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HomeMy WebLinkAboutStudies APPLICANT 6/30/2022Stormwater Management Plan and Drainage Study 824 S 70" Street Springfield, Oregon For 836S 70`h, LLC 939 Riverstone Lane Springfield, OR 97477 FD PRO, cc�S�F NG I NEF �O� R69162PE DIGITALLY SIGNED OREGON R 'qN. lo, 2pO6 �T SEE CI0J EXPIRES: 12/30/23 June 30, 2022 E'N'GINEERING_ since 1999 civil - transportation structural - geotechnical SURVEYING www.BranchEngineering.com 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 2 3.0 Proposed Development.........................................................................................2 4.0 Stormwater Constraints........................................................................................2 5.0 Proposed Development Stormwater Description.......................................2 6.0 6.0 Facility Sizing.....................................................................................................3 7.0 7.0 Maintenance.......................................................................................................3 8.0 8.0 Conclusion...........................................................................................................3 Appendices Appendix A Appendix B © Branch Engineering, Inc. Description Site Soils and Map Basin Map 1.0 INTRODUCTION This Stomnwater Management Plan (SWMP) report has been prepared for the analysis of subdividing a property located at 824 S 70'" Street. The subject site is south of Glacier Drive, north of Ivy Street, and east of S 69'" Place. The subject property is identified as Tax Map 18-02-02-23 Tax Lot 4300. The stormwater runoff will be directed to the public stormwater system within the S 70" Street right-of- way. 2.0 EXISTING CONDITIONS The subject property is located in Springfield, Oregon in the Hillside Development Overlay Zone and consists of 1.76 acres of private property. The subject property's southwesterly boundary abuts the northerly terminus of S 69"' Place and the easterly boundary abuts the westerly margin of S 70`" Street. The subject property is surrounded on all sides by fully improved properties. There are fences of various heights and character around the site where it abuts other properties. 2.1 TOPOGRAPHY The subject property is sparsely wooded with clusters of trees which have diameters at breast height ranging from 6- to 28 -inches and grassed slopes of up to 12.5 percent. The elevations on the subject property range from 710 -feet in the southwesterly corner of the site to 678 -feet in the northeasterly comer of the site. 2.2 SOILS The Natural Resources Conservation Service (NRCS) Web Soil Survey maps the subject property as containing approximately 92.5% of Urban land-Hazelair-Dixonville complex (127C) and 7.5% Hazelair silty clay loam (52D) which resides in the northeasterly portion of the property. Urban land-Hazelair-Dixonville complex can have key features of both Hazelair and Dixonville soil types, while Hazelair silty clay loam only has Hazelair characteristics. They are differentiated below, • Hazelair is described as a moderately well -drained colluvium derived from sedimentary rock and is in Hydrologic Soil Group D; the soil has a limiting drainage capacity between 0.00- and 0.06 -inches per hour. The typical profile of the soil type has silty clay loam between 0- to it - inches, silty clay from 11- to 15 -inches, clay from 15- to 36 -inches, and weathered bedrock from 36- to 46 -inches; 5 test pits were dug for the associated Geotechnical Report, and no bedrock was found. • Dixonville is described as a well -drained colluvium and residuum derived from basalt and is in Hydrologic Soil Group D; the soil has a limiting drainage capacity between 0.06- and 0.20 - inches per hour. The typical profile of the soil type has silty clay loam between 0- to 14 -inches, silty clay from 14- to 26 -inches, and weathered bedrock from 26- to 36 -inches; S test pits were dug for the associated Geotechnical Report, and no bedrock was found. A soils map and individual soil information is provided in Appendix A. 2.3 EXISTING STORM SYSTEM There is currently a roadside ditch located on the westerly edge of the pavement inside the S 70th Street right-of-way. Additionally, a 36 -inch public stormwater line exists along the easterly edge of the same right-of-way. An 8 -inch public stormwater line also exists to the southwest of the site inside the S 69t' Place right-of-way, but is uphill from the subject property, and continues westerly away from the site. 3.0 PROPOSED DEVELOPMENT An 8 -lot subdivision is proposed. Each lot ranges from 7,300- to 7,600 -square feet. It is intended to dedicate and extend S 696 Place (a public road) from its northerly terminus, curving through the site to intersect S 70" Street. Included within the dedicated right-of-way will be a 20 -foot -wide roadway with curb and gutters with a 4.5 -foot planter strip and 5 -foot sidewalk on both sides of the road. The dedicated road will include underground utilities for the lots on its northerly and southerly margin. The frontage along S 70" Street will also be improved to the centerline of the right-of-way. This improvement will consist of 9.5 feet of road pavement with curb and gutters with a 4.5 -foot planter strip and 5 -foot sidewalk. Based on conversations with staff at the City of Springfield, the site willnot include on-site stormwater quality management facilities. Most treatment facilities require water to slowly move through a growing medium. Having water housed in facilities for longer time spans on slopes exceeding 10 percent may be harmful by promoting erosion and slides. it is assumed that individual lot improvements will either weephole/drain to the road or stub into the stormwater main. 4.0 STORMWATER CONSTRAINTS Connecting to the existing 36 -inch stormwater line within S 70'h Street will require an oversized manholeor connection further downstream at an existing manhole. The average slope across the site prevents the use of infiltration facilities to reduce stormwater discharge. S 69" Place will be extending through the subject property. The easterly half on the existing improvements on S 69`h Place will drain to the site (see Appendix B for total basin map). S 69'" Place extends approximately 420 -feet to the south. This accounts for approximately 6,720 -square feet of asphalt, 2,000 -square feet of sidewalk, and up to 22,369 -square feet of private residential improvements, which will be draining to the curb inlets being installed in this project. 5.0 PROPOSED DEVELOPMENT STORMWATER The proposed stormwater development for the subject property will consist of four curb inlets along the northerly portion of the S 69'° Place and one along S 70ih Street. The road will shed water to the north to match the existing direction of the slope on-site, while the improvements on S 70t' Street will shed to the westerly curb and gutter. Below is an explanation of what pipe size is required to convey runoff. The impervious surface of the proposed road consists of 10,312 square feet of asphalt and S,725 - square feet of sidewalk. All the homes' impervious areas will be draining to the stormwater system as well as some of the pervious area on the south side of the road. If each lot maximizes its lot coverage, there will be an additional 20,857 square feet of impervious area and 19,442 square feet of pervious area entering the system. Note, that the calculated impervious area was taken by using the maximum buildout of the entire proposed site (59,591 SF x 35%= 20,857 SF). Utilizing the areas calculated in the last paragraph and in Section 4.0 above, the total flows during 2-, 10-, and 25 -year storm events through the most restrictive stormwater pipe in the new system are listed in the table below: The most restrictive stormwater pipe is located near the beginning of the new stormwater drainage system and will not house the amount of water shown in the table above, but was used as a model to ensure that any pipe in the system would be able to handle the maximum amount of stormwater runoff from the site. It consists of a 262.4 -foot length of piping at a 4.1% slope. The existing downstream system consists of a 36" stormwater pipe within the S 701h Street right-of- way. The city of Springfield Stormwater Master Plan list this as manhole 1166. Future hydraulic performance of this pipe lists the 25 -year storm as having a flow of 23 cis. Adding the additional 1.89 cis from the proposed system to this value increases the water volume in the pipe from 25.6% full to 26.7% full at peak flow. This shows plenty of capacity in the existing system to accommodate the new impervious area runoff. 6.0 6.0 FACILITY SIZING All stormwater structures and pipes are to be installed per the City of Springfield's Engineering Design 8- Procedures Manual Chapter 4.00. 7.0 7.0 MAINTENANCE The facilities will be maintained publicly as they are located within the public right-of-way and are being installed through a Public Improvement Project (PIP). 8.0 8.0 CONCLUSION Feasibility has been established for the stormwater system outlined herein for the Tentative Subdivision. It will be necessary to update the design and calculations at the time of the PIP permitting. Flow (cfs) Depth of water in 12" pipe (inches) Pipepercent full 2 -year 1.24 3.1 20.7% 10 -year 1.67 3.6 25.2% 25 -year 1.89 1 3.8 27.6% The most restrictive stormwater pipe is located near the beginning of the new stormwater drainage system and will not house the amount of water shown in the table above, but was used as a model to ensure that any pipe in the system would be able to handle the maximum amount of stormwater runoff from the site. It consists of a 262.4 -foot length of piping at a 4.1% slope. The existing downstream system consists of a 36" stormwater pipe within the S 701h Street right-of- way. The city of Springfield Stormwater Master Plan list this as manhole 1166. Future hydraulic performance of this pipe lists the 25 -year storm as having a flow of 23 cis. Adding the additional 1.89 cis from the proposed system to this value increases the water volume in the pipe from 25.6% full to 26.7% full at peak flow. This shows plenty of capacity in the existing system to accommodate the new impervious area runoff. 6.0 6.0 FACILITY SIZING All stormwater structures and pipes are to be installed per the City of Springfield's Engineering Design 8- Procedures Manual Chapter 4.00. 7.0 7.0 MAINTENANCE The facilities will be maintained publicly as they are located within the public right-of-way and are being installed through a Public Improvement Project (PIP). 8.0 8.0 CONCLUSION Feasibility has been established for the stormwater system outlined herein for the Tentative Subdivision. It will be necessary to update the design and calculations at the time of the PIP permitting. APPENDIX A Site Soils and Map Soil may fano county urea, Oregon a repsme: t:ss.nwm+on aamre m�.e.sy� § N ,a p NaW2INBeeumesweb Soil Survey ]2Vzozz cone—m n Service Nanora! Onogeaeve son survey vaget M3 Soil Map—Lane County Area, Oregon Li Natural Resour¢s Web Soil Survey WI 0022 Conasrvaflon Service National Cooperative Soil Survey Page 203 MAP LEGEND MAP INFORMATION Area0 Wereat(AOp Ar Gm The soft suroeys that comprise your Are were mapped at Q .. MlMa ,., 1:20,000. Smn vspd &olio Ven, very am Warning. Soil Map may not be valid at Nis scale. Wil Man Mnrt Poryynna war Spd Enlargement of maps beyond Ne scalping mapping N Shc Map up, Gres 1 untlersrantll Tof and of Soil mapping antlacaracyinas C.hav ema do net maps Oove show the small f e Shc Map aril PolMe .� 9pcial Gne Features contrane cting ontrasting stills tat could have been shown atamore aetatle0 sats to moms Spacial PalMFog Nrx a CeIB. (eJ BloarM Water Feawes Streams and Canals Pleaygmmon Na barscale on each map beat My map ® AA -11 measurements. T fee Car Snd Polls Souralof Map: Natural Resoums Conservafion Service Q trims Common Web Soil Survey URL ry '. N ] menta m wglv+aYs ..sooner. System: Web MY—tnr(EPGG. 3851 �a Orspe Pit US reomes Malay from the Web Soil Survny are based On Ne Web Mercator ..W, Spd melon aaees %oleaen. YyNdb preserves eimmon and shape but dicers, loselll Eislance am area. A projection Nat preserves area, such as the ® Local Rcaes Albers equalarea-or. plutonium shcule be wood it more A b.. Floe, a�nm_oa eWalons cl distance nr area aro Mquirea. evxe.ouna Morh on swamp . Aarial PhdamaphV This pro isg paudded from Ne USDA- all certified data as of the sorrier drams) listed below. +jtr MmettDuairy Soil Survey Ama: (ane COUnty Area. Olegon Q mlamlkmouewulsr Survey Area Data: Version 19, Gd 2I, 2021 P .... ..m VAN, Sall map ands are labeled (as space other for map scales q. Rod OuAmp 1: 50, 000 or larger. .� Syin. Spd Catalog aerlellmages sham metc,phi Jun 12, 2018 un 19 2019 Surly spm The oehophon Or elver base map an which the axil lines were .ua seeprely Fimm spot compiled and di9dided probably diRen firm the w Wgmund Ani imagery displayed on Nese mapsas a result, same minor an thing of map unit boundaries may be evident, jy slid, or slip see:8par Li Natural Resour¢s Web Soil Survey WI 0022 Conasrvaflon Service National Cooperative Soil Survey Page 203 Soil Map—Lane County Area, Oregon Map Unit Legend Map Unit Symbol Map Unit Name Acres in AO] Percent of AOI 52D Hazelair silty clay loam, 7 to 20 0.1 7.5% percent slopes 127C Urban land-Hazelair-0ixonville 1.8 92.5% complex, 3 to 12 percent slopes Totals for Area of Interest 2.01 101 usuix Natural Resources Web Soil Survey 312112022 211111111 Conservation Service National Cooperative Soil Survey Page 3 of Map Unit Description: Hazelair silty clay loam, 7 to 20 percent slopes--- Lane County Area, Oregon Lane County Area, Oregon 52D—Hazelair silty clay loam, 7 to 20 percent slopes Map Unit Setting National map unit symbol: 237c Elevation: 200 to 2,000 feet Mean annual precipitation: 30 to 60 inches Mean annual air temperature: 50 to 55 degrees F Frost -free period.., 160 to 235 days Farmland classification: Farmland of statewide importance Map Unit Composition Hazelair and similar soils: 85 percent Minor components: 3 percent Estimates are based on observations, descriptions, and transects of the mapunit. Description of Hazelair Setting Landform: Mountains, mountains Landform position (two-dimensional): Footslope Landform position (three-dimensional): Mountainbase Down-slope shape: Linear Across -slope shape: Linear Parent material: Colluvium derived from sedimentary rock Typical profile Hl - 0 to 11 inches: silty clay loam H2 - 11 to 15 inches: silty clay H3 - 15 to 36 inches: clay H4 - 36 to 46 inches: weathered bedrock Properties and qualities Slope: 7 to 20 percent Depth to restrictive feature: 20 to 40 inches to paralithic bedrock Drainage class: Moderately well drained Capacity of the most limiting layer to transmit water (Ksat): Very low to moderately low (0.00 to 0.06 in/hr) Depth to water table: About 12 to 24 inches Frequency of flooding: None Frequency of ponding: None Available water supply, 0 to 60 inches: Low (about 4.8 inches) Interpretive groups Land capability classification (irrigated): None specified Land capability classification (nonirrigated): 4e Hydrologic Soil Group: D Ecological site: R002XCOl OOR - Claypan Low Hill Group Forage suitability group: Moderately Well Drained < 15% Slopes (G002XY0040R) usm Natural Resources Web Soil Survey 3/21/2022 41111111 Conservation Service National Cooperative Soil Survey Page 1 of 2 Map Unit Description: Hazelair silty clay loam, 7 to 20 percent slopes--- Lane County Area, Oregon Other vegetative classification: Moderately Well Drained < 15 Slopes (G002XY0040R) Hydric soil rating: No Minor Components Panther Percent of map unit: 3 percent Landform: Swales Hydric soil rating: Yes Data Source Information Soil Survey Area: Lane County Area, Oregon Survey Area Data: Version 19, Oct 27, 2021 ai Natural Resources Web Soil Survey 3/21/2022 Conservation Service National Cooperative Soil Survey Page 2 of 2 Map Unit Description: Urban lard-Hazelair-Dixonville complex, 3 W 12 percent slopes ---Lane County Area, Oregon Lane County Area, Oregon 127C—Urban land-Hazelair-Dixonville complex, 3 to 12 percent slopes Map Unit Setting National map unit symbol: 2341 Elevation: 200 to 2,000 feet Mean annual precipitation: 30 to 60 inches Mean annual air temperature: 50 to 55 degrees F Frost -free period. 160 to 235 days Farmland classification: Not prime farmland Map Unit Composition Urban land: 35 percent Hazelair and similar soils: 25 percent Dixonville and similar soils: 20 percent Minor components: 3 percent Estimates are based on observations, descriptions, and transects of the mapunit. Description of Urban Land Interpretive groups Land capability classification (irrigated): None specified Land capability classification (nonirrigated): 8 Hydric soil rating: No Description of Hazelair Setting Landform: Hills Landform position (two-dimensional): Summit, shoulder, toeslope Landform position (three-dimensional): Interfluve, nose slope, base slope Down-slope shape: Linear Across -slope shape: Linear Parent material. Colluvium derived from sedimentary rock Typical profile H1 - 0 to 11 inches: silty clay loam H2 - 11 to 15 inches: silty clay H3 - 15 to 36 inches: clay H4 - 36 to 46 inches: weathered bedrock Properties and qualities Slope: 3 to 12 percent Depth to restrictive feature: 20 to 40 inches to paralithic bedrock Drainage class., Moderately well drained Capacity of the most limiting layer to transmit water (Ksat): Very low to moderately low (0.00 to 0.06 in/hr) Depth to water table: About 12 to 24 inches Natural Resources Web Soil Survey 3/21/2022 conservation Service National Cooperative Soil Survey Page 1 of 3 Map Unit Description: Urban land-Hazelair-Dixonville complex, 3 to 12 pementslopes--Lane County Area, Oregon Frequency of flooding: None Frequency of ponding: None Available water supply, 0 to 60 inches: Low (about 4.8 inches) Interpretive groups Land capability classification (irrigated): None specified Land capability classification (nonirrigated): 4e Hydrologic Soil Group: D Ecological site: R002XCO10OR - Claypan Low Hill Group Forage suitability group: Moderately Well Drained < 15% Slopes (G002XY0040R) Other vegetative classification: Moderately Well Drained < 15 Slopes (G002XY0040R) Hydric soil rating: No Description of Dixonville Setting Landform: Hills Landform position (two-dimensional): Summit, shoulder, toeslope Landform position (three-dimensional): Interfluve, nose slope, base slope Down-slope shape: Linear Across -slope shape: Linear Parent material. Colluvium and residuum derived from basalt Typical profile H1 - 0 to 14 inches: silty clay loam H2 - 14 to 26 inches: silty clay H3 - 26 to 36 inches: weathered bedrock Properties and qualities Slope: 3 to 12 percent Depth to restrictive feature: 20 to 40 inches to paralithic bedrock Drainage class: Well drained Capacity of the most limiting layer to transmit water (Ksat): Moderately low to moderately high (0.06 to 0.20 in/hr) Depth to watertable: More than 80 inches Frequency of flooding: None Frequency of ponding: None Available water supply, 0 to 60 inches: Low (about 4.6 inches) Interpretive groups Land capability classification (irrigated): None specified Land capability classification (nonirrigated): 3e Hydrologic Soil Group: D Ecological site: R002XCO11 OR -Low Hill Group Forage suitability group: Well drained < 15% Slopes (G002XY0020R) Other vegetative classification: Well drained < 15% Slopes (G002XY0020R) Hydric soil rating., No gaNatural Resources Web Soil Survey 3121/2022 i Conservation Service National Cooperative Soil Survey Page 2 of 3 Map Unit Description: Urban land[ -Ha lair-Dixonville complex, 3 to 12 percent slopes ---Lane County Area, Oregon Minor Components Panther Percent of map unit: 3 percent Landform: Swales Hydric soil rating: Yes Data Source information Soil Survey Area: Lane County Area, Oregon Survey Area Data: Version 19, Oct 27, 2021 Natural Resources Web Soil Survey 3121/2022 Conservation Service National Cooperative Soil Survey Page 3 of 3 I_\MON Basin Map L J 3" i irf K _ Y -i V TOTAL en IN MlEA i p -Til f e _ 1 f•