HomeMy WebLinkAboutDrainage Reports - 01/12/2000Apr-20-99 05:05P Northern Engineering 970 221 4159 P-01
`,.
M
April 19, 1998
Mr. Basil Hamdan
City of Fort Collins Stormwater Utility
Development Review Division
235 Mathews
Fort Collins, Colorado 80522-0580
RE: Harmony Safeway Marketplace
Proposed Village Inn
Dear Basil,
ENGINEERING
S
MAYA'1999
As a part of the approved plan set for the Harmony Safeway Marketplace, there is a lot located
in the southwest corner of the property, designated as Lot 7. There is a Village Inn proposed for
this lot. This letter is intended to provide you with an overview of what the impacts development
of this lot will have on the existing drainage design, approved for the site.
As is typically the case with pad sites designed with this type of project, the ultimate user
generally likes to make minor changes to the approved plan. I have attached copies of the plan
which was the basis of our original runoff calculations, and what the proposed site modifications
include. As you can see, the changes they are requesting are minor in nature, and the original
intent of our engineering and drainage design has remained in tact. Our original runoff design
calculations did account for areas of imperviousness at these location. The differences are listed
below.
Attached is a copy of a portion of the drainage basins that were associated with the Harmony
Safeway Marketplace site. As you can see Lot 7 is partially located within basins N-2, NA, N-5
and S-2. I have also attached copies of the proposed improvements to Lot 7, which served as the
basis of our original calculations with the impervious areas highlighted. Also attached is a copy
of the proposed improvements for Village Inn, with the impervious areas highlighted
To recap, subbasins designated with an N prefix were designed such that developed runoff
would be directed towards the northeast comer of the site to a proposed detention pond, and then
ultimately to the Golden Meadows Regional detention pond. Basins designated with a S prefix
were design to release into an open channel, through proposed culverts under McMurray and
Innovation Drive, and also ultimately to the Golden Meadows regional detention facility.
Subbasins with an S prefix were not designed to be detained on -site, should sufficient capacity
exist in the downstream conveyance system.
420 SOUTH HOWES, SUITE 202, FORT (O(EINS. (O(ORADO OOS21, (910) 221-41SO, FAX (9701 221-4159
Apr'-20-99 05:05P Northern Engineering 970 221 4159 P.02
I I
Those impervious areas of this project located within basins N-2, 4, and 5 are virtually identical,
and therefore will not have any impacts on the existing drainage conveyance systems, or the
existing detention pond located at the northeast corner of the site. There is however, a slight
increase in the impervious area located within basin S-2. This increase is approximately 4000
square feet, or approximately 0.09 acres.
I have attached a copy of the calculations for the composite runoff coefficient for basins S-2, as
approved, as well as the revised calculations. As you can see, are assumptions were for a
composite C value of 0.68 for basin S-2. The revised composite C value would be approximately
0.70. This increase would result in approximately 0.23 cfs increase for the 10 year event (7.92
cfs as opposed to 7.7 cfs in the approved study), and approximately 0.51 cfs increase in the 100
year event (17.31 cfs as opposed to 16.8 cfs).
If the proposed Village Inn site is also compared with the proposed Blockbuster Video and
Farmers Bank, the assumed composite C value is still greater than the proposed. Therefore, it is
my assumption that these proposed pad site configurations are generally in compliance with the
concepts approved in the original Harmony Safeway Marketplace study.
With final design, site grading for the revised site should comply with the original basin
boundary lines, and that the site is graded so as not to disturb the interception swale and culverts
to the south of the site. Downstream capacity should also be verified during the design process.
Please call if you have any questions regarding this project, or require additional information.
Sincerely,
I Q P
1 At X-19-
27362
Roger Curtiss P.E. - Northern Engineering Servir�s nc.
cc: Michael Davison - Wyatt & Associates
Apr-20-99 05:06P Northern Engineering 970 221 4159 P.03
No Text
No Text
Apv— 0-99 05:07P Northern Engineering
970 221 4159
P.06
U
7
�R.r-
El) In.
o
000
f/
�
§§
�/
o
2Q
¢
B
kA
tn &
$ƒ
a
as
B�
§7
M
kq
&ƒ
a
77
\
r32
dda
d
2\
�/
Q
■
2R$
�§
d
ddo
g
�2
3
eu
g
r
tnin
��l
§�
C5
22
a_
ou
cI
<
«
tn%n
cis all
�
¢�
Q
00
�
�
( ■
g
w ^
m%$
dew
L k
.
»
�
ƒ
�2\
/\k
S
m
I
�
t
��C�/2�\
�/
96