FLOOD02961

• County: Weld
• Community: Severance
• Basin: South Platte
• Title: FIS - Severance
• Date: 9/22/1999
• Prepared For: Severance
• Prepared By: FEMA
• Floodplain - Doc Type: Current FEMA Regulatory Floodplain Information

2.2 Community Description

The Town of Severance is located in Weld County, in the north-central portion of Colorado,
The average annual precipitation over the study basin is approximately 12 inches, of which
approximately 60 percent occurs during the period from May through September. Mean
temperatures range from a high of 73 OF in July to a low of 24 OF in January, with an annual
mean temperature of 48 of, The basin is predominantly agricultural, with scattered
cottonwood ttees along the stream bottoms. Native vegetation present is primarily sagebrush
and prairie grasses (Reference 1), Scattered urbanization is occurring in the basin, According
to the U.S. Bureau of the Census, the 1990 population of the Town of Severance was 106
(Reference 2),

2.3 Principal Flood Problems

Little flood history is available for The Slough in the Town of Severance. Interviews with
local residents indicate the only floods of any consequence occurred during the summers of
1971 or 1972, 1975, and 1977. The flood of either 1971 or 1972 resulted from
approximately 4 inches of rainfall in the basin upstream from the Town of Severance. The
plugged culvert at the Great Western Railroad south of the Town of Severance aggravated the
flooding, Flood damages were mostly to crops. The floodwater went over County Road 74
in both 1975 and 1977 according to local authorities. It was also stated that the culvert
beneath County Road 74 was plugged and debris forced against the fence caused backwater
in this area until the debris was removed from the fence (Reference 1).

Flooding in the Town of Severance area from The Slough has not been frequent, probably
due to the irrigation development in the basin consisting of cauals and reservoirs. Systems
such as these have a large effect on floods having a short recurrence interval, but have little
effect on floods with a longer recurrence interval such as the l00-year flood. A false sense
of security occurs in such situations (Reference 1).

2.4 Flood Protection Measures

There are no known flood-protection measures in place in the Town of Severance,

3,0 ENGINF.F.RlNG MEmODS

For the flooding sources studied by detailed methods in the community, standard hydrologic and
hydraulic study methods were used to determine the flood-hazard data required for this study, Flood
events of a magnitude that are expected to be equaled or exceeded once on the average during any 10-,
50-, 100-, or 500-year period (recurrence interval) have been selected as having special significance
for floodplain management and for flood insurance rates. These events, commonly termed the 10-,
50-, 100-, and 500-year floods, have a 10-, 2-, 1-, and 0.2-percent chance, respectively, of being
equaled or exceeded during any year, Although the recurrence interval represents the long-term,
averat:e period between floods of a specific magnitude, rare floods could occur at short intervals or
even wilhin the same year. 1he risk of experiencing a rare flood increases when periods greater than
1 year are considered. For example, the risk of having a flood that equals or exceeds the lOO-year
flood (I-percent chance of annual exceedence) in any 5O-year period is approximately 40 percent (4
in 10); for any 9O-year period, the risk increases to approximately 60 percent (6 in 10). The analyses
reported herein reflect flooding potentials based on conditions existing in the community at the time
of completion of this study. Maps and flood elevations will be amended periodically to reflect future
changes.

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