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

Reaches 4 and 5 are primarily agricultural land with isolated development.
A multi-use detention facility with an overflow section can effectively detain
and transmit flows through the Larkborough Subdivision at the lower end of these
reaches. Drainage crossing at Shields Street and at Pleasant Valley and La ke
Canal as well as a grass-lined channel throughout Reaches 4 and 5 will be a re-
sult of future development. Benefits of uninterrupted traffic flows, water qua-
lity and aesthetics will be realized by these improvements.

Interpretation and Use
TheZ-, 5-, 10-, Z5-, 50-, and 100-Yearfloodevents were used as th eflood
frequencies for this floodplain analysis. These various flood events have an
average occurrence of one in the number of years as indicated. Forexample,the
lOO-year flood occurs, on the average, once a 100- year period, and has a one (1%)
percent chance of being equaled or exceeded in any given year.
The particular uses for the various flood events in addition to those
stated above are as follows:

Reach 6 is at the present all agricultural except for Reach 6C which is
being developed for residential and mu1'ti-family uses. The drainage improvements
which are proposed as a part of this development can adequately pass the lOO-year
design flows. A box culvert under Timberline Road at the lower limit of Reach SA
effectively passes the lCD-year storm event. A pipe culvert jacked under the
Union Pacific Railroad is highly beneficial improvement because of the inadequacy
of the existing pipe to pass flows equal to the 10.year storm and greater. Major
interruptions to railway service could occur if the existing culvert is not en-
larged. A grass-lined channel with varying capacity will be constructed from
Timberl1neRoad to Hannony Road once development occurs in this area. Floodplain
management will be an effective means to control the layout and scheduling of
improvements to provide continuity in this reach.

2-Yearthrough50-YearFloodEvents_
lnformation regarding these lower frequency floods is especially useful for
future engineering studies and land use planning purposes related to minor road
systems, minor channel impro~ements, the location of parks and recreational faci-
lities, agricultural lands, and appurtenant structures. For structures and use
of this type of the smaller tributaries and in areas where the high risk of struc-
tural failure is economically feasible and the hazard to life and property non-
existent, the use of the lower frequency floods may be considered.
100-YearFloodEvent_
The 100-Year flood event may also be used for engineering design purposes
where a lower risk of failure than the 10- or 50-year flood is desired. However,
the most important use of the lOa-year flood e~ent lies in floodplain designation
and land use reoulation as set forth in the state statutes. The State of Colorado
considers the lOo-year frequency flOOd as the flood event to be used in designing
and protecting structures and dwellings for human occupation. Therefore, all flood-
plain regulations are based upon the lOO-year flood.

Benefitjcostcalculations fortherecOOlllended plan are listed in Ta blel-1.
In addition to reducing flood damage, benefits of the overall plan are that of
providing park and recreation opportunities in the floodplain, enhanced land
values and improved visual impact of the floodplain, improvement of water quality,
preservation of the floodplain ecology, and reduced potential for public incon-
venience. Also, several improvement schemes have a benefit/cost ratio greater than
one, thus effectively utilizing public funds.

Mappinq and SurveyS
Topographic mapping for this study was furnished by the City of Fort Collins
and Larimer County. The scale is 1" ~ 100', and the contour interval is 2-feet.
All mapping north of Harmony Road is orthophotography of quarter sections. South
of Harmony Road. the topography was taken from the manuscript prepared for the
Corps of Engineers' Fossil Creek flood study, verified by field surveys.
FlOOdplain information prepared for this report is presented on this topographic
mapping at a scale of 1" ~ 100', and copies of these flood plain maps are avail-
able from the City of Fort Collins or from Larimer County. The floodplain maps
printed in this report are reduced to a scale of 1 n ~ 200' for eaSY presentation.
Field surveys were completed by !ntermill land Surveying to supplement the
topographic mapping as required. These cross.sections were taken at stream cros.
sing structures, at locations where the channel was obscured by trees or foliage,
and at other locations where elevations were critical to the hydraulic computations.
In general, field information was consistent with the topographic mapping.

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