FLOOD06672

• County: Logan
• Community: Crook
• Stream Name: South Platte River
• Basin: South Platte
• Title: South Platte River at Crook
• Date: 10/1/1993
• Prepared For: Crook
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Floodplain Report/Masterplan

floats so as not 1:0 interfE!re with their operation.

5.2.2 Discussion of the Ufle of flood warning times
5.2.2.1 Use of Alarms
Tables 3 and 4 are provided to give the flood coordinator
a general idea of how thu sys'tl!m will perform in floods of
different sizes. The times given in the tables are estimates.
They were developed using a hydrologic model of the basin and
hypothetical balanced floods on the South Platte River.

The information in Tables 3: and 4 should be interpreted to
mean the followin~Jl

1. The sounding of the upper alarm in sterling is a
signal that, a significant flood wave is moving
downstruam toward the Crook area.
2. If the 1ilpper alarm was sounded only 0.4 hours after
the low.ar (first) alarlD, then the flood iEI around a
500-year event. The flood wave should reach the
height of the Highwa:y 55 closure structure within
about 1:1.5 hours" The structure should be closed by
then but if the flood is in fact a 500-year event,
then significant flooding will occur in Crook
regardless of the structure being in place. It is
understclod that tohe fU.t'ilre levee will not be designed
for a 500-year event.
3. If the 1ilpper ala,rm was sounded 0.6 hours after the
lower alarm, thEm th-e flood wave should reach the
height of the Highwa:y 55 closure structure within
about 18.4 hours; this corresponds to a 100-year
event.
4. If the upper alelrm i:l. sounded 0.8 hours after the
lower alarm, thEm th,5! flood wave should reach the
height of the closure structure within about 20.0
hours, ~,hich is equival'5!nt to a 50-year event.
5. If the lilpper alarm was sounded 3.8 hours after the
lower al.arm, then the flood wave may never reach the
height of the closure structure, using an invert
elevation of 3711.8 feet, msl. The highway itself
would be overtopped. Visual observations at the
Sterling flood warning gage would be required to
determine the max:imUlD flood peak moving downstream.

All of the above results are based upon HEC-1, a
mathematical model, and a number of assumptions. It is
recommended that a more simple approach be used in the early
phases of the Crook flood warning program until such time as
more confidence can be plclced illl the HEC-l results and until
more experience iSI gained with rei!l.-life floods. Specifically,
work should begin immediately on closing the Highway 55
structure in Croo~: anytime a valic1 upper alarm is received from
the Sterling gage. The structure should be in place (or ready
to be closed) within 10 hours from when the upper alarm is
received. The warning time of 10 hours lis based on the average

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