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Home My WebLink About2014-05-14 Stormwater Narrative & Drainage Calcs D E S I G N L A B 901 N 3rd St, Suite 120 Minneapolis, MN 55401 612.260-7980 www.elanlab.com May 14, 2014 Christopher W. Long,PE Senior Project Manager Stantec 2335 Highway 36 West St. Paul,MN 55113 PALMER STATION RE:Stormwater Narrative Dear Mr. Long: Attached are the stormwater narrative and drainage calculations for this project. You should consider these preliminary subject to completion of the geotechnical investigation to confirm infiltration rates. Please feel free to contact me should you have questions or need additional information, 612- 260-7982. Sincerely, Elan Steve Johnston,PE Principal Engineer Cc: Elan File #CHI14001 Mick Lynskey Nick Hackworthy Site Analysis& Planning I Civil Engineering I Landscape Architecture I Construction Services PALMER STATION Single Family Residential Neighborhood Oak Park Heights, Minnesota STORM WATER NARRATIVE May 14, 2014 Developer: (Tea 1'V° Nt1Ee'. E ti Nick Hackworthy President 707 Commerce Drive Suite 410 Woodbury, MN 55125 651-501-6503 Direct 612-790-8194 Cell www.CreativeHCl.com D E S I G N L AB PROPOSED DEVELOPMENT The proposed development consists of 13 residential lots and an outlot served by a public street approximately 600 feet in length. The total parcel is 6.76 acres, with 0.48 acres of wetland, 0.83 acres of new and 0.41 acres of"implied 33' Oakgreen Ave. ROW". This results in a gross density of 1.92 units per acre and a net density of 2.58 units per acre. The site is located South of Oak Park Blvd. and East of Oakgreen Avenue North in Oak Park Heights, Minnesota. The surrounding land uses to the east, south and west are single family residential homes of comparable size to proposed development. The properties to the north and Northwest are developed with medium to higher density residential. The project is proposed to be developed as a PUD to address minor reductions in lot and street right of way widths. This reduction, along with a minimized street width of 28 feet is requested in part to reduce impervious area within the site. The reduced right of way has the added benefit of moving the homes closer to the street resulting in less tree loss. Access to the site is accomplished through a connection to Oak Park Boulevard, the street/driveway serving City Hall. EXISTING SITE CONDITIONS The site is presently occupied by a single family home with a variety of outbuildings. A portion of the site flows to the north to a wetland in the northeast corner of the site and to a depression that runs parallel to Oak Park Blvd. These two basins are drained by the same storm sewer system. The balance of the site flows to the south through a series of small wetlands and depressions and leaves the site via a swale between two homes. There is approximately 16 feet of overall relief. Significant portions of the site are wooded native trees as well a number of trees planted by the property owner. A geotechnical report has not been prepared for the project. The Web Soil Survey indicates the prevalence of Santiago Silt Loam and Kingsley Sandy Loam, hydrologic soil group (HSG) B and Freer Silt Loam HSG C. A copy of the soil map is attached. There are 4 wetlands on the site that were delineated by Kjolhaug Environmental Services Company, Inc. and approved by a Technical Evaluation Panel in the fall of 2013. All three wetlands were determined to meet the criteria for Managed 2 classification resulting in the requirement for a 30 feet buffer. DESIGN LAB Pagelof8 3/27/2014 JURISDICTIONAL BODIES AND REQUIREMENTS The City of Oak Park Heights and the Middle St. Croix Watershed Management Organization (MSCWMO) have jurisdiction over storm water management, wetland preservation and erosion control for this site. The Watershed District's requires that an applicant for a stormwater management permit demonstrate to the District that the proposed land-altering activity meets the following policies and performance standards applicable to this project: 1. Direct discharge of stormwater to wetlands and all other water bodies without water quality treatment is prohibited. WIMP Section 5.1.4, Poliy Item 1 2. All hydrologic studies shall analyze the 24-hour two- (2.75 inch), ten- (4.15 inch) and one hundred-year (5.9 inch) critical rainfall event, with the critical duration defined as that event causing either the highest water surface elevation or the largest peak discharge in an area or both. Any study must use consistent methodology for the pre-development and post- development land use conditions. The methodology must be approved by MSCWMO. 1VMP Section 5.1.4, Policy Item 3 3. Newly developed or redeveloped areas will be limited to the predevelopment or existing rate of runoff or to a rate within the capacity of downstream conveyance systems, whichever is less, and no increase in the volume or rate of runoff from newly developed areas will occur in areas where natural outlets do not exist. In sub-areas of a landlocked watershed, development shall not increase the predevelopment volume or rate of discharge from the sub-area for the 10-year return period event. WIMP Section 5.1.4, Policy Item 4 4. Runoff will be minimized through on-site infiltration or prevented through reduction in impervious surfaces to the largest extent possible before detention ponds are used. WMP Section 5.1.4, Policy Item 5 5. Site design practices that may have only a minor negative impact on peak flow and volume, such as the use of buffer strips along receiving waters and drainage swales,will be promoted to achieve compliance with the water quality performance standard. WIMP Section 5.1.4, Poliy Item 6 6. Project must meet the Design Standards for Structural Stormwater Management Measures of the State of Minnesota Stormwater Manual, November 2005, or the most recent edition at www.pca.state.mn.us/water/stormwater/stormwater-manual.html. WMP Section 5.1.4, Performance Standard 1; WIMP Section 5.2.4, Performance Standard 1 Design requirements for bioretention basins: • Adequate pretreatment upstream of bioretention practice(s) • Construction phased so that impervious area construction is complete and pervious areas have established vegetation before stormwater is introduced into bioretention practice(s) • Planting plan detailing establishment of vegetation suitable for hydrology of basin Z DESIGN LAS Page 2 of 8 5/14/14 • The design permeability rate through the planting soil bed must be high enough to fully drain the stormwater quality design storm runoff volume within 48 hours. • The design permeability rate through the planting soil bed must be high enough to fully drain the stormwater quality design storm runoff volume within 48 hours • Easy access provided for routine maintenance of infiltration practice(s) Maintenance plan clearly identifies long-term maintenance schedule and responsibility • Secondary outlet provided • Separation from seasonally saturated soils or bedrock is 3 feet or more 7. Enhanced volume runoff controls will be designed to retain on-site the first 1/2 inch of runoff for all impervious areas plus 1/4 inch of runoff for areas with compacted soils. WMP Section 5.1.4, Performance Standard 2 • Impervious areas or surfaces are defined as a surface in the landscape that impedes the infiltration of rainfall and results in an increased volume of surface runoff. • Compacted soils are defined as surfaces in the landscape where the underlying soils are compacted by construction or other activity that impedes the infiltration of rainfall and results in an increased volume of surface runoff. • In redevelopment scenarios,volume control standards apply only to areas of new construction,where soils are compacted,runoff characteristics are changed, or vegetation is removed. 8. No increase in rate of runoff leaving the site from pre-development to post-development conditions generated by the 24-hour two- (2.75 inch), ten- (4.15 inch) and one hundred-year (5.9 inch) critical rainfall events. WIMP Section 5.1.4,Performance Standard 3;VMP Section 5.2.4 Performance Standard 2 9. A Stormwater Pollution Prevention Plan (SWPPP) that meets the National Pollutant Discharge Elimination System (NPDES) requirements must be submitted to and approved by the MSCWMO for all projects undertaking grading, filling, or other land alteration activities which involve movement of earth or removal of vegetation on greater than 10,000 square feet of land (See Appendix J for NPDES requirements). 1VMP Section 5.1.4, Performance Standard 4;WIMP Section 5.2.4, Performance Standard 3 10. Direct discharge of stormwater to wetlands and all other water bodies without water quality treatment is prohibited. WIMP Section 5.1.4, Performance Standard 5 11. Predevelopment conditions shall assume "good hydrologic conditions" for appropriate land covers as identified in TR-55 or an equivalent methodology. The meanings of"hydrologic soil group" and "runoff curve number" are as determined in TR-55. WIMP Section 5.1.4, Performance Standard 6 DESIGN r 4'...:8 Page 3 of 8 - 5/14/14 12. Lowest floor elevations of structures built adjacent to stormwater management features and other water bodies must be a minimum of two feet above the 100-year flood elevation and a minimum of two feet above the natural overflow of landlocked basins. The landowner or developer is required to provide MSCWMO with the location of the 100-year flood elevation, natural overflow elevation, and lowest floor elevations. I-VMP Section 5.1.4, Performance Standard 8 13. Buffer zones of unmowed natural vegetation shall be maintained or created upslope of delineated wetland edges, ordinary high water elevation, or floodplain of all water bodies (wetlands, streams, lakes) in accordance with the Performance Standards relating to wetlands, Section 5.3.4. IVMP Section 5.1.4, Performance Standard 9 14. Detention facilities will be designed to attenuate peak flows and provide on-site infiltration in high-permeability soils, natural depressions, and swales. WIMP Section 5.1.4, Performance Standard 10 The proposed project meets all of these criteria. PROPOSED ON-SITE STORM WATER MANAGEMENT The runoff from the front half of the homes, and front yards is directed to the street where it flows north in the gutter to approximately station 1+15 where it is directed by concrete flumes to pretreatment forebays, designed to remove the majority of the suspended solids. These forebays are interconnected with a 12" RCP culvert. Once the forebays reach a depth of approximately 2 feet they overflow to an infiltration basin. For eventsproducing runoff in excess of the capacity of the larger than The infiltration basins have effectively managed to the predevelopment rates and volumes as demonstrated in the HydroCAD model which is attached. As required by watershed requirements the curve numbers for disturbed areas have been increased to reflect a change in hydrologic soil types (i.e. B becomes C, C becomes D). As tabulated below, the model shows a decrease in the rate and volume leaving the site in all three events. There is no surface discharge from the infiltration areas for the 2 and 10 year events. Basin/Event Existing Rate Proposed Rate Existing Volume Proposed Vol. Southwest 2-yr, 24 hr 0.09 CFS 0.06 CFS 0.025 Ac-Ft 0.009 Ac-Ft 10-yr,24 hr 0.66 CFS 0.31 CFS 0.100 Ac-Ft 0.028 Ac-Ft 100-yr,24 hr 3.01 CFS 1.12 CFS 0.362 Ac-Ft 0.210 Ac-Ft East 2-yr, 24 hr 0.53 CFS 0.36 CFS 0.117 Ac-Ft 0.079 Ac-Ft 10-yr, 24 hr 3.68 CFS 1.6 CFS 0.450 Ac-Ft 0.303 Ac-Ft 100-yr,24 hr 15.80 CFS 4.7 CFS 1.573 Ac-Ft 1.348 Ac-Ft Northwest 2-yr, 24 hr 0.25 CFS 0.23 CFS 0.060 Ac-Ft 0.055 Ac-Ft 10-yr,24 hr 2.01 CFS 1.84 CFS 0.243 Ac-Ft 0.223 Ac-Ft ZZ! DESIGN (AB: Page4of8 5/14/14 100-yr, 24 hr 9.16 CFS 8.39 CFS 0.876 Ac-Ft 0.803 Ac-Ft Central 2-yr,24 hr 0.50 CFS 0.14 CFS 0.074 Ac-Ft 0.026 Ac-Ft 10-yr,24 hr 2.30 CFS 0.89 CFS 0.234 Ac-Ft 0.162 Ac-Ft 100-yr,24 hr 8.13 CFS 7.85 CFS 0.733 Ac-Ft 0.638 Ac-Ft Combined 2-yr,24 hr 1.37 CFS 0.79 CFS 0.276 Ac-Ft 0.169 Ac-Ft 10-yr,24 hr 8.65 CFS 4.64 CFS 1.027 Ac-Ft 0.716 Ac-Ft 100-yr, 24 hr 36.10 CFS 22.06 CFS 3.544 Ac-Ft 2.999 Ac-Ft Shown below are the modeled high water elevations for each computed rainfall event. The column labeled "time is the duration that runoff will remain after an event, assuming an infiltration rate of 0.3 inches per hour. The ponds have been oversized to account for a misassumption in the infiltration rate. Basin Forebay Infiltration 2 yr,24 hr 10 yr,24 hr 100 yr,24 hr 2 yr,24 hr Time 10 yr,24 hr 100 yr,24 hr SW 912.3 912.6 915.4 911.2 48 912.6 913.8 East 917.8 918.5 918.9 913.5 60 914.7 915.7 Central n/a n/a n/a 914.3 52 914.6 915.2 NW n/a n/a n/a 912.2 8 912.4 912.6 WATER QUALITY TREATMENT During the 2-year and 10-year events 100% of the runoff from impervious areas is directed to the infiltration basins where 100% of the nutrients are trapped and infiltrated. For this reason calculation of the nutrient discharge is deemed unnecessary. WETLANDS There is no direct wetland impacts as a result of this proposed project. The four wetlands are all classified as Managed 2 resulting in 30 foot buffers. The required buffers are shown on the grading plan. It is proposed to place limited stormwater treatment facilities within the buffers. It is also proposed to build a retaining wall within the wetland #1 buffer. EROSION CONTROL AND CONSTRUCTION BMPS The plans for the proposed project include numerous Best Management Practices (BMP) to minimize erosion and the transport of sediment during and after construction. A construction entrance is indicated at the only access point to the site. The disturbed areas of the site that flow offsite are protected with silt fence. All disturbed areas will be seeded and protected with fiber blanket or mulch. See plans for additional details. BASEMENT FLOOR ELEVATIONS The proposed basement floor elevations are at least 2 feet above the high water level of the nearest infiltration basin,pond or wetland. ZZr DESIGN AE, Page 5 of 8 5/14/14 CONCLUSION We believe that our design meets the city and watershed requirements for rate and volume control and water quality treatment. Please direct any questions regarding the design of the storm drainage system to Steve Johnston at 612-382-4804 This report was prepared by me and I am a duly Licensed Professional Engineer under the laws of the State of Minnesota. iI♦ Stephen Johnston 3/27/2014 Registration #18914 ZZrtiff,,� DESIGN 4A$. Page6of8 5/14/14