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Home My WebLink AboutFORT COLLINS MOTORSPORTS - BDR200009 - SUBMITTAL DOCUMENTS - ROUND 2 - DRAINAGE REPORTDRAINAGE LETTER FORT COLLINS MOTORSPORTS FORT COLLINS, CO 1501 ACADEMY CT. STE 203 | FORT COLLINS, CO 80524 | 970-530-4044 | www.unitedcivil.com July 21, 2020 City of Fort Collins Stormwater Utility 700 Wood Street Fort Collins, Colorado 80521 RE.: FORT COLLINS MOTORSPORTS FORT COLLINS, COLORADO PROJECT NUMBER: U19044 Dear Staff, United Civil Design Group, LLC. is pleased to submit this Drainage Letter for the Fort Collins Motorsports site in Fort Collins, Colorado. In general, this summary serves to document the stormwater impacts associated with the proposed site improvements associated with the Minor Amendment for the Fort Collins Motorsports Subdivision. SITE LOCATION AND PROJECT DESCRIPTION The Fort Collins Motorsports site (referred herein as ”the site”) is situated in the Northwest Quarter of Section 22, Township 7 North, Range 68 West of the 6th Principal Meridian, City of Fort Collins, Larimer County, State of Colorado. More specifically, the site is located along Frontage Road, immediately east of Interstate 25, and south of the Prospect Road. The planned improvements for the site will consist of the following: • The addition of a gravel storage yard with associated fencing. • The addition of gravel drives for an emergency access easement. • Removal of recycled asphalt area for proposed gravel storage yard EXISTING CONDITIONS Hydrology The site, located east of the Boxelder Creek Basin, is approximately 3.3 acres and currently consists of one commercial building, a paved parking lot, associated utilities, and drainage infrastructure. The site was originally developed in 1998 when the property was located in unincorporated Larimer County prior to annexation into the City of Fort Collins. The existing site is largely designed to drain via curb, concrete pans, and overland flow to an existing detention pond located at the southwest corner of the site. The existing site imperviousness is calculated at 48.7%. Floodplain The site is not located within a FEMA floodplain or City-regulatory floodplain. PROPOSED CONDITIONS Drainage patterns associated with site are largely unchanged with the site improvements. The proposed site improvements include modifications to the storage area north of the existing building in addition to the addition of gravel drives for an emergency access road. The proposed site imperviousness is 49.7% and is documented on the Proposed Site Impervious Exhibit found in Appendix A. The improvements increase the effective impervious area of the site by 1,706 square feet. The table on the following page summarizes the modifications to the impervious area. In addition, a Modified Site Impervious Exhibit can be found in Appendix A. DRAINAGE LETTER FORT COLLINS MOTORSPORTS FORT COLLINS, CO 1501 ACADEMY CT. STE 203 | FORT COLLINS, CO 80524 | 970-530-4044 | www.unitedcivil.com MODIFIED IMPERVIOUS AREA SUMMARY TABLE Area Imperviousness Effective Impervious Area (sf) (%) (sf) Proposed EAE Area to the North (Gravel*) 2,128 40% 851.2 Proposed EAE Area to the West (Gravel*) 1,447 40% 578.8 Proposed Yard Area (Gravel*) 5,000 40% 2,000 Proposed Pan to the West (Concrete*) 40 90% 36 Less Yard Area to be Removed (Recycled Asphalt*) -2,200 80% -1,760 Effective Impervious Differential 1,706 * - Percent imperviousness for different area types based on Fort Collins Stormwater Criteria Manual recommended values. DETENTION & WATER QUALITY It is understood that the City may require stormwater detention and water quality/LID for an increase in imperviousness of over 1,000 square feet with a Minor Amendment. On this 3.3-acre site an increase in imperviousness of 1,706 square feet increases the imperviousness of the site 1% from 48.7% to 49.7%. It is our opinion that this minor increase in impervious area will create negligible increases in runoff that do not warrant additional stormwater detention volume. In addition, the new impervious areas are being minimized by the use of gravel which is considered 40% impervious. The proposed gravel storage yard and northern portion of the gravel emergency access road will drain to a grass swale that is 95 feet in length at a slope of 0.40%. Then it will drain across a concrete pan for 22’ prior draining along an existing grass swale that is approximately 130 feet at an average slope of 0.40%. The new southern portion of the gravel emergency access road will drain south along an existing swale that is approximately 380 feet at an average slope of 0.40%. The shallow longitudinal slopes are less than typical standards for new construction (1% to 2%), but they are an existing condition that cannot be fixed. In this case, the flatter slope will provide better water quality and is not in a location that will cause drainage issues on the site. EROSION CONTROL The site disturbance is less than 1 acre; therefore, a CDPHE Stormwater Management Plan (SWMP) is not required. However, erosion control measures are included with the site improvements. Refer to the Grading and Erosion Control Plan under separate cover for additional information. CONCLUSIONS In relation to the existing site and drainage system, the proposed site improvements are negligible. Existing drainage patterns are unaltered with the site improvements; developed stormwater runoff from the proposed improvements continues to drain downstream to an existing detention pond and then downstream. It is our opinion that onsite and downstream storm drainage systems will not be affected by these minor improvements. We understand that review by the City of Fort Collins to assure general compliance with standardized criteria contained in the Fort Collins Stormwater Criteria Manual (FCSCM). This summary was prepared in compliance with DRAINAGE LETTER FORT COLLINS MOTORSPORTS FORT COLLINS, CO 1501 ACADEMY CT. STE 203 | FORT COLLINS, CO 80524 | 970-530-4044 | www.unitedcivil.com technical criteria set forth in both the Urban Drainage and Flood control District (UDFCD) Urban Storm Drainage Criteria Manual and the FCSCM. If you should have any questions or comments as you review this drainage summary, please feel free to contact us at your convenience. Sincerely, United Civil Design Group, LLC Sam Eliason, P.E. Principal APPENDICES APPENDIX A – Calculations and Exhibits -Runoff Coefficients and % Impervious Chart -Proposed Site Impervious Exhibit -Modified Site Impervious Exhibit APPENDIX A CALCULATIONS AND EXHIBITS RUNOFF COEFFICIENTS AND % IMPERVIOUS Fort Collins Motorsports, Fort Collins, CO Basin Composite Total Total Roof Asphalt Concrete Recycled Asphalt Gravel Lawns Imperviousness C2 C100 %I = 90% %I = 100% %I = 100% %I = 80% %I =40% %I=2% (%I) C=0.95 C=0.95 C=0.95 C=0.80 C=0.50 C=0.20 acres sf sf sf sf sf sf sf Existing Site 3.31 144,090 19,785 33,083 6,422 5,837 18,768 66,032 48.7% 0.58 0.73 Basin Composite Total Total Roof Asphalt Concrete Recycled Asphalt Gravel Lawns Imperviousness C2 C100 acres sf %I = 90% %I = 100% %I = 100% %I = 80% %I =40% %I=2% (%I) C=0.95 C=0.95 C=0.95 C=0.80 C=0.50 C=0.20 sf sf sf sf sf sf Proposed Site 3.31 144,090 19,785 33,083 6,462 3,655 27,133 57,627 49.7% 0.59 0.73 Notes: - (1) Recommended % Imperviousness Values per Table 4.1-3 Surface Type - Percent Impervious in Fort Collins Stormwater Criteria Manual (2) Runoff C is based Table 3.2-2. Surface Type - Runoff Coefficients and Table 3.2-3. Frequency Adjustment Factors in Fort Collins Stormwater Manual (3) Runoff C for Lawns based off of Lawns, Clayey Soil, Flat Slope < 2% Existing Areas Composite Runoff Coefficients (2) Areas Composite Runoff Coefficients (2) Proposed Date: 7/15/2020 P:\U19044 - Fort Collins Motorsports\Reports\Drainage\Calculations\U19044-Impervious Calcs X X X X X X X X X X X X X X X X X X X X X X X X ST ST X X X X X X X X X X X X X X X X X X X ELEC ELEC TELE ELEC PROPOSED GRAVEL AREA PROPOSED GRAVEL YARD PROPOSED EMERGENCY ACCESS EASEMENT AREA (GRAVEL) PROPOSED EMERGENCY ACCESS EASEMENT AREA (GRAVEL) PROPOSED CONCRETE PAN PROPERTY BOUNDARY EXISTING BUILDING EXISTING X X X X X X X X X X X X X X X X X X X X X X X ST ST X X X X X X X X X X X X X X X X X X ELEC ELEC TELE ELEC RECYCLED ASPHALT TO BE REMOVED PROPOSED GRAVEL YARD PROPOSED EMERGENCY ACCESS EASEMENT AREA (GRAVEL) PROPOSED EMERGENCY ACCESS EASEMENT AREA (GRAVEL) PROPOSED CONCRETE PAN PROPERTY BOUNDARY EXISTING BUILDING EXISTING GRAVEL PARKING AREA EXISTING ASPHALT STORAGE YARD EXISTING CONCRETE PAN LIMITS OF DISTURBANCE LIMITS OF DISTURBANCE EXISTING DETENTION POND PROPOSED EMERGENCY ACCESS EASEMENT 0' 40' 80' SCALE: 1" = 40' 20' PATH: P:\U19044 - FORT COLLINS MOTORSPORTS\CADD\ET\SITE IMPERVIOUS EXHIBIT.DWG SITE IMPERVIOUS EXHIBIT DRAWING NAME: SHEET SIZE: 17 x 11 FORT COLLINS MOTORSPORTS SUBDIVISION MODIFIED SITE IMPERVIOUS EXHIBIT DATE: July 14, 2020 PREPARED FOR: FORT COLLINS MOTORSPORTS JOB NUMBER: U19044 NOTE: THIS EXHIBIT WAS PREPARED FOR INFORMATION ONLY AND THE ENGINEER PREPARING THESE PLANS WILL NOT BE RESPONSIBLE FOR UNAUTHORIZED USE OF THEM. 1501 ACADEMY COURT, SUITE 203 FORT COLLINS, CO 80524 (970) 530-4044 WWW.UNITEDCIVIL.COM SHEET 1 OF 2 UNITED CIVIL Design Group LLC NORTH ASPHALT PAVEMENT CONCRETE PAVEMENT/ SIDEWALK PROPOSED GRAVEL DRIVE EXISTING RECYCLED ASPHALT EXISTING GRAVEL DRIVE PAVEMENT LEGEND PROPOSED GRAVEL YARD GRAVEL PARKING EXISTING ASPHALT AREA STORAGE YARD EXISTING CONCRETE PAN LIMITS OF DISTURBANCE EXISTING DETENTION POND PROPOSED EMERGENCY ACCESS EASEMENT EXISTING ASPHALT PARKING AREA 0' 40' 80' SCALE: 1" = 40' 20' PATH: P:\U19044 - FORT COLLINS MOTORSPORTS\CADD\ET\PROPOSED SITE IMPERVIOUS EXHIBIT.DWG PROPOSED SITE IMPERVIOUS EXHIBIT DRAWING NAME: SHEET SIZE: 17 x 11 FORT COLLINS MOTORSPORTS SUBDIVISION PROPOSED SITE IMPERVIOUS EXHIBIT DATE: July 14, 2020 PREPARED FOR: FORT COLLINS MOTORSPORTS JOB NUMBER: U19044 NOTE: THIS EXHIBIT WAS PREPARED FOR INFORMATION ONLY AND THE ENGINEER PREPARING THESE PLANS WILL NOT BE RESPONSIBLE FOR UNAUTHORIZED USE OF THEM. 1501 ACADEMY COURT, SUITE 203 FORT COLLINS, CO 80524 (970) 530-4044 WWW.UNITEDCIVIL.COM SHEET 2 OF 2 UNITED CIVIL Design Group LLC NORTH ASPHALT PAVEMENT CONCRETE PAVEMENT/ SIDEWALK PROPOSED GRAVEL DRIVE EXISTING RECYCLED ASPHALT EXISTING GRAVEL DRIVE PAVEMENT LEGEND PROPOSED GRAVEL YARD