HomeMy WebLinkAbout504 S WHITCOMB STREET, CARRIAGE HOUSE - FDP210006 - DOCUMENT MARKUPS - ROUND 3 - DRAINAGE REPORT
Highland Development Services, Inc.
6355 Fairgrounds Avenue, Suite 100 | Windsor, CO 80550 | 970.674.7550
Memorandum
TO: Robert Kirkpatrick
FROM: Adele Bartel, E.I.T., Jason Claeys, P.E.
DATE: 10/26/21
PROJECT: 504 S. Whitcomb St.
HDS JOB NO: 20-1039-00
SUBJECT: Drainage Summary
This memo has been prepared to describe the existing conditions at 504 S. Whitcomb St, the
adjacent alley, and the drainage impacts of the proposed alley house and change in impervious
area. Below is a vicinity map showing the general location of the property:
The property is approximately 9,000 sqft (50’ wide by 180’ deep, 0.21 acres) and is located along
the east side of S. Whitcomb St and south of W. Mulberry St. A shared alley is located along the
east property line and single-family residences are adjacent to the north and south. The existing
single-family residence is located toward the west side of the lot with a shed adjacent to the alley
to the east and a small garden shed along the south property line. The shed adjacent to the alley
is proposed to be removed, along with portions of the existing sidewalk, to accommodate the
proposed addition of a detached alley house, and additional gravel parking area.
10/26/21
504 S. Whitcomb St.
10/26/21
Page 2 of 3
Based on aerial topography and a couple site visits, it appears there is a highpoint approximately
midway along the east-west property lines of the existing single-family residence. Approximately
1/3rd of the site drains west to Whitcomb St and 2/3rd of the site drains east to the alley. The alley
appears to be paved with asphalt, though the surface is degrading and there are a few potholes.
A topographic survey was completed to confirm the existing alley slopes. The alley has slopes
ranging from 1.5% to 2.0% draining to the north, and an approximate cross slope of 1.5% draining
to the west side of alley. An existing residence is located adjacent to the alley along the east side
of the alley. Based on the survey, drainage is directed away from the existing residence. In its
current condition, during a 100-yr storm event, the runoff depth would inundate the residence’s
existing crawlspace vent (see attached Appendices). Measures have been taken to ensure that
the proposed improvements do not exacerbate the current alley drainage, described on the
following page. Below are photos for reference describing the alley:
504 S. Whitcomb St.
10/26/21
Page 3 of 3
Upon the second site visit after measuring slopes with a digital level, it was discovered that there
is a highpoint at the midpoint of the south side of the single-family residence, as well as a
lowpoint between the sidewalk and the parkway at the front of the lot. The property owner will
modify the parkway landscaping to provide positive drainage relief to S. Whitcomb St and add
roof gutters along the south roofline and east porch cover to redirect roof drainage toward S.
Whitcomb St.
The existing property at 504 S. Whitcomb that drains to the alley is approximately 27.9%
impervious. Accounting for the existing improvements to remain, redirection of the existing
single-family residence roof drainage, removal of the existing shed, and the addition of the alley
house and gravel parking area, the site is approximately 27.7% impervious, an approximate
decrease of 0.2% imperviousness. See the attached calculations for reference.
Runoff rates were determined utilizing the rational method. The existing peak runoff rate is 0.56
cfs and the proposed peak runoff rate is unaffected at 0.56 cfs.
With the redirection of the existing single-family residence roof drainage toward S. Whitcomb St,
the proposed improvements should not adversely impact the existing drainage conditions within
the shared alley. Please do not hesitate to call (970.674.7550) or email (jclaeys@highland-
ds.com) if there are any additional questions.
Design Engineer:
Design Firm:
Project Number:
Date:
%
Impervious
Runoff
Coefficient C
Return
Period
Frequency
Adjustment
Factor (Cf)
100% 0.95
2-year to
10-year 1.00
90% 0.95 100-year 1.25
90% 0.95
40% 0.50
15% 0.35
* Lawn area assumed to be 15% imperv and 0.35 coeff to account for misc walks
C2 to C10 C100
Ex Alley 22,098 0.507 718 3,458 0 1,958 15,964 31.7% 0.48 0.60
Ex 504 5,818 0.134 0 761 0 723 4,334 27.9% 0.45 0.56
Prop 504 5,818 0.134 0 648 0 1,006 4,164 27.7% 0.44 0.55
504 S. Whitcomb Street
DEVELOPED IMPERVIOUS AREA CALCULATION
DESIGN CRITERIA:
Fort Collins Stormwater Criteria Manual
BASINS:
A.McKenna
Highland Development Services
21-1039-00
October 22, 2021
% Impervious values from Table 3.2.2 in the Fort Collins Stormwater Criteria Manual
Land Use
Paved
Roof
Walks
Runoff Coefficients and Frequency Adjustment Factors for City of Fort Collins - Storm Water Criteria Manual
Gravel/Pavers
Lawns*
(Clayey, Flat < 2% Slope)
Sub-basin
Designation
Atotal
(sq feet)
Atotal
(acres)
Weighted %
Impervious
COMPOSITEApaved
(sq feet)
Aroof
(sq feet)
Awalk
(sq feet)
Agravel/pavers
(sq feet)
Alawn
(sq feet)
504 S. Whitcomb St - Rational Calcs.xlsx Page 1 of 4 Highland Development Services