=7,937 sf 37.13% Impervious Runoff Depth=2.99" Flow Length=100' Tc=3.5 min CN=83 Runoff=0.70 cfs 1,980 cf Subcatchment 122S: PR-WEST Runoff Area=21,303 sf 52.57% Impervious Runoff Depth=3.28" Flow Length=80' Tc=0.8 min CN=86 Runoff=2.24 cfs 5,824 cf Subcatchment 123S: PR-NORTH Runoff Area=11,931 sf 93.86% Impervious Runoff Depth=4.45" Flow Length=70' Tc=0.8 min CN=97 Runoff=1.53 cfs 4,422 cf Subcatchment 127S: PR-TRENCH Runoff Area=6,868 sf 67.68% Impervious Runoff Depth=3.68" Tc=1.0 min CN=90 Runoff=0.78 cfs 2,108 cf Pond 117P: BIOFILTER 1 Peak EIev=133.60' Storage=1,624 cf Inflow=1.96 cfs 8,553 cf Discarded=0.03 cfs 2,724 cf Primary=1.76 cfs 5,830 cf Outflow=1.79 cfs 8,553 cf Pond 120P: BIOFILTER 2 Peak Elev=133.38' Storage=193 cf Inflow=0.70 cfs 1,980 cf Discarded=0.00 cfs 317 cf Primary=0.66 cfs 1,663 cf Outflow=0.67 cfs 1,980 cf Pond 126P: Stone Trench Peak EIev=135.02' Storage=496 cf Inflow=0.78 cfs 2,108 cf Discarded=0.00 cfs 479 cf Primary=0.82 cfs 1,356 cf Outflow=0.83 cfs 1,835 cf Link 119L: Total Surficial Flow leaving limit of watershed analysis Inflow=6.27 cfs 22,347 cf Primary=6.27 cfs 22,347 cf Link 124L: Flow to the west Inflow=2.95 cfs 11,654 cf Primary=2.95 cfs 11,654 cf Link 125L: Flow to wetlands to southeast Inflow=2.04 cfs 6,270 cf Primary=2.04 cfs 6,270 cf Total Runoff Area=86,455 sf Runoff Volume=26,139 cf Average Runoff Depth =3.63" 35.58% Pervious=30,763 sf 64.42% Impervious= 55,692 sf rshed Analysis (With Exfiltration in Biofilters) - 122-130 South Main Street , Middleton MA Revised March 26, 2013 proposed_r2(exfiltration) Type ///24-hr 100 yr Rainfall=8.60" Prepared by Microsoft Printed 4/2/2013 HydroCAD®9.10 s/n 06611 @2010 HydroCAD Software Solutions LLC Page 3 Time span=0.00-40.00 hrs, dt=0.01 hrs, 4001 points x 3 Runoff by SCS TR-20 method, UH=SCS Reach routing by Dyn-Stor-Ind method - Pond routing by Dyn-Stor-Ind method Subcatchment 116S: PR-131 Runoff Area=24,968 sf 84.92% Impervious Runoff Depth=7.88" Flow Length=230' Tc=15.0 min CN=94 Runoff=3.64 cfs 16,393 cf Subcatchment 120S: PR-SE Runoff Area=13,448 sf 33.45% Impervious Runoff Depth=6.43" Flow Length=70' Tc=9.2 min CN=82 Runoff=2.03 cfs 7,207 cf Subcatchment 121S: PR-132 Runoff Area=7,937 sf 37.13% Impervious Runoff Depth=6.55" Flow Length=100' Tc=3.5 min CN=83 Runoff=1.48 cfs 4,334 cf Subcatchment 122S: PR-WEST Runoff Area=21,303 sf 52.57% Impervious Runoff Depth=6.91" Flow Length=80' Tc=0.8 min CN=86 Runoff=4.55 cfs 12,274 cf Subcatchment 123S: PR-NORTH Runoff Area=11,931 sf 93.86% Impervious Runoff Depth=8.24" Flow Length=70' Tc=0.8 min CN=97 Runoff=2.77 cfs 8,192 cf Subcatchment 127S: PR-TRENCH Runoff Area=6,868 sf 67.68% Impervious Runoff Depth=7.40" Tc=1.0 min CN=90 Runoff=1.52 cfs 4,233 cf Pond 117P: BIOFILTER 1 Peak Elev=133.69' Storage=1,983 cf Inflow=3.64 cfs 16,393 cf Discarded=0.03 cfs 2,966 cf Primary=3.38 cfs 13,427 cf Outflow=3.41 cfs 16,393 cf Pond 120P: BIOFILTER 2 Peak Elev=133.46' Storage=246 cf Inflow=1.48 cfs 4,334 cf Discarded=0.00 cfs 349 cf Primary=1.43 cfs 3,984 cf Outflow=1.44 cfs 4,334 cf Pond 126P: Stone Trench Peak Elev=135.02' Storage=496 cf Inflow=1.52 cfs 4,233 cf Discarded=0.00 cfs 512 cf Primary=1.53 cfs 3,448 cf Outflow=1.53 cfs 3,960 cf Link 119L: Total Surficial Flow leaving limit of watershed analysis Inflow=12.51 cfs 48,534 cf Primary=12.51 cfs 48,534 cf Link 124L: Flow to the west Inflow=6.00 cfs 25,701 cf Primary=6.00 cfs 25,701 cf Link 125L: Flow to wetlands to southeast Inflow=4.21 cfs 14,640 cf Primary=4.21 cfs 14,640 cf Total Runoff Area=86,455 sf Runoff Volume= 52,634 cf Average Runoff Depth =7.31" 35.58% Pervious=30,763 sf 64.42%Impervious = 55,692 sf Operation& Maintenance Plan "250 Clark Street " 250 Clark Street, North Andover, MA 01845 February 7, 2013 Revised: March 26, 2013 This Operation&Maintenance Plan has been prepared to comply with the provisions set forth in the Massachusetts Department of Environmental Protection(DEP) Stormwater Management Standards. Structural Best Management Practices (BMPs) require periodic maintenance to insure proper function and efficiency in pollutant removal from stormwater discharges that would otherwise reach wetland resource areas untreated. Maintenance schedules found below are as recommended in Department of Environmental Protections Massachusetts Stormwater Handbook and as recommended in the manufacturer's specifications. The following BMPs shall be utilized for project at 250 Clark Street for pollutant removal from stormwater discharge as well as providing additional groundwater recharge on site: • Biofilters with stone diaphragms • Stone Trench for roof runoff Biofilters: • Remove any sediment from the top of the stone diaphragms and from the pretreatment sideslope annually. • Mow the pretreatment sideslopes once a month during the growing season. • Repair areas of erosion and revegetate as needed by no less than once per year. • Cleanings may be taken to a landfill or other facility permitted by MassDEP to accept solid waste,without any prior approval by MassDEP. However,some landfills require catch basin cleanings to be tested before they are accepted. For information on all of the MassDEP requirements pertaining to the disposal of catch basin cleanings go to www.Mass. og v/dep/recycle/laws/cafacts.doc • Inspections shall be performed at least twice a year,or after a major storm event which is defined as a storm that is equal to or greater than the 2-year storm event. • During these inspections,record and map the following information: The types and distribution of the dominant wetland plants, The presence and distribution of invasive wetland species (invasives must be removed), Stability,of the sideslopes-and berms,... _.. Accumulation of sediment, Survival rate of plantings (dead plantings must be replanted) Operation &Maintenance Plan 250 Clark Street North Andover, MA 01845 February 7, 2013 Stone Trench for roof recharge: • Leaf screens shall be installed in the gutters to prevent debris from reaching the trench. • Install three (3) inspection ports to be located at the beginning,middle and end of trench. • Should standing water be present in bottom of system during a routine inspection or more than twenty-four (24)hours after the end of a rainfall event,it may be as a result of the stone bed layer being clogged. The amount of elapsed time required for the water to infiltrate completely should be recorded. If after seventy-two (72)hours from the end of a rainfall event there is still standing water in the bottom of the trench,the affected area should be excavated and the stone replaced with clean stone. • We recommend the following schedule for system maintenance Monthly in first year of service: -Check inlets for clogging and remove any debris as necessary -Check for depressions in areas over and surrounding the system Spring and fall in the second year service: -Check inlets for clogging and remove any debris as necessary -Check for depressions in areas over and surrounding the system One year after commissioning and every year following -Check inlets for clogging and remove any debris as necessary -Inspect the interior of the trench via inspection ports and measure sediment or depth of standing water -Check for depressions in areas over and surrounding the system -Confirm that no unauthorized modifications have been performed to the site • It should be noted that most failures occur in subsurface systems such as these due to inadequate pre-treatment which leads to clogging. However,this system shall only receive roof runoff from the roof,which is considered clean by DEP and does not require pre- treatment. Therefore,we do not expect there to be any adverse impacts to the system due to debris accumulation. 2 d' p Cull O 5= W 0) 0 t a I I—cn u _ J Q Q O- ... -.....- -._ -�..._. Q -..' 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