Overall Drainage Report

Transcription

1 Overall Drainage Report Date: May 3, 2016 Project: Attn: Gateway at Prospect Overall Development Plan Fort Collins, Colorado Mr. Shane Boyle Fort Collins Utilities 700 Wood Street Fort Collins, Colorado Mr. Boyle: This letter report accompanies the submittal for the Gateway at Prospect Overall Development Plan (ODP). Specifically, this letter report serves to document the overall drainage impacts associated with the proposed Gateway at Prospect Development. The proposed development site is located northwest of the I-25/Prospect Road interchange in Fort Collins, Colorado. The site is situated along the existing I-25 west frontage road, just south of the existing Crossroads East Business Center and just east of the existing Boxelder Estates 1 st Replat. The overall Gateway at Prospect site is roughly 177 acres in size. Zoning across the site includes Low-Density Mixed Use (LMN), Employment (E), Commercial (C), Urban Estate (UE). An additional permitted use (APU) is proposed within Parcels K and L. These parcels are zoned as LMN and E, respectively. Multi-family land use is proposed on these parcels. The proposed development site is in the City of Fort Collins Cooper Slough/Boxelder Creek Basin. Detention requirements for this basin are to detain the difference between the 100-year developed inflow rate and the historic 2-year release rate. As this location is optimal for the use of full spectrum detention, the City has suggested the option of full spectrum detention with additional SWMM modeling to verify no substantial increases in flow to Boxelder Creek. No modeling has been done at the present. Modeling will be conducted at Project Development Plan submittal to verify detention volumes and viability of full spectrum detention. The current submittal does show proposed full spectrum on-site detention. 1

2 Full Spectrum detention utilizes a staged historic release rate based on the storm recurrence. Total developed stormwater release from the site will adhere to the release rates indicated in the Urban Drainage and Flood Control District Volume 2, Table SO-1 (December 2011). Approximate detention volumes are shown on the Master Drainage Plan. These volumes were calculated utilizing the Percent Imperviousness Relationship to Land Use (Table RO-14, Fort Collins Amendments to the Urban Drainage and Flood Control District Criteria Manual) and the Urban Drainage Detention Basin Volume Estimating Workbook, Version These detention volumes will be adjusted as the site further develops and proposed percent impervious are further refined with Project Development Plan submittals. The proposed Gateway at Prospect site will drain into on-site detention ponds which will release into the Boxelder Creek. All water quality treatment requirements and LID requirements will be satisfied with the proposed development. Means of providing for these requirements will be worked through at the with each respective Project Development Plan submittal. A significant portion of the subject property is located within the existing Boxelder Creek flood fringe and floodway. FEMA FIRM Panel Number 0984H for Larimer County, Dated May 2, 2012 is referenced in this study. Base (100-year) flood elevations in the vicinity of the proposed project range from to (elevations referenced to the City of Fort Collins NAVD 88). Please see the attached FIRMette with the approximate site boundary superimposed. The Boxelder Creek floodway bisects the site from north to south. A CLOMR has been submitted to FEMA and was approved for the Boxelder 6 CLOMR prepared for Boxelder Basin Regional Stormwater Authority (Case Number R). Improvements include an overflow channels, box culverts, dams, and bridge replacement. These improvements are proposed to remove much of this property from the flood fringe and floodway. Limits for both the existing FEMA Floodway/Floodplain and the proposed Floodplain/Floodway are shown on the Master Drainage Plan. These master plans were prepared utilizing the proposed floodplain/floodway. The majority of on-site grading will tie-in with existing grades outside of the proposed floodway boundary. Improvements including the proposed Boxelder Creek roadway crossing (west of the proposed roundabout) and the widening of Prospect Road may trigger a CLOMR and No-rise certification. It is acknowledged that the site design must satisfy the standards of Chapter 10 of City Code; Final design documents will be completed accordingly. 2

3 In summary, this Overall Drainage Report letter adequately addresses any potential stormwater changes associated with the proposed Overall Development Plan Amendment. In general, there are no significant changes proposed at a major drainage level. The ODP complies with the governing City of Fort Collins Master Drainage Plans, and the previously approved drainage plans specific for the subject property. Please feel free to contact me with any questions you may have. Sincerely NORTHERN ENGINEERING Stephanie Thomas, PE Project Engineer Enc 3

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