HomeMy WebLinkAboutDrainage Reports - 01/26/1989LAND
EUOPMENT
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Ms. Susan Hayes January 23, 1989
Engineering Department
Storm Drainage Division
City of Fort Collins
235 Mathews
Fort Collins, CO. 80524
RE: Clarendon Hills Third Filing
Amendment to Final Storm Drainage Study
Dear Susan,
Pursuant to my phone conversation with Mike Hertzig and you
on Friday, January 20th, we have revised the design for the
referenced subdivision to eliminate the need for an easement for
storm drainage on properties east of the project.
A Swale one (1) foot deep and ten (10) feet wide has been
added to the rear of those lots abutting the easterly property
line of the third filing. A copy of Sheets 18 and 20 of the
Utility Plans, as well as a copy of the sheet on the Final Plat
for the affected lots, is provided to reflect the revised design.
The easement provided is 15 feet in width so that the 10
foot wide drainage swale can be built westerly of, and without
adversely affecting, the telephone and cable television system.
Mr. Ed Heinze of U.S. West Communications has approved the
improvements as shown on Sheet 18 at Section A —A.
The effect of this change on the storm drainage system
design that was previously provided to you is minimal. Subbasin
9 is the drainage basin in the third filing that had been thought
to be appropriate for sheet flow onto the properties east of
Clarendon Hills Third Filing. The redesign allows for
intercepting the storm runoff that would otherwise enter the
properties to the east and directing the flows from subbasin 9
southerly into other improvements provided with the third filing
— and ultimately to Pond E in Detention Area A of the first
filing.
The most conservative approach to the redesign calls for
adding the 100 year storm discharge from Subbasin 9 (5.4 cubic
feet per second) directly to the 32.7 cubic feet per second of
flow that would occur during a 100 year storm in the previously
approved Swale that the new interceptor swale empties into near
he common lot line of Lots 30 and 31, Clarendon Hills Third
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Lan Development Services 117 East Mountain Avenue • Suite 201 Fort Collins, Colorado 80524 303/493.4624
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Disregarding, then, routing proceedures that would predict a
lesser total flow, the rating curve for the originally designed
Swale downstream of its confluence with the newly created 10 foot
wide interceptor Swale shows that the depth of flow required to
accommodate the combined 100 year flow of 38.1 cfs is only 0.08
feet more, or 1.80 rather than 1.72 feet deep.
As for the new interceptor Swale, the enclosed rating curve
and related information show that the minimum slope of the new
Swale is 2 percent, and that the channel capacity is about 11
cfs. A portion of subbasin 9 is not yet contributing upstream of
the confluence with the original Swale, and therefore 5.4 cfs
during a 100 year storm is a conservative estimate.
Even though the 5.4 cfs of estimated flow during a 100 year
storm in subbasin 9 does not all enter the new interceptor swale
upstream of the confluence, a conservative approach shows that
the Swale would be less than 0.9 feet deep if it were carrying
5.4 cfs.
This redesign effectively handles storm runoff from Subbasin
9, does not have an adverse impact on the remainder of the
drainage system, and eliminates any need for an drainage easement
from property owners to the east of the subject property.
Thank you for Your consideration of this matter.
Sincerely,
Dennis L. Donovan, P.E.
A ' L
1301 .1 .c7-, 1 —aweiana Avef
LOVELAND, C9LORADO 80537
14 (303) 669.0516
CALCULATED BY -ATE
CHECKED BY DATE
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