HomeMy WebLinkAboutRIDGEWOOD HILLS FIFTH FILING - FDP200024 - SUBMITTAL DOCUMENTS - ROUND 3 - VARIANCE REQUESTApril 13, 2021Marc VirataEngineering Development ReviewCity of Fort Collins281 N College AvenueFort Collins, CO 80524RE:Ridgewood Hills Filing No. 5 - Variance Request to Stormwater
Development Construction StandardsDear Marc,Please find this letter as our formal request for an administrative variance to the requirements set forth within the Fort Collins Utilities
Stormwater Development Construction Standards as they relate to depth of cover over proposed storm sewer pipes. The project, Ridgewood Hills Filing No. 5, is located at the southwest
corner of the intersections of Triangle Drive and US 287 in the City of Fort Collins. The project proposes a network of proposed storm sewer to convey runoff generated on-site to a
series of water quality and detention ponds before that runoff is discharged into the adjacent public storm sewer system. Various other utilities, including public sanitary sewer, public
water, and a private irrigation ditch, are proposed with the development as well. Storm Sewer Line B is proposed to convey captured runoff that is detained in Detention Pond B to the
site’s outfall into the public storm sewer system. This portion of storm sewer runs from Pond B to the north within Strasburg Drive and discharges into the Robert Benson Lateral at
the northeast corner of the site. Portions of the proposed storm sewer pipe have less than 3’ of cover from top of pipe to finished grade within Strasburg Drive.We would like to request
a variance to Stormwater Development Construction Standards to allow for the reduced depth of cover on the proposed storm sewer pipe. The vertical profile of Strasburg Drive is constrained
between its connection point in Triangle Drive and its southern terminus at the property line. Storm Sewer Line B is proposed to meet minimum pipe size (15”) and slope requirements
(0.5%) for stormwater utilities within the Right-of-Way. These minimum pipe constraints allow for adequate conveyance while still providing at least 30” of cover on top of the pipe,
with the exception of approximately 15’ where the pipe first passes under Strasburg Drive. In this area cover will not be less than 24”. Further, the proposed profile of the Storm
Sewer Line B is restricted due to conflicts with other portions of the storm sewer system that it crosses. Minimum clearances must be maintained across these utility crossings that
prevent a minimum of 3’ of cover. Particularly, the crossing of Storm Sewer Line B with Storm Sewer Line A-11, prevents Line B from achieving 3’ of cover. A hydraulic analysis of Storm
Sewer Line A-11, which only meets minimum profile slope requirements, determined that adjustments of this line to lower the crossing and achieve more cover for Line B is not feasible.
According to Fill Heigh Tables developed by the American Concrete Pipe Association, Class III 15” Reinforced Concrete Pipe is suitable for a 2’ minimum depth of cover. In lieu of the
3’ depth of cover, it is proposed to increase the pipe class material to Class IV Reinforced Concrete Pipe. This increase in pipe class will provide an extra measure of safety against
failure of the structural pipe. Therefore, it is not anticipated that the proposed limitations on depth of cover would pose any additional detriment to safety or the design life or maintenance
costs of the proposed storm sewer. Thank you for your consideration of this variance request. Should you have any questions, please do not hesitate to contact me 970.800.3300.Sincerely,GALLOWAYDonald
B. Cecil, P.E.Civil Project Engineer970.800.3300DonaldCecil@GallowayUS.comcc: