FLOOD08945

• County: Larimer
• Community: Fort Collins
• Stream Name: Dry Creek
• Basin: South Platte
• Title: Hydrologic Analysis Major Drainageway Planning Dry Creek
• Date: 7/1/1979
• Prepared For: Larimer County
• Prepared By: Gingery Associates, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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system on the hydrology of the basin, and all were helpful in pro-
viding information on the operation of their canals and reservoirs.
Information pertinent to this study was that in most cases the
irrigation canals have a normal operating capacity, defined by their
water rights, and a greater bankful capacity. The difference between
the bankful and normal discharges is the capacity generally available
to intercept drainage flows during operation season, which coincides
with the cloudburst flood season. The irrigation canals most likely
to affect flood flows along Dry Creek are the Larimer and Weld Canal
(Eaton Ditch), the Larimer County Canal, Terry Inlet, the poudre
Valley Canal, and the North Poudre Ditch. Only the canals down-
stream of Douglas Reservoir, that is, the Larimer and Weld Canal,
the Larimer County Canal and the Terry Inlet, would contribute to
a reduction in peak flows in Fort Collins.
The approximate normal operating and bankful discharges for
these canals are shown in Table I.

TABLE I
Approximate Ditch Capacities

Normal Bankful Flood Interception
Canal Capacity Capacity Capacity
Larimer and Weld 700 cfs 1,000 cfs 300 cfs
Terry Inlet 100 cfs 200 cfs 100 cfs
Larimer County 575 cfs 700 cfs 125 cfs

The Irrigation Ditch Companies also stressed that in the event
of flooding within the Dry Creek Basin, the canal systems would be
shut down so that flood flows could be intercepted and peaks reduced
downstream. Good information was available on the response time of
the ditches at Dry Creek, which would be the time required for the
ditch to become noncontributing. This concept will receive further
analysis later in the Master Drainageway Planning Study; however, a
flood warning system utilizing these ditches could significantly
reduce the potential for flooding at Fort Collins, if the integrity
of the system could be insured. Without a warning system, however,
timely and proper shutdown of these canals should not be relied upon
to reduce potential flooding downstream. "Canal shutdown" was not
considered in this analysis.

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