FLOOD04704

• Designation Number: 212
• County: Larimer
• Community: Fort Collins
• Stream Name: McClellands and Mail Creek
• Basin: South Platte
• Title: Major Drainageway Planning - McClellands and Mail Creek
• Date: 12/1/1980
• Designation Date: 3/1/1983
• Floodplain - Doc Type: Floodplain Report/Masterplan

SECTlONIV

Flood Hazard Areas

HydraulicD-eterminations

The Mail Creek and McClellands Basins are presently in different stages of
useanddevelopmentvaryingfrOOlexistingagriculturaltofullydevelopedcondi-
tions. With the exception of Larimer County Voc-Tech Center and portions of the
basins adjacent to College Avenue, the developed reaches are in residential use.
Those portions of the basins which are most subject to the hazards of flooding are;
Reach No. 1 (Plates2,3and4)

Analyses of the hydraulic characteristics of Mall Creek and McClellands
Basins were performed to provide estimates of the water surface elevations to each
of the design floods for the existing channel conditions. A total of approximately
5.9 miles of channel was included in the analysis, with 3.6 miles in Mail Creek and
2.3 miles in McClel1ands Basin. All floodplains were delineated by routing the peak
flood flow through the channel and calculating the depth of flow in the channel and
adjacent floodplain. Thedepthofaflood,orwatersurfaceelevation, is directly
related to the conveyance characteristics of the channel and floodplains: channel
geometry, roughness, longitudinal slope, and presence of obstructions such as bridges,
houses and large trees.
Water surface profiles and floodplains were computed using the most recent
version of the U.S. Army Corps of Engineers HEC-2 computer program (Reference i3).
Cross.sections were digitized from available topographic maps and have been augmented
by field surveys at regularly spaced intervals and at Channel obstructions (culverts,
roads, etc.). Locations of the cross-sections are depicted on the existing fl ood-
plain plates with a more detailed description contained within the Technical Addendum.
Cross.sections that were field surveyed are noted with an asterick (*) on the flooded
area plates.
Channel roughness coefficients (Manning's "n"values) for these computations
were assigned on the basis of fie~ inspection of the floodplain areas (tabulated in
the Technical Addendum). Hydraulic losses through culverts have been determined with
the use of nomographs published by the Bureau of Reclamation and input into HEC.2
program. In developing rating curves for the culverts in the study reache s,zero
blockage was assumed for culverts greater than 2.feet in diameter. The normal bridge
routing of the HEC-Z program was used to calculate losses through non.standard cul-
verts. The structural integrity of existing culverts, bridges, detention dams and
other Channel features has not been evaluated in this analysis, except FainolayDam.
Fairway Dam has been re.evaluated by the State Engineer's office. The ori.
ginal design of Fairway Dam was based on a significantly smaller drainage area and a
lower intensity of development whiCh indicated the dam, spillway and culverts under
Palmer Drive were sufficient to handle the flood flows. However, present re-evalua.
tion indicates a need for further study and probably structural improvements to acco-
modate larger flood flows whiCh have resulted from new development in the contributing
drainage area. The enlarged basin is the result of the diversion of flows in the
upper portion of the historic McClel1ands Basin at the C & S Railroad into and
through the New Mercer DitCh to Mail Creek.
Computed water surface profiles and the floodplain delineation for the 100-
year existing and future discharges are shown on the existing floodplain plates for
designated reaches at 1" = 200' scale. These plates a~ at the end of this section.
Floodplains for the 2., 10-, 25., and 50.year events have been plotted on working
drawings and are included in the Technical Addendum. Water surface elevations for
these storms have also been tabulated on Tables IV-l and IV-2.

Mail Creek drainageway within Fairway Estates Subdivision, although deep
and subject to some erosion, does not pose a significant floodlng ha~ard. For the
most part, the runoff from the 100-year storm is contained within the existing
channel and sufficient right-of-way for flood flows was provided at the time of
deve~opment. There are two (2) exceptions to the containment of the 100-year run-
off 1n this reach. One 1S at the Passway Drive crossing of Mail Creek. The insuf-
ficient culvert capacity under Passway Drive causes a backwater condition which
results in water overtopping the road and some flooding of adjacent residential
properties. The other exception is at Palmer Drive, where the box culverts whiCh
are part of the Fairway Dam spillway do not have enough capacity for more than the
25-yearstorm.

Reach No.2 (Plates 4, 5 and 6)

Both the access road to Nordic Construction Company and Crest Road in the
lower end of Reach No.2 are overtopped during even small storms causing erosion
and the potential for loss of access. In addition, the existing house in between
these.tworoadsisthreatenedbythelOO-yearflood. Eventhoughthe100-year
flow 1S below the first flood elevation, backwater behind the Nordic Construction
Company access road and the C & S Railroad may cause structural damages to the
building.

Prior to the construction of the imprOVelnellts for Theioioodldlld sPUDdnddl
the time of initiation of this study, Harmony Road from about 500.feet west of
Crest Road to Shields Street was subject to inundation during the 100.year runoff.
Three of Larimer County Voc-Tech Center's buildings and their contents have asso-
ciated damages from this major storm as well. The drainage swale parallel to
Harmony Road which has been constructed by Woodlands PUD was designed to contain
the loa-year flows, but the structures for crossings over this swale and the
crossing at Harmony Road were designed for the 2-year historic runoff.
All four (4) legs of the intersection of Shields Street and Harmony Road
ar~ overtopped dUrlng even th~ minor storm Cdusing the potentidl for serious in-
terruption to traffic in this part of Fort Collins. Upstream of Shields Street,
there is a wide floodplain resulting from the damming effect of this intersection.
A little further west, the Pleasant Valley and Lake Canal is overtopped causing
damage to ditch banks and the potential introduction of silt and excess water into
the irrigatjon 5yste~.

IV-l

lV-2