HomeMy WebLinkAboutDrainage Reports - 03/21/1988STORM DRAINAGE REPORT
AMENDED MINING PERMIT #77-184
MOBILE PREMIX COMPANY
FORT COLLINS, COLORADO
ADDENDUM TO:
FINAL STORM DRAINAGE REPORT
CACHE LA POUDRE INDUSTRIAL PARK
FORT COLLINS, COLORADO (DEC 1986)
John R. Clark P.E.
Consulting Engineer
Fort Collins, Colorado
March 21, 1988
Page 1
1. Scope
The study area of this report is in the northeast 1/4 of section 18,
Township 7 North, Range 68 West in the City of Fort Collins, Colorado. It
is covered by thick grasses and old abandoned farm structures; it contains
6 acres more or less. It is in the flood plain of the Cache La Poudre
River, further described in Exhibit 1 and 2.
The purpose of the report is to supplement basic work done in the
Final Storm Drainage Report, Cache La Poudre Industrial Park, and
Pikin-Riverside Storm Drain of December 1986. The additional study area
shall be an extension of the existing Mobile Premix Company. The report
addresses drainage considerations on the subject parcel.
2. Site Investigation and Soils Considerations
The site is relatively flat but has a bluff rising to the west and
south. Soils are free draining sands and gravels overlain by 1 to 3 feet
of loam and organic overburden. The area will be used for gravel
extraction.
3. Drainage Considerations and Computations
Minor Storm System . The minor storm system consists of overland flow
from offsite and on -site runoff into the gravel extraction areas. The
contributing drainage basin of about 9 acres of pasture land contains a
length of 800 feet, a slope of 2.6 percent and an assumed runoff
coefficient of 0.2. Design storm period of 10 years is used. Resulting
flowrate is 4.7 cfs using the rational method. See Exhibit 3.
Major Storm System . The major storm is a 100 year return period.
This is not analysed because the parcel is in the flood plain. Runoff
from upslope is not significant in comparison to the flood plain
considerations.
4. Provisions for Site Drainage
The erosion control improvements shall be made as required by Mobile
Premix prior to and during the extraction operations. The extraction
permit requires mitigation of operations and restoration of the site,
including slope repair and stabilization. The 5 foot levee referred to in
the Mobile Premix mining permit amendment is not considered; discussion
with submitting parties indicates that it would not be in the public
interest to construct such a levee at the present time.
The safety considerations due to the flood hazard are that no habitable
structures will be erected on the site. Mobile machinery will be used in
the operations. No utility improvements are planned for the site.
Construction and maintenance of higher ground levels at the south end of
the parcel which ajoins the Industrial Park to a minimum elevation of 4922
Page 2
will minimize flooding threats to the Industrial Park from high waters of
the flooding Poudre River. Detention in the excavation area will mitigate
hazards from flooding due to localized storm runoff of lesser storms.
Although no current watertable information is available, it is
estimated that natural watertable may fluctuate around the 4910 level
based upon observation of the immediate area. Since natural ground is
currently at 4916 to 4919, on -site detention capacity will be
considerable.
Should habitable structures be considered for this site in the future,
further study is recommended. However, based upon Exhibit 2, preliminary
foundation elevation of such structures should exceed elevation 4924 to
meet current standards.
5. Engineers Certification
I hereby certify that this report for the drainage design of the subject 6
acre parcel was prepared by me for the owners thereof and meets or exceeds
the criteria in the City of Fort Collins Storm Drainage Design Criteria of
May 1984.
hn R. Clark P.E.
Colo No. 13740
March 21, 1988
Attachments: Exhibit 1 Vicinity Map
Exhibit 2 FIRM Map Larimer County of March 1986 (portion)
Exhibit 3 Storm Drainage Preliminary Design Data Form 11-3
Page 3
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