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

SECTION I

5. Evaluate potential flooding hazard areas and associated damage s
within each of these floodplains.
7. Develop alternative improvement plans and preliminary designs
toaccoroodatetnelOO-yearstormrunoff.
8. Evaluate the economic consequences of the alternatives and develop
a cost-benefit analysis for use in selecting an improvement plan.
g. Prepare a final report documenting the results of the study.

INTROOUCTION

Purpose
The City of Fort Collins and surrounding portions of Larimer County have
experienced rapid growth in recent years resulting in increased drainage and
flooding problems. Local government officials are under pressure to manage this
urban development in a way whiCh will accomodate this growth while at the same
time protecting the health, safety and welfare of the area's citizens. Such a
situation now exists in the McClellands and Mail Creek Drainages of Fort Collins.
The Purpose of this study is to provide a means for making decisions re-
garding development and floodplain regulation in McClellands and Mail Creek
Drainageways. This purpose was accomplished by determining the flooded areas
associatedwithvariousfrequencystonlls. Once this was done, an economically
feasible and practical plan was fOnllulated for mitigating the drainage and flood-
ing problems. The infonllation in this report is intended for the use ~f state,
county and local governments involved in planning and land use reguh.tlons. It
is also intended for use by developers within the basin so that they will under-
stand the problems and solutions and thus be able to design and construct their
develo~nts in a manner whiCh will utilize the plan and facilities fully to the
benefit of all.

Summary of Study Findinqs
There is no way of predicting when a flood will occur in the Mail Creek
and McClellands Drainageways. Should a flood of even minor magnitude occur under
existing channel conditions, considerable damage to homes, businesses, railroads
and streets could result. Implementation of the recommended improvement plan
would prevent potential damage caused by floods greater than a 2-year event.
In selecting the recommended plan, a syst~ approach was used az well
as evaluating isolated improvements. We initially evaluated the cast (including
maintenance and operation) of a wide variety of improvements, and combinations
of these improvements, in order to find the most effective plan. We then added
to this, those major drainage improvements which are likely to be built when un-
improved ground is developed. Therefore, economically beneficial improvements
in conjunction with improvements likely to occur as a direct result of develop-
ment are part of the final plan. Continuity of the plan from reach-to-reach
was a major consideration, as were the environmental and aesthetic aspects of
the alternatives.
Flooding prOblems in Reach 1 occur only during intense storms because of
the well-defined channel. With peak flow reductions affected by detention in
Reach 2B, a~ ir.proved spillway at Fairway ~ar. is all that is ~ecessary to elir.i-
nate flood damage. Reach ZA and 26 exhibit extensive flood damage potential due
to the lack of a well-defined channel for even the Z-year storm flows. Because
of the limited amount of development in this area, a grass-lined channel with
box culvert crossings of Harmony Road. Crest Road and Nordic access road.will
effectively pass the 100.year storm event. The proposed upstream detentlon sys-
tem will allow a reduction in the size and construction costs of downstream im-
provements. Also, since the recommended detention is in an undeveloped area,
final location and configuration can be coordinated at the time ( if conve~ient)
of development and possibly be utilized as a multi-use facility. Grass.lined
chan~els betwee~ ir.prover.e~ts are likely to occur at ti~e o~ develo~ent wit~
final locations established in conjunction with the developer's plan.
Reach J, the New Mercer Canal, has the capacity to carry approximately the
25-year storm in addition to the ditch's irrigation flows. Peak flow reductions
due to detention in Reach 4 enables the canal to carry the lOa-year storm event
within its existing banks. An improved overflow spillway section in the existing
Larkborough detention pond safely passes the 100-year storm flows.

Scopeof'"ork
The scope of work defined for the study of McClellands and Mail Creek
Drainaoewavs follows the guidelines and specifications for basin studies prepared
by the'City of Fort Collins. Ingenera1,thestudyconsistsofthefol10win<j:
1. Research and collection of base data and information pertaining
to the study area. This includes contact with the local govern-
ment bodies, ditch company representatives and other public.and
private agencies along with comments from property owners wlthin
the basin limits.
2. Define the basins' characteristics by means of available topo-
graphical mapping as well as in-field investigation.
3. Prepare a flood history for the area.
4. Conduct a hydrologic analysis for determining runoff charac-
teristics and flow quantities associated with the 2-, 5., 10-,
25- 50- and 100-year storm events under both existing and fully
dev~loped conditions. AlsO a brief comparison to historic con.
ditions will be made.
5. Hydraulic routing of peak flows developed in item 4 above to
determine flooded areas by water surface profiles. Once deter-
mined. delineate the 100-year floodplain on available mapping.

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