HomeMy WebLinkAboutMemorandums - Drainage - 10/15/2025
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748 Whalers Way
Suite 200
Fort Collins, CO 80525
970.226.0557 main
970.226.0204 fax
ideas@tstinc.com
www.tstinc.com
October 15, 2025
City of Fort Collins
700 Wood Street
Fort Collins, CO 80521
Re: Drainage Memo – 3206 Fairplay Court, Fort Collins, CO 80525
Project No. 9000.0128.00
TST, Inc. Consulting Engineers (TST) are pleased to submit this Drainage Memo for
3206 Fairplay Court property. The purpose of this letter is to evaluate the addition of
an accessory dwelling unit (ADU) for the existing property.
The project is located in the southwest quarter of Section 26, Township 7 North,
Range 69 West of the 6th Principal Meridian, more specifically Lot 20, Block 19 of
Woodwest Seventh Filing. within City of Fort Collins, Larimer County, Colorado. The
project is in the Foothill Basin according to Fort Collins Master Drainage Plan. The
existing lot is approximately 0.22 acres that consists of an existing house, driveway
and open space. A proposed ADU with an approximate 578.5 s.f. building footprint
is being proposed within the open space on the north side of the lot.
TST conducted a site visit on 07-24-2025 to evaluate the existing drainage patterns.
The runoff from the site, where the ADU will be located, flows to the north and east
and is collected in the existing swales that run between the adjacent lots and
ultimately into Blue Mesa Avenue. The runoff is then carried along Blue Mesa
Avenue to the existing storm infrastructure within the street. See Appendix A for a
drainage map showing existing drainage patterns, and original subdivision grading
plans.
To determine if the increase in impervious would have any negative impacts
downstream, we compared the allowed lot impervious area to the proposed. The
site is Zoned Low Density Residential and according to City Criteria an allowed
percent impervious of 50% is allowed/assumed for this type of zoning. An
impervious area calculation was conducted to ensure our compliance with this
requirement. The lot was analyzed with a portion of Fairplay Court adjacent to the
property, which would be standard practice when designing the major subdivision.
The calculation shows that the new imperviousness is 42.13%, which is less than
what the development would have designed for and therefore there would be no
negative impacts from the increase in impervious from this ADU. This increase in
runoff from this ADU is negligible. See attached Appendix B for calculations and
map.
According to the City criteria, detention and water quality requirements can be
waived if the ADU adds less than 1,000 s.f of new impervious area. Therefore, no
detention or water quality will be necessary for the proposed ADU.
3206 Fairplay Court, Fort Collins Drainage Memo
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We greatly appreciate your time and consideration reviewing this memo. If you should have
any questions or require additional information, please contact us.
Respectfully,
TST, INC. CONSULTING ENGINEERS
Derek A. Patterson, P.E.
DAP
I hereby attest that this letter for the drainage design of 3206 Fairplay Court was prepared by
me or under my direct supervision, in accordance with the provisions of the Fort Collins
Stormwater Criteria Manual. I understand that the City of Fort Collins does not and will not
assume liability for drainage facilities designed by others.
___________________________________
Derek Andrew Patterson
Registered Professional Engineer
State of Colorado No. 48898
3206 Fairplay Court, Fort Collins Drainage Memo
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Index of Appendices
The following appendices are included with this memo:
APPENDIX A Existing Grading Plan
APPENDIX B Proposed Drainage Plan and Impervious Area Calculation
Appendix A:
Existing Grading Plan
Appendix B:
Proposed Drainage Plan &
Impervious Area Calculations