ETL 1110-2-540
30 Jun 96
(c) The time needed to observe the developing event,
be grouped and response actions defined based on the
Tobs , is a balance between waiting long enough to get enough
warning time.
information to accurately determine the flood threat and acting
soon enough to have a valuable forecast. A reasonable
(e) The time from the onset of rainfall to the critical
estimate of Tobs is about 15 to 25 percent of the time of
flood stages is determined directly from each event. If the
concentration of the watershed, depending on how the data
distribution of maximum potential warning times of the set
are observed.
of critical hydrographs is highly variable, then many
events could have maximum potential warning times
(3-3)
significantly less than the mean. In those cases, a lower
0.25Tc $ Tobs $ 0.15Tc
value may be chosen as a more realistic and conservative
(d) Maximum warning times can be estimated using any
estimate of maximum potential warning time.
one of several techniques. The most appropriate technique
is to examine flood hydrographs developed from historical
(f) All flood warning - preparedness programs
and hypothetical storm events. The range and variability of
should include stream stage monitoring (stages and rates of
flood warning time at key locations should be determined
rise) near the locations where response actions are
based on the hydrologic simulation of historical and
required. This should include a staff gage and more
hypothetical storm events.
Events should include the
sophisticated systems as applicable. Additional warning
variation of rainfall intensity and distribution as well as
time may be obtained by monitoring upstream conditions.
antecedent conditions likely to be encountered in the
Stream monitoring is always desirable and significantly
watershed.
Warning time will vary depending on the
more reliable than rainfall data. However, for small flash
location (elevation) of damageable properties in the
flood conditions, rainfall monitoring is normally required
floodplain. Properties with common warning times should
to extend the warning time.
3-2
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