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Home My WebLink AboutFRCC HEALTH CARE CAREERS CENTER SITE PLAN ADVISORY REVIEW - SPA180002 - SUBMITTAL DOCUMENTS - ROUND 2 - DRAINAGE RELATED DOCUMENT (3)CRESTONE CONSULTANTS, LLC - 14145 West Warren Circle - Lakewood, CO - 80228 303 · 997 · 6113 - www · crestonellc · com - crestone@crestonellc.com April 23, 2018 Mr. Dennis DeRemer Director of Facilities – Larimer Campus Front Range Community College 4616 S. Shields Street Fort Collins, CO 80526 Re: Larimer Campus - Northwest Detention Pond Status Crestone Project No.: 13020 Dear Mr. DeRemer: Per your request on April 17, 2018, we’ve prepared this letter to identify the status of the existing detention pond located west of the Sunlight Peak (SP) building and north of the Observatory and Harmony Library Parking Lot at the northwest corner of the Larimer Campus. We’ve reviewed the following Master Drainage Studies for the Larimer Campus in the preparation of this letter: 1. Master Drainage Study – Front Range Community College Campus, prepared by Interwest Consulting Group, dated December 1, 2010 revised. (AKA 2010 Master Study) 2. Master Drainage Study – Front Range Community College Campus, prepared by Interwest Consulting Group, dated May 22, 2014. (AKA 2014 Master Study) A summary of the 2010 Master Study as it pertains to the subject detention pond has been provided as follows: 2010 Master Study Summary: The subject detention pond was constructed with the Harmony Library Parking Lot project in 2010/2011 to help mitigate any effects from the parking lot development on the existing storm sewer system. The 2010 Master Study identified the existing storm sewer system which the Harmony Library Park Lot is tributary to (i.e. Storm System 1) as being undersized. The subject detention pond is located within Basin A1 and is required to be installed to attenuate developed flows from the interim condition Basin A1 until improvements to the campus’ storm drainage conveyance and storage systems are constructed. Required storm drainage conveyance improvements include the construction of a new storm pipe (36”) located on the north side of the SP building which will convey the developed flows from Basin A1 east to the existing riprap channel adjacent to Harmony Road which outfalls to the campus’ North Detention Pond located at the northeast corner of the campus. Required storage improvements include increasing the North Detention Pond volume by raising the pond’s spillway elevation and providing additional pond excavation to account for the additional April 23, 2018 Dennis DeRemer; Front Range Community College – Northwest Detention Pond Status Page 2 imperviousness proposed throughout the campus and to provide additional water quality capture volume. Additionally, the 2010 Master Study includes modifying the North Detention Pond outlet structure to accommodate the additional water quality capture volume and detained volumes. Please refer to select excerpts and maps attached to this letter from the 2010 Master Study. In 2011, the storm drainage conveyance and storage systems identified in the 2010 Master Study were constructed. This includes the 36” storm pipe conveyance for Basin A1 as well as the North Detention Pond storage improvements and outlet structure modifications. A summary of the 2014 Master Study as it pertains to the subject detention pond has been provided as follows: 2014 Master Study Summary: The 2014 Master Study states, “In the current condition, Basin A1 drains to a temporary pond. This temporary pond drains north to the existing 36” RCP Storm System located to the northeast that conveys the flow east to the Northern Channel adjacent to Harmony Road at the northeast corner of the SP building and finally to the North Detention Pond. This temporary pond can be removed at any time, the existing storm pipes and swales are adequate to receive undetained water now and with future building. This entire basin will drain to the existing 36” RCP Storm System.” Please refer to select excerpts and maps attached to this letter from the 2014 Master Study. Based on the 2014 Master Study, all improvements which were required to be installed for the removal of the subject detention pond have been constructed and the subject detention pond may be removed at any time. Please Note - The 2014 Master Study identifies conveyance and storage capacity allowances provided by the existing storm drainage system for future identified planned improvement areas. The study also identifies triggers for future improvements to the storm drainage conveyance and storage systems, including improvements for North Detention Pond. We recommend that Front Range Community College provide a complete copy of the 2014 Master Study to all perspective designers for their review and use in preparing their respective designs of future improvements throughout the Larimer Campus. Should you have any questions, please feel free to call me at your convenience at (303) 997-6113. Sincerely, Crestone Consultants, LLC Joseph M. Erjavec, P.E. Principal Attachment(s) Attachment A – 2010 Master Study Select pages from Master Drainage Study ‐ Front Range Community College Campus, prepared by Interwest Consulting Group, dated December 1, 2010 revised Attachment B – 2014 Master Study Select pages from Master Drainage Study ‐ Front Range Community College Campus, prepared by Interwest Consulting Group, dated May 22, 2014 MASTER DRAINAGE STUDY Front Range Community College Campus Prepared for: Front Range Community College 4616 S Shields St Fort Collins, Colorado 80526 Prepared by: Interwest Consulting Group 1218 West Ash, Suite C Windsor, Colorado 80550 (970) 674-3300 May 22, 2014 Job Number 1124-086-00 6 the current peak runoff from the site to the pond is about 178.4 cfs. The existing pond, based on a current topographic survey, has 3.1 Ac-ft of storage (not including water quality capture volume). The 100-yr water surface elevation is at elevation 5063.8’ and spillway is at elevation 5063.9’. With today’s pond size and the historic 100-year release rate of 42 cfs, 0 cfs would spill over the spillway to the drainage channel adjacent to the Coventry Subdivision. Please refer to Appendix D for the North Detention Pond calculations. The South Detention Pond is located in the southeast portion of the campus adjacent the Clarendon Hills and Coventry Subdivisions. This pond treats water quality and detains flow from the newest parking lot. The pond releases to the Southern channel that directs flow to the North Detention Pond. The South Detention Pond releases 0.6 cfs and is a negligible impact to the overall release from the North Pond. 5. DRAINAGE FACILITY DESIGN 5.1 Specific Flow Routing A summary of the drainage patterns within each basin is provided in the following paragraphs. The Master Drainage Plan Current Condition map is located in the back pocket of this study. Basins A and A1 include the northwest portion of the site. These basins contain portions of HL, BP and RP buildings and all of SP and CP buildings. With current conditions, basins A and A1 consist of 8.0 acres. Basin A is 3.8 acres and flow from this basin enters Storm System 1 and is conveyed east to the North Detention Pond. Basin A1 is 4.2 acres and is the area west of the SP building, consists of a parking lot and has space for an additional impervious area in the future. Once an improvement adding hard surface is completed, it will then be considered completely built out. For the purposes of this report, basin A1 will be considered in its current condition. In the current condition, basin A1 drains to a temporary pond. This temporary pond drains north to the existing 36” RCP Storm System located to the northeast that conveys the flow east to the Northern Channel adjacent to Harmony Road at the northeast corner of the SP building and finally to the 7 North Detention Pond. This temporary pond can be removed at any time, the existing storm pipes and swales are adequate to receive undetained water now and with future building. This entire basin will drain to the existing 36” RCP Storm System. Basin B is 3.2 acres and includes the center portion of the site. This basin includes portions of buildings BP, RP, LP and MA. Flows from this basin enter existing Storm System 2 and are conveyed east to the North Detention Pond. Basin C is 14.3 acres and includes the student parking lots located in the center of the site. Flows in this area will be routed east in existing Storm System 3 to the North Detention Pond. Basin D is a total of 5.5 acres and consists of the south central portion of the site. It consists of parking lots and the existing Southern Channel. This basin drains north via curb cut and inlet in the parking lot to the existing grass lined Southern Channel located between the existing student parking lots. Culverts are sized to pass this flow and the additional offsite flow from the Westbury Pond and Shields under the two drive aisles that access the parking lot. Basin E is 4.2 acres and is located in the east portion of the site. The majority of this basin is a parking lot. Flow from this basin sheets to the northeast and into the existing North Detention Pond. If an addition of a building does occur in the future, it will occur in the existing parking lot and will include removal of pavement and will not affect the overall site imperviousness of this basin. Basin F is 6.7 acres and consists of the northeast portion of the site and includes the Northern Channel and the North Detention Pond. It also contains the MP and FB buildings. Basin G is 5.2 acres and is located in the southwest portion of the campus. This basin includes the southern parking lot and is north of the irrigation ditch and is considered the Southwest Parking Lot Expansion. The basin drains via a self-contained storm system to a detention and water quality pond located in the southeast portion of the campus adjacent the Clarendon Hills and Coventry Subdivisions. The pond detains this basin’s 100-year event to historic 2-year flows and is sized to also handle runoff from a future phase of 8 parking area. The total 100-year developed release rate for the Southwest Parking Lot Expansion is slightly over the 10-year historic runoff. Construction of a parking lot and South Detention Pond in this basin is currently underway (2014). Design of this area is included in JVA Consulting Engineers, “FRCC Larimer Campus—South Parking Lot (Phase I & II) Final Drainage Letter”, dated January 12, 2014 in Appendix E of this document. Because of the detention provided in Basin G, this acreage is not included in the North Detention Pond volume calculations. 5.2 Proposed Improvements The current conditions of the portions of the site draining to the existing North Detention Pond include 41.9 acres at 61% imperviousness. The existing pond, based on a current topographic survey, has 3.1 ac-ft of storage (not including water quality capture volume). The required water quality capture volume is 1.0 ac-ft and is achieved at elevation 5061.8’ based on the as-built shots of the water quality outlet structure. With today’s pond size and the historic 100-year release rate of 42 cfs, the 100-yr water surface elevation is at elevation 5063.8’ and spillway is at elevation 5063.9’. The entire site is detained and 0 cfs spills over the spillway to the drainage channel adjacent to the Coventry Subdivision. Currently, the pond has capacity for 70,000 square feet of additional impervious area allowing for Future Impervious Areas A and B to be completed without needing to upgrade the North Detention Pond or water quality outlet structure. These improvements equate to the 41.9 acres having 65% imperviousness. With these conditions, the required water quality capture volume will be 1.1 ac-ft at elevation 5061.8’. With today’s pond size and the historic 100-year release rate of 42 cfs, the 100-year water surface and spillway elevation will be at elevation 5063.9’. Please refer to Appendix D for pond calculations. Please refer to the Master Drainage Plan Current Conditions map located in the back pocket for two delineated areas where future impervious areas could be located. Area A is located in the northwest corner of the site within Basin A1. Area B is in Basin F and is located in the flat area adjacent to the Mount Antero Building. If changes to the 41.9 acres that drain to the North Detention Pond cause a percent imperviousness to be greater than 65%, the pond must be reanalyzed using EPA SWMM 5.0 software and a new water quality and detention volume must be calculated. Changes