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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and Southern Railroad ha:i ~:Ieveral separate channels thr.tt are one foot or leBa

in depth with an average velocity of 1.5 feet per second. At State Highway

14, a ponded area will form due to an undersized culvert resulting in flood

flows overtoppirtg the highway. South of State Highway 14, two separate

floodplains are plotted. A channel that carries 300 cfs follows Cooper Slough

and the remainder of the flow spills over the highway and flows through the

commercial area to the south. The floodplain in this commercial area is

approximately 1300 feet wide with an average velocity of three feet per second

and average depths less than one foot. Near the confluence with Boxelder

Creek, the floodwaters will overtop the Lake Canal and flow between the Lake

Canal and the Cache la poudre Reservoir Inlet to Boxelder Creek. The flow

along the Cooper Slough channel flows south overtopping both the Lake Canal

and the Cache la poudre Reservoir Inlet and eventually ponding at Prospect

Street where it enters the Boxelder Creek floodplain.

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v . ~'LOOD DAMAGES

Damages due to flooding were estimated to establish a baseline condition

for economic analysis. Damages were estimated on a reach by reach basis for

both existing and future developed basin conditions using flood events from

future developed basin conditions. Only the damages for the existing basin

conditions were presented because the damages in dollars of future develop-

ments are difficult to predict. The reaches are defined in Section 3.2.

The damages caused by floods of 2-, 10-, 100-, and 500-year return

periods were estimated for each reach by property value. The floodplain asso-

ciated with each flood event was delineated and the indivldual floodplains

were divided into areas of equal flood depth. Velocities are relatively low

(less than three feet per second) in the floodplain and were considered to

have no significant effect on damages. The flooded areas were then measured

to determine the total area covered by each depth and land use for each flood

frequency .

Property values were estimated for each property in the Boxelder

Creek-Cooper Slough floodplains. Property values (in 1973 dollars) for com-

mercial property in the study area were obtained from the Larimer COunty

Assessor's Office. A six percent annual inflation factor was used to estimate

the current value of the land and improvements. Only one industry was located

in the study area. The appraised value was considered low so industrial land

was estimated to cost about $2.25 per square foot with improvem,mts valued at

$200,000 per acre for a total estimated value of $300,000 per acre. The value

of residential properties are based on $75,000 per dwelling. A mobile home

was valued at $35,000.

Flood damage to structures and contents were estimated by applying the

appropriate damage curve developed by the Federal Insurance Administration

(reference 1B). These curves give a percentage of total property or content

value damages versus flood depths above first floor elevations. Commercial,

residential and industrial buildings were assumed to have first floor eleva-

tions one foot above the ground surface. Mobile homes were assumed to have

the first floor two feet above the ground surface. Content value was esti-

mated at 50 percent of total property values based on practices of the

insurance industry.

Indirect damages that may result from flooding include lost business and

survices, lost time for employees, clean-up of debris and sediment, lost

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