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HomeMy WebLinkAboutStudies Applicant 6/5/2024 Stormwater Management Plan and Drainage Feasibility Study Robert Kemp Springfield, Oregon For Robert Kemp 904 Filbert Lane Springfield, OR 97478 June 4, 2024 © Branch Engineering, Inc. Contents Page No. 1.0 Introduction ............................................................................................................... 1 2.0 Existing Conditions ................................................................................................. 1 2.1 Topography 1 2.2 Soils 1 2.3 Existing Storm System 1 3.0 Proposed Development ......................................................................................... 1 4.0 Stormwater Constraints ........................................................................................ 2 5.0 Proposed Development Stormwater Description ....................................... 2 5.1 Single Family Home or Duplex 2 5.2 Middle Housing Developments 2 6.0 Maintenance ............................................................................................................... 3 7.0 Conclusion .................................................................................................................. 3 Appendices Description Appendix A Stormwater Management Drainage Basin Map Appendix B Site Soils and Map 1 1.0 INTRODUCTION This Stormwater Management Plan (SWMP) has been prepared in support of a Tentative Partition Application for the property located at the southwest corner of the intersection of Filbert Lane and North Street, identified as 904 Filbert Lane, Springfield, Oregon (Tax Map 18-02-06-14, Tax Lot 6200). This partition will result in the creation of two distinct parcels: Parcel 1, which will retain all current improvements including the home and driveway, and Parcel 2, which also has a driveway and is designated for future residential development. This SWMP outlines considerations for stormwater treatment and discharge for any future development on Parcel 2 in compliance with the City of Springfield's regulations and guidance from the City of Eugene Stormwater Management Manual. The purpose of this report is to document the feasibility of future development. 2.0 EXISTING CONDITIONS The property is currently underdeveloped, with a majority of the improvements located on the northerly half of the property. There is a small gravel driveway along its southerly edge that is utilized by the property owner and the neighbor to the south. 2.1 TOPOGRAPHY The existing improvements on the site include a home and two driveways. Filbert Lane, located along the easterly property line, drains southward toward Jasper Road, while North Street, along the northerly property line, drains eastward toward Filbert Lane. Within the site there is a natural low spot at the southwest corner. Additionally, a ridge line running north-south along the center of the property diverts runoff from the site to the east and west. 2.2 SOILS The Natural Resources Conservation Service (NRCS) Web Soil Survey maps the development site as composed entirely of Chapman-Urban land complex (25). This soil type is described as a well- drained alluvium and falls within Hydrologic Soil Group B. A soils map and individual soil information are provided in Appendix B. 2.3 EXISTING STORM SYSTEM The existing home does not connect to the public stormwater system. As the site is very flat, most rainfall infiltrates on-site with minimal off-site runoff. According to the City of Springfield’s Stormwater Basin Master Plan, there are no downstream deficiencies. Curb inlets at the intersection of Filbert Lane and North Street feed into the Filbert Lane stormwater system. 3.0 PROPOSED DEVELOPMENT As outlined in the tentative partition plan shown in Appendix A, the division of the property at 904 Filbert Lane will result in the creation of two parcels. Parcel 1 will retain all existing improvements in the northerly half of the property, including the home and driveway, and encompasses 7,288 square feet. Parcel 2, consisting of 8,661 square feet, is currently vacant but is designated for future residential development. Parcel 2 will include a 20.00-foot-wide driveway along the southerly boundary that benefits the neighbors to the south and west. This driveway will be widened and improved at the time of home 2 construction on Parcel 2. A majority of vehicular access for Parcel 2 will share use of this driveway with the neighbors, ensuring efficient ingress and egress from the properties. Public improvements along North Street are proposed as part of this development. These improvements will include the construction of a new sidewalk and a roadside ditch. 4.0 STORMWATER CONSTRAINTS Two constraints affect the site: 1. Connection Point Proximity and Depth - Any proposed development on the site needs to consider the location and depth of the connection point for stormwater discharge. The desired connection point within the intersection of Filbert Lane and North Street has a maximum depth of approximately 4 feet below the rim of the curb inlet, and is approximately 100 feet north of Parcel 2. The elevation of Parcel 2 is 1-2 feet above the rim of this inlet. The distance that the stormwater pipe would need to extend, along with the depth to the connection point, may pose limitations on the feasibility of certain stormwater management solutions. 2. Stormwater Pipe Size Limitations - The existing stormwater pipe within Filbert Lane is a 24- inch pipe. Connecting to this pipe may necessitate a public improvement permit if a blind connection is not feasible or permitted. This requirement would add a layer of complexity to the development process, potentially impacting the timelines and types of permissible stormwater infrastructure enhancements. 5.0 PROPOSED DEVELOPMENT STORMWATER DESCRIPTION The stormwater facilities for future development on Parcel 2 will be designed to meet Chapter 3 of the City of Springfield’s Engineering Design Standards and Procedures Manual for water quality, Chapter 4 of the City of Springfield Development Code, and comply with the City of Eugene Stormwater Management Manual. This report will demonstrate the feasibility of managing stormwater effectively for any type of permitted home construction on the site, ensuring compliance with all applicable local regulations and standards. 5.1 SINGLE FAMILY HOME OR DUPLEX For the purpose of this analysis, this report will assume conditions of maximum lot coverage for the home and general impervious coverage for driveways and patios on Parcel 2. Assuming a maximum lot coverage of 45%, and a projected additional impervious area for driveways and parking (including an improved driveway along the joint use access easement), it is possible the development could include up to 6,000 square feet of impervious area. According to the simplified method in the City of Eugene Stormwater Management Manual, treatment facilities like stormwater planters or sand filters would require 180 square feet of space, based on a scaling factor of 0.03. This setup ensures compliance with Springfield’s requirements that all impervious surface stormwater runoff is treated prior to discharge for the first 1.4 inches of rainfall. 5.2 MIDDLE HOUSING DEVELOPMENTS For Middle Housing Developments (such as triplexes, fourplexes, or cottage clusters) on Parcel 2, the stormwater management system will also need to meet flow control standards. Making similar assumptions as above but increasing the impervious area to account for additional parking and pathways, the total projected impervious area is estimated to be approximately 7,000 square feet. Flow control facilities such as stormwater planters and sand filters would require approximately 3 490 square feet of space, based on a scaling factor of 0.07. This will ensure compliance with Springfield’s flow control standards, providing adequate stormwater treatment and detention to support a Middle Housing development. Both scenarios must also consider soil conditions which, based on the Chapman-Urban land complex's characteristics, suggest a potential for high infiltration rates. An infiltration test is recommended to validate this assumption and to determine the feasibility of infiltration-based stormwater solutions. Additionally, the total impervious area for each scenario and the space needed for their respective stormwater solution is less than the square footage of Parcel 2, ensuring that both development options are feasible within the constraints of the available land. 6.0 MAINTENANCE The stormwater facilities will be maintained privately due to receiving water only from private improvements and will be constructed under a Building Permit. Maintenance will be per Chapter 3 of the City of Springfield’s Engineering Design Standards and Procedures Manual. Repairs to storm facilities shall be made in accordance with an Operations and Maintenance Agreement associated with future development. 7.0 CONCLUSION Feasibility has been established for a stormwater system for the partition at 904 Filbert Lane. Water quality and, if needed, flow control can be accomplished through various stormwater management systems. This ensures that the development will meet all necessary local and regional environmental regulations while effectively managing stormwater runoff. This approach supports the sustainability and compliance of any permitted development on the property, aligning with community goals and regulatory standards. APPENDIX A Stormwater Management Drainage Basin Map APPENDIX B Site Soils and Map Soil Map—Lane County Area, Oregon Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 6/4/2024 Page 1 of 34875900487591048759204875930487594048759504875960487597048759004875910487592048759304875940487595048759604875970502660502670502680502690502700502710 502660 502670 502680 502690 502700 502710 44° 2' 12'' N 122° 58' 0'' W44° 2' 12'' N122° 57' 58'' W44° 2' 10'' N 122° 58' 0'' W44° 2' 10'' N 122° 57' 58'' WN Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 10N WGS84 0 15 30 60 90 Feet 0 5 10 20 30 Meters Map Scale: 1:388 if printed on A portrait (8.5" x 11") sheet. Soil Map may not be valid at this scale. MAP LEGEND MAP INFORMATION Area of Interest (AOI) Area of Interest (AOI) Soils Soil Map Unit Polygons Soil Map Unit Lines Soil Map Unit Points Special Point Features Blowout Borrow Pit Clay Spot Closed Depression Gravel Pit Gravelly Spot Landfill Lava Flow Marsh or swamp Mine or Quarry Miscellaneous Water Perennial Water Rock Outcrop Saline Spot Sandy Spot Severely Eroded Spot Sinkhole Slide or Slip Sodic Spot Spoil Area Stony Spot Very Stony Spot Wet Spot Other Special Line Features Water Features Streams and Canals Transportation Rails Interstate Highways US Routes Major Roads Local Roads Background Aerial Photography The soil surveys that comprise your AOI were mapped at 1:20,000. Warning: Soil Map may not be valid at this scale. Enlargement of maps beyond the scale of mapping can cause misunderstanding of the detail of mapping and accuracy of soil line placement. The maps do not show the small areas of contrasting soils that could have been shown at a more detailed scale. Please rely on the bar scale on each map sheet for map measurements. Source of Map: Natural Resources Conservation Service Web Soil Survey URL: Coordinate System: Web Mercator (EPSG:3857) Maps from the Web Soil Survey are based on the Web Mercator projection, which preserves direction and shape but distorts distance and area. A projection that preserves area, such as the Albers equal-area conic projection, should be used if more accurate calculations of distance or area are required. This product is generated from the USDA-NRCS certified data as of the version date(s) listed below. Soil Survey Area: Lane County Area, Oregon Survey Area Data: Version 22, Sep 8, 2023 Soil map units are labeled (as space allows) for map scales 1:50,000 or larger. Date(s) aerial images were photographed: May 17, 2023—Jun 3, 2023 The orthophoto or other base map on which the soil lines were compiled and digitized probably differs from the background imagery displayed on these maps. As a result, some minor shifting of map unit boundaries may be evident. Soil Map—Lane County Area, Oregon Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 6/4/2024 Page 2 of 3 Map Unit Legend Map Unit Symbol Map Unit Name Acres in AOI Percent of AOI 25 Chapman-Urban land complex 0.4 100.0% Totals for Area of Interest 0.4 100.0% Soil Map—Lane County Area, Oregon Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 6/4/2024 Page 3 of 3 Lane County Area, Oregon 25—Chapman-Urban land complex Map Unit Setting National map unit symbol: 2360 Elevation: 300 to 1,000 feet Mean annual precipitation: 40 to 60 inches Mean annual air temperature: 52 to 54 degrees F Frost-free period: 165 to 210 days Farmland classification: Farmland of statewide importance Map Unit Composition Chapman and similar soils:45 percent Urban land:40 percent Estimates are based on observations, descriptions, and transects of the mapunit. Description of Chapman Setting Landform:Stream terraces Landform position (three-dimensional):Tread Down-slope shape:Linear Across-slope shape:Linear Parent material:Alluvium from mixed sources Typical profile H1 - 0 to 8 inches: loam H2 - 8 to 42 inches: loam H3 - 42 to 50 inches: gravelly sandy loam H4 - 50 to 60 inches: very gravelly sandy loam Properties and qualities Slope:0 to 3 percent Depth to restrictive feature:More than 80 inches Drainage class:Well drained Capacity of the most limiting layer to transmit water (Ksat):Moderately high to high (0.57 to 1.98 in/hr) Depth to water table:More than 80 inches Frequency of flooding:Rare Frequency of ponding:None Available water supply, 0 to 60 inches: Moderate (about 8.1 inches) Interpretive groups Land capability classification (irrigated): 1 Land capability classification (nonirrigated): 1 Hydrologic Soil Group: B Ecological site: F002XC003OR - Low Flood Plain Group Forage suitability group: Well drained < 15% Slopes (G002XY002OR) Map Unit Description: Chapman-Urban land complex---Lane County Area, Oregon Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 6/4/2024 Page 1 of 2 Other vegetative classification: Well drained < 15% Slopes (G002XY002OR) Hydric soil rating: No Description of Urban Land Properties and qualities Slope:0 to 3 percent Frequency of flooding:Rare Interpretive groups Land capability classification (irrigated): None specified Land capability classification (nonirrigated): 8 Hydric soil rating: No Data Source Information Soil Survey Area: Lane County Area, Oregon Survey Area Data: Version 22, Sep 8, 2023 Map Unit Description: Chapman-Urban land complex---Lane County Area, Oregon Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 6/4/2024 Page 2 of 2