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

2.2.2 Present.,tion of Travel Times
Travel tiDies :F.or discharges of 10-, 5(1-, 100- and SOO-year
magnitude are presfm.ted in Table 1.

TABLE 1.
TRAVEL TIME FOR HIGH FLOWS IN HOURS *

REF. POINTS

DIS'JrANCE
.ill} .!.mi)

10-YR

50-YR

100-YR

SOO-YR

Hwy 6 to
Hwy 55

15~.,OO(:1 29.3

14.4

14.4

12.7

9.4

· Travel tiJ~es are based on average fle:.w velocities of 3.0
feet/sec for J..O- and 50-year floods, 3.4 feet/sec for 100-
year floods and 4.6 feet/sec for SOb-year floods (see
Appendix J\). A flood wave moves dowrtstream at a faster
rate than the Clverage flow velocity. A rule of thumb is
that the flood wave will move 1. 5 ti:ales faster than the
average flow velocity in natural chamitels. The time for
the flood pea:k: to move from Sterling to Crook would then
be about 10 hours for the 10-and 50-year floods and less
for the IOO-and SOO-year floods.

2.3 Relationship Between Discharges at Sterling and Crook
2.3.1 Discussion elf. stage Relationships
A firm relationship between stage at t:he Highway 6 bridge
and the stage at the Highway 55 bridge carlI'lot be established.
Hydrograph volume as well as peak discha:r!1e enters into any
stage relationship between separate loc~tions on a river.
Hydrograph charactE,ristics are a function ()~ rainfall patterns
as well as bas in ':l.nd channel features. Since there are an
infinite number of storm patterns wh~ch can generate
hydrographs, a rel.,tionship between discharljes at two sites on
the South Platte River 29 miles apart would have to be
express,ed as statistical averages with apprclpriate standard
deviations.

Stage rela.tionships between sites involves an additional
level of abstract:i.on. The relationship would also have to
include the sit:e ri:l.t:ing curves, which are subject to shifting
and seasonal backwater variations.

2.3.2 Discussion of Rate of Change Relatio;rl~hips
From our elCperi.ence with flood warning systems in Nebraska
and Iowa, we have found that a combination '~f stage and rate of
change in stage at the warning gage has mOl:e bearing on flood
severity at do~rnstream locations than the stage alone at the
warning gage. RatE' of rise data relates directly to the shape
of the incident hytir.ograph. It is possiblE" tel better evaluate
an approaching flood wave by using an upstr,afim gage or even two
or more upstream gages. Factors which are important to the
f),ood coordinator's decision-making process I such as flood wave

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