HomeMy WebLinkAboutDrainage Reports - 07/03/2013Quality
Engineering,,
August 28, 2012
Mr..Glen Schlueter
City of Fort Collins Stormwater Utility
700 Wood Street
Fort Collins, Colorado 80521
RE: Maldonado Subdivision - Drainage Letter
412 Whitcomb Street, Fort Collins, Colorado
QE Project No. 6054-001
Dear Mr. Schlueter:
W W W.QUALITY-ENGINEERING.BIZ
City of Ft%,,C4WMAPPr0v0 Plans
Approved By
Date
This letter addresses drainage impacts due to subdividing a single family lot with an existing
single family residence into two single family lots along to include a future residence located at 412
Whitcomb Street. The site is situated in the northeast'/4 (quarter) of Section 11, Township 7 North,
Range 69 West of the 6 h P.M. in the City of Fort Collins, Latimer County, Colorado. More
specifically, the site is located northeast of Whitcomb Street and Cherry Street. The site is bounded
by Whitcomb Street on the west and residences to the north, east, and south. Please see the attached
vicinity map. The site is located within the City's Old Town Master Drainage Basin and is not
located within a floodplain as shown in the attached FEMA flood insurance rate map exhibit.
Lot I of the proposed subdivision currently consists of a residential house with mostly mature
turf grass and an existing gravel driveway. The additional lot created by the subdivision, Lot 2,
consists primarily of existing turf and landscaping with no existing buildings. We understand that an
approximate 1,200 SF footprint house will be constructed in the future on Lot 2 along with an
approximate 700 SF footprint, zero lot line garage split between both lots. Each lot will have a new
gravel driveway constructed to the new garage. The site currently drains partially to the east via
sheet flow at approximately 2% and partially to with west at an existing slope of approximately 2-
7%. The high point of the lot appears to be generally at the existing fence line and building line
shown on the drainage plan. The total area of the property is 0.21 acres (9,000 SF) not including
Whitcomb right-of-way.
The City of Fort Collins Stormwater Criteria Manual (Manual) and Denver Urban Drainage
and Flood Control District Manual (UDFCDM) has been utilized for drainage design guidance. The
rational method was utilized to compute existing and proposed flows for the 2-year and 100-year
storm events per the Manual. Rational method Composite "C" values were utilized from Table RO-
11 and percentage imperviousness values were taken from Table RO-3, Recommended Percentage
Imperviousness Values, UDFCDM. Runoff precipitation data was utilized from Tables RA-7 and
RA-8.
Onsite runoff was calculated to determine the runoff differential between historic (existing)
and developed conditions. The difference in hydrologic basin parameters and runoff rates for the site
is shown below in Table 1:
QUALITY ENGINEERING, LLC (970) 416-7891 1612 LAPORTE AVE, #7 FORT COLLINS, COLORADO
Page 2
RE: Maldonado Subdivision - Drainage Letter
412 Whitcomb Street, Fort Collins, Colorado
QE Project No. 6054-001
Table 1- Existing and Developed Conditions Basin Flows Comparison
QZ
Q100
Difference
Design Point
Basin
Area (acre)
% Imp.
CZ
C100
(cfs)
(cfs)
Qioo (cfs)
Historic
El
E1
0.1246
21.0
0.33
0.41
0.10
0.43
-
Developed
PI
P1
0.1929
38.0
0.48
0.60
0.23
1.09
+0.66
Historic
E2
E2
0.1312
10.0
0.24
0.30
0.07
0.32
-
Developed
P2
P2
0.0628
26.0
0.38
0.48
0.06
0.26
-0.06
The system is designed to safely convey site developed condition storm flows primarily by
historic sheet flow paths to historic conditions. It is our understanding that the City of Fort Collins
requires detention and a release rate not greater than the 2-year stone with new developments with an
increase in impervious area above 5,000 square feet. As shown in the attached impervious area
exhibit, the subdivision will generate approximately 1,900 S.F. of impervious area and does not
require detention. Flows to the east (Basins E2 and P2) are decreased by 0.06 cfs due to the increase
in area from the existing condition (Basin E2) to the developed condition (Basin P2). Basin El flows
to the west towards Whitcomb street are increased by 0.66 cfs (Basin P1).
The general layout of the proposed site plan lends itself well to water quality best
management practices. All impervious areas are bordered (disconnected) on their downstream sides
with either grass or gravel buffers. First flush runoff will be subject to good infiltration through long
sheet flow paths across the site.
Since the individual lots are less than 10,000 square feet, we understand that a full erosion
control plan is not required per amendment 1.1 to Chapter 7, Volume 3 of the UDFCDM (page 3-11
of the Manual). However, standard individual lot erosion control measures should be followed at
time of construction and typical notes are included on the drainage plan.
In summary, the grading, drainage, and water quality design for Maldonado Subdivision
complies with the requirements of the City of Fort Collins Stormwater Criteria Manual. We are
confident that we have addressed your concerns. If you have any questions or require additional
information please contact the undersigned.
Sincerely,
QUALITY ENGINEERING, LLC _apDO Ucx
1326 m)
Attachments
Cc: Christine Maldonado, Dennis Sovick
0
/VICINITY MAP
08/27/12
QUALITY I
ENGINEERING
1612 Laporte Ave. Suite 7
Fort Collins. W 80521
(970) 416-7891
VICINITY MAP
MALDONADO SUBDIVISION
FORT COLLINS, CO
NO.
REVISIONS
BY
DATE
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^IMPERVIOUS AREAS
', 1 =20-
Existing (S.F) Proposed (S.F) Total (S.F.)
Sidewalk 400 -
Roofs 832 1900
Total 1232 1900 3132
Difference 1900 S.F. Additional Impervious Area
08/27/12
QUALITY
lIMPERVIOUS
ENGINEERING
1812 Laporte Ave, Suite 7
Fort Collins. CO 80521
(970) 418-7897
AREA
EXHIBIT
MALDONADO SUBDIVISION
FORT COLLINS, CO
N0.
REVISIONS
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EXISTING DRAINAGE EXHIBIT
STANDARD EROSION AND SEDIMENT CONTROL CONSTRUCTION PLAN NOTES
SEPTEMBER 2003 (REVISES JULY 2 10)
21
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