HomeMy WebLinkAboutGATEWAY AT PROSPECT AMENDED ODP - ODP160001 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORT1
Overall Drainage Report
Date: May 3, 2016
Project: Gateway at Prospect
Overall Development Plan
Fort Collins, Colorado
Attn: Mr. Shane Boyle
Fort Collins Utilities
700 Wood Street
Fort Collins, Colorado 80524
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 1st
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.
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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 2.33. 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 4905.0 to 4918.0 (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 15-08-0329R). 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.
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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
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