HomeMy WebLinkAboutTHORLAND SUBDIVISION PDP - PDP - 9-08 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTRUNOFF COEFFICIENTS & % IMPERVIOUS
LOCATION: Thorland Suhdivision
PROJECT NO: 39580.00
COMPUTATIONS BY: jmj
DATE: 312712008
Recommended Runoff Coefficients from Table 3-3 of City of Fort Collins Design Criteria
Recommended % Impervious from Urban Storm Drainage Criteria Manual
Runoff %
)efficienA Impervious
C
Streets, parking lots (asphalt): 0.95 100
Sidewalks (concrete): 0.95 96
Roofs: 0.95 90
Lawns (flat <2%, sandy soil): 0.10 0
Equations
- Calculated C coefficients & % Impervious are area weighted
C; = runoff coefficient for specific area, A;
A; = areas of surface with runoff coefficient of C;
n = number of different surfaces to consider
A t = total area over which C is applicable; the sum of all A is
OVERALL SITE IMPFRVInI1SNFSS
BASIN
TOTAL
TOTAL
ROOF
PAVED
SIDEWALK
LANDSCAPE
RUNOFF
%
DESIGNATION
AREA
AREA
AREA
AREA
AREA
AREA
COEFF.
Impervious
NUMBER
(ac.)
(sq.ft)
(sq.ft)
(sq.ft)
(sq.ft)
(sq.ft)
(C)
1
2.80
121,929
4,194
16,482
0
282,686
0.39
16.61
2
3.89
169,320
6,000
4,343
0
158,977
0.15
5.75
March 27, 2008
City of Fort Collins
Stormwater Utility
700 Wood Street
Fort Collins, CO 80521
RE: Thorland Subdivision Drainage Study
To whom it concerns,
J•R ENGINEERING
A Westrian Company
We are pleased to submit for your review and approval, this letter describing the drainage
design requirements for the Thorland Subdivision. All computations within this report
have been completed in compliance with the City of Fort Collins Storm Drainage Design
Criteria dated May 1984, revised April 1997.
As a result of a preliminary design review requested by the Thorland family, the city
responded with a letter dated June 11, 2007. In said letter, the stormwater department
states "since this is a very large site, the property owner's design engineer can compare
the overall site imperviousness, including '/z of Kechter Road, to the 35% imperviousness
used in the drainage master plan. If it is equal to or less than the 35%, then on -site
detention and water quality treatment are waived since the 35% imperviousness is
accounted for in the drainage master plan."
The attached calculations indicate very conservative estimates of future impervious
coverage including '/z of the ultimate ROW for Kechter Road being fully paved, the full
width of the ROW required by the city in the middle of the site being fully paved, all
existing structures, and a very conservative estimate of the proposed future building.
Since the calculations indicate that under this conservatively estimated condition the total
site impervious substantially less than 35%, JR Engineering respectfully requests that the
detention and water quality requirements imposed on the property be waived as
promised.
We greatly appreciate your time and consideration i
have any questions or required additional information
Sincerely,
JR Engi eri
Prepare y,
John Meyers Jr.
Design Engineer
Michael S. Brake, PE
Principle
If you
2620 Fast Prospect Road, Suite 190, Fort Collins, CO 80525
970-491-9888 • Fax: 970-491-9984 • w jrengineering.com