FLOOD09485

• Designation Number: 206
• County: Larimer
• Community: Fort Collins
• Stream Name: Fossil Creek Drainage Basin
• Basin: South Platte
• Title: Master Drainageway Planning - Fossil Creek Drainage Basin, Fort Collins, Larimer County, Colorado
• Date: 8/1/1982
• Designation Date: 1/1/1983
• Floodplain - Doc Type: Floodplain Report/Masterplan

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IV. FLOOD PLAIN INFORMATION

The 100-year flood plains for existing and developed conditions were

delineated on the topographic maps using the results from the water-surface

profiles. The laO-year flood occurs, on the average, once in 100 years, or

has a one percent chance of occurring in any given year.

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

Colorado Statutes. The State of Colorado considers the lOa-year flood to be

used in designing channels 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

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bill.

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4.1 Flooded Areas

The lOa-year flood plains for Fossil Creek, Stanton Creek, Lang Gulch,

Smith Creek and Burns Tributary for existing and developed conditions are

shown in Plates 76 through 104. (Plate 75 is the Index Sheet.) The dif-

ference between the flood plain elevations for existing and developed con-

ditions ranges from zero to two feet, with most differences being less than

one foot.

The flood plain along Fossil Creek ranged from about 2,000 feet in width

upstream of Trilby Road (County Road 34) to about 20 feet in the upstream

reach. The channel velocities ranged from about five feet per second (fps) to

ten. OVerbank velocities ranged from one fps to four fps except where ponded

conditions created velocities less than one fps.

The floou plain along Stanton Creek ranged from about 1,000 feet, due to

the backwater from the pond at Fossil Creek and the Union Pacific Railroad, to

20 feet. Channel velocities ranged from six fps to ten fps; overbank veloci-

ties were on the order of two to four fps. Velocities in ponded areas were

less than one fps.

The flood plain along Lang Gulch was variable, with top widths ranging

from 40 feet in the downstream portion to 600 feet in ponded reaches behind

Shields Street. Channel velocities varied from three to six fps, with over-

bank velocities and velocities in ponded areas being less than three fps and

one fps, respectively.

The flood plain for Smith Creek is almost 600 feet wide upstream of

Shields Street. In general the flood plain ranges in width from 100 to 200

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feet. Channel velocities are from three to six fps, with overbank velocities

between one and two fps.

Burns Tributary has a narrow flood plain with top widths varying from

about 40 feet to 200 feet upstream of Shields Street. Channel velocities

range from three to seven fps with velocities less than two fps in ponded

areas. Overbank flow was very limited in extent and velocities were generally

about one fps.

The laO-year frequency flood has been selected by the State of Colorado

as the flood event to be used for flood plain delineation and regulations.

Thus the Flooded Area plates show only the boundary of the lOa-year flood

plain. Since the base maps for the Flooded Area plates are unrectified aerial

photos or anlarged U.S. Geological Survey Quadrangle maps, the flooded area

limits on the plates should be considered approximate. When exact limits of

flooded areas are needed, the water surface profiles and/or tabulated eleva-

tion data (Table 3) should be used in conjunction with accurate topographic

maps of the area in question.

The area delineated on the Flooded Area plates as the lOa-year flood

plain meets the requirements of H.B. 1041 as an area of state interest. Also

upon official approval of this report by the Colorado Water Conservation Board

the area outlined by the lOa-year flood boundary will be designated as a flood

plain area and may be regulated accordingly by the local officials.

4.2 General use of Report

Flood elevations for 2-, 5-, 10-, 50-, 100-, and 500-year floods may be

found in two separate locations in this report. The Flood Elevation Data

Tables (Tables 10 through 19) list water surface elevations at selected

reference points. The flood profile plates (Plates 1 through 74) show the

water surface elevations for the 10-, 50-, 100- and 500-year floods.

The flood profiles may be used in areas where controversy arises over the

100-year flood boundary on the Flooded Area plates (Plates 76-104). Since the

Flood Profile plates give the elevations and distance or stationing from a

known point, the high water elevations can be surveyed on the ground to alle-

viate any discrepancies on the base map.

Elevation data for the City topographic mapping is documented in

reference 26. No floodways were developed along the channels because the

flood plain is narrow and the flow is generally confined to a valley channel.