HomeMy WebLinkAboutMAPLE HOLLOW PARK (FORMERLY MAPLE HILL PARK) - BASIC DEVELOPMENT REVIEW - BDR170002 - REPORTS - DRAINAGE REPORTFebruary 17, 2017
Mr. Wes Lamarque
City of Fort Collins
Storm Water Utility
700 Wood Street
Fort Collins, CO 80522
Re: Maple Hill Park
ICG Project No. 1165-012-89
Dear Wes,
Please accept the following letter report on behalf of The City of Fort Collins Park Planning and
Development to demonstrate the site’s compliance with the previously approved Final Drainage
Report for Maple Hill Subdivision, as prepared by TST Consulting Engineers, dated March 26,
2003.
INTRODUCTION
The proposed park is legally known as Tract I of Maple Hill Subdivision and is 7.14 acres. The
park is bounded on the north by Maple Hill Drive, the east by Bar Harbor Drive, and existing
single family homes on the west and south. The majority of the park currently drains to an
existing low point located near the eastern property line. The original runoff coefficient
calculations and stage-storage table are attached for reference and all detention and water quality
requirements for this site have been met by the overall Maple Hill development.
EXISTING CONDITIONS
The site is currently vacant and contains a stand of natural grasses. The Overall Drainage Plan
that was included in the previously approved Final Utility Plans for Maple Hill designates the
park area and the back yards of the surrounding single family homes that are tributary to the park
as Basin B13. This basin drains to a low point on the park property where it is collected by an
area inlet. The area inlet is connected to the storm system that discharges into the existing water
quality and detention pond located just east of Bar Harbor Drive. The northeast and southeast
corners of the park property are designated as portions of Basin B9 and B14, respectively. These
areas sheet flow directly onto Bar Harbor Drive and are collected by the existing storm system
which discharges into the existing pond. Basin B13 has a minor storm runoff coefficient of
0.28.
Mr. Wes Lamarque
February 17, 2017
Page 2 of 2
PROPOSED DEVELOPMENT
The proposed park primarily follows the original drainage patterns described in the original
plans. Basin 1, 2 and 3 make up the park property. Basin OS accounts for the off-site tributary
area that was originally included with Basin B13 from the approved plans. Basin 1 and 3
continue to sheet flow directly on to Bar Harbor Drive as initially intended; however, these basin
areas are slightly increased. Basin 2 and Basin OS drain to a low point in the park which is the
proposed irrigation pond. This pond will be fed by a proposed raw water connection from the
existing irrigation main located off-site. All stormwater runoff from Basin 2 and Basin OS will
sheet flow directly into this pond. A Type C inlet structure will be placed 6 inches below the top
bank of the pond setting the permanent water surface elevation to 5009.50. This inlet will be
connected to the existing area inlet (which will be replaced with a proposed manhole) and be
conveyed to the existing storm system which outfalls to the existing pond constructed with the
subdivision.
Basin 1, 2, 3 and OS have a combined minor storm Runoff Coefficient of 0.23 which is less than
the Runoff Coefficient for Basin B13 (0.28) from the approved plan. Therefore, it is requested
that the site be deemed in substantial conformance with the previously approved Final Drainage
Report. Please see the attached excerpts from the Maple Hill Subdivision Final Drainage Report
detailing the assumptions and original flow calculations for the park area.
LOW IMPACT DEVELOPMENT REQUIREMENTS
Runoff reduction practices (LID techniques) are also required. No less than seventy-five percent
of any newly added impervious area must be treated using one or a combination of LID
techniques. The development of the park adds 28,857 sf of new impervious area. Approximately
81% of that area (23,303 sf) will sheet flow over a grass buffer prior to entering the irrigation
pond. City Stormwater has confirmed that the LID requirements are met with the grass buffers
and the wet irrigation pond. The new impervious area and turf areas will be treated with these
two methods.
It is also important to note that there is no pavement or pollution producing impervious area
associated with this project. Additionally, any suspended solids associated with the runoff from
the park will be captured in the irrigation pond prior to exiting the site via the proposed Type C
inlet.
If you have any questions or comments please contact me at (970) 460-1091.
Sincerely,
Skylar Brower, P.E.
Colorado Professional Engineer 44248
Interwest Consulting Group
RUNOFF COEFFICIENTS & % IMPERVIOUS
LOCATION: Maple Hill Park
PROJECT NO: 1165-012-69
COMPUTATIONS BY: sb
DATE: 1/10/2017
Recommended Runoff Coefficients from Table RO-11 of City of Fort Collins Stormwater Code, Volume I
Recommended % Impervious from Table RO-3 Urban Storm Drainage Criteria Manual, Volume I
Type B Soils Runoff %
coefficient Impervious
C
Streets, parking lots (asphalt): 0.95 100
Sidewalks (concrete): 0.95 96
Roofs: 0.95 90
Gravel or Pavers: 0.50 40
Lawns, sandy soil (Flat <2%) : 0.10 0
SUBBASIN TOTAL TOTAL ROOF PAVED SIDEWALK LANDSCAPE RUNOFF %
DESIGNATION AREA AREA AREA AREA AREA AREA COEFF. Impervious REMARKS
(ac.) (sq.ft) (sq.ft) (sq.ft) (sq.ft) (sq.ft) (C)
1 1.02 44,495 0 0 570 43,925 0.11 1
2 5.88 256,144 0 0 27,271 228,873 0.19 10
3 0.24 10,448 0 0 1,016 9,432 0.18 9
Park Total 7.14 311,087 0 0 28,857 282,230 0.18 9
OS 1.70 74,218 30,800 0 0 43,418 0.45 37 Assumed Roof Area for Ex homes
TOTAL 8.85 385,305 30,800 0 28,857 325,648 0.23 10
Equations
- Calculated C coefficients & % Impervious are area weighted
C = Σ (Ci Ai) / At
Ci = runoff coefficient for specific area, Ai
Ai = areas of surface with runoff coefficient of Ci
n = number of different surfaces to consider
At = total area over which C is applicable; the sum of all Ai's
Final FLOW(01-10-17).xls
This unofficial copy was downloaded on Jul-30-2015 from the City of Fort Collins Public Records Website: http://citydocs.fcgov.com
For additional information or an official copy, please contact City of Fort Collins Utilities 700 Wood Street Fort Collins, CO 80524 USA