HomeMy WebLinkAboutDrainage Reports - 08/22/1977James. H. Burt and Associates, Inc!
Consniting Engineers
August 22, 1977
DRAINAGE REPORT
FOOTHILLS SQUARE
FORT COLLINS, COLORADO
Foothills Square is a proposed shopping center on 16.1 acres at the
Northeast corner of College Avenue and Horsetooth Road at the present
southern limits of the City of Fort Collins, Colorado.. This report is of
the engineering investigation procedure, results of calculations and
expected hydrologic and hydraulic performance, of the watershed and the
proposed facilities.
There is concern on the part of the irrigation company owning Warren
Lake into which the runoff from this site will enter as expressed in a
letter from their attorney, William Stover, to the City of Fort Collins
recently. The City of Fort Collins requires detention on all developments
with their criteria derived from the "Report on Stormwater Drainage
Facilities For Fort Collins, Colorado" as prepared by Black & Veatch (B&V)
in 1971. That report was used in the development of this study as was more
recent hydrologic and meteorologic data for the City of Fort Collins as.
contained in the "Preliminary Design Report, Rolland W. Moore Memorial Park,
Fort Collins, Colorado" by Land and Water Consultants (L&W) in 1976. The
main building of the proposed Foothills Square Shopping Center lies directly
in the thalweg of its sub -watershed. The developers of Foothills Square,
HCI, are working in conjunction with Elmer Brune, the owner of the abutting
property (east of the proposed Plaza Drive) along the north side of Horse -
tooth Road.
During the development of the Rolland W. Moore Memorial Park by L&W it
was found using the then recently available data that the 100-year return
frequency meteorologic conditions approximated the 50-yearoresults of the
earlier B&V study. In the development of this work, we placed more emphasis
on the later L&W results as they are more current and more familiar to us.
The City's criteria for detention storage is based on the B&V 50-year data
which we feel would be somewhat comparable to the L&W 100-year results.
OFFICE-214NORTH HOWES • P.O.ROx429 • FORT COLLINS. COLORADO 80522 • TFIXTHO.NE AREA 70y1d2.93:11
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Upstream from the Foothills Square site are such manmade barriers to
natural drainage as College.Avenue, the Larimer County No. 2 Irrigation
Canal, the Colorado & Southern Railway, and the New Mercer Irrigation Canal.
The Black & Veatch study visualized all these manmade barriers being
breached in their 50-year runoff calculations. Immediately upstream from
the site is College Avenue. There is a raised (curbed) median along
College Avenue. The general slope of College Avenue is to the south and
the pavement elevation at the center of College Avenue at the south end of
the raised median on the north side of Horsetooth Road is elevation 5031.9.
Further upstream the top -of the east bank of the Latimer No. 2 Canal at its
low point is elevation 5030.8,,65 ft. south of the centerline of Horsetooth
Road. Continuing upstream in this sub -watershed there is a 24" corrugated
metal pipe culvert 30 ft. north of the centerline of Horsetooth Road under
the C&S Railroad. The invert of that culvert is 5034.2. The low point of
the railroad fill is located 103 ft. north of the centerline of Horsetooth
Road. The top of the ties at this point are at elevation 5037.7. Continuing
upstream to the last manmade barrier to natural drainage is the New Mercer
Canal. The low point on the top of the east bank of this canal is 40 ft.
north of the centerline of Horsetooth Road with an elevation of 5040.0.
For the purposes of this study of Foothills Square itself, we
visualize these natural barriers being breached during the 100-year
occurence and the entire sub -watershed activated in its natural condition
as outlined on Sheet 1/4, 8-12-77. This we have referred to as Watershed
No. 1, containing 172 acres with point of concentration at Horsetooth Road
in the natural drainage northwest of the northwest corner of Warren Lake.
Sheet 2/4, 8-12-77, shows the downstream portion of the watershed constrained
by the Larimer County No. 2 Irrigation Canal as it might function during
the 2-year runoff occurence. This Sheet 2/4 shows the smaller portions of
the sub -watershed such as No. 3 (22 acres) which lies upstream from the
east boundary of Foothills Square and No. 5 (19 acres) lying downstream
from that point, both of these combined total 41 acres and are referenced
as Watershed No. 2. The sub -watershed No. 5 represents the Brune property
lying north of Horsetooth Road and south of the north divide of this overall
sub -watershed under consideration here. Watershed No. 4 comprising 16.1
acres is the Foothills Square tract itself as shown on Sheet 4/4, 8-12-77.
3 -
That portion of the sub -watershed being considered in this work lying north
of the old Dixon Lateral has previously been included in the drainage
considerations for.the Foothills Mall Shopping Center and Soutbmoor
Subdivisions.
Following discussions with the Planning Department of the City of Fort
Collins, we considered the future zoning for this area as highway business
one -quarter mile east and one -quarter mile west of College Avenue and for
the remaining portions of the watershed east and west of those limits as
dense residential. For calculation purposes, we used a percent impervious
factor of 10% for existing natural ground conditions, 50; for high density
residential, and 80% for the shopping center. Utilizing the Colorado Urban
Hydrograph procedure,outflow hydrographs were prepared for sub -divides of
this sub -watershed for the 2-year, 100-year, and probable maximum precipi-
tation (PMP) conditions for various stages of development as summarized on
Sheet 2/12, 8-16-77.
Dentention basin storage volume requirements were determined by the
difference in the volume of runoff for a 100-year situation between
existing conditions (natural ground) and expected future full development.
One detention basin was sized to cover both the HCI 16.1 acre Foothills
Square and the 19 acres of the Brune property within this sub -watershed.
The Brune property is. -for the purposes of this study considered to be high
density residential once fully developed. The volume then is the difference
between 3.6 acre feet of runoff for Foothills Square when fully developed
and 2.6 acre feet in its present condition, or, 1.0 acre feet The volume
of storage provided for the Brune 19 acres, is the difference between 3.7
acre feet of runoff when fully developed and 3.1 acre feet of runoff in its
present condition, or, 0.6 acre feet. The total required storage is 1.6
acre feet.
A detention basin is planned along the north side of'Horsetooth Road
on the existing thalweg of this sub -watershed as shown on Sheet 2/3,
8-16-77. This basin considers the use of the existing 15" corrugated metal
pipe culvert across Horsetooth Road as part of its outlet work. In addition
to that culvert, an additional 12" reinforced concrete pipe culvert is
proposed at a lower elevation with the invert of the inlet of the 12"
precast end section at elevation 4992.8. This elevation is headwater
elevation equal 0 in computations of..routing through this basin. The
drainage from Foothills Square in the basin is along the natural ground
from the west. A 1 ft. ditch is proposed along the southern limits of the
Brune property from the high point of the sub -watershed to the east. This
ditch is to collect water flowing south on this parcel and divert it into
the detention basin facilities. It is our understanding that the City
intends to maintain the present gradient.of Horsetooth Road. The pavement
of Horsetooth Road then is proposed as the overflow spillway of this
detention facility.
The previous highwater mark that was found at Warren Lake was
elevation 4990.4. The crest of the earthen dam at Warren Lake is at
approximately elevation 4992.8. A V-notch in the crest of the concrete
section of the dam containing the release gate valve has an elevation of
4991.3 in the bottom of the notch. The elevation of the proposed 12"
reinforced concrete pipe culvert outlet works from the Foothills Square/
Brune drainage detention basin has been set so as to not be affected by
backwater from Warren Lake.
Several of the previously determined sub -watershed outflow hydrographs
were routed through the detention facility with the results as follows:
Watershed
No.
Area
Condition
Runoff
Fre .
Peak
Disch.
c.f.s.
HW
Ft.
Stor.
Vol.
Ac. Ft.
.1
172 Ac.
Fully Developed
P.M.P.:
2227
6.77
4.3
1
172 Ac-.
Fully Developed
100-yr.
506
5.74
2.9
2
41 Ac.
Fully Developed
100-yr.
167
5.3
2.4
2
41 Ac.
Fully Developed
2-yr.
9.82
4.45
1.6
2
41 Ac.
Exist. Condition
2-yr.
4.47
3.07
0.7
This report assumes the upstream barriers to be breached during the fully
developed conditions for the 100-year runoff. The Watershed No. 2, 41 acre
fully developed routing for the 100-year storm above was prepared tb see
what might happen if these barriers held. The routing calculation showed
the spillway to be activated under those conditions for a period of about
10 minutes (peaking at 167 c.f.s.) with the remaining time of the runoff
event occuring at water elevations below a headwater of 4.7 which is the
10
• _ 5
pavement of Horsetooth Road (the spillway crest). As a comparison, note .
the 2-year, 41 acres, existing condition, peak outflow of 12 c.f.s. from
Sheet 2/12, 8-16-77,.against the 41 acres fully developed 2-year routed
flow above. Here the desired 1.6 acre feet of storage is obtained all
below the spillway elevation and the peak discharge of the culverts is less
than the 12 c.f.s..
A subdivision called The Landings is proposed on the south side of
Horsetooth Road opposite the detention basin location for this project.
That Landings project contemplates the installation of sanitary sewers in
the vicinity of the outlet of the culverts from the Foothills Square/Brune
detention basin. The plans as presently drawn show the sanitary sewer for,
The Landings project to be encased in concrete at the vicinity at the
crossing of our 12" reinforced concrete pipe outlet culvert. There would
be a grade conflict in that case. We have discussed the matter with the
engineer for the developer of The Landings. Our suggestion was that in
lieu of concrete encasement, a section of ductile iron pipe be used with
lesser wall thickness to facilitate clearance between the two lines. In
addition, The Landings project contemplates the widening of the south half
of Horsetooth Road. As the existing 15" corrugated metal pipe culvert
under Horsetooth Road is a part of the total hydraulic configuration of the
/Brune
outlet works for the Foothills Square detention basin, it would be necessary
to extend that culvert on its present gradient to outlet into the natural
drainage swale.
The peak outflow at the east side of the proposed Foothills Square
shopping center at Plaza Drive is 37 c.f.s. for the fully developed 2-year
runoff from the 22 acres of the confined sub -watershed above that point as
shown on Sheet 2/12, 8-16-77. Runoff from the buildings and pavement of
Foothills Square is proposed to flow overland to a low point at about the
existing thalweg on the west side of the proposed Plaza Drive. Undulations
in the finished grade of the parking area will be "blotted up" by area
drain inlet structures flowing to a small diameter storm sewer discharging
J
at the low point on Plaza Drive where a drop inlet structure is proposed
to feed a 30" reinforced pipe discharging into the thalweg on the east
side of Plaza Drive out through a precast end section with an outlet invert
elevation of 5005. The expected headwater elevation at the drop inlet
- 6 ^ is
to the culvert to carry the design flow is 5009.2. The flow line of the
Plaza Drive gutter section is proposed to be at elevation 5012 at this
point and the grating over the W' diameter drop inlet at elevation 5011.
All excess flows over the design amount are proposed to run over the
Plaza Drive pavement and continue in their natural path.
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