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HomeMy WebLinkAboutStudies Applicant 3/27/2024 Stormwater Management Plan and Drainage Study Teresa Schoonover Springfield, Oregon For Teresa Schoonover 1010 Kintzley Avenue Springfield, OR 97478 February 20, 2024 DIGITALLY SIGNED © 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 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) report has been prepared for a Tentative Partition application to divide an existing lot at the property currently known as 1010 Kintzley Avenue. The subject site is at the southwesterly corner of the Jasper Road and Kintzley Avenue intersection, and is identified as Tax Map 18-02-06-24, Tax Lot 3800. The stormwater runoff from future roof drains and drive surfaces will be directed to treatment facilities prior to being discharged to the existing stormwater system in Kintzley Avenue or Jasper Road. The purpose of this report is to document feasibility for future development. 2.0 EXISTING CONDITIONS The subject site is located in Springfield, Oregon at the southwesterly corner of the Jasper Road and Kintzley Avenue intersection, and consists of 0.84 acres of private property. The development site is underdeveloped with a majority of the improvements on the southerly half of the property. 2.1 TOPOGRAPHY The existing improvements consist of a single-family home, carport, shop, and gravel vehicular circulation area. An existing garage on site will be removed. Jasper Road is fully improved; Kintzley Avenue is partially improved with 21.5 feet of pavement and roadside ditches. The whole site is between 478-479 feet in elevation above sea level. 2.2 SOILS The Natural Resources Conservation Service (NRCS) Web Soil Survey maps the development site as containing mostly Chapman Loam (24); the southeasterly corner of the site contains Cloquato Silt Loam (29). Both Chapman Loam and Cloquato Silt Loam are described as a well-drained alluvium and are in Hydrologic Soil Group B. A soils map and individual soil information is provided in Appendix B. 2.3 EXISTING STORM SYSTEM The northerly portion of the lot is open and vegetated with grass. 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. The public stormwater system in Kintzley Avenue was recently constructed as part of the Mountainvale Subdivision. A 72-inch stormwater main is located within Jasper Road and there are no noted downstream deficiencies. Curb inlets within the adjacent intersection feed into the 72-inch main. 3.0 PROPOSED DEVELOPMENT As shown on the attached Basic Map in Appendix A, the partition consists of three parcels. Parcels 1 and 2 lie within the undeveloped portion of the property. Each parcel is larger than 4,500 square feet and can have a maximum building footprint of 45% of the total lot area. Parcel 2 includes a 20.00 foot-wide access easement for the benefit of Parcel 1. Both parcels will take access through this easement with a single access connection on Kintzley Avenue. Parcel 3 consists of the existing improvements on site. The home, shop, and carport encompass 30% of the total parcel area. Removal of the existing garage on the line between Parcels 1 and 3 is the only construction to be made on-site 2 as part of this development. Parcel 3 will continue to take access along Kintzley Avenue via a separate a driveway from Parcels 1 and 2. Public improvements along Kintzley Avenue are proposed in accordance with the Annexation Agreement associated with the land. The public improvements consist of sidewalk, curb and gutter, a street light, planter strip, and widening the existing asphalt. The public improvements will be designed and approved for construction prior to platting. No public stormwater improvements are required as a part of this development. 4.0 STORMWATER CONSTRAINTS Two significant constraints affect the site: 1. Wellhead Time of Tavel Zone - The site lies along the 2- and 5-Year Time of Travel Zones to the nearest wellhead. Per SDC 3.3.233(B)(5) treatment of stormwater runoff must occur prior to conveyance to an infiltration facility. 2. Site Elevations - The site is extremely flat with little to no grade change across the site. There are high points throughout the site which may be the product of in-fill. There is a shallow ditch along Kintzley Avenue which is the assumed discharge point for the existing site runoff. This appears to drain southward toward the improvements constructed for the Mountainvale Subdivision. 5.0 PROPOSED DEVELOPMENT STORMWATER DESCRIPTION The stormwater facilities will be designed to meet Chapter 3 of the City of Springfield’s Engineering Design Standards and Procedures Manual for water quality. As stated previously, the proposed development consists of 3 parcels (one with existing improvements, and two without). The two undeveloped parcels (Parcels 1 and 2) are both large enough for the City of Springfield’s Land Use Code to allow a variety of development types including: single family home, duplex, triplex, fourplex, or cottage cluster. As it is unknown at this time which type of development will be constructed, the following is a conceptual analysis to determine feasibility. Nothing in this analysis is intended to be definitive or binding on future development. For the purpose of this analysis, a duplex on each parcel will be considered. It should be noted that Parcel 3 (Basin 3 in the Basin Map in Appendix A) is not being analyzed here as it is fully improved. Per SDC 3.2.250, duplexes do not adhere to the stormwater infrastructure standards for middle housing. Each parcel will need to treat all impervious surfaces prior to discharge to the public system for the first 1.4-inches of rainfall. It is conservatively assumed that each parcel being developed (Basin 1 and 2 in the Basin Map in Appendix A) utilizes the maximum lot coverage of 45% and includes approximately 1,900 square feet of additional impervious area for the driveway and parking area. The simplified approach in the City of Eugene Stormwater Management Manual, utilizes a scaling factor of 0.03-0.06 for treatment facilities including stormwater planters, sand filters, rain gardens or swales resulting in 180 to 360 square feet of area needed per parcel. Between building, vehicular circulation and easement areas, there is between 2,400 and 2,700 square feet remaining on each parcel for stormwater treatment facilities. Additionally, the soils on site have the potential to allow infiltration of stormwater. Prior to the development of an infiltration facility, it is recommended that the site have an infiltration test 3 performed. Also, as the site is located within the 2- and 5-Year Wellhead Time of Travel Zones, any infiltration facility will need to be approved by the City of Springfield. The proposed public improvements associated with the partition will be graded to match the existing improvements within Kintzley Avenue. The existing road profile drains southerly, and an existing ditch lies along the westerly edge of the road. Beyond this ditch to the south are new improvements associated with the development of the Mountainvale Subdivision. These improvements include curb and gutter and curb inlets along Kintzley Avenue. Per the Annexation Agreement and conversation with the City of Springfield’s Public Works Department, no additional capture systems will be needed along this site’s frontage. 6.0 MAINTENANCE The stormwater facilities will be maintained privately since they receive 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. Water quality can be accomplished through various treatment facilities. 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 1/10/2024 Page 1 of 3487578048757904875800487581048758204875830487584048758504875860487578048757904875800487581048758204875830487584048758504875860501670501680501690501700501710501720501730 501670 501680 501690 501700 501710 501720 501730 44° 2' 9'' N 122° 58' 45'' W44° 2' 9'' N122° 58' 41'' W44° 2' 6'' N 122° 58' 45'' W44° 2' 6'' N 122° 58' 41'' WN Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 10N WGS84 0 20 40 80 120 Feet 0 5 10 20 30 Meters Map Scale: 1:439 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: Jun 12, 2019—Jun 19, 2019 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 1/10/2024 Page 2 of 3 Map Unit Legend Map Unit Symbol Map Unit Name Acres in AOI Percent of AOI 24 Chapman loam 0.8 96.5% 29 Cloquato silt loam 0.0 3.5% Totals for Area of Interest 0.9 100.0% Soil Map—Lane County Area, Oregon Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 1/10/2024 Page 3 of 3 Lane County Area, Oregon 24—Chapman loam Map Unit Setting National map unit symbol: 235z 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: All areas are prime farmland Map Unit Composition Chapman and similar soils:85 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 loam---Lane County Area, Oregon Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 1/10/2024 Page 1 of 2 Other vegetative classification: Well drained < 15% Slopes (G002XY002OR) 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 loam---Lane County Area, Oregon Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 1/10/2024 Page 2 of 2 Lane County Area, Oregon 29—Cloquato silt loam Map Unit Setting National map unit symbol: 2365 Elevation: 290 to 800 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: All areas are prime farmland Map Unit Composition Cloquato and similar soils:85 percent Estimates are based on observations, descriptions, and transects of the mapunit. Description of Cloquato Setting Landform:Flood plains Landform position (three-dimensional):Tread Down-slope shape:Linear Across-slope shape:Linear Parent material:Alluvium from mixed sources Typical profile H1 - 0 to 14 inches: silt loam H2 - 14 to 50 inches: silt loam H3 - 50 to 60 inches: stratified sand to silt 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:Occasional Frequency of ponding:None Available water supply, 0 to 60 inches: High (about 11.0 inches) Interpretive groups Land capability classification (irrigated): None specified Land capability classification (nonirrigated): 2w Hydrologic Soil Group: B Ecological site: F002XC003OR - Low Flood Plain Group Forage suitability group: Well drained < 15% Slopes (G002XY002OR) Other vegetative classification: Well drained < 15% Slopes (G002XY002OR) Map Unit Description: Cloquato silt loam---Lane County Area, Oregon Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 1/10/2024 Page 1 of 2 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: Cloquato silt loam---Lane County Area, Oregon Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 1/10/2024 Page 2 of 2