HomeMy WebLinkAboutGrading Plans - 09/29/2015September 25, 2015
Mr. Dan Mogen
City of Fort Collins
Water Utilities Engineering
700 Wood Street
Fort Collins, CO 80521
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RE: Drainage Memorandum-307 Riverside Cottage, 307 Riverside Avenue, Fort Collins
Dear Dan:
The purpose of this drainage memorandum is to summarize the proposed drainage design and
summarize the proposed improvements for construction of the 307 Riverside Cottage. The existing
0.21 Acre site is currently a single family residential lot. The lot is bounded by Riverside A venue to
the north, a gravel alley to the west and residential lots to the south and east. The lot is occupied by
a single house on the north end and a shed at the southwest comer. Parking is provided on a gravel
surface adjacent to the alley, between the shed and house. The balance of the lot is vegetated with
grass and weeds. A wood fence is constructed along the west, east and south lot lines.
The proposed lot improvements will include removal of the existing shed in the southwest comer
and wood fencing north of the shed along the alley. The existing house will remain in place.
The limits of disturbance delineated with these improvements will be approximately 2,928 square
feet (0.067 acres) which represents approximately 31 % of the lot area. Currently, the impervious
area of the lot is 1,635 square feet (SF). The new impervious area with the proposed improvements
will be 2,165 SF resulting in an increase of 530 SF.
The runoff coefficient, c-value was calculated for the existing and proposed conditions and a peak
flow calculated to evaluate the difference in runoff under both conditions. The results are tabulated
below.
Condition Area (Ac) C-value Intensity Peak Flow
(in/hr) (cfs)
Existing 0.21 0.38 6.0 0.48
Proposed 0.21 0.42 6.0 0.53
As indicated by the runoff coefficient and peak runoff calculations, the impact of the improvements
on surface runoff from the site is negligible.
The proposed drainage patterns will remain similar to existing conditions and maintain historic
patterns. The majority of the runoff will drain northeast in the existing grass lined drainage path
towards Riverside Avenue, as historically occurs. The west side and one-half of the south side of the
cottage will drain to the alley, which drains to Riverside Avenue. Please refer to the attached
Grading and Drainage Plan, C-001 for the site grading information and drainage paths.
Disturbance to existing vegetation should be minimized during construction to improve runoff water
quality. Runoff to the northeast will flow within an existing grassed lined drainage path, which will
enhance filtration of sediment before discharging to Riverside A venue. The gravel alley will
provide access to the site. Mud tracked from the site to the public right-of-way must be cleaned by
the Contractor. A portable concrete wash out should be used on site to capture wash water following
concrete deliveries; or concrete washing provided elsewhere, in accordance with state and City
statutes, as applicable.
In summary, the existing home will remain in place and the existing shed will be removed for
construction of a new cottage. Disturbance will be approximately 0.07 acres, with a net increase of
approximately 530 SF of impervious area. An additional 529 SF of gravel area will be added on the
south side of the lot; however, the net impact is negligible, as today the area is primarily dirt and
existing gravel. Drainage patterns and discharge points will remain similar to existing and historic
conditions. Therefore, the proposed improvements appear to have minimal impact, and will provide
equal or better drainage facilities for the site.
Should you have any questions or concerns, please contact me.
Sincerely,
Anthony Willkomm, P .E.
Aspen Engineering
Attachments: Grading and Drainage Plan
Drainage Calculations
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Developed Weighted Runoff Coefficients
307 Riverside Cottage
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Bas 111 Roof Are.1 (sqft) Asphalt A,ea (sqft) Concrete Area (sqft) r~ravel Area (sqft) L.Jwn Arr>a (sqft) Total Area Composite
No. C•0 90' C•0 95 C•0 90 C•0 41 C•0 25 (sqtt, C
otal SITE 2,165 0 0 f, ,902 9,27 O.ll2
Total Acreage 0.00 0.00 0.00 0.03 0.14 0.21
Impervious Ratio
Bas in Roof Area (sqft) Asphalt A,ea (sqft) Conc,ete Area (sqft) Gr.Jve/ Area fSL/ftJ Lawn A,ea (sqft) Tot.1/ Area Imperviousness
No (sqft) Ratio
TotllSITE '65 0 1,204 5,902 9,271 29%
Total Acreage 0.05 0.00 0.00 0.03 0.14 0.21
Land Use or Surface Characteristics Percent lrnpervJousness
Business:
Commercial areas 95
Neiahborhood areas 85
Residential:
Sinale-famitv
Multltmlt fdetachedl 60
Multiunlt lattachedl 75
Half-acre lot or laraer
Aru1rtments 80
Industrial:
Uahtareas 80
Heavv areas 90
Parks cemeteries: 5
Plavarounds : 10
Schools : 50
Railroad vard areas: 15
Undevelonad areas:
Historical Flow Ana~ 2
Greenbelts, agricultural
Off-site flow analysis 45
{when land use not defined)
Streets:
Paved 100
Gravel /oacked\ 40
Orivewavs and sidewalks: 90
Roofs: 90
Lawns, sandv soil 0
Lawns clavev soil 0
•Refer to Figures R0-3 through R0-5 In Runoff Chapter
ASPEN ENGINEERING
10:51 /Wr
9/2512015