HomeMy WebLinkAboutCANYON PLACE (331 S. MELDRUM) - FDP - FDP120024 - REPORTS - DRAINAGE REPORT (3)Page 1 of 2
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Drainage Letter Report
Date: December 26, 2012
Project: Canyon Place Project No. 838-002
Fort Collins, Colorado
Attn: Mr. Wes Lamarque
City of Fort Collins Stormwater Utility
700 Wood Street
Fort Collins, Colorado 80521
Dear Wes:
This letter serves to address the stormwater impacts of the above referenced project. The site currently
consists of rooftop, concrete walks and patio, asphalt parking areas, some turf, trees, and other vegetation.
The majority of the site presently drains to an area inlet located in the center of the main drive aisle. This
area inlet releases into the City’s storm sewer system within West Magnolia Street. Historic drainage
patterns will generally be preserved. The total impervious area within the limits of construction of the
existing site is approximately 23,563 square-feet.
The proposed redevelopment will completely remove all existing above-grade structures and adjacent
hardscape. The new project will consist of a larger building, as well as a courtyard area and modified
parking area. Concrete pedestrian connections will be maintained along South Meldrum Street, West
Magnolia Street and Canyon Avenue. The total impervious area within the limits of construction of the
proposed redevelopment is roughly 22,572 square-feet.
The two existing curb cuts and concrete aprons to West Magnolia will be removed. Two proposed curb
cuts are designed within Canyon Avenue to meet current code requirements and minimize the amount of
area draining across the pedestrian path. The area within the court yard has been designed with a ridge
routing stormwater north and south. The stormwater routed to the south is intercepted by a trench drain
which outfalls into the City’s stormwater system. The stormwater routed to the north is intercepted by a
trench drain which out falls into Canyon Avenue. The area of pavement sheet flowing across the Canyon
sidewalk will be approximately 662 square-feet, which is less than the maximum 750 square-feet allowed
by LCUASS. The remaining area within the northern parking lot will be concentrated within a proposed
curb chase and routed into Canyon Avenue.
The site is located in the Old Town master drainage basin, and therefore, on-site detention is not required
so long as the increase in impervious area is less than 5,000 square-feet. The total impervious area
proposed with this development will be decreased by approximately 991 square-feet (23,563 – 22,572).
A detailed breakdown of the existing and proposed impervious areas is attached to this letter.
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Although stormwater quantity detention is not required, stormwater quality mitigation will be addressed by
both temporary and permanent Best Management Practices (BMPs). During construction, the Contractor
will follow the appropriate and applicable City of Fort Collins standards for erosion and sediment control.
A comprehensive Stormwater Management Plan has been prepared for this project, and a copy has been
supplied to the Environmental Regulatory Specialist with Fort Collins Utilities. Post construction water
quality and erosion control will be achieved by a fully established and stabilized site. All areas disturbed
during construction will receive permanent hardscape, landscape, or building structure.
This portion of the Old Town basin does receive regional water quality treatment; therefore, on-site water
quality mitigation is not required. However, much of the existing landscaped buffers on the north, south
and east sides of the building will be preserved as permeable areas to promote infiltration and treat surface
runoff prior to entering the adjacent gutters and inlets.
The project site is also located in the Old Town Basin floodplain. This is a City of Fort Collins regulatory
floodplain. Since a small portion of the development falls within a City of Fort Collins regulatory floodplain,
a Floodplain Use Permit (FUP) shall be obtained prior to commencing with any construction within the
floodplain. The proposed improvements within the floodplain consist of site work only, no structures are
proposed within the 100-year floodplain. As such, this project seeks to obtain a FUP with the initial
Development Construction Permit (DCP), which would then cover all work to take place within the
floodplain.
It is important to note that the floodway currently follows the back of sidewalk along the project’s West
Magnolia Street frontage. This means that some of the driveway and streetscape modifications will occur
within the floodway. A No-Rise Certification is required for all work within the floodway. Further details
and supporting documentation for the No-Rise Certification shall accompany the FUP application.
The proposed grading concept closely matches existing elevations and drainage patterns, and only varies in
an effort to improve the existing condition. On-site detention is proven to be unnecessary. On-site
stormwater quality is not required as runoff will be treated at the Udall Natural Area water treatment
facility. The project is designed in accordance to the governing floodplain regulations contained within
Chapter 10 of the City Code, and the Floodplain Use Permit and necessary certifications shall ensure that
construction remains compliant with Chapter 10. Therefore, it is my professional opinion that the new
building and site improvements proposed with Canyon Place satisfy all applicable stormwater criteria.
Please do not hesitate to contact me if you have questions or require additional information.
Sincerely,
Nicholas W. Haws, PE
Project Manager
Cody N. Snowdon
Project Engineer
Canyon Place
CHARACTER OF SURFACE:
Runoff
Coefficient
Percentage
Impervious Project: Canyon Place
Streets, Parking Lots, Roofs, Alleys, and Drives: Calculations By: C. Snowdon
Asphalt ……....……………...……….....…...……………….………………………………….0.. 95 100% Date:
Concrete …….......……………….….……….………………..….………………………………… 0.95 90%
Gravel ……….…………………….….…………………………..……………………………….0.. 50 40%
Roofs …….…….………………..……………….…………………………………………….. 0.95 90%
Pavers…………………………...………………..…………………………………………….. 0.40 22%
Lawns and Landscaping
Sandy Soil ……..……………..……………….…………………………………………….. 0.15 0%
Clayey Soil ….….………….…….…………..………………………………………………. 0.25 0% 2-year Cf
= 1.00 100-year Cf = 1.25
Basin ID
Basin Area
(s.f.)
Basin Area
(ac)
Area of
Asphalt
(ac)
Area of
Concrete
(ac)
Area of
Roofs
(ac)
Area of
Gravel
(ac)
Area of
Lawns and
Landscaping
(ac)
2-year
Composite
Runoff
Coefficient
10-year
Composite
Runoff
Coefficient
100-year
Composite
Runoff
Coefficient
Impervious
Area
(sq ft)
Composite
% Imperv.
Existing 30,781 0.71 0.34 0.12 0.07 0.00 0.17 0.76 0.76 0.95 23,563 74%
Proposed 30,781 0.71 0.07 0.18 0.26 0.00 0.19 0.74 0.74 0.92 22,572 67%
COMPOSITE % IMPERVIOUSNESS AND RUNOFF COEFFICIENT CALCULATIONS
December 26, 2012
**Soil Classification of site is Sandy Loam**
10-year Cf = 1.00
Runoff Coefficients are taken from the City of Fort Collins Storm Drainage Design Criteria and Construction Standards, Table 3-3. % Impervious taken from UDFCD USDCM, Volume I.
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