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HomeMy WebLinkAboutMIDTOWN ARTS CENTER (3750 S. MASON ST.), CHANGE OF USE - PDP - 1-10/A - SUBMITTAL DOCUMENTS - ROUND 1 - CORRESPONDENCE-CONCEPTUAL REVIEW (4)%'. r . 7 Page 2 of 2 Installation of the new inlet in the landscaped area just south of the existing parking lot would allow space to install the gravel surround and to set everything to the appropriate grades, without impacting the ultimate parking area. The outlet piping from the new inlet could be connected to the existing pipe leading to the public storm drain in Mason St., thus minimizing the amount of disruption to the existing surface improvements in the right-of-way and on -site. b. The existing impervious area is grandfathered, and there aren't any new impervious areas being proposed. If that changes and there is an increase in impervious area greater than 1,000 square feet, a drainage and erosion control report and construction plans are required and must be prepared by a Professional Engineer registered in Colorado. A grading plan is required when there is between 1,000 and 350 square feet of new impervious area, but no drainage study is required. Response: No new impervious areas are proposed as part of the site redevelopment. In fact, several new parking lot islands will likely be added, which will reduce the overall imperviousness of the site. c. If there is an increase in impervious area greater than 1,000 square feet, onsite detention is required with a 2 year historic release rate for water quantity. Parking lot detention for water quantity is allowed as long as it is not deeper than one foot. If there is less than 1,000 but more than 350 square feet of new impervious area, a site grading plan is required along with the impervious area documentation. Response: See response to item "b" above. d. The design of this site must conform to the drainage basin design of the Mail Creek Master Plan Update as well the City's Design Criteria and Construction standards. Response: The proposed installation of a new inlet to enhance the water quality of runoff from the site is consistent with the design criteria set out in the Mail Creek Master Plan Update, as well as the City's Design Criteria and Construction Standards. e. The City-wide development fee is $4,420/acre ($0.1015/sq.ft.) for neu impervious area over 350 sq.-ft. No fee is charged for existing impervious area. This fee is to be paid at the time each building permit is issued. Since there is no proposed increase in the impervious area of the site, it is expected that storm drainage development fees will be payable in connection with the redevelopment of this site. CG OWEN CONSULTING GROUP, INC. MEMORANDUM To: Glen Schlueter, City of Fort Collins Stormwater Utility CC: Steve Olt, Planning Department, Randy Shortridge, RB+B, Deanne Frederickson, The Frederickson Group, LLC From: Larry C. Owen, P.E. Date: January 6, 2010 Proj. No.: 10-345 Project Name: 3750 S. Mason Subject: Responses to Concept Review Comments: Midtown Arts Center Follows are comments with responses regarding Storm Water Utility for the Midtown Arts Center. a. Since the site needs to be updated to conform to the Land Use Code (LUC) for the change -of -use, the site drainage system needs to be modified to treat the runoff. Water quality treatment methods are described in the Urban Storm Drainage Criteria Manual, Volume 3 — Best Management Practices (BMPs). Please have the design engineer contact Stormwater staff to discuss their proposed methods of water quality treatment. As discussed, the water quality treatment methods could be shown on the site plan so that a separate construction plan set is not needed. Response: It is proposed that the water quality of storm runoff be enhanced through the installation of a new inlet in the southwest region of the site, replacing the existing inlet. The design of the new inlet will allow for primary filtration of virtually all runoff from minor storm events and the initial runoff, or 'first flush", from more significant storm events, without diminishing the overall capacity of the inlet to convey storm flows away from the site and into the municipal storm drain system. It is proposed that a new, replacement inlet be installed near the location of the existing inlet, in the landscaped area immediately south of the curb along the south perimeter of the parking lot. The walls of the new inlet would be perforated with a series of relatively small diameter (approx. 1" dia.) holes along a line 12" below the top of the inlet structure. The backfill around the upper 12" of the inlet structure would be a layer of gravel filter material, approximately 2" wide, providing a 2' wide x 12" deep annular ring, through which the initial runoff flow would pass prior to entering the inlet via the perforations. The perforations would serve to inhibit the flow, thereby reducing the velocity and allowing the sediments to settle out in the gravel. For the most part, the runoff would accumulate in the gravel and not back up into the parking lot. In the case of a more significant storm event, with heavier runoff, the accumulating runoff would become deeper to the point where the water surface rose above the top of the inlet, at which point the flow would enter the inlet through a standard grate in the lid of the inlet, thus dissipating the accumulated flow and minimizing the potential for encroachment of the ponded water into the parking area. The initial runoff, which typically carries the majority of the pollutants, would still be routed, for the most part, through the filtering gravel surround, thus enhancing the water quality of the runoff before it enters the downstream storm drain system. A detail showing the new inlet and gravel surround in plan view and in section is included on the submittal drawings. 3715 Shallow Pond Dr., Fort Collins, CO 80528 Phone (970) 226-0264 Fax (970) 226-3760