HomeMy WebLinkAboutBUCKING HORSE, THIRD FILING - FDP - FDP140015 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTJune 11, 2014
City of Fort Collins
Stormwater Utility
700 Wood Street
Fort Collins, Colorado 80521
RE: Drainage Summary Letter for
Bucking Horse Filing 3
Dear Staff:
Northern Engineering is pleased to submit this Drainage Summary Letter in support of the
proposed Bucking Horse Filing 3 project for your review. The proposed project involves the
addition of 14 lots along Nancy Gray Avenue and 5 lots along Yearling Drive within the Bucking
Horse Subdivision.
Revisions Along Nancy Gray Avenue
As shown on the Bucking Horse Filing 3 Grading Plan, the lots along Nancy Gray Drive have
been graded such that the front half of each lot will drain to Nancy Gray Avenue and the back
half of each lot will drain towards the open space designated as “Outlot A” along the rear of
each lot. A low point in Nancy Gray Avenue will capture all drainage from the front halves of
the lots and convey drainage via an existing pair of sump inlets and storm line (existing Storm
Drain 3, Bucking Horse Filing 2). This drainage pattern is slightly modified from the drainage
pattern anticipated for this area in the Filing 2 submittal. There will be a net decrease in area
draining to the sump inlet on Nancy Gray Avenue of 0.14 acres resulting from the revised
grading that will occur with the proposed Filing 3 project (compared with the Filing 2 design);
thus, there will be a reduction in flow to the sump at Nancy Gray Avenue.
All drainage from the additional 14 lots will ultimately be captured by the major drainage swale
running along the north boundary of the Bucking Horse site. Storm Drain 9 (designed with
Filing 2) conveys all flow from “Outlot A” under Nancy Gray Avenue. Runoff from the additional
14 lots will have no adverse impact on the major drainage swale or storm line or the
downstream receiving detention pond (Pond 215, designed with Filing 2), as the overall basin
this area is encompassed by has experienced a net decrease in basin imperviousness, which is
discussed further below.
Revisions Along Yearling Drive
The 5 lots along Yearling Drive will be graded such that the front halves of lots along Yearling
Drive will drain into the roadway and the back halves of lots will drain to “Tract H” along the
rear of each lot. Drainage in Yearling Drive will be conveyed in street curb and gutter southeast
to “Outlot A”. Drainage in “Tract H” will be conveyed via swale flow into “Outlot A”. This
drainage pattern is generally consistent with drainage patterns anticipated for this area in the
Filing 2 submittal.
As with the additional 14 lots along Nancy Gray Avenue, all drainage from the additional 5 lots
along Yearling Drive will ultimately be captured by the major drainage swale running along the
north boundary of the Bucking Horse site. Storm Drain 9 (designed with Filing 2) conveys all
flow from “Outlot A” under Nancy Gray Avenue. Similar to the additional 14 lots along Nancy
Gray, runoff from the additional 5 lots will have no adverse impact on the major drainage swale
or storm line or the downstream receiving detention pond (Pond 215, designed with Filing 2),
as the overall basin this area is encompassed by has experienced a net decrease in basin
imperviousness, which is discussed further below.
Overall SWMM Modeling Revisions
Previously, we submitted a drainage summary letter dated May 7, 2014 discussing the minor
modification that the currently proposed Filing 3 would create for the current effective SWMM
model reviewed and approved with the Bucking Horse Filing 2 project (“Final Drainage Report
for Bucking Horse Filing 2”, December 17, 2012, by Northern Engineering).
As we stated previously, the proposed Filing 3 project will modify basin percentage
imperviousness for SWMM Basin 105. The previously approved SWMM model showed this
basin with a percent impervious value of 75%. With the proposed site plan for Bucking Horse
Filing 3, the averaged percent impervious value will be reduced by 6% to a value of 69%. This
will be beneficial for the storm system design and detention pond design by creating a small
decrease in both peak 100-year runoff rates and volumes.
If you should have any questions as you review this, please feel free to contact us.
Sincerely,
NORTHERN ENGINEERING SERVICES, INC.
Aaron Cvar, PE
Project Engineer