HomeMy WebLinkAboutDrainage Reports - 12/14/2010NORTHERN
ENGINEERING
Drainage Letter Report
City of Ft. Coil! jApp ad Plans
Approved
Date
ADDRESS: PHONE:970.221.4158 WEBSITE:
200 S. College Ave. Suite 100
Fort Collins,C080524 FAX:970.221.4159 www.nodhernengineering.com
Date: December 13, 2010
Project: Appleblossom Subdivision Project No. 574-001
Replat of Lots 17 and 18, Block 5, Westlawn Addition
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 residential
redevelopment. The existing home is situated towards the western portion of the site, but does
occupy portions of both lots. The site currently consists of rooftop, concrete walks and patio,
turf, trees, and other vegetation. It presently functions similar to a 'B' lot, with historic drainage
being split towards the northeast and southeast. The unpaved alley adjacent to the western
properly line has a high point such that it too drains to the north and towards Laurel Street on
the south. The total impervious area of the existing site is approximately 2,223 square -feet.
The proposed redevelopment will completely remove all existing above -grade, structures and
hardscape. The new project will consist of a duplex on the western lot (Building A), a triplex on
the eastern lot (Building B), and a permeable paver courtyard between the two. A new parking
lot will be constructed north of the residential units, over half of which will consist of permeable
pavers. Concrete pedestrian connections will be made from the units south to the Laurel Street
sidewalk. The total impervious area of the proposed redevelopment is roughly 7,196 square -
feet.
The off -site alley adjacent to the project will get paved with a cross-section according to LCUASS
Figure 7-12F. The existing curb cut and concrete apron to Laurel Street will remain as -is. The
area of paved alley draining south across the Laurel sidewalk will be approximately 650 square -
feet, which is less than the maximum 750 square -feet allowed by LCUASS.
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
increase in impervious area proposed with this development is approximately 4,973 square -feet
(7,196 — 2,223). A detailed breakdown of the existing and proposed impervious areas, along
with graphical site depictions can be found on the attached Exhibit 1.
Page 1 of 2
D:1Projecls,574-001\DrainagOPeports\Drainage ReporBDminage-Lettrs 12-13-10.dacx
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. 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 receives regional water quality mitigation in the Udall Natural
Area; therefore, on -site water quality treatment in not required. However, sustainable design
practices and environmental stewardship are part of the goals and vision of this project. As
such, this development will remain sensitive to quality of stormwater runoff leaving the site. A
permeable paver system will be employed, not only to limit the increase in total impervious area,
but also to help cleanse and treat runoff prior to leaving the site. Landscaped buffers on either
side of the concrete walk along Laurel Street will further treat surface runoff and applicable roof
drainage prior to entering the adjacent curb and gutter.
While a good portion of historic runoff does travel north and east towards existing single-family
residences, a slight modification to current drainage patterns for the site is proposed in an
attempt to improve this condition. First and foremost, on -site infiltration will be encouraged to
the maximum possible extent. However, sub -surface soils conditions, combined with the
proximity to building foundations, demand an underdrain system. Therefore, a shallow
underdrain will relieve near surface waters and discharge into the curb and gutter along Laurel
Street. Additionally, areas that historically sheet flowed across adjacent properties are now
minimized to strictly the pervious 6' utility easements along the northern and eastern property
lines. All downspouts and runoff from impervious surfaces will be discharged away from off -site
properties. Exhibit 2 further documents that there will be no adverse increase in runoff to the
adjacent properties east of Appleblossom Subdivision.
The proposed grading concept closely matches existing elevations and drainage patterns, and
only varies in an effort to improve the existing condition and to help ensure that adjacent
properties are not adversely impacted. On -site detention is proven to be unnecessary.
Stormwater quality has been considered, and will be addressed in a manner beyond the
minimum requirements. Therefore, it is my professional opinion that the triplex and duplex
proposed at the Appleblossom Subdivision (Replat of Lot 17 and Lot 18, Block 5, Westlawn
Addition) satisfy all applicable stormwater criteria.
Please do not hesitate to contact me if you have questions or require additional information.
Sincerely
"QWmm=%0P" LEED
Nicholas W. Haws, PE AP
enc.
cc: Dave Derbes, LEED AP — Brinkman Construction
Page 2 of 2
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