HomeMy WebLinkAboutGrading Plans - 07/16/2021���
ENGINEERiNG
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June 14, 2021
Mr. Wes Lamarque
City of Fort Collins Stormwater Department
700 Wood Street
Fort Collins, CO 80521
(970) 416-2418
Re: Drainage Memorandum for 119 Shields Street
Dear Mr. Lamarque,
Grading/Stormwater Plan Accepted
(Building Permit Released)
Accepted by: Matt Simpson
Date: 07/16/2021
119 N Shields Street
Grandview
Lot 9 Block A
B2102847 - addition
B2101300 - garage
The project site is located 1 l9 Shields Street, Fort Collins, Colorado, and shown as Lot 9, Block "A"
Grandview Addition to the City of Fort Collins, consisting of approximately 032 +/- acres. The proposed
improvements for the 119 Shields Street project will occur over approximately 0.11 acres of the 0.32 acre site
and will entail a new addition to the e�isting residence, a detached garage, minor re-grading of the existing
gravel driveway area, a new concrete trickle pan added along tlle south fence to convey rtuloff east and out to
Shields Street, as historical(y occurs, and a new gravel driveway extension west and to the proposed detached
garage. The proposed improvements will add approximately: 2,017 square feet of new roof area (for home
addition and detaclled garage); 1,716 square feet of new gt•avel area (1,340 sf for driveway area, 376 sf for
patio area); and 372 square feet of concrete tricl:le pan area. The proposed improvements will ceplace e�isting
grass, gt•avel, and dirt areas.
The existing drainage pattern foc the majority of the lot is from the northwest to southeast, with runoff flowing
east and out to Shields St�•eet. A portion of the lot, along the northern boundary of the property, flows east
along the north propei-ty line and out to Shields Street. The eYisting lot is relatively flat with grades ranging
from approxiinately 0.2% to 3%.
Proposed grading improvements, internal to the site and within the improvement area, will provide for grades
ranging from 0.5% to 2.7%. Historic drainage patterns and flows along the northern and southern property
boundaries will continue to be discharged out to Shields Street, as historically occurs.
The proposed grading nnprovements and proposed concrete trickle pan along the so��th boundary will improve
eYistilig conditions by providing for a continuous positive conveyance of attributable runoff east and out to
Shields St�-eet. Grading along and near the northein property line will be veiy minin7al, «�ith the exception of
grading neYt to the new addition's foundation and against the north side of the detached garage to provide ior
positive conveyance of iunoff away from the new structures, and then east and out to Shields Street.
Aspen has modeled the existing and proposed conditions of the site, with rational calculations attached to this
memorandum. Please refer to the attached rational calculations for impervious calculations and flow data.
The existing imperviousness of the 0.32 acre lot is approaimately 17%, with e�isting raiional 2-yeac and 100-
vear flows of 0.11 and 0.47 cfs, respectively. Tlie proposed improvements will provide for the 032 ac�•e lot
having an imperviousness of approximately 48%, with rational2-year and 100-year tlows being 0.37 and 1.79
cfis. respectively. The rational `c' value for the existing and proposed conditions are 0.20 and 0.56, respectively.
It's important to note that the proposed rational calculations provide for the entire elisting gravel area and new
gravel area for the driveway exteiision and patio area bellig modeled as eonerete, to allow the o��ners to pave
theu- driveway and patio in concrete ar other like sut•face, now or in the future. Aspen understands the owners
plan to only install gravel for tl�ese areas at this time.
Roof drain downspouts will discharge to green/landscape areas, thus pcoviding LID treatment for the project.
Runoff from majoc stonn events, that could exceed the landscaping a»d green areas, will follow historic runoff
patterns to the north and south property boundaries and then east and out to Shields Street.
Considering the draina;e masterplan for this area, it appears that even with the proposed improvements, the
impervjousness of 48% is less than that allowed for the lot. Therefore, the proposed improvements appear to be
in conformance with the maste�plan and Aspen does not anticipate any adverse impacts to the esisting sh-eet's
capacity or to other adjacent oi• downstream properties, due to the �ninor flow increase fi�om the lot
improvements.
Thank you for your time and consideration in reviewing this addendum. If you have any further questions,
please feel free to contact me.
subbasin HW B150
Sincerely,
/�.,'
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JoHry GoocH, PE
ASPEN ETIGINEERING
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