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Home My WebLink AboutHIGH TOP CULTIVATION FACILITY - BDR200006 - SUBMITTAL DOCUMENTS - ROUND 2 - DRAINAGE RELATED DOCUMENT7200 South Alton Way, Suite C400 Centennial, CO80112 303-740-9393 x Fax 303-921-7320 130 East Kiowa Street, Suite 400 Colorado Springs, CO80903 719-593-2593 x Fax 303-921-7320 2900 South College Avenue, Suite 3D Fort Collins, CO80525 970-491-9888 x Fax 303-921-7320 MEMORANDUM JR Engineering, LLC has completed a weighted imperviousness based storm drainage evaluation for the High Top Cultivation Facility. The following is a summary of the findings and determinations made while performing an analysis of the proposed emergency access paving addition, concrete mechanical pad, and sidewalk. It is important to note three things while reviewing this memo. First, we believe that this project will not be required to provide a full drainage report, water quality or detention facilities, based on the proposed site condition having a net reduction in impervious area from the sites existing condition. Second, this analysis has been performed using a “weighted impervious area” calculation, which is the square footage of each impervious surface type, multiplied by the surfaces weighted impervious coefficient values for infiltration characteristics of different materials (< 1). The impervious coefficient values were derived from the City of Fort Collins Stormwater Criteria Manual Table 4.1-3 “Surface type – Percent Impervious”. Lastly, the High Top Cultivation Facility site area’s drainage pattern was originally included in the “Stormwater & Erosion Control Report for Kindcare” prepared by Forbes Engineering and approved by the City of Fort Collins September 26, 2016. The “Proposed Impervious Area Exhibit” and “Proposed Drainage Sub- Basins” maps from the original report can be referenced in Appendix C. The High Top Cultivation Facility project site is located along the west side of South College Avenue and south of West Trilby Road as seen on the vicinity map in Appendix A. The property currently supports an existing metal building, which covers an approximate footprint of 18,270 sf. Just south of the building lies an existing asphalt parking area and a vacant earth/gravel area to the west. The site is composed primarily of type C soils and generally, slopes from 0 to 2 percent towards the lot’s eastern property line. A specific breakdown of weighted impervious areas that compares existing areas to proposed improvement areas can be found on the “Impervious Area Exhibit” within Appendix C. All flows generated in the existing condition can be found in the “Stormwater & Erosion Control Report for Kindcare” as previously mentioned. The proposed improvements will include the addition of 5,578 sf of concrete/asphalt area and the removal of 842 sf of existing asphalt for landscape planters. The proposed improvements are composed of the addition of 2,390 sf of asphalt surface for emergency access, replacing a portion of existing parking with landscape planters and sidewalk, the addition of 869 sf concrete mechanical pad, 2,319sf of sidewalk, and removal of the on-site gravel area to be replaced with seed and landscaping. To: Mr. Wes Lamarque, PE From: Tim Halopoff, PE, CFM Date: June 9, 2020 Subject: High Top Cultivation Facility – Weighted Impervious Drainage Analysis 7200 South Alton Way, Suite C400 Centennial, CO80112 303-740-9393 x Fax 303-921-7320 130 East Kiowa Street, Suite 400 Colorado Springs, CO80903 719-593-2593 x Fax 303-921-7320 2900 South College Avenue, Suite 3D Fort Collins, CO80525 970-491-9888 x Fax 303-921-7320 All of the site improvements as well as their weighted impervious values can be seen on the “Impervious Area Exhibit” within Appendix C. The summation of newly added impervious areas as well as the same areas in their existing condition can be found within the “Impervious Area Exhibit” within Appendix C. The table on the “Impervious Area Exhibit” shows that the site has a net reduction in the impervious area. The original approved site drainage report by Forbes Engineering sized the existing detention pond to include runoff generated on the High Top Cultivation Facility Site. The proposed site condition has a net impervious area that is less than the condition within the approved report demonstrating that the existing drainage facilities are adequate. The Fort Collins Stormwater Criteria Manual (FCSCM) specifies that, “75% of all newly added or modified impervious area must be treated by LID techniques”. The impervious area being treated can be seen on the “Impervious Area Exhibit” within Appendix C. Approximately 10,540 sf of impervious area is being treated by the proposed 14’ grass buffer which is more than the 100% of the new impervious area. All runoff generated on the site is being safely conveyed via sheet flow and a natural swale in conformance with the approved “Stormwater & Erosion Control Report for Kindcare”. Mile High Flood District spreadsheet UD-BMP_v3.07 was utilized to design the grass buffer and can be found in Appendix D. Rational method calculations based on Fort Collins Storm Drainage Criteria were utilized to calculated the 2-year flow rate for the design of the grass buffer and those calculations and basing map can be found in Appendix D. Finally, this Drainage Memo demonstrates two key points with respect to this site’s storm drainage and the adjacent properties. First, the proposed site condition has a net reduction in impervious area from the sites existing condition. Second, the grass buffer is treating more than 100 percent of the newly added and/or modified impervious area on the site. We have also shown that the site improvements prevent any adverse drainage impacts to adjacent sites from new impervious area, in the form of minimal grading, addition of more landscape area, drainage swales and the proposed grass buffer. Thus, the High Top Cultivation Facility improvements have been strategically designed to mitigate the potential for adverse drainage affects to adjacent properties and maintaining the established drainage pattern within the approved “Stormwater & Erosion Control Report for Kindcare”. Sincerely, Tim Halopoff, PE, CFM 970.817.1010 HIGH TOP CULTIVATION FACILITY BASIC DEVELOPMENT REVIEW VICINITY MAP NTS LOT 2 OF DINGS SUBDIVISION, 6629 S. COLLEGE AVE., FORT COLLINS, COLORADO 80525 LEGEND A Westrian Company HIGH TOP CULTIVATION FACILITY BASIC DEVELOPMENT REVIEW IMPERVIOUS AREA EXHIBIT SITE X:\3970000.all\3978300\Drawings\Presentations\3978300 Impervious Exhibit.dwg, BDR Site Plan, 6/12/2020 9:32:15 AM, FC