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Home My WebLink AboutPLANETSCALE LIVING - PDP200015 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTJuly 30, 2020 City of Fort Collins Public Works and Development (Stormwater Department) 201 Laporte Ave, Fort Collins, CO 80521 RE: Drainage Memorandum – Low Impact Development Project: 2020-006 (TT) Serdar Badem, 906 E Stuart St, Fort Collins, CO 80525 To Whom it may Concern: This Drainage Memorandum is for analysis of on-site drainage for Low Impact Development for Project 2020-006 (TT) Serdar Badem, 906 E Stuart St, Fort Collins, CO 80525. The project is in Fort Collins, Colorado and will entail converting (remodeling) an existing childcare center into an affordable housing 4- Plex. The site (parcel #9724106924) is located 850 feet west of S. Lemay Avenue and 1,200 feet south of E. Prospect Road. Access is taken from E. Stuart Street directly to the south. The property is within the Low Density Mixed-Use Neighborhood District (L-M-N) zone district and is subject to Administrative (Type 1) Review. The existing drainage on site has no drainage structures and minimal surface flow (on-site or off- site). Most of the existing site (Figure 1) consists of Concrete access and driveway; most of the southern portion of the site, the concrete footprint of building, concrete patio and the remaining consists of grass/dirt/gravel mixture with trees. The existing site is flat with minimal to zero slopes throughout the site. To the east there is an HOA Easement that parallels the site and Springmeadows Court. The HOA will not allow any construction and/or stormwater work within this area. Because of this limitation, the project will be implementing Low Impact Development drainage solutions using City of Fort Collins Stormwater Criteria Manual – Chapter 7, Section 6.0 and Appendix C – LID Implementation Manual. Because of the inability to go of site with any of the on-site run-offs; the project will design LID solutions that will improve the entire site. Table 1 Below shows that over seventy percent of the existing site is impervious (Areas 2 & 4) and the remaining pervious areas (1 & 3) are limited because of no positive slope and random native material mixed with dirt, grass, gravel and rocks. The proposed solution will be to implement Pavers and Landscape Beds/Areas throughout most of the site, which will improve and decrease the overall impervious area 30%. (Figure 2) Table 1 Figures 1 (existing) and Figure 2 (Proposed) [correspond with Table 1] Figure 1 - Existing Figure 2 - Proposed The proposed project includes 2,812 sf of new impervious area. The project will treat 79.27% of this area (6,782 sf) using permeable pavers and run-on to the pavers. The proposed project provides 66.4% of new impervious area as permeable pavers (4,234 sf) located in parking areas within the site. The project will treat 79.27% of the total new impervious area using a combination of LID treatments (both permeable pavers and landscape beds/areas). A summary LID Compliance Table is provided below. The project will be requesting and implementing a Dry Well system since no offsite runoff or detention can be met. The client is requesting that the City of Fort Collins will allow for a Dry Well solution, since there are no solutions for offsite run-off. Summary In summary, the proposed project incorporates permeable pavers as the primary component of stormwater LID treatment. It also includes a single rain garden. The LID treatment standard is very closely met with 66.4% of the site as permeable pavers and 79.27% of runoff treated. This project and memo are in compliance with the City of Fort Collins Stormwater Criteria Manual (December 2018) and associated references to the latest version of the Urban Drainage and Flood Control District Manual. If you have any questions regarding this submittal, please do not hesitate to contact me by email at apowell@pec-inc.co or by phone at (720) 409-2454. Thank you. Sincerely, Adam A. Powell, PE PEC Inc (From Fort Collins Stormwater Criteria Manual - 6.0 Low Impact Development) This Section of this Chapter presents information that is specific to the City of Fort Collins and may be a significant deviation from the information presented in the UDFCD Manuals. Utilizing UDFCD methodologies for Low Impact Development (LID) designs may not be accepted by FCU. In February 2013, Fort Collins City Council adopted Ordinance No. 152, 2012, to incorporate provisions implementing LID principles; with the goal to declare that the purpose of the City Stormwater Utility is to provide an integrated, sustainable stormwater management program that reflects the community’s values of protecting and restoring the City’s watersheds. This was subsequently modified and updated in January 2016 with Ordinance No. 007, 2016 to allow for some added flexibility in the implementation of the LID policy. Reference: Both the initial LID ordinance, Ordinance No. 152, 2012, and the subsequent ordinance, Ordinance No. 007, 2016, can be found on the City of Fort Collins website. LID is simply defined as an integrated, sustainable stormwater management program that requires a distributed, closer to the source stormwater runoff control that simulates natural processes and relies mainly on filtration and infiltration to locally treat and manage stormwater runoff. Integration of LID systems into the drainage design is required for all development projects in order to comply with the City’s policies on LID, the requirements of this Manual, the City Code and the Land Use Code. LID systems provide a higher degree of stormwater quality treatment than that provided with standard water quality design. The implementation of LID systems requires one of the following two options: 1) 50% of the newly added or modified impervious area must be treated by LID techniques and 25% of new paved (vehicle use) areas must be pervious. 2) 75% of all newly added or modified impervious area must be treated by LID techniques. Impervious surfaces are defined as hardscape surfaces that do not allow stormwater to infiltrate into the ground. Impervious surfaces include asphalt and concrete surfaces, concrete curbs, gutters, sidewalks, patios and rooftops. (Impervious surface areas must be assumed for single family residential lots when overall impervious areas are being determined for residential developments. The assumed areas must then be included in LID calculations.) “Added” impervious area stated in the two options above is further defined as existing vegetation (or pervious) areas becoming hardscape (or impervious) areas. “Modified” impervious area stated in the two options above is further defined as existing impervious areas on an existing site being removed and replaced with other impervious surfaces through a redevelopment process (i.e. existing asphalt surface becoming a rooftop surface). Mill and overlay of asphalt areas is not considered a “modified” impervious area. “Paved” areas, as stated in option 1 above are generally considered to be private vehicle use areas only. Reference: Refer to the City of Fort Collins LID Implementation Manual in Appendix C for detailed information and requirements on LID systems. (From Fort Collins Stormwater Criteria Manual - 6.2 Permeable Pavement) The term “permeable pavement” is a general term to describe any one of several pavement systems that allow infiltration of water into the layers below the pavement through openings within the pavement surface. Use of permeable pavements is an accepted Low Impact Development (LID) practice in Fort Collins and is often used in combination with other BMPs to provide full treatment and slow release of the WQCV. In addition, there are some installations in Fort Collins that have also been designed with an outlet control and increased depth of aggregate material in order to provide quantity detention in excess of the water quality (80th percentile) storm event. Design considerations for permeable pavement systems are presented in the LID Implementation Manual, included in Appendix C. LID Summary for Affordable Housing Project Location: 1651 Skyway Drive, Fort Collins, Colorado Design by: AAP Date: 7/29/2020 IMPERVIOUS PERCENTAGE 41.46% TOTAL AREA 6782.30 LANDSCAPE BEDS/AREAS B 2548.30 PAVERS A 4234.00 TOTAL AREA 2812.00 PERVIOUS AREA SUMMARY AREA DESCRIPTION AREA # AREA-SF FOOTPRINT C 2028.00 PROPOSED AFFORDABLE HOUSING IMPERVIOUS AREA SUMMARY AREA DESCRIPTION AREA # AREA-SF CONCRETE D 784.00 IMPERVIOUS PERCENTAGE 70.86% TOTAL AREA 5455.10 NATIVE GRASS/GRAVEL (2%) 1 4858.70 FRONT ISLAND (DIRT/SW) 3 596.40 TOTAL AREA 7698.28 PERVIOUS AREA SUMMARY AREA DESCRIPTION AREA # AREA-SF EXISTING FOOTPRINT IMPERVIOUS AREA SUMMARY EXISTING SITE LOT 4 4980.60 AREA DESCRIPTION AREA # AREA-SF CONCRETE (EXSITING) 2 2717.68 LID Summary for Affordable Housing Project Location: 1651 Skyway Drive, Fort Collins, Colorado Design by: AAP Date: 7/29/2020 50% of the newly added or modified impervious area must be treated by LID techniques and 25% of new paved area must be pervious Area (SF) Total New Impervious 2812.00 Required area to be treated 1406.00 Actual area being treated by pavers 4234.00 Actual area being treated by Landscape Beds 2548.30 Difference -5376.30 Percent being treated 79.27% or treat 75% of all newly added or modified impervious area must be treated by LID techniques Total new Impervious 2812.00 Required area to be treated 2109.00 Actual area being treated 6782.30 Difference -4673.30 Precent being treated 69.00%