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13-073 Storm Drainage analysis RockbridgeSTORM DRAINAGE ANALYSIS AND DESIGN for the proposed improvements to "Rockridge at Laurel Park on 25 Coles Meadow Road Northampton, Massachusetts 01060 Prepared For New England Deaconess Association 80 Deaconess Road Concord, Massachusetts 01742 Project No. 60- 155.00 N ARRATIVE / 0&M PLA ONLY Prepared by. Coler & Colantonio, Inc. 1 Sugarloaf Street South Deerfield, MA 01373 Tel: (413) 665 -5300 Fax: (413) 665 -53 March 5, 2003 New England Deaconess Association 80 Deaconess Road Concord, Massachusetts 01742 STORM DRAINAGE ANALYSIS AND DESIGN for the proposed improvements to "Rockridge at Laurel Park" 25 Coles Meadow Road in Northampton, Massachusetts 1.0 INTRODUCTION Coler & Colantonio, Inc. has pkr {d drainage Park" as design assisted living tr ac lityin proposed improvements to the Roc g Northampton, Massachusetts. The property is shown on Assessor's Map 13, Lot ach and is l n owned and operated by New England D at the s A s s intersection of Coles Meadow Road and The existing 11.09 acre site is located North King Street, adjacent to the Massachusetts land of the Laurel Park Association, on, on the East bounded on the North and West by la East by North King Street, and on the South by Coles Meadow Road (refer to project locus in Appendix I). assisted The project proposes the expansion and redevelopment of the o ckr i d se ut living facility with improvements to the access drive, parking, round stormwater landscaping, and a closed-conveyance ancludes construc { t th on ee story bui d ng detention system. The proposed expansion ate and twelve one -story cottages with Iderp g cottages to the existing site for a assited living units and 12 attached elderl total of approximately 88 units. The existing main access drive has been lengthened and terminates in a cul -de -sac near the new access the one - bedroom cottages and terminates { in { ag parking area has been proposed to emergency hammer -head style turnaround. New parking areas have been added for and visitors and staff near the main entrance from service Coles Meadow Road endent serve the new sewer connections have been propose facility. The property is located in the suburban residential district (SR) within the planned f mixed village overlay. The surrounding areas along ed North King site is pre de�eloped { and o on sts residential and commercial uses. The prop o f of an existing three - story/one -story combination building a king lot for vehiclesrTher approximately 38,800 sf (14,500 sf ± {n pla ) and a p and are two existing access points to the site o s the main entrance includes a s king ne on Nrth Kin staff, t other off of Coles Meadow Road areas. A gravel access drive extends from the main entrance to Laurel Park and serves primarily as a secondary access to that facility. The site is Topographic relief across the existing moderately from the of ma ?n entrance to the nearly level near North King Street and climbs western property line near Coles Meadow bordering nadvegetated northeastern portion of the site wetlands separated by small contains two separate narrow bands of 9 1 New England Deaconess Association 80 Deaconess Road Concord, Massachusetts 01742 ridge. The tributary drainage area is M 9 e West. and encompasses the land between the property line and Coles Meadow Road to The purpose of this report is to compare the existing drainage conditions with proposed conditions relative ter mana ementt will be gradient prrovided according to policy y estabshed improvements. Storm 9 by the Massachusetts Department 0997 and thetcurPrent City of P lan Volumes 1 and 2 issued March 1 approval regulations. The propose' p n that will match or provide approach ess less impact development of the site through a c areful 9 for compared to the pre - development conditions. The storm o�e e 100 year saorm event ha impacts to the 2 -year, 10 -year, and 25- ear evaluated against adverse impact to downstream properties and will not substantially increase off -site flooding impacts. The closed- conveyance drainage system has been designed with deep sump catch basins, water quality inlets, smooth-walled theewly developed D n� newly g S exest i n emp collect and direct the surface w ater from areas to a series of corrugated u steel pipe underground o the etet practicable storage tl an d s feature a cont have been retrofitted and upgraded Stormceptor® water quality inlet s been rerouted re outed ntoothe the new conveyance system T foe e 44 car parking area has treatment of suspended solids designed to minimize the controlled release. The rate of discharger control management system has been erosion and sedimentation, provide for recharge of groundwater, and reduce the runoff characteristics for the 2 -year, 10 -year, and 25 -year storm events. The proposed improvements are shown on the plans entitled "Proposed Site Improvements, Rockridge at Laurel Park, Coles Meadow Road and North King Street, Northampton, Massachusetts'; date March 0 , 003 as prepared by Coler & Colantonio, Inc., 1 Sugarloaf Street, South Deerfield, 1.1 METHODOLOGY HydroCAD Stormwater Modeling System computer program by Applied es and volumes for the Microcomputer Systems was used t develop t s site. The software t uses Soil Conservation existing and proposed conditions the project Service (SCS) methodology. The SCS method is based on rainfall observations that were used to develop the Intensity- Du ratio re relationship SCS deterD ned u m y as9 the Weather Bureau's Rainfall Fr q uenc y Atlases, curves" could be used to represent the ens dimensionless distribution on of ala distributio over h the country. The mass curve is a indicates the fraction of the rainfall n develops peak ates of varyg precipitation event. This synthetic distributio durations and intensities. The SCS distribution provides a cumulative rainfall at any point in time and allows volume dependent routing runoff calculations to occur. to The HydroCAD software is a CS r methodology. Th HydroCAD p software sl ha s the TR -20. Both programs utilize the SCS New England Deaconess Association 80 Deaconess Road Concord, Massachusetts 01742 additional capability to describe shallow concentrated flow. The "NEH -4 Upland Method' included in the HydroCAD software is applicable for conditions which occur in the headwaters of a watershed up to 2000 acres. The NEH -4 Upland Method allows the Time of Concentration (T,) to reflect ground conditions such as overland flow, grassed waterways, paved areas and upland gullies. The T is the time required for water to flow from the most distant point on a runoff area to the measurement or collection point. This results in a model that more accurately reflects the ground surface for shallow concentrated flow conditions, than TR -20 which is limited to distinguishing only paved and unpaved surfaces. In instances where the watersheds are small and impervious, Tc has been directly entered as a 5- minute minimum. This is consistent with the Rational Method, TR -55 Urban Hydrology for Small Watersheds, and standard engineering practice. The lower boundary of 5 minutes will yield a conservative, yet practical measure of stormwater runoff flow. 1.2 SOIL MORPHOLOGY Soils are grouped according to their potential runoff characteristics. Soils are assigned to four groups, A through D. Group A contains soils with high infiltration rates when thoroughly wet and have low runoff potential. This group includes mainly sands and gravels consisting of deep, well to excessively drained media (sand, loamy sand or sandy loam). Group D, at the other end of the spectrum, has slow infiltration rates and high runoff potential. High water table, clay layers, and nearly impervious material are all characteristics associated with this group. A USDA Soil Conservation Service map can be found in the appendix indicating soils present on the project site according to the Soil Survey of Hampshire County Massachusetts, Central Part, December 1981. Soils throughout the site were classified into the following soil groups: Soil Group /Symbol Hydrologic Group Hinckley loamy sands (HgA) A Charlton sandy loams -Rock Outcrop Hollis complex (CrC) B The Soil Conservation Service (SCS) defines Hinckley loamy sands as deep and excessively drained. The soils are formed in glacial outwash deposits and are commonly found on outwash plains, kames, and terraces. Typical profiles of these soils indicate a very dark grayish brown loamy sand about 8 inches thick, and a 21 -inch subsoil consisting of brown loamy and gravelly sand. The substratum extends to a depth of 60 inches or more and is classified as a loose brownish yellow stratified sand, coarse sand, gravelly sand, and gravel. Permeability of these soils is rapid in the subsoil and very rapid in the substratum. These soils are well suited to cultivated crops, hay, pasture, and on -site absorption systems. Hinckley soils are commonly found on level to gently sloping areas on the sides of hills, ridges, and small drainage ways. Approximately 40% of the total tributary drainage area or 5.96 acres± consists of the Hinckley series. The Charlton series consists of deep, well- drained soils on uplands formed in glacial till. This soil is commonly found on hills and the side slopes of small drainage ways. 3 New England Deaconess Association 80 Deaconess Road Concord, Massachusetts 01742 A typical profile of Charlton soil is contained beneath wooded areas with a surface layer of about 7 inches containing dark fine sandy loam, a 15 inch thick subsoil of yellowish- brown, fine sandy loam and /or light -olive brown gravelly fine sandy loam, and an olive colored substratum that extends from a depth of 60 inches or more containing a massive, firm, gravelly sandy loam. Permeability of Charlton soil is moderately rapid in the subsoil and substratum. These soils are well- suited to trees and most areas are wooded. Slope is the main limitation of these soils for small commercial buildings n absorption fields. ton remaining 60% of the tributary drainage area or approximately soils. 1.3 DESIGN CRITERIA This drainage analysis was developed utilizing a Type III, 24 -hour storm as developed by the Soil Conservation Service. Two distinct design points were chosen at down gradient points in the drainage area to compare development conditions for each of the following storm frequencies. The design storm frequencies and corresponding rainfall depths were compiled from the "Atlas of Precipitation Extremes for the Northeastern United States and Southeastern Canada" and Technical Paper No. 40, Rainfall Frequency Atlas of the United States for Durations from 30 Minutes to 24 Hours and 1 to 100 Years" and have been estimated as follows: Storm Frequency (Years) Rainfall Depth (Inches) 100 7.3 25 5.4 10 4.4 2 3.0 1.4 EXISTING CONDITIONS The project site consists of an existing three -story building with one -story addition and parking facilities for approximately 44 vehicles. All of the existing developments are confined to the southern end of the site along Coles Meadow Road and North King Street. The grounds around the facility consist of light woods and grassed areas. The remaining land to the North and East of the existing buildings is wooded uplands. The site is bisected by a bordering vegetated wetland and depression to the North of the existing facility. It was determined through site visits, aerial photography, and soil surveys that the majority of the site is woods, lawn, and woods /lawn mix in good condition. The tributary drainage area has a weighted average CN value of 57. The attached Existing Drainage Plan in Appendix C delineates the watersheds for the project and has been divided into two distinct design points for runoff comparison. Design Point 1 is the area tributary to the 24" Box Culvert that passes under Route 5 &10 (North King Street) and eventually travels into the Interstate 91 drainage system. Design Point 2 is the area tributary to the 30" twin culverts to the extreme North of the site. This design point remains unchanged in the post drain condition as no improvements are proposed within this watershed. A limited stormwater conveyance system is contained 4 New England Deaconess Association 80 -Deaconess Road Concord, Massachusetts 01742 onsite and routes two catch basins from the main parking area under the building to the northeast side where roof drains from the existing facility and a catch basin in the service parking area terminate into an 12" RCP outfall. The outfall travels to an area upstream of Design Point#1. There appears to be no existing infiltration or water quality BMP's utilized in the current layout. The 100 -year storm was routed through the existing facility for illustrative purposes only. The *HydroCAD model confirms that this limited system cannot handle the expected flows from this storm event. * (see summary Section 1.7) 1.5 PROPOSED CONDITIONS The Applicant proposes to construct a three -story addition to the existing facility . including twelve one - bedroom cottages with a new building footprint area of approximately 0.67 acres. The associated site improvements include driveways, parking facilites, site utilities, ADA compliant walkways, landscaping, and a state -of- the -art underground stormwater management facility utilizing innovative treatment technologies. The existing entrance drive will be enlarged and extended to provide access to the new addition. A new internal driveway will stem from the existing access drive to provide vehicular entry to the cottages. Additional parking, including private parking areas and carports for the cottages will be incorporated into the site design to provide parking areas for approximately 90 total vehicles. Pedestrian access will be achieved through a system of lighted asphalt walks and features a wooded park and formal garden area. The service area entrance will be reconfigured and enlarged to provide parking spaces for staff including relocation of a storage building from the south side of the facility. A boardwalk and footpath will be constructed to provide access across the bordering wetland, making the existing community garden site accessible to the residents of the assisted - living facility. Impervious pavement areas will increase by approximately 1.1 acres have reduced calculated open space from 89% to 71%. The attached Proposed Drainage Plan contains the subcatchments for each tributary watershed. The weighted average CN value for the tributary watershed has increased to 63. The closed- conveyance stormwater management system consists of smooth - walled high- density polyethylene (HDPE) and corrugated steel drain pipe (CSP) connected between a series of precast manholes and deep sump hooded catch basins. The proposed underground detention facility is fabricated from perforated 60" diameter aluminized type 2 corrugated steel pipe (CSP) with 3 "x1" corrugations (12 ga.) to provide the required detention volume. The underground stormwater management facility is backfilled with 3 /4 angular crushed stone and wrapped with an engineered non -woven filter fabric to provide additional storage area and to facilitate infiltration into the surrounding soils. 36" diameter CSP connects the facility to a 30" diameter riser outlet structure contained within a 72" diameter manhole to control release rates of various design storms. The proposed stormwater management detention structure outlets to a level spreader to dissapate energy and control erosion. The existing 12" metal pipe /RCP drainage system and outlet will continue to convey roof runoff from the existing three -story building. To guard against possible impacts from the proposed, erosion and sedimentation control measures will be incorporated into the sequence of construction. This will include both modified construction procedures and temporary construction installations to control the movement of sediment into protected areas. Upon completion of all work, restoration and stabilization of disturbed areas will commence. Erosion /sedimentation control New England Deaconess Association 80 Deaconess Road Concord, Massachusetts 01742 installations include placement of hay bales and filter- fabric fencing around disturbed areas and at storm drainage inlet points. On -going stabilization of I disturbed areas will be completed as the work progresses. Restoration/stabilization measures include seeding, mulching, and placement good vegetative a c ' over ' s established and required. s accepted by the Northampton removed once g Conservation Commission. The proposed drainage improvements have been designed to meet the City of Northampton site plan approval requirements as well as the Massachusetts Department of Environmental Protection's Stormwater Management Policy. The stormwater conveyance and detention system has been designed to mitigate the increases in the peak rates of runoff from the site for the 2 -year, 10 -year, and 25 -year 24 -hour design storm events. The 100 -year design storm has been evaluated for the site and exceeds the flow of the pre - development condition", but does not appear to affect downstream properties. 1.6 SUMMARY OF RESULTS AND CALCULATIONS The stormwater management for the proposed project includes: stormwater collection, treatment, and infiltration systems. These proposed mitigation measures were designed in accordance with the Massachusetts Department of Environmental Protection " Stormwater Management Volume 1 & 2, the Vortech® stormwater system, and the Stormceptor® system. In accordance with these regulations and standard engineering practice, the potential impacts of this project on downstream properties are mitigated and summarized in the tables below. The following is a summary of the calculation results for runoff encompassing the areas tributary to the Rockridge at Laurel Park improvements for the 2 -year, 10 -year, 25- year, and 100 -year design storm events. Detailed routing and volume calculations can be found in various sections appended to this narrative. Existing Conditions Summary Design Point 1 - 24" Box Culvert "Design Point 2/20 - Twin 30" Culverts 3.0 2.16 0.356 0.950 1.493 # 2.726 0.01 0.19 0.61 2.41 0.003 0.040 0.089 0.220 4.4 8.33 5.4 13.30 7.3 # 22.65 1 New England Deaconess Association 80 Deaconess Road Concord, Massachusetts 01742 Proposed Conditions Summary Design Point 10 - 24" Box Culvert UNMITIGATED UNMITIGATED " MITIGATED ** MITIGATED . r 3.0 6.94 0.724 1.90 0.403 )- r 4.4 14.57 1.477 7.00 1.082 i- r 5.4 21.47 2.116 11.97 1.688 0- r 7.3 36.70 3.486 34.09 3.005 Notes: Design Point #2 is identical to Design Point #20. No improvements are proposed in the area tributary to this point, therefore no comparison between existing and proposed is warranted. ** Mitigated runoff rates and volumes as shown include the benefit of BMP's, Detention, and Infiltration Measures. # The 100 -year storm values as shown are for illustrative purposes only. The HydroCAD model for the existing storm drainage condition indicates that the current system cannot handle this storm event. The tables above indicate that peak rates have remained consistent or decreased between post- drainage and pre- drainage conditions when summed at the design points down gradient of the site. The new stormwater system conveyances will not discharge untreated runoff into any resource or buffer areas and have been designed to infiltrate into the surrounding soils through partial exfiltration in the underground stormwater management system. A presumed increase in runoff is shown for the 100 -year post condition at Design Point 10. The 100 -year storm is conveyed through a 30" horizontal orifice emergency outfall emptying into the wetland upstream of the 24" square box culvert. The existing 12" RCP outfall has been retained to convey clean roof runoff from the existing three -story building to the wetland. The groundwater recharge volume for this site has been calculated based upon type A and type B hydrologic soils. The overall tributary drainage area consists of a 60 -40 split for group B and A soils respectively. Only 5% of the proposed impervious area covers Type A soil. The recharge calculations have been adjusted to reflect a decrease in required volume to compensate for existing impervious areas. The adjusted recharge volume is calculated by reducing the total required recharge volume by the existing required volume to total 0.037 acre -feet or 1,600 cf. Groundwater recharge is achieved through partial exfiltration in the underground stormwater management system below the outlet structure and in the stone and perforated pipe infiltration trench near the main 7 New England Deaconess Association 80 Deaconess Road Concord, Massachusetts 01742 entrance. The 2ft x 3ft x145 ft long infiltration trench has been incorporated into the design to help convey snowmelt and runoff from Subcatchment 26. This area was removed from Subcatchment 3 to help reduce the burden of the existing drainage system and to increase the "dead storage" available for recharge. A total of approximately 1,600 cf of "dead storage" is available for infiltration in these BMP's prior to any release. It is important to note that these BMP's will continue recharge to groundwater as flow rates increase above outlet inverts. The post- drainage conditions will not reduce groundwater recharge as compared to the existing site and should improve as much of the existing impervious areas have been directed towards the detention facility for partial infiltration. A long -term infiltration rate of 2.2 inches per hour or 0.003 feet per minute was estimated using the USDA soil classification of gravelly sandy loam and loamy sands presumed in the detention area. This infiltration rate is consistent with the rapid permeability in the substratum as listed in the USDA Soil Survey Engineering Properties and concurs with published permeability rates for Charlton (CrC) and Hinckley (HgA) soils ranging from 0.6 to 20 inches /hour. The depth to groundwater and long -term infiltration rate will be confirmed through exploratory borings scheduled in for building and foundation design. Treatment of stormwater runoff volume equal to 0.5" of rainfall over the proposed impervious areas tributary to the drainage area has been provided by the stormwater management system for the purpose of meeting water quality standards. An increase of 1.76 acres + of impervious area is proposed in the current layout (3.24a total - 1.48a existing = 1.76a). The total impervious area of 1.76 acres produces the required total volume of 0.073 acre -feet. A separate storm drainage conveyance system has been designed to covey approximately 0.67 acres (29,000 sf ±) of new clean roof runoff to the detention system for partial exfiltration. The actual volume to be treated has been reduced to 0.045 acre -feet or 1,960 cubic feet by factoring in the reduction of roof top runoff volume (1.76a - 0.67a =1.09a x 0.5" depth or 1,960 cf) because it is separated and conveyed separately to the stormwater management facility. The required water quality volume has been met through innovative technology by incorporating Stormceptor ® and VortechsTM treatment systems. These water quality inlets incorporate baffles, weirs, and hydrodynamic forces to enhance gravitational separation of floating and settling materials from stormwater flows without conventional sediment traps. The Stormceptor® has been sized based upon proprietary software which calculates sediment removal efficiency based upon particle size distribution. The VortechsTM system has been specified as an off -line system that diverts low flows to the unit for treatment. Each system has a high flow by -pass to safely convey large, low probability storm events. The proposed stormwater management system has been designed to provide in excess of the required "calculated" 80% removal of Total Suspended Solids (TSS) tributary to the conveyance system on the proposed site per the Massachusetts D.E.P. Stormwater Management Policy. Discretionary street sweeping, deep sump catch basins, inlet Stormceptor® and off -line Vortechs® systems have been utilized in the design to remove an average of 87% of the Total Suspended Solids (TSS) from the tributary site runoff. The performance of the Vortech® and Stormceptor® systems have been verified by the Massachusetts Envirotechnology Partnership (STEP) and are approved for wide spread use. The effect of the underground detention facility was not factored into the calculations because the removal of accumulated sediment is more difficult than a conventional at- grade system. M New England Deaconess Association 80 Deaconess Road Concord, Massachusetts 01742 The existing asphalt parking and service areas were retrofitted and routed through water quality inlets and detention facilities to reduce impact from the site. A chart illustrating each proposed conveyance to the detention facility is provided in the supplemental calculations. The supporting calculations indicate that the proposed facilities are more than sufficient to mitigate the effects of the development on stormwater runoff quality entering the resource areas. 1.7 STORMWATER MANAGEMENT STANDARDS The following is an explanation on how the proposed project meets the Stormwater Management Standards as prepared by the MA Department of Environmental Protection and the MA Office of Coastal Zone Management. Please refer to the Stormwater Management Form in Appendix B. Untreated Stormwater (Standard 1) - No new stormwater system conveyances will discharge untreated runoff into the resource or buffer areas or cause erosion in the onsite wetlands. A closed- conveyance drainage system has been designed to direct stormwater runoff from the newly developed areas to Stormceptor® and Vortechsim water quality inlets for pre- treatment prior to partial exfiltration of first flush runoff through the underground stormwater management system. A riprap outlet spreader will dissipate runoff energy from significant storm events before release into a wetland resource area. The wetland will transmit the clean runoff to the 24" Box Culvert eventually traveling into the Interstate 91 drainage system. Post - Development Peak Discharge Rates (Standard 2) — The peak discharge rates were calculated with the aid of a hydrograph routing program using TR -20 methodology. A multi -stage outlet structure has been sized to meet the run -off rates for the 2 -yr, 10 -yr and 25 -yr storm events and reduce the volume of runoff over unmitigated levels. The stormwater detention system will partially detain the 100 -year storm event to provide a buffer against off -site flooding impacts. A 30" diameter emergency overflow orifice has been incorporated into the outlet structure to convey the 100 -yr storm event. Exfiltration will occur in the underground stormwater management system after pre- treatment through deep sump catch basins and water quality inlets. Recharge to Groundwater (Standard 3) - The majority (95 %) of the impervious areas for the tributary drainage area are contained within Hydrologic Grouping B as shown on the drainage area maps. Recharge to groundwater will be provided through partial exfiltration in the underground stormwater management system and through a 145 -foot long infiltration trench. An exfiltration rate of 2.2 inch /hour (0.003 feet/minute) is utilized in the calculations and is consistent with sandy loam /loamy sand soils mapped onsite. The proposed development and expansion will not reduce groundwater recharge as compared to the existing site and will approximate current conditions to the extent feasible. Supplemental drainage calculations found in the appendix. Removal of 80% TSS (Standard 4) - The proposed stormwater management system has been designed to provide the required "calculated" removal of the Total Suspended Solids (TSS) for the conveyance system on the proposed site per the Massachusetts D.E.P. Stormwater Management Policy. Discretionary street sweeping, deep sump hooded catch 9 New England Deaconess Association 80 Deaconess Road Concord, Massachusetts 01742 basins, innovative water quality inlets, and a partial exfiltration detention facility has been utilized in the design to remove an average of at least 80% of the Total Suspended Solids (TSS) from the total site storm runoff. Large portions of the existing impervious parking and service areas have been captured and treated by the proposed system to provide an increase in treatment for the redeveloped portions of the site. A chart illustrating each proposed conveyance outfall is provided in the supplemental calculations. Land Uses with Higher Pollutant Loads (Standard 5) — The property is zoned suburban residential (SR) district within the planned village overlay. The proposed use as an assisted living facility is not considered a land use with high pollutant loads. Critical Areas (Standard 6) — The project is not located within a critical area as defined the Stormwater Management Policy. For the purposes of this design, an equivalent Water Quality Volume of 0.5" x new impervious area was maintained and treated through innovative technologies to preserve and protect the Bordering Vegetated Wetland resource areas and buffer zones. Redevelopment (Standard 7) — The project proposes a combination of redevelopment and new development on the site. The associated improvements have been designed with BMP's to the extent practicable to meet the performance standards in the Stormwater Management Policy including retrofitting and expanding the existing system to meet TSS removal, water quality, and mitigation of peak discharge rates. Erosion and Sediment Controls (Standard 8) — Staked hay bales and filter- fabric fencing will be used during construction as outlined in the Operation & Maintenance Plan and shown on the plan set. Direction of silt -laden runoff shall be directed towards vegetated areas, temporary sedimentation basins, and diversion swales. Operation and Maintenance Plan (Standard 9) — An Operation and Maintenance plan has been customized to fit the design of the proposed Rockridge site improvements. Provisions to maintain runoff control devices have been assured through structural, non- structural, and construction management approaches. Please see the attached O &M Plan. 1.8 CONCLUSION In conclusion, the results of this report indicate that the proposed stormwater management plan appears to meet all requirements of the City of Northampton and the Massachusetts Department of Environmental Protection's " Stormwater Management Policy" within the scope and parameters of the proposed project. 10 New England Deaconess Association 80 Deaconess Road Concord, Massachusetts 01742 STORMWATER OPERATION AND MAINTENANCE PLAN OWNER AND RESPONSIBLE PARTY: Construction Phase Activities: New England Deaconess Association 80 Deaconess Road Concord, Massachusetts 01742 Contact: Mr. Walter Bartkus, Director of Engineering Services Phone: (978) 369 -5151 Day -to -day Operation and Maintenance: Rockridge at Laurel Park 25 Coles Meadow Road Northampton, Massachusetts 01060 Contact: Mr. Walter Jones Phone: (413) 586 -2902 PROJECT OVERVIEW The proposed project includes construction of a three -story addition (37,500 sf gross floor area) to the existing facility plus 12 one- bedroom cottages and associated site improvements including parking facilites, paved access drives, utilities, landscaping, ADA compliant walkways, and a closed- conveyance stormwater managememt system. CONSTRUCTION MANAGEMENT A construction manager with adequate knowledge and experience on projects of similar size and scope shall be employed to oversee all sitework related construction. During construction, silt -laden runoff or discharge from dewatering operations (if necessary) will be prevented from entering wetlands and resource areas untreated. Siltation barriers consisting of a filter fabric silt fence, hay bales will be erected in advance of construction along the downstream edge of all disturbed areas and maintained throughout the construction period. Check dams will be used in temporary constructed drainage ways as necessary to reduce erosion. If dewatering is required during the construction, discharges may be directed toward vegetated areas for settlement of suspended solids. Outflow of silt -laden runoff shall not be permitted to flow directly into the wetlands or resource areas. Upon completion of site stabilization, the catch basins and existing conveyance system 11 New England Deaconess Association 80 Deaconess Road Concord, Massachusetts 01742 shall be thoroughly cleaned of silt and sediment and made ready for the proposed operation. Siltation barriers, temporary settling basins, and diversion berms shall be constructed and inspected by the resident project Engineer on a monthly basis or as necessary, after any significant (0.5" or more) storm event and daily while dewatering operations are proceeding. Care should be taken when constructing stormwater control structures. Light earth - moving equipment shall be used to excavate in the vicinity of the infiltration areas. Use of heavy- equipment causes excessive compaction of the soils beneath the basin resulting in reduced infiltration capacity. At no time shall temporary infiltration areas or settling basins be constructed in the vicinity of the proposed infiltration system in order to prevent the soils from becoming clogged with sediment. NON - STRUCTURAL APPROACHES PARKING LOT /DRIVEWAY SWEEPING The property owner shall maintain a program of parking lot/driveway sweeping to reduce sediment accumulation in the deep sump catch basins. It is recommended that sweeping occur at least once in the fall and spring prior to cleaning and inspection of the catch basin and manhole structures. GRADING The impervious areas of the site shall be graded as gently as possible, generally not more than 4% or 5% slopes to reduce runoff velocities. Steep slopes will be permanently vegetated to dissipate energy and reduce potential erosion. No constructed vegetated slopes should exceed 2H:1 V. Steep slopes may require soil reinforcement and additional vegetation. FLOW OVER VEGETATED AREAS Wherever possible, runoff from paved areas and snowmelt shall be directed over vegetated areas to promote settlement of suspended solids before entering a wetland or resource area. STRUCTURAL BEST MANAGEMENT PRACTICES DEEP SUMP CATCH BASINS, AREA DRAINS, AND MANHOLE STRUCTURES Catch basins shall be cleaned, in dry weather, when half of the sump capacity is filled or at a minimum of twice a year. Cleaning will take place at the completion of construction, in early spring after sanding of roadways has ceased, and in the fall. All manholes shall be inspected at least once annually. Any obstructions, sediment, and debris that could potentially cause clogs shall be removed within the 12 New England Deaconess Association 80 Deaconess Road Concord, Massachusetts 01742 conveyance system as necessary. Inverts, grates, and hoods shall be checked and replaced as necessary to maintain hydraulic effectiveness. DETENTION FACILITY WITH PARTIAL EXFILTRATION The underground detention system has been designed with riser structures at grade to aid the removal of sediment and debris accumulating in the structure. The detention facility contains a multi -stage outlet structure to meet individual storm events. Once the system goes online, inspections should occur after each storm event for the first few months to ensure proper stabilization, function, and to ensure that the outlets remain free of obstructions. Preventative maintenance shall be performed at least twice per year and after every major storm event (> 1" of rainfall) and shall include removal of accumulated sediment, inspection of the detention structure, and monitoring of groundwater and infiltration rates to ensure proper operation of the system. Important items to check for include differential settlement, cracking, breakout, clogging of outlets and vents, and root infestation. Water levels should be checked and recorded against rainfall amounts to verify that the drainage system is working properly. ENERGY DISSAPATORS AND LEVEL SPREADERS During the construction phases of the project, the energy dissipaters and level spreaders that receive significant runoff shall be inspected monthly and cleaned as necessary and /or after major storms events (> 1" of rainfall). Thereafter, these structures will be cleaned at least twice per year, in the spring and fall, or as needed depending on the frequency of major storm events (> 1" of rainfall). The riprap shall be inspected, cleaned of sediment and debris, and reinstalled as necessary to maintain effectiveness. GRASS -LINED SWALES Swales shall be inspected on a semi - annual basis in the early spring and fall. Additional inspections shall be performed during the first few months after construction to ensure that adequate vegetation has been established. Regular maintenance shall include mowing, fertilizing, pruning, debris removal, and weed /pest control. Swales shall be mowed at least once per year to a minimum of 4 inches so as not to reduce the effectiveness of pollutant removal and energy dissipation. Accumulated sediment and debris shall be manually removed at least twice per year. WATER QUALITY INLETS Water quality inlets shall be inspected and cleaned in accordance with the manufacturer's recommendations. Documentation from the manufacturer is attached. 13