HomeMy WebLinkAboutDrainage Reports - 10/24/1994STEWART &ASSOCIATES
October 24, 1994 Consulting -Engineers and Surveyors
a
Mr. Glen Schlueter
Stormwater Utility
City of Fort Collins
P.O. Box 580
Fort Collins, CO 80522
Dear Glen:
Re: Jerry Wuerker's 407 S. Grant Duplex Addition.
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Mr. Wuerker asked me to prepare,an analysis of the impact of his proposed
addition on storm drainage.
The new addition results in a net increase of 453 square feet of impervious
surface. With the addition of the 453 square feet of hard surface, the grass
site, including the dirt alley and half of the Grant Street pavement, has a
runoff coefficient of 0.41. I have used a C of 0.20 for the grassed yards,
which are very flat, and the rear yard, which is by far the largest grassed
area, has a low area of about 1000 square feet which acts as a retention area.
The runoff coefficient used for this area by the Old Town Report is 0.40, so
I believe that even with the additional hard surface of the duplex addition,
the site runoff will remain within the parameters used for the Old Town Study,
especially since the rear yard retention area probably lowers the C below 0.41.
The additional volume of runoff generated by the 453 square feet of hard
surface during a 100 year storm will be 118 cubic feet. This is a very small
amount when spread out over a 3 hour storm and will have no adverse impact
on downstream facilities.
Since the building addition generates a very small amount of additional
runoff, and also since the runoff coefficient seems to comply with the
coefficient used for the Old Town Report, we request that a variance be granted
from the requirement for on site detention.
I am attaching a plan of the site along with copies of my calculations.
Also attached are copies of pages form the Old Town Report showing percent
imperviousness used for the area.
Please call if you need additional information.
Sincerely,
Phillip I. Robinson
a
4502
Vice President
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/r sc James H. Stewart
Associates, Inc.
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and
103 S. Meldrum Street
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enclosures P.O. Box429
Ft. Collins, CO 80522
303/482-9331
Fax 303/482-9382
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Table 4.3.
Differences in Impervious Area Percentages for
Existing
and Future Conditions.
Percent; Impervious
Number:
: x
E isting
......... ...
tum
6
40
70
102
50
70
108
40
60
109
40
60
201
40
60
203
50,
70
204
60
70
205
50
70
206
40
60
211
40
60
304
40
60
305
40
60
312
40
50
360
1 20
40
361
60
100
ry o
r'eG S 2-0 tv 2)l I.
¢O 7 S. Gra,,J06,1eJ 1,i cAf—e-c— 207
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