FLOOD05064

• Designation Number: 205
• County: Larimer
• Community: Fort Collins
• Basin: South Platte
• Title: Master Drainageway Planning - Cooper Slough, Boxelder Creek, Fort Collins, Larimer County, Colorado
• Date: 8/1/1981
• Designation Date: 1/1/1983
• Floodplain - Doc Type: Floodplain Report/Masterplan

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1" ~ 100', At the reaches where water surface elevations rise more than one-

half foot in the floodway, encroachment on the floodplain should be accom-

panied by increased channel capacity. The channel cross section should be

enlarged so that the, one-half foot rise in water surface elevation-criteria

can be met.

A detailed: description of the hydraulic analysis performed is included in

the "Technical Addendum" (reference 1S),

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IV. FLOODPLAIN INFORMATION

The 100- and SOO-year floodplains were delineated using the information

contained in the water surface profiles and the topographic maps of the study

area. These floods have an average occurrence of once in the number of years

indicated. For example, the 100-year flood occurs on the average of once in a

100-year period and has a one percent chance of occurring in any given year.

The 100-year flood is used for floodplain designation as set forth in

Colorado Statutes. The State of Colorado considers the 100-year recurrence

interval flood as the flood event to be used in designing and protecting

structures and dwellings for human occupation. Also, the area inundated by

the 100-year flood may be delineated as an area of State interest as set forth

in H.B. 1041 - the State's land use bill.

The SOO-year flood event is useful in making the public aware that flood

evente larger than the 100-year flood can and do occur. The SOO-year flood

boundary can also be used to limit high risk developments within the

floodplain.

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Flooded Areas

The Boxelder Creek floodplain for the 100- and SOO-year events and the

100- and SOO-year flood boundaries for Cooper Slough developed basin con-

ditions and 100-year flood boundaries for existing basin conditions for Cooper

Slough are shown in Plates 13 through 2S.

The 100-year flood for Boxelder Creek will result in flooded areas an

average of 380 feet wide. Channel velocities average about five feet per

second and overbank velocities about three feet per second. Channel depths

are about six feet on the average; overbank depths vary, but in general are

three feet or less. The flood flows are restricted by roads, canals, and the

Colorado and southern Railroad tracks in some locations which result in

floodplain widths over 1000 feet in the vicinity of these restrictions.

The Cooper Slough floodplain is different from Boxelder Creek as the

area is very flat and the channel capacity is limited in places resulting in

increased overbank flows and divided flow conditions. The floodplain averages

over 1000 feet in width with channel velocities from one-half to eight feet

per second. The depth of flooding is generally three feet or less except for

areas that are ponded. The flow is restricted at Vine Drive, the Colorado and

Southern Railroad, and Colorado Highway 14. The area south of the Colorado

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