FLOOD01901

• County: Larimer
• Community: Larimer County Unincorporated
• Title: Flood Insurance Study - Larimer County Unincorporated Areas, Volume 1 of 3
• Date: 3/23/1999
• Prepared For: Larimer County
• Prepared By: FEMA
• Floodplain - Doc Type: Historic FEMA Regulatory Floodplain Information

Numerous irrigation canals cross the Dry Creek channel and directly
intercept drainage flows, as mentioned previously. Several of
these canals, including the Larimer and Weld Canal (Eaton Ditch).
the Larimer County Canal. the Terry Inlet, the Poudre Valley Canal.
and the North Poudre Ditch. ~ave large enough flow capacity to
significantly affect flood magnitudes on Dry Creek. For the more
frequent flood events, the Larimer and Weld Canal (Eaton Ditch).
the Larimer County Canal, and the Terry Inlet were assumed to
intercept all Dry Creek flows reaching their channels
(Reference 1).

As development within the basin continues, these canals will
intercept greater flows on a more frequent basis.

This will increase the probability of sporadic overflows from the
canals, which can cause severe damage, particularly if an
embankment fails. For this reason, separation of drainage flows
and irrigation flows should be considered far at least the lower
frequency events (Reference 1).

There are no flood-control structures on the Cache La Poudre River
in the study area, although there are two irrigation dams on the
North Fork Cache La Poudre River. The effect of the two dams is
considered negligible in the study area. There are no flood-
control structures that reduce discharges on Buckhorn Creek,
Redstone Creek, Fish Creek, Black Canyon Creek, or the Fall River.

3.0 ENGINEERING METHODS

For the flooding sources studied in detail 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 which
are expected to be equaled or exceeded once on the average during any
10-, 50-, 100-, or SOO-year period (recurrence interval) have been
selected as having special significance for flood plain management and
for flood insurance rates. These events, commonly termed the 10-. 50-.
100-, and SOO-year floods, have a 10, 2, I, and 0.2 percent chance,
respectively, of being equaled Or exceeded during any year. Although
the recurrence interval represents the long term avera2e period between
floods of a specific magnitude, rare floods could occur at short
intervals or even within the same year. The risk of experiencing a rare
flood increases when periods greater than 1 year are considered. For
example, the risk of having a flood which equals or exceeds the IOO-year
flood (1 percent chance of annual exceedence) in any SO-year period is
approximately 40 percent (4 in 10), and, for any 90-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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