HomeMy WebLinkAbout504 S WHITCOMB STREET, CARRIAGE HOUSE - FDP210006 - SUBMITTAL DOCUMENTS - ROUND 2 - DRAINAGE REPORT
Highland Development Services, Inc.
6355 Fairgrounds Avenue, Suite 100 | Windsor, CO 80550 | 970.674.7550
Memorandum
TO: Robert Kirkpatrick
FROM: Jason Claeys, P.E.
DATE: 7/19/21
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.
504 S. Whitcomb St.
7/19/21
Page 2 of 3
Based on aerial topography and a site visit, 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. There is
concern that the residence’s existing crawl space vent could potentially be inundated during a
major storm event. To ensure that the additional impervious area proposed does not adversely
impact the existing drainage conditions, a small detention area is proposed. This detention area
is sized to limit the proposed 100-yr stormwater runoff rate to the existing 100-yr runoff rate.
Below are photos for reference describing the alley:
504 S. Whitcomb St.
7/19/21
Page 3 of 3
The existing property at 504 S. Whitcomb is approximately 30.5% impervious. Accounting for the
existing improvements to remain, removal of the existing shed, and the addition of the alley
house and gravel parking area, the site is approximately 35.0% impervious, an approximate
increase of 4.5% imperviousness. See the attached calculations for reference.
Runoff rates were determined utilizing the rational method. The existing peak runoff rate is 0.64
cfs and the proposed peak runoff rate is increased by 0.12 cfs to a total of 0.76 cfs. Reducing the
proposed runoff rate to match the existing, 80 cu-ft of detention volume is needed within what
is called Pond 504. Pond 504 has a proposed capacity of 210 cu-ft which exceeds the needed
volume and provides 30%+ of additional storage to account for freeboard. For water quality
consideration, runoff is directed to a landscape area, detaching the impervious area, prior to
being released into the alley. This will allow sediment to settle and be removed by landscape
materials. See attached calculations for reference.
With the incorporation of the proposed detention area and allowing a release rate equal to the
existing runoff rate, 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.