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Home My WebLink AboutPROSPECT 7-ELEVEN STORE NO. 35506 - PDP - PDP120002 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTJanuary 25, 2012 Mr. Glen Schlueter City of Fort Collins Stormwater Engineering Division 700 Wood Street Fort Collins, CO 80521 RE: Prospect 7-Eleven Store No. 35506 Drainage Letter REG No. 111101 Dear Mr. Schlueter: The purpose of this letter is to discuss the drainage system improvements and erosion control measures proposed for the development of this project located in the southeast quadrant of the intersection of South Overland Trail and West Prospect Road. More specifically this project is comprised of the Elliot-Miller Foothills West Subdivision, Third Filing, located in the northwest 1/4 of the northwest ¼ of Section 21, Township 7 North, Range 69 West of the 6 th Principal Meridian, City of Fort Collins, County of Larimer, State of Colorado. A vicinity map is included in Appendix A. As defined in the City of Fort Collins, Stormwater Utility, Storm Drainage Design Criteria and Construction Standards, Section 1.3: “All new multi-family and commercial development with an increase in impervious area of greater than 350 shall submit drainage and erosion control reports and plans to be approved by the Stormwater Utility”. This project will decrease the imperviousness from approximately 98% impervious to approximately 75% impervious with the addition of landscape treatment on portions of the project. An Impervious Area Comparison is included in Appendix B. This percentage is based upon the net remaining area after a right of way dedication is conveyed. If the right of way land were included the impervious value would drop even more, as much of the existing right of way which is paved will be replaced with landscaped surface. As a formal drainage and erosion control is not required this letter will detail site stormwater runoff and conveyance capacity conditions in addition to the construction and post construction water quality considerations. This letter is divided into two sections. 1) will describe the historic and proposed stormwater characteristics and detail the efforts proposed to provide permanent storm water runoff quality This letter and accompanying calculations describe the impacts to the drainage system, quantify the 100 year stormwater runoff through the site, and propose actions to mitigate the impacts and safely convey the storm water runoff through and from the site. The design is compliant with City of Fort Collins (City) requirements and regulations. and, 2) will detail the erosion control planning and effective and performance standards. Prospect 7-Eleven Store No. 35506 January 25, 2012 Drainage Letter DRAINAGE The Prospect 7-Eleven Store No. 35506 (Project) is a 0.315 acre (after right of way is dedicated to the City, which is a condition of the development) parcel of land. The land is zoned NC and is currently functioning as a convenience store and fueling station and is almost exclusively covered by paving and building. The land falls gently (1% – 2%) from the south to the north. A small strip of landscaped area (about 5 feet by 125 feet) contributes stormwater runoff from the south. The adjacent parcel to the east has a connecting parking lot to the Project and may contribute minor surface runoff onto the site. The proposed development will remove all existing improvements (parking lot, structure and fuel service and build a new 7-Eleven convenience store with appurtenant parking, utility services and landscape. This letter and the supporting documentation are based upon the following assumptions: 1) No detention storage is required for this site. 2) Onsite water quality mitigation is required for this site, following the recommendations and criteria of the Urban Drainage and Flood Control Districts, Storm Drainage Criteria Manual (Manual), Volume 3, to the maximum extent practical. The full development of this project will be consistent with the approved Zoning for NC. This zoning requires a five foot parking lot setback from adjacent parcels and a 10 foot – 25 foot build to line along the two arterial frontages. Furthermore the requirements for maximum parking counts and interior parking landscape requirements have been followed. The predominant soil is Altvan- Staanta loams, according to the NRCS, which is Type B hydrologic soil group. Soil maps and soil properties are included in Appendix C. The Project does not propose to change the existing drainage patterns of the site. The Project is considered, for hydrological purposes, a single subbasin that is tributary to the inlet located just off site in the southeast corner of the intersection of Prospect Road and Overland Trail. Runoff calculations and supporting documentation are included in Appendix D at the back of this report, and a Drainage Map is included in Appendix E. Any tributary offsite runoff from the east will be intercepted by a shallow grasslined swale and conveyed to Prospect Road, the last fifteen feet within a curb and gutter. The tributary runoff from the south will pass through the site and will enter the water quality swales, or if it enters far enough to the west it will be conveyed to the Overland Trail right of way street, curb and gutter. The runoff from the site will drain overland and via downspout or curb and gutter will enter a shallow grass lined swale (designed per the Manual’s criteria for a grass lined swale BMP) on either the west or north side of the convenience store. This water will be conveyed to the northwest corner of the site where it will be intercepted by a concrete inlet which will be constructed to tie to the existing inlet in the south east corner of the intersection. While there is no 100-year storm detention requirement, the capacity of all of the conveyances to this inlet can Prospect 7-Eleven Store No. 35506 January 25, 2012 Drainage Letter handle the 100-year developed runoff of approximately 2.4 cfs. In the event of inlet clogging, the water will overtop the channel bank and spill into the right of way prior to reaching the finish floor level of the convenience store. POST CONSTRUCTION WATER QUALITY The Manual defines a four step process to minimize the adverse impacts of urbanization. While this project is a redevelopment of an urbanized parcel, the proposed improvements have considered and employed the recommendations of this process and provide significant improvement to the existing condition. Step 1 is to “Employ Runoff Reduction Practices”. To reduce runoff peaks, volumes and pollutant loads this project has implemented Low Impact Development (LID) strategies, including Minimizing Directly Connected Impervious Area (MDCIA). Historically, all runoff from this site, which is almost 100% paved , and the small amounts of offsite drainage from paved surfaces, drained overland to the public right of way conveyance system, as Directly Connected Impervious Area to the public storm system. This project will increase the pervious area by about 25% from existing conditions. In addition, this project will direct all on-site and offsite tributary flows into and through landscaped areas and grass lined swales. The effective impervious is reduced, and runoff velocities are slowed within the grass swales which promotes infiltration, thereby potentially reducing the capacity requirements of downstream facilities. Step 2 is to “Implement BMP’s that Provide a Water Quality Capture Volume with Slow Release”. This project does not include water quality capture volume ponds. However, the grass lined swales are designed with specified soil and underdrains to facilitate infiltration. This will allow for some retention volume with in the soil in the form of infiltration. Step 3 is to “Stabilize Drainageways”. This project does not include modification to any existing drainageways. It is anticipated that the LID measures defined in Step 1, above will decrease the amount of runoff leaving the site. This will place less of a burden on downstream public facilities. A visual inspection of the nearby receiving storm system and roadside ditches did not indicate existing erosion concerns. The on site grass lined swales will be stabilized with thick stands of blue grass. CONSTRUCTION WATER QUALITY The project will follow City standards to prevent erosion and sediment transport during construction. An Erosion Control Plan has been prepared for approval by the City. The project will include perimeter controls to prevent sediment transport including silt fence and straw wattles. Also included is a vehicle tracking control entry at the access from Overland Trail. The concrete wash out and stabilized staging area will be located in the vicinity of the vehicle tracking control. Prospect 7-Eleven Store No. 35506 January 25, 2012 Drainage Letter A sediment trap will be located on the site to capture sediment laden runoff. The site is 0.357 acres of disturbed land and requires 100 cubic yards per acre. Therefore the sediment trap must be 964 cubic feet. Calculations for the sediment trap are included in the Appendix D The Project will disturb approximately 0.357 acres. Therefore no Colorado Discharge Permit will be required with this project. CONCLUSION The Site has been laid out to accommodate the offsite and site generated storm water runoff up to the 100-year recurrent event. Further, water quality mitigation in the form of grass lined swales has been considered for the Project Site. The drainage system has been designed to safely convey the tributary offsite and onsite generated storm runoff to the existing conveyance system within Overland Trail and Prospect Road. Thank you for your consideration of this matter. Should you require any additional information or have any questions, please do not hesitate to contact me at 303-841-9365. Sincerely ROTH ENGINEERING GROUP, LLC Kevin S. Roth, P.E., LEED AP Managing Member Prospect 7-Eleven Store No. 35506 January 25, 2012 Drainage Letter APP A Appendix A – Vicinity Map SITE SITE VICINITY MAP SCALE 1" = 1000' N Prospect 7-Eleven Store No. 35506 January 25, 2012 Drainage Letter APP B Appendix B – Impervious Area Comparison X X X X T C:\Users\Kevin Roth\Roth Engineering Group\Projects\111101\CADD\Exhibits\iMPERVIOUS cOMPARRISON.dwg tab: Layout1 Jan 20, 2012 - 1:20pm Kevin Roth ROTH ENGINEERING GROUP, LLC SHEET NUMBER: DESIGNED BY: DRAWN BY: JOB NUMBER: DATE KSR KSR 111101 IMPERVIOUS AREA COMPARRISON 01/25/12 PROSPECT 7-ELEVEN STORE # 35506 1 EXISTING PERVIOUS AREAS AREA= 274.8 SQUARE FEET (NOTE ONLY AREAS WITHIN PROPOSED SITE BOUNDARY IS INCLUDED) PROPOSED PERVIOUS AREAS AREA= 3,197.7 SQUARE FEET (NOTE ONLY AREAS WITHIN PROPOSED SITE BOUNDARY IS INCLUDED) PERVIOUS AREAS ARE SHADED Prospect 7-Eleven Store No. 35506 January 25, 2012 Drainage Letter APP E Appendix C – Soils Information Pleasant Valley and Lake Canal Prospect Rd Lake St Overland Trl Banyan Dr Azalea Dr Sumac St Pecan St Somerville Dr Larch St Cedarwood Dr Golden Currant Blvd Palm Dr Holly Way Virginia Dale Dr Holly St Deerfield Dr Rams Ln Charleston Way Etton Dr Flintridge Pl Bradbury Ct Real Ct La Reatta Ct Ross Ct Wells Fargo Dr Spruce Dr Wildwood Rd Bangor Ct Hollingbourne Dr Dorset Dr Indian Peaks Pl Zirkels Ct Reedgrass Ct Honeysuckle Ct Belmar Dr Fleetwood Ct Covington Ct Palm Ct Holly Pl Golden Currant Ct Wildwood Ct Somerville Ct Rawahs Way Azalea Ct Wheatgrass Ct Knotwood Ct Burning Bush Ct Woodrose Ct 3 4 4 74 74 4 54 76 65 74 4 MAP LEGEND MAP INFORMATION Area of Interest (AOI) Area of Interest (AOI) Soils Soil Map Units Soil Ratings .02 .05 .10 .15 .17 .20 .24 .28 .32 .37 .43 .49 .55 .64 Not rated or not available Political Features Cities Water Features Streams and Canals Transportation Rails Interstate Highways US Routes Major Roads Local Roads Map Scale: 1:7,520 if printed on A size (8.5" × 11") sheet. The soil surveys that comprise your AOI were mapped at 1:24,000. 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 accurate map measurements. Source of Map: Natural Resources Conservation Service Web Soil Survey URL: http://websoilsurvey.nrcs.usda.gov Coordinate System: UTM Zone 13N NAD83 This product is generated from the USDA-NRCS certified data as of the version date(s) listed below. Soil Survey Area: Larimer County Area, Colorado Survey Area Data: Version 7, May 1, 2009 Date(s) aerial images were photographed: 8/6/2005 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. K Factor, Whole Soil–Larimer County Area, Colorado (7-Eleven Prospect) Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 1/19/2012 Page 2 of 3 K Factor, Whole Soil K Factor, Whole Soil— Summary by Map Unit — Larimer County Area, Colorado (CO644) Map unit symbol Map unit name Rating Acres in AOI Percent of AOI 3 Altvan-Satanta loams, 0 to 3 percent slopes .28 93.7 43.6% 4 Altvan-Satanta loams, 3 to 9 percent slopes .28 60.3 28.0% 54 Kim loam, 3 to 5 percent slopes .32 5.2 2.4% 63 Longmont clay, 0 to 3 percent slopes .17 0.8 0.4% 65 Midway clay loam, 5 to 25 percent slopes .24 2.1 1.0% 67 Minnequa-Laporte complex, 3 to 15 percent slopes .43 0.2 0.1% 74 Nunn clay loam, 1 to 3 percent slopes .20 48.2 22.4% 76 Nunn clay loam, wet, 1 to 3 percent slopes .17 4.5 2.1% Totals for Area of Interest 215.1 100.0% Description Erosion factor K indicates the susceptibility of a soil to sheet and rill erosion by water. Factor K is one of six factors used in the Universal Soil Loss Equation (USLE) and the Revised Universal Soil Loss Equation (RUSLE) to predict the average annual rate of soil loss by sheet and rill erosion in tons per acre per year. The estimates are based primarily on percentage of silt, sand, and organic matter and on soil structure and saturated hydraulic conductivity (Ksat). Values of K range from 0.02 to 0.69. Other factors being equal, the higher the value, the more susceptible the soil is to sheet and rill erosion by water. "Erosion factor Kw (whole soil)" indicates the erodibility of the whole soil. The estimates are modified by the presence of rock fragments. Rating Options Aggregation Method: Dominant Condition Component Percent Cutoff: None Specified Tie-break Rule: Higher Layer Options: Surface Layer K Factor, Whole Soil–Larimer County Area, Colorado 7-Eleven Prospect Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 1/19/2012 Page 3 of 3 Report—Engineering Properties Absence of an entry indicates that the data were not estimated. The asterisk '*' denotes the representative texture; other possible textures follow the dash. Engineering Properties– Larimer County Area, Colorado Map unit symbol and soil name Depth USDA texture Classification Fragments Percentage passing sieve number— Liquid limit Plasticity index Unified AASHTO >10 inches 3-10 inches 4 10 40 200 In Pct Pct Pct 3—Altvan-Satanta loams, 0 to 3 percent slopes Altvan 0-10 *Loam CL, CL- ML A-4 0 0 100 100 85-95 60-75 25-30 5-10 10-18 *Clay loam *Loam *Sandy clay loam CL A-6 0 0 95-100 95-100 85-100 60-80 25-35 10-20 18-30 *Loam *Fine sandy loam *Silt loam ML A-4 0 0 90-100 85-100 60-95 50-75 20-25 NP-5 30-60 *Gravelly sand *Gravelly coarse sand *Coarse sand SP-SM, SP A-1 0 0 75-95 50-90 25-35 0-10 — NP Satanta 0-9 *Loam CL-ML, CL A-4 0 0 100 95-100 85-95 60-75 25-30 5-10 9-18 *Loam *Clay loam *Sandy clay loam CL, CL- ML A-4, A-6 0 0 100 95-100 75-100 50-80 20-35 5-20 18-60 *Loam *Clay loam *Fine sandy loam SC, CL A-4 0 0 100 95-100 65-100 40-80 25-30 5-10 Engineering Properties–Larimer County Area, Colorado 7-Eleven Prospect Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 1/19/2012 Page 3 of 6 RUSLE2 Related Attributes This report summarizes those soil attributes used by the Revised Universal Soil Loss Equation Version 2 (RUSLE2) for the map units in the selected area. The report includes the map unit symbol, the component name, and the percent of the component in the map unit. Soil property data for each map unit component include the hydrologic soil group, erosion factors Kf for the surface horizon, erosion factor T, and the representative percentage of sand, silt, and clay in the surface horizon. Report—RUSLE2 Related Attributes RUSLE2 Related Attributes– Larimer County Area, Colorado Map symbol and soil name Pct. of map unit Slope length (ft) Hydrologic group Kf T factor Representative value % Sand % Silt % Clay 3—Altvan-Satanta loams, 0 to 3 percent slopes Altvan 45 — B .28 4 42.4 38.1 19.5 Satanta 30 — B .28 5 43.0 39.5 17.5 4—Altvan-Satanta loams, 3 to 9 percent slopes Altvan 55 — B .28 4 42.4 38.1 19.5 Satanta 35 — B .28 5 43.0 39.5 17.5 54—Kim loam, 3 to 5 percent slopes Kim 90 — B .32 5 41.6 37.4 21.0 63—Longmont clay, 0 to 3 percent slopes Longmont 85 — C .17 5 22.1 27.9 50.0 65—Midway clay loam, 5 to 25 percent slopes Midway 90 — D .24 2 33.3 31.7 35.0 67—Minnequa-Laporte complex, 3 to 15 percent slopes Minnequa 55 — C .43 3 11.3 67.7 21.0 Laporte 30 — D .20 2 43.8 40.2 16.0 74—Nunn clay loam, 1 to 3 percent slopes Nunn 85 — C .20 5 35.4 33.6 31.0 76—Nunn clay loam, wet, 1 to 3 percent slopes Nunn, wet 90 — C .17 5 35.4 33.6 31.0 RUSLE2 Related Attributes–Larimer County Area, Colorado 7-Eleven Prospect Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 1/19/2012 Page 1 of 2 Prospect 7-Eleven Store No. 35506 January 25, 2012 Drainage Letter APP E Appendix D – Drainage Calculations PROJECT NAME: DATE: 1/10/2012 PROJECT DESCRIPTION: REVISED: PROJECT NO.: DESIGN BY: KSR ELEVATION AREA INC. VOL INC. VOL. TOTAL VOL. (FT2) (FT3) CY (AC-FT) 21.00 107 0.00 0.0000 0.000 22.00 457 261.71 261.7104 261.710 22.90 915 605.59 867.3051 1,129.016 Disturbed area 0.32 acres Water Surface El Sediment Pond 0.003 ac-ft 5670.9 0.085 cf 5671.5 100-YR Volume 0.04 ac-ft 5671.80 Total Volume 0.13 111101 ROTH ENGINEERING GROUP DETENTION POND SIZING STAGE VERSUS STORAGE 7-11 Ft. Collins TSB K = K (0.95I = (1.- 78I-1.90).002I2 /1000 - 20.5 21.0 21.5 22.0 22.5 23.0 0.000 200.000 400.000 600.000 800.000 1,000.000 1,200.000 Elevation Storage (ac-ft) Stage vs. Storage Prospect 7-Eleven Store No. 35506 January 25, 2012 Drainage Letter APP E Appendix E – Drainage Plan X X X T 5123 5122 5122 5123 5122 5122 5122 5122 5124 PROPOSED ROW 57.5' MEASURED FROM SECTION LINE/ CENTER LINE) 8' UTILITY ESMT 6' CONC. WALK 8' UTILITY ESMT 6' CONC. WALK BIKE RACK EXISTING TRANSFORMERS EX BLDG (TYP) EXIST 8' UTILITY ESMT EXIST 5' UTILITY ESMT PROPERTY LINE PROPERTY LINE EXISTING ROW EXISTING ROW EXIST FH EXISTING 18" PVC 2 3 3 3 1 B B A A A A 1/4 SECTION LINE PROPOSED 12" CP BLDG FACE PL UTIL ESMT GRASS SWALE SIDE SLOPES= 10% (MIN) 4:1 MAX 8' 2' 6' 6' CONC WALK 11.1' +/- TREE LAWN 2% 2% 4" THICK SANDY LOAM PL UTIL ESMT 6" DEEP GRASS SWALE SIDE SLOPES= 10% (MIN) 5:1 MAX ADJACENT PARKING LOT SLOPE VARIES PARKING LOT 5' SHEET NUMBER: OF ISSUE DATE: DATE REVISIONS BY ROTH ENGINEERING GROUP, LLC PREPARED FOR: DESIGNED BY: DRAWN BY: JOB NUMBER: Know what's R CALL 2-BUSINESS DAYS IN ADVANCE BEFORE YOU DIG, GRADE OR EXCAVATE FOR THE MARKING OF UNDERGROUND MEMBER UTILITIES. 111101 JANUARY 25, 2012 DP KSR KSR N SECTION AA - GRASS LINED SWALE SCALE 1" =5' SECTION BB SCALE 1" =5' INLET: 2' SQUARE OLDCASTLE 2 X 2 CATCH BASIN WITH 24" SQUARE FRAME & GRATE (OR APPROVED EQUAL) 4" PERFORATED HDPE PIPE. SLOPE TO MATCH SWALE. SIDEWALK CHASE RE: PAVING PLAN FOR DETAIL RIGHT OF WAY DEDICATION 1 2 3 A RUNOFF SUMMARY A AREA DETENTION OFFSITE EXISTING C:\Users\Kevin Roth\Roth Engineering Group\Projects\111101\CADD\Sheet Sets\Construction\DRAIN PLAN.dwg, 1/22/2012 11:47:52 PM, 1:1 63 67 74 488000 488000 488100 488100 488200 488200 488300 488300 488400 488400 488500 488500 488600 488600 488700 488700 488800 488800 488900 488900 489000 489000 489100 489100 489200 489200 489300 489300 489400 489400 489500 489500 4490200 4490200 4490300 4490300 4490400 4490400 4490500 4490500 4490600 4490600 4490700 4490700 4490800 4490800 4490900 4490900 4491000 4491000 4491100 4491100 4491200 4491200 0 400 800 1,600 2,400 Feet 0 100 200 400 600 Meters 40° 34' 18'' 105° 7' 23'' 40° 33' 44'' 105° 7' 23'' 40° 33' 44'' 40° 34' 18'' 105° 8' 30'' 105° 8' 30'' Map Scale: 1:7,530 if printed on A size (8.5" x 11") sheet. K Factor, Whole Soil—Larimer County Area, Colorado (7-Eleven Prospect) Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 1/19/2012 Page 1 of 3