HomeMy WebLinkAboutSADDLEBACK SUBDIVISION, PHASES II, III & IV - COUNTY REFERRAL - 55-88, A - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTf
•
•
PRELIMINARY DRAINAGE REPORT
for
SADDLEBACK PHASE II. III & IV
LARIMER COUNTY,
COLORADO
May 18. 1988
Submitted by:
NORTHERN ENGINEERING SERVICES, INC.
418 S. HOWES, SUITE D
FORT COLLINS, COLORADO 80521
(303) 221-4158
Project Number 8812.00
s
I. GENERAL
PRELIMINARY DRAINAGE REPORT
for
SADDLEBACK PHASE II, III & IV
This report summarizes the methods and results of a
stormwater runoff study for a proposed 35-acre residential
subdivision in Larimer County, northwest of Fort Collins.
Methods presented in Larimer County's "Storm Water Management
Manual" have been followed in performing the calculations for
this report. These calculations are contained in the appendix to
this report.
• The rational method has been used to determine stormwater
runoff on and adjacent to the project site. Detention volume has
been calculated using a runoff hydrograph produced from the
rational method peak flow and time to peak.
II. SITE DESCRIPTION
The proposed phases of Saddleback Subdivision are on 35
acres located in the east half of section 4. Township 7 North.
Range 69 West of the 6th Principal Meridian, in Larimer County,
Colorado. It is situated west of Taft Hill Road, approximately
one half mile north of Vine Drive.
The proposed site is currently in pasture land which slopes
gently down from west to east at slopes from 0.5X to
approximately 3.5% with a steeper ridge in the northeast portion
of the site. The entire site drains to a low area at the north -
central portion of the project. Ground cover is mostly dry -land
grasses. A small tree nursery has been planted in the southwest
portion of the site to provide landscaping for the project as it
develops.
The site is bounded on the north and east by the New Mercer
• Canal, to the south by undeveloped land in light agricultural use
and to the west by Lin -Mar Subdivision and by agricultural land.
•
III. PROPOSED LAND USE
The project site is proposed to be divided into 71 single-
family residential lots with a detention pond to be located in
the low area to the north. (See attached Drainage Plan.)
Stormwater runoff will be conveyed through the site in road -side
swales with gutter pans or storm sewers at street crossings.
IV. HISTORIC CONDITIONS
Stormwater runoff from the catchment which includes the
proposed site currently drains to the low area near the center of
the project. During storm events of approximately five-year or
greater return period, this low area fills up and over -flows into
the New Mercer Ditch. The low area has a capacity of
approximately 146.031 cubic feet before over -flow occurs, while
the two- and one hundred -year runoff volumes are 109.275 and
347.540 cubic feet. respectively.
• Off -site flows reaching the site are mostly from the
undeveloped area to the west and from a portion of Lin -Mar
subdivision, also to the west. Runoff from areas south of the
site either flow directly east into the New Mercer Ditch or are
intercepted by an existing irrigation lateral along the southern
boundary of the site.
V. DEVELOPED CONDITIONS
After the site has been completely developed. runoff will
still be conveyed to the low area discussed above, which will be
graded to act as a detention pond for the project. Since this
pond will be too low to gravity -flow into the New Mercer Canal,
it is proposed that stormwater will be released, instead, into
the Larimer No. 2 Canal to the north. which is at a lower
elevation. The proposed detention pond has been sized to
accommodate the entire 100-year runoff from the catchment area
with no release. A headgate would be provided in the detention
pond which releases into a pipe to convey the detained runoff
into the Larimer No. 2 Canal. The canal company could then
release stormwater from the pond at a time and rate that would
fit with their ability to take the extra water.
•
•
Street sections are being proposed which would match that
used in Saddleback First Filing. This section has 36 feet of
pavement at one side of the Right -Of -Way and an 8-foot wide
concrete walk on the other. Between the two would be a ten foot
wide Swale to carry stormwater runoff. Street crossings will
carry flow either in gutter pans or storm sewers, as shown on the
drainage plan. As in the first phase of Saddleback. no driveway
culverts will be allowed in the swales and drives will "dip'•
through the swale to allow continuous flow.
Due to the proximity to the New Mercer Canal, all lots will
be raised to a minimum of 5061 feet to be above the potential
water level should the ditch over -flow and flood the area.
•
Michael F. Jones. P.E.
Colorado Licence 25033
•
•
APPENDIX
•
�J
IS"o1Zt G �W No �
• 1. UMoi=F COeFF`c-1.``4
u l.tN - t�lA
A1'LG� i S51
i
o
-
d)
s
- -- -- --
-
- ---
o
,
J
L . { l M C dF _. _ GoN GE h-� "�fZA. �._� D �-1 --
-- - •
_.-_-
- _
Lc = 24,Z
T
t
•
i e
_
1 ,,
1
7
i
- 2U N o
►-i E �.
�-
3 4-7, Soo
+
V O l.. o F' . ZY-(S —P w �
C.9 K1 .
to ►ZZra ; to
F- -
4
I
1. C_..0NOFF Cam=T=
S�rz,�-5
Vc,t-Rac„►c5 ; 3� 5�:� c.l= ?� ��'� ' =�'
-3. $$ Ae _ - -
P�t-D3� DTL? Va) Wa6dlcS ; ? f Y. z oo=?
:, S, _
11 C�
a
W 3 a i Na 7
7 13.4
-- -_
.119
. G.S - - -
No Text
C- LA ec-14- zo% A of =
'a- (,. ") = o.oZS
1,4
I*
S