HomeMy WebLinkAboutDrainage Reports - 04/30/2013 (2)City of Ft Colfns Ap rove, Flans
Approved Bq.
Date 1 -13
NORTHERN
ENGINEERING
Drainage Letter Report
Date: April 22, 2013
ADDRESS: PHONE:970.221.4158 WEBSITE:
200 S. College Ave. Suite 10 www.northernerigineedng.com
Fort Collins, CO 80524 FAX: 970.221.4159
Project: Lot 8, Thunderbird Estates Filing 61 Subdivision Project No. 874-001
Major Amendment .
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 Major Amendment. The
existing building is situated towards the western portion of the site. The site currently consists of rooftop,
concrete walks, asphalt parking areas, some turf, trees, and other vegetation. It presently functions similar
to a 'B' lot, with historic drainage being split towards the frontage road to the west and towards the private
drive to the east. The total impervious area of the existing site is approximately 26,606 square -feet. The
existing percent impervious for the entire site is 86.1%.
The proposed redevelopment will leave the existing building footprint unchanged, while adding a vehicle
display area on the west side of the building constructed of concrete. Portions of existing paving along the
north side of the building will be modified, and a bio-retention area will be added on the east side and
south side of the existing parking area. While very minor grading changes will occur, existing drainage
patterns will be maintained. The total imperious area of the proposed redevelopment is roughly 26,585
square -feet. This is a slight decrease from the existing condition. The proposed percent imperious for the
entire site also decreases slightly to 85.2%.
The site is located in the Spring Creek Master Drainage Basin. On -site detention is not required so long as
there is no increase in imperious area. Since the redevelopment proposes a slight decrease in percent
imperious, detention storage is not required. A detailed breakdown of the existing and proposed
imperious areas is attached to this letter, as is an exhibit depicting the areas.
There are no regulatory floodplains associated with the project.
Page 1 of 2
D:\Proiects\874-001\Drainage\Reports\Drainage Report\874-001 Drainage-Letter.docx
NORTHERN
ENGINEERING
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.
Since the approximately 8,500 sf area of disturbance for the project is less than one acre, a
comprehensive Stormwater Management Plan will not be prepared for this project. 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.
There are no existing stormwater BMP's currently on -site. Since the percent of site imperviousness is
decreasing from the existing condition, formal water quality treatment is not required. While formal water
quality treatment is not required for this site, an effort has been made to employ low -impact development
strategies to treat the majority of stormwater runoff from the site. This treatment occurs through a variety
of methods. The first method is the use of a bio-retention area along the eastern property line that
intercepts runoff from the existing asphalt parking lot, which allows for the settling of particulates to occur
prior to drainage leaving the site. Another method that is being utilized is to discharge roof drains into
landscaped areas where available. The final method of treatment is the use of permeable pavers along the
western portion of the site, which allows minor rain events to infiltrate into the soil rather than leaving the
site.
To summarize, the proposed grading concept very closely matches existing elevations and drainage
patterns. On -site detention is proven to be unnecessary. Stormwater quality is not required, but has been
provided anyways. Therefore, it is my professional opinion that the remodeled building proposed with Lot
8 of the Thunderbird Estates Sixth Subdivision satisfies all applicable stormwater criteria.
Please do not hesitate to contact me if you have questions or require additional information.
Sincerely, O�p.D0 L/C
Vim° 64,Z 67 y'�O
YV
`S�nNAL
Andrew Reese Nicholas LEED
Project Engineer Project Manager qp
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Page 2 of 2
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