Loading...
Burts Pit Road stormwater excerpt.pdfBurts Bog Limited Development – Burts Pit Road April 18, 2017 (Revised June 6, 2017) Northampton, Massachusetts Stormwater Management Report The Berkshire Design Group, Inc. Page 1 The City of Northampton is proposing the construction of 10 single-family homes as part of a Conservation and Limited Development project on Burts Pit Road in Northampton. The Berkshire Design Group has prepared a Stormwater Management and Erosion and Sedimentation Control Plan for the site, in compliance with the Massachusetts Stormwater Handbook and the Code of the City of Northampton. This report documents the proposed Stormwater Management Plan and that plan’s compliance with those standards. I. Introduction The proposed project will construct 10 single-family houses (including 2 zero-lot-line houses separated by a party wall) on 10 newly-created residential lots. The project is located on Burts Pit Road between Emerson Way and Woods Road, at the south end of the Burts Bog Greenway on land currently owned by Tofino Associates. The project location is shown in Figure 1. The work includes construction of the proposed buildings, as well as associated site, utility and stormwater improvements. The existing site is undeveloped, and primarily wooded, with a small amount of grass cover near the street. A low point carries through the site on an east-west axis, creating a gradual swale sloping to the east and west from a high point near the southeast corner of the site. A hillside extends from the northern site limit, resulting in a significant volume of run-on from land outside the site boundary. Wetland areas exist outside the site limits to the east, west and north. None of these wetlands or their associated buffer areas are impacted by the work. There is an existing storm drainage system in Burts Pit Road; however, the southern edge of the site generally slopes away from the street onto the site, and it appears that a negligible amount of runoff from the site reaches Burts Pit Road. Soil Data NRCS Soil Survey The NRCS Soil Survey reports that the soils at the site consist of fine sandy loam of Hydrologic Soil Group C. The USGS Surficial Geology survey reports that the site is underlain by a relatively thin (<10’ thick) layer of glacial till. The NRCS Soil Report for the site is attached in Appendix A. Subsurface Exploration A total of 4 test pits were conducted at the site in order to classify existing soils and determine the relevant engineering properties. A summary of these explorations is included in Appendix B. Soils were found to be highly consistent across the site. The typical test pit revealed 6-8 inches of topsoil underlain by sandy loam subsoil. The site is underlain by a dense layer of red glacial till that is classified as sandy loam. In general, all soils have a significant gravel content, and are dotted with larger stones and cobbles, but soil texture becomes slightly more fine with depth. Seasonal high groundwater was estimated to occur at a depth of 24-30” across the site. Burts Bog Limited Development – Burts Pit Road April 18, 2017 (Revised June 6, 2017) Northampton, Massachusetts Stormwater Management Report The Berkshire Design Group, Inc. Page 2 Site Limits Site limits were delineated for the purpose of establishing drainage areas within the site. Site limits were based on property lines, as adjusted to account for up-gradient areas where stormwater is expected to run on to the site. II. Existing Conditions An Existing Conditions Plan is shown on Figure 2. A plan of the Existing Hydrology is shown on Figure 4. Drainage Areas The existing site was divided into eight drainage areas. The majority of the site is collected by a gentle swale traversing the site from east to west, which discharges at the western property line at the point labeled CP1 (Control Point 1) on Figure 4. The remainder of the site is collected by a swale sloping in the opposite direction, toward the east, which discharges at the point labeled CP2 (Control Point 2). The overall drainage area flowing toward CP1 was divided into 7 sub-drainage-areas (A1 through A7). Because the stormwater model can be affected by the way in which drainage areas are divided, these sub-drainage-areas were drawn to match the approximate divisions that are created by the proposed work, which allows for a more accurate comparison of the pre-development and post development condition. III. Proposed Conditions The proposed site plan is shown on the Site Plan in Figure 3. The proposed project will construct 10 single family houses on 10 building lots. Two of the houses are constructed as zero-lot-line houses sharing a party wall (see Lots 5 and 7 in Figure 3). Seven houses are planned to include an attached two-car garage, and each lot provides for the construction of a shed or other accessory structure. Four paved, shared driveways are proposed to serve the ten homes. As the developer proposing the project, the City of Northampton directed Berkshire Design to provide for stormwater management, to the greatest extent possible, through decentralized systems to be maintained by individual homeowners. A series of rain gardens and swales are proposed to attenuate peak runoff, improve water quality and promote groundwater recharge. These features are shown on Figure 3, and highlighted on Figure 5. It is notable that a significant portion of this attenuation is achieved by intercepting run-on stormwater from the hillside above the site. Because of the large area and relatively poor soils, the hillside produces a significant amount of stormwater runoff which enters the site in the existing condition. Four large rain gardens are to be built along the northern edge of the site to intercept, infiltrate, and redirect this runoff. The work of these rain gardens partially offsets the increase in runoff created by development of the interior of the site. The system has been designed to reduce peak runoff. The proposed detention basin is designed to attenuate the 2, 10, and 100-year storms, and to produce a reduction in peak runoff from the site. Burts Bog Limited Development – Burts Pit Road April 18, 2017 (Revised June 6, 2017) Northampton, Massachusetts Stormwater Management Report The Berkshire Design Group, Inc. Page 3 The prosed design is also designed to reduce both peak runoff and runoff volume that discharge to adjacent properties to the east and to the west. The soil evaluation revealed soils of marginal permeability and relatively shallow groundwater. Nevertheless, opportunities for infiltration and groundwater recharge were maximized in the design. Rain gardens will retain 9” of water, with an overflow weir built with crushed stone. The bottom of each rain garden is proposed at an elevation 24” above the estimated seasonal high groundwater at the nearest test pit. The soil evaluation revealed that surface soils are primarily sandy loam, which has a standard “Rawls” infiltration rate of 1.02 inches per hour1, which would allow the retained water to dissipate within the 72 hours required by the Massachusetts stormwater handbook. Rain gardens include an engineered outlet weir to reduce peak flow rates during intense rainfall. The plans call for a weir to be carved out of the berm enclosing the rain gardens, which provides a 24” wide opening for water to overflow from the rain gardens. The weir would be armored with 1” to 2” crushed stone. The invert of the weir is set 9” above the bottom of the rain garden (which is 3” below the crest elevation of the gardens).The rain gardens would be planted with a simple woodland seed mix, which would require annual mowing to prevent the growth of woody plants. Western Discharge Point A critical location in the design is the discharge point at the western end of the site. In the existing condition, approximately 75% of the site drainage areas collect at a point along the western property line at a low point that is consistently wet, and appears to impound water during times of high groundwater. There is no obvious discharge location for this impounded water. The topographic survey shows that surrounding land to the north, south and east is at least 6” higher than the low point. The topographic survey does not show the elevations of the neighboring property (829 Burts Pit Road); however, based on field observation, the grass lawn to the north of the house on that property appears to be higher than the low point by 6-12” or more. Based on this information, it is likely that this low area floods during intense rainfall, which ultimately forces water to flow to the north of the lawn at 829 Burts Pit Road. According to GIS information, land north and west of 829 slopes more steeply to the west, ultimately forming a small stream/drainage channel that is carried by a culvert under Woods Road. The Burts Bog development project proposes to leave this area undisturbed, and to allow existing drainage patters to continue. The limit of disturbance on Lot 2 maintains a 30’ buffer at the western edge of the site, which is extended to 70’ where water is carried under the Lot 1/2 common driveway. The proposed stormwater management system has been designed to mitigate increases in both peak flow rate and volume from the developed site verses the existing condition. The stormwater calculations predict that the existing drainage condition in this part of the site will be no worse in the proposed condition. Additionally, a portion of the run-on to the site from the hillside above is discharged north of the proposed houses, where it can more directly enter the proposed conservation land north and west of the site (see Rain Garden 2A on Sheet C-3). 1 For calculation purposes, the infiltration rate was reduced to a more conservative value of 0.25 inches per hour in the stormwater model. The model is discussed further in Section IV. Burts Bog Limited Development – Burts Pit Road April 18, 2017 (Revised June 6, 2017) Northampton, Massachusetts Stormwater Management Report The Berkshire Design Group, Inc. Page 5 near Burts Pit Road discharge to the road, with runoff from one drainage area entering the storm drain system flowing toward the west, and the other entering the storm drain system flowing toward the east. IV. Calculations and Design Water Quantity – Flow Rate Drainage calculations were performed on Hydrocad Stormwater Modeling System version 10.0 using Soil Conservation Service (SCS) TR-20 methodology. The SCS method is based on rainfall observations, which were used to develop the Intensity-Duration-Frequency relationship, or IDF curve. The mass curve is a dimensionless distribution of rainfall over time, which indicates the fraction of the rainfall event that occurs at a given time within a 24-hour precipitation event. This synthetic distribution develops peak rates for storms of varying duration and intensities. The SCS distribution provides a cumulative rainfall at any point in time and allows volume-dependent routing runoff calculations to occur. These calculations are included in the appendices. The watershed boundaries for calculation purposes are divided according to the proposed site grading and the natural limits of the drainage areas. The curve numbers (CNs) and times of concentration for the existing and proposed subcatchment areas are based on the soil type and the existing and proposed cover conditions at the site. Calculations were performed for the 2-, 10-, and 100-year frequency storms under existing and proposed conditions. The results of the calculations are presented in Table 1 below. Appendix C presents the HydroCAD output reports. Table 1. Runoff Summary Table for Individual Control Points Condition & Point of Analysis 2-Year Storm 3.00” 10-Year Storm 4.50” 100-Year Storm 6.40” Peak Flow Rate(cfs) Peak Flow Rate(cfs) Peak Flow Rate(cfs) Existing To West Property Line (CP1)* 2.54 6.92 13.49 Proposed To West Property Line (CP1)* 2.33 6.75 12.96 Existing To East Property Line (CP2)* 0.35 0.91 1.75 Proposed To East Property Line (CP2)* 0.34 0.80 1.68 Burts Bog Limited Development – Burts Pit Road April 18, 2017 (Revised June 6, 2017) Northampton, Massachusetts Stormwater Management Report The Berkshire Design Group, Inc. Page 6 Condition & Point of Analysis 2-Year Storm 3.00” 10-Year Storm 4.50” 100-Year Storm 6.40” Peak Flow Rate(cfs) Peak Flow Rate(cfs) Peak Flow Rate(cfs) Existing To Roadway, Flowing West (CP3)* 0.05 0.11 0.20 Proposed To Roadway, Flowing West (CP3)* 0.13 0.27 0.47 Existing To Roadway, Flowing East (CP4)* 0.06 0.15 0.28 Proposed To Roadway, Flowing East (CP4)* 0.05 0.11 0.20 *Names in parentheses refer to HydroCad model and calculations. Table 2. Runoff Summary Table For Total Site Condition & Point of Analysis 2-Year Storm 3.00” 10-Year Storm 4.50” 100-Year Storm 6.40” Peak Flow Rate(cfs) Peak Flow Rate(cfs) Peak Flow Rate(cfs) Existing Total Runoff (CPT)* 2.91 7.91 15.41 Proposed Total Runoff (CPT)* 2.78 7.77 15.07 The model predicts that peak flow rate from the total site will be reduced relative to the existing condition under each design storm. The model also predicts that peak flow rate crossing the east and west property lines will be reduced relative to the existing condition. Flow rate to the roadway (CP3 and CP4) is predicted to increase. The drainage area to the roadway is small and this runoff is primarily generated by driveway aprons, which must be sloped toward the road to maintain gutter flow. Water Quantity – Volume The site is underlain entirely by soils belonging to Hydrologic Soil Group C. Therefore, the Northampton Stormwater Ordinance requires only that stormwater infiltration be achieved to the greatest extent practicable. Due to the use of rain gardens as a primary means of stormwater management, the proposed system is anticipated to result in relatively high levels of infiltration. The results of the HydroCAD model for stormwater runoff volume in the existing and proposed condition are presented in Table 2 for the 2-year design storm.