HomeMy WebLinkAboutDrainage Reports - 05/01/1981DRAINAGE REPORT FOR
i GSM WEST MOUNTAIN P.U.D.
Certification:
I, David M. Frick, do hereby certify that this drainage
report for GSM West Mountain P.U.D. was prepared under my
supervision in accordance with City of Fort Collins
specifications.
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David Frick,
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Resource Consultants, Inc.
P. 0. Box Q
Fort Collins, Colorado 80522
rilay- 1981
Ref: 1063
DRAINAGE REPORT FOR GSM WEST MOUNTAIN P.U.D.
INTRODUCTION
This report presents the drainage plans for the GSM West Mountain
P.U.D. office complex. The report presents drainage information for the
site and the proposed drainage plans for the development.
SITE DESCRIPTION
The site is located at 408 and 412 West Mountain Avenue, Fort Collins,
Colorado. Currently two houses are on the site which are being used as
offices and there is no paved parking area; however, the rear portion of
the "412" property is being used as an unpaved parking lot. The property
is adjacent to the alley between Meldrum and Sherwood streets and the
drainage from the area west of the alley flows north along the alley to
LaPorte Street.
Fig. 1 shows the existing drainage patterns of the site. !Essentially-
the front half of the site drains towards West Mountain Avenue while the
rear half drains to the northeast corner of the property.
The proposed site development which includes renovation of the west
structure and reconstruction of the east structure will also have a parking
lot along the rear or northern portion of the property, as shown in Fig. 2.
This parking lot will be connected with the parking lot at 402 West
Mountain Avenue in order to provide access from Meldrum Street in addition
to the alleyway off of West Mountain Avenue. The site sloping to the east
would require drainage water to flow into the parking lot at 402 West
Mountain Avenue which would be accepted by the owners at that address --a
copy of a letter is attached which indicates their willingness to accept
this water. The front or southern portions of the site which drain towards
West Mountain Avenue will not be changed significantly with the site
development; therefore the drainage as it now exists and as planned for
full development would flow into West Mountain Avenue.
RESOURCE CON ULTMTS INC
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DRAINAGE CALCULATIONS
Computations of site drainage releases for the 2-year and 100-year
storms were made utilizing the rational method. Table.1 summarizes the
results of these calculations and shows the peak flows along the northeast
corner and along the southern border of the property. The results show
that the total increase in site discharges for the 2-year flows at full
development conditions would be 0.27 cfs over the discharges under existing
conditions, and for the 100-year flood it would be 1.19 cfs greater than
under existing conditions. The total volume of increased runoff during the
100-year storm would be 850 cubic feet over the.100-year storm under
existing conditions. This change in runoff is entirely due to the con-
struction of the paved parking lot at the rear of the property.
Table 1. Summary of Drainage Calculations
A C Q Vol.1/
(ac.) (cfs) (cu-ft)
South 11 of site,
Existing and
Fully Developed
2-year 0.187 0.40 0.17 250
100-year 0.187 0.50 0.63 880
North �i of site,
Existing Conditions
2-year '0.197) 0.36 0.16 240
100-year 0.197 0.45 0.60 840
North =j of site,
Fully Developed
2-year 0.197 0.72 0.43 480
100-year 0.197 0.91 1.79 1690
1/Assuming a 1-hour storm.
2 RESOURCE CONSULTANTS INC
The only method of reducing these runoffs would be to develop some
sort of storm -water detention plan for the property. However, there are
several problems with this. One is that with the connection of the parking
lot at 402 West Mountain Avenue with this proposed parking lot there is no
convenient location for storm -water detention. The letter of acceptance of
the drainage at 402 We,5t Mountain indicates they would not accept the water
if they were required to detain water for this site. Therefore, with the
proposed plan as it is, it would be virtually impossible to construct
storm -water detention facilities.
SUMMARY AND CONCLUSIONS
Based on the results of this study the proposed drainage plan would be
to release waters from the parking lot in the GSM West Mountain P.U.D. into
the parking lot to the east behind 402 West Mountain Avenue, with the
remainder of the drainage waters flowing into West Mountain Avenue as they
do currently. The release of these storm waters would result in an addi-
tional 1.2 cfs of increased flows during the 100-year storm and less than
0.3 cfs increase during the 2-year storm.
The site's steep slope to the east and its connection with the parking
lot to the east makes it almost impossible to construct storm -water deten-
tion facilities for this parking lot. If storm -water detention facilities
are required of the P.U.D., major modifications would be necessary and
these modifications would make it impossible to connect the parking lot
with the one at 402 West Mountain, thus limiting access and creating
traffic flow problems. (Therefore it is requested that this drainage plan
be approved with the requirement for onsite detention being eliminated for
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EDWARDS HOUSE PARTNERSHIP
P. 0. Box Q
Fort Collins, CO 80522
May 8, 081
Mr. Peter Gunderson
Mr. Tom Smith
320 West Olive Street
Fort Collins, CO 80521
Gentlemen:
This letter is in reference to your request to release drainage waters
from your proposed parking lot behind 408 and 412.West Mountain Avenue into
our parking lot at 402 West Mountain Avenue. We have reviewed your site
plan and have evaluated the effect this runoff would have on our parking
area.
Based on our review of the site information we can see no problem with
the drainage plan as designed. This approval is based on the assumption
that the City of Fort Collins will not require additional storm -water
detention on our site at 402 West Mountain Avenue to accommodate drainage
from your site or for any additional drainage waters created by the modifica-
tions to our parking area necessary to connect with your parking area. If
the City requires modifications of our drainage plan to include additional
storm -water detention, then we would not be able to accept your drainage
water.
Sincerely yours,
nEDWARDS HOUSE PARTNE HIP
David M. Frick
DMF:js. General Partner
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