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Home My WebLink AboutNotes Applicant 11/15/2024 (6)1 Drainage Memorandum To: Timberview Construction From: Scott Morris, PE Date: 11/15/2024 Taxmap & T.L.: 17-02-32-41-00400 Re: Tentative Stormwater Design for Parcel 3 of Cherry Springs Partition (Potential Development) Project Overview The applicant is proposing to partition to create 3 lots. One lot on each side of "B" Street as it extends to 48th Street through the city-approved Cherry Springs Apartments and one lot fronting Main Street for future commercial development. The project site is located at 4800 B Street in Springfield. The storm system for the existing approved apartment development on parcels one and two have approved stormwater management facilities that are currently under construction. Therefore, this memorandum does not cover the drainage on the parcels one and two. Proposed parcel three is vacant and is being created for potential future development, with no proposed site plan at this time. Therefore, the intent of this memorandum is to demonstrate there is enough space on the parcel to place a stormwater management facility that will adequately serve the impervious area and meet City requirements. Site Soil Characteristics Per the web soil survey, parcel 3 is expected to be Salem-urban land complex. The attached printout from the web soil survey estimates this soil to be hydrologic group B with infiltration rates between 0.57-1.98 inches per hour. A field infiltration test should be completed during site review application process to determine the actual onsite design infiltration rate. The soil is listed as well drained and the depth to water table is more than 80 inches. Stormwater Calculation There is no proposed site plan for parcel 3 at this time. So, the analysis was to confirm enough space is available onsite to support a future mixed used development with a stormwater management facility. 380 Q Street, Ste 200 Springfield, Oregon 97477 (541) 302-9790 destinta@ao-engr.com  Page 2 We assumed that as much as 85% of the parcel could be covered with impervious surfaces, which equates to a total of 45,542 square feet. We assume this impervious area could be served by a stormwater planter (or planters). Given this, there are two potential design routes depending on the field infiltration testing. If the design infiltration rate is more than 2 inches per hour, it may be feasible to design an infiltration only facility. If the design infiltration rate is less than 2 inches per hour, then the facility should have an overflow to the public system in Main Street. Water quality and flow control only facility – overflow to public system Utilizing the SIM form from the ESMM, this equates to a minimum facility area required of 3,188 square feet (see attached SIM form calculation). The 15% non-impervious area on this parcel is adequate to place this type of facility within the site design. Water quality, flow control and flood control facility – infiltration only Utilizing the SIM form from the ESMM, this equates to a minimum facility area required of 5,010 square feet (see attached SIM form calculation). The 15% non-impervious area on this parcel is adequate to place this type of facility within the site design. Once a proposed site plan is determined and a field infiltration test completed, the actual facility can be designed during the site review application process. It may be determined at that time to perform a presumptive analysis to size the facilities. Conclusion There is adequate room on the property to construct either facility outlined above to adequately serve the a proposed mixed use development. The actual storm management facility will be designed during the site review process when the actual proposed site development plan is known. Appendix • SIM form for stormwater planter sizing – overflow to public system • SIM form for stormwater planter sizing – infiltration only facility • Existing soil properties from the Web Soil Survey SIM FORM: 2014 (Simplified Approach for Stormwater Management) Application Building Permit # Address Residential/Commercial Tax Lot #(Circle One) NRCS Soil Type or Measured Infiltraiton Rate Facility Sizing Total Proposed New or Replaced Impervious Surface Area Box 1 Impervious Area Reduction Permeable Pavements sf Eco‐Roof sf Contained Planter sf Tree Credit sf Total Impervious Area Reduction Box 2 Total Impervious Area Requiring Stormwater Managemment Box 3 (Box 1 ‐ Box 2) Facility Sizing for Water Quality Only Surface Facilities Impervious Area Managed Sizing Factor Faclity Surface Area Rain Garden sf x0.05 = Stormwater Planter sf x0.03 = Swale sf x0.06 = Vegetated Filter Strip sf x0.2 = Sand Filter sf x0.03 = Facility Sizing for Water Quality and Flow Control Surface Facilities Impervious Area Managed Sizing Factor Faclity Surface Area Rain Garden sf x0.11 = Stormwater Planter sf x0.07 = Sand Filter sf x0.07 = Facility Sizing for Water Quality, Flow Control and Flood Control *** Only for use in Type A & B Soils Surface Facilities Impervious Area Managed Sizing Factor Faclity Surface Area Rain Garden sf x0.13 = Stormwater Planter sf x0.11 = Sand Filter sf x0.11 = Sum of Total Impervious Area Managed Box 4 Point of Discharge (check one) (Box 4 must be equal or greater than Box 3) Overflow to gutter (weephole) Overflow to public storm drain pipe Overflow to Open Drainage Subsurface Infiltration 2014 SIM FORM: Tree Credit and Rainwater Harvesting Worksheet See "Tree Credits" section for more information regarding the use of trees to meet Stormwater Impervious Area Reduction. New Evergreen Trees To receive Impervious Area Reduction Credit, new evergreen trees must be planted within 25 feet of the new or replaced impervious surfaces. New trees cannot be credited against rooftop areas. Minimum tree height (at the time of planting)to receive credit is 6 feet Enter number of new evergreen trees that meet qualification requirements in Box ABox A Multiply Box A by 200 and enter result in Box B Box B New Deciduous Trees To receive Impervious Area Reduction Credit, new large deciduous trees must be planted within 25 feet of the new or replaced impervious surfaces and new small deciduous trees must be plantd within 10 feet of new or replaced impervious surfaces. New trees cannot be credited against rooftop areas. Minimum tree caliper (at the time of planting)to receive credit is 2 inches. Enter number of new deciduous trees that meet qualification requirements in Box CBox C Multiply Box C by 100 and enter result in Box D Box D Existing Tree Canopy To receive Impervious Area Reduction Credit, existing large tree canopies must be within 25 feet and existing small tree canopies must be within 10 feet of ground‐level impervious surfaces (cannot be credit against roof top surfaces). Existing tree canopy credited towards Impervious Area Reduction must be preserved during and after construction throughout the life of the development. Minimum tree caliper to receive credit is 4 inches. No credit will be given to existing tree canopy located within environmental conservation areas. Enter square footage of existing tree canopy that meet qualification requirements in Box E. Box E Multiply Box E by 0.5 and enter result in Box F.Box F Total Tree Credit Add Boxes B, D and F and enter the result in Box G Box G Multiply Box 1 of Form SIM by 0.1 and enter the result in Box H. Box H Enter the lesser of Box G and H in Box I. (This is the amount to be entered as "Tree Credit" on Form SIM.)Box I SIM FORM 2014 Instructions 1. Enter square footage (sf) of total impervious area being developed into BOX 1. 2. Enter square footage (sf) for impervious area reduction techniques. 3. Enter sum of the impervious area reduction techniques into BOX 2. 4. Subtract BOX 2 from BOX 1 to find BOX 3, the amount of impervious area that requires stormwater management. 5. Select appropriate stormwater management facilities. 6. Enter the square footage of impervious area managed that will flow into each facility type. 7. Multiply each impervious area managed by the corresponding sizing factor. Enter this area as the facility surface area, This is the size of faclity required to manage runoff 9. Where selecting facilities that will overflow, select the point of discharge location. 10. Enter the sum of the total of all the impervious area managed into BOX 4. BOX 4 must be greaer than or equal to BOX 3. SIM FORM: 2014 (Simplified Approach for Stormwater Management) Application Building Permit # Address Residential/Commercial Tax Lot #(Circle One) NRCS Soil Type or Measured Infiltraiton Rate Facility Sizing Total Proposed New or Replaced Impervious Surface Area Box 1 Impervious Area Reduction Permeable Pavements sf Eco‐Roof sf Contained Planter sf Tree Credit sf Total Impervious Area Reduction Box 2 Total Impervious Area Requiring Stormwater Managemment Box 3 (Box 1 ‐ Box 2) Facility Sizing for Water Quality Only Surface Facilities Impervious Area Managed Sizing Factor Faclity Surface Area Rain Garden sf x0.05 = Stormwater Planter sf x0.03 = Swale sf x0.06 = Vegetated Filter Strip sf x0.2 = Sand Filter sf x0.03 = Facility Sizing for Water Quality and Flow Control Surface Facilities Impervious Area Managed Sizing Factor Faclity Surface Area Rain Garden sf x0.11 = Stormwater Planter sf x0.07 = Sand Filter sf x0.07 = Facility Sizing for Water Quality, Flow Control and Flood Control *** Only for use in Type A & B Soils Surface Facilities Impervious Area Managed Sizing Factor Faclity Surface Area Rain Garden sf x0.13 = Stormwater Planter sf x0.11 = Sand Filter sf x0.11 = Sum of Total Impervious Area Managed Box 4 Point of Discharge (check one) (Box 4 must be equal or greater than Box 3) Overflow to gutter (weephole) Overflow to public storm drain pipe Overflow to Open Drainage Subsurface Infiltration 2014 SIM FORM: Tree Credit and Rainwater Harvesting Worksheet See "Tree Credits" section for more information regarding the use of trees to meet Stormwater Impervious Area Reduction. New Evergreen Trees To receive Impervious Area Reduction Credit, new evergreen trees must be planted within 25 feet of the new or replaced impervious surfaces. New trees cannot be credited against rooftop areas. Minimum tree height (at the time of planting)to receive credit is 6 feet Enter number of new evergreen trees that meet qualification requirements in Box ABox A Multiply Box A by 200 and enter result in Box B Box B New Deciduous Trees To receive Impervious Area Reduction Credit, new large deciduous trees must be planted within 25 feet of the new or replaced impervious surfaces and new small deciduous trees must be plantd within 10 feet of new or replaced impervious surfaces. New trees cannot be credited against rooftop areas. Minimum tree caliper (at the time of planting)to receive credit is 2 inches. Enter number of new deciduous trees that meet qualification requirements in Box CBox C Multiply Box C by 100 and enter result in Box D Box D Existing Tree Canopy To receive Impervious Area Reduction Credit, existing large tree canopies must be within 25 feet and existing small tree canopies must be within 10 feet of ground‐level impervious surfaces (cannot be credit against roof top surfaces). Existing tree canopy credited towards Impervious Area Reduction must be preserved during and after construction throughout the life of the development. Minimum tree caliper to receive credit is 4 inches. No credit will be given to existing tree canopy located within environmental conservation areas. Enter square footage of existing tree canopy that meet qualification requirements in Box E. Box E Multiply Box E by 0.5 and enter result in Box F.Box F Total Tree Credit Add Boxes B, D and F and enter the result in Box G Box G Multiply Box 1 of Form SIM by 0.1 and enter the result in Box H. Box H Enter the lesser of Box G and H in Box I. (This is the amount to be entered as "Tree Credit" on Form SIM.)Box I SIM FORM 2014 Instructions 1. Enter square footage (sf) of total impervious area being developed into BOX 1. 2. Enter square footage (sf) for impervious area reduction techniques. 3. Enter sum of the impervious area reduction techniques into BOX 2. 4. Subtract BOX 2 from BOX 1 to find BOX 3, the amount of impervious area that requires stormwater management. 5. Select appropriate stormwater management facilities. 6. Enter the square footage of impervious area managed that will flow into each facility type. 7. Multiply each impervious area managed by the corresponding sizing factor. Enter this area as the facility surface area, This is the size of faclity required to manage runoff 9. Where selecting facilities that will overflow, select the point of discharge location. 10. Enter the sum of the total of all the impervious area managed into BOX 4. BOX 4 must be greaer than or equal to BOX 3. Lane County Area, Oregon 119—Salem-Urban land complex Map Unit Setting National map unit symbol: 2341 Elevation: 300 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: Farmland of statewide importance Map Unit Composition Salem and similar soils:50 percent Urban land:40 percent Estimates are based on observations, descriptions, and transects of the mapunit. Description of Salem Setting Landform:Stream terraces Landform position (three-dimensional):Tread Down-slope shape:Linear Across-slope shape:Linear Parent material:Gravelly mixed alluvium Typical profile H1 - 0 to 7 inches: gravelly silt loam H2 - 7 to 26 inches: gravelly clay loam H3 - 26 to 60 inches: very gravelly sand 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:None Frequency of ponding:None Available water supply, 0 to 60 inches: Low (about 4.8 inches) Interpretive groups Land capability classification (irrigated): 2s Land capability classification (nonirrigated): 2s Hydrologic Soil Group: B Ecological site: R002XC006OR - Stream Terrace Group Forage suitability group: Well drained < 15% Slopes (G002XY002OR) Other vegetative classification: Well drained < 15% Slopes (G002XY002OR) Map Unit Description: Salem-Urban land complex---Lane County Area, Oregon Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 11/14/2024 Page 1 of 2 Hydric soil rating: No Description of Urban Land 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 23, Aug 30, 2024 Map Unit Description: Salem-Urban land complex---Lane County Area, Oregon Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 11/14/2024 Page 2 of 2