HomeMy WebLinkAboutDrainage Reports - 07/17/2014NORTHERN
ENGINEERING
Drainage Memorandum
Date: July 16, 2014
Project: Replat Lot 1, Harvard Subdivision
2801 S. College Avenue
Fort Collins, Colorado
Attn: Mr. Wes Lamarque
City of Fort Collins Stormwater Utility
700 Wood Street
Fort Collins, Colorado 80521
Dear Wes:
City of Ft.. Collins rov Plans
Approved By -
Date -7--17 -1
Project No. 1020-001
This memorandum serves to address the stormwater impacts of the above referenced project.
The site currently consists of an existing building with associated sidewalks, parking areas and
drive aisles. The proposed improvements are to construct a building addition of 1170 square feet
in size (29'x3O'). In addition to the building addition, a new perimeter walk will be construction
along with a paver section running down the property line. The overall area proposed to be
disturbed is 3,360 square feet.
Replat of Lot 1, Harvard Subdivision was approved in December of 1977 and required a
detention volume of 3,000 cubic feet for on -site detention at a high water surface elevation of
5024.3. The detention was designed to be achieved through use of two ponding areas; one in
the western parking lot designed to detain 1,240 cubic feet and the other in the eastern parking
lot designed to detain 2,002 cubic feet. The overall designed detention was 3,242 cubic feet
resulting in 242 cubic feet above the required volume.
The proposed building will cause for a portion of the perimeter sidewalk, existing shed and back
parking area to be regraded. The building addition and proposed perimeter walk will encroach
within the ponding limits, therefore causing a slight decrease in volume of 126 cubic feet. Due to
different datums used between the original survey and the proposed design, the adjusted high
water surface elevation equates to 5027.19 through this memorandum and design. The decrease
in volume will result in an overall designed volume of 3,116 cubic feet, which still exceeds the
amount required within the original design by 116 cubic -feet. Since the building addition is
proposed to replace portions of existing concrete walk and pavement, there will be no increase to
the amount of impervious area. However, the proposed paver section will replace concrete
paving, decreasing the total percent imperviousness of the site. In addition, pervious pavers have
been proposed to increase the amount of detention volume available. The storage volume
available within the reservoir areas (i.e., No. 57 and No. 2 open -graded aggregate) of the MBP
section is ±441 cu. ft. The MBP section is designed with an increased No. 2, open -graded
aggregate, material depth in order to provide storage for storm events in excess of the water
quality (80th percentile) storm event. Without out taking into consideration the previous design,
the amount of storage generated within the paver section exceeds the volume displaced by the
building addition and perimeter sidewalk.
In conclusion, the construction of the building addition, perimeter walk and paver section will
result in a decrease of impervious area. The encroachment into the ponding limits will result in a
volume still exceeding the original design. The additional detention volume within the paver
section exceeds the volume displaced by the proposed design.
Please do not hesitate to contact me if you have questions or require additional information.
Sincerely,
Codl Snowdon
Project Engineer
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