30 Sep 96
tornado warnings. If sirens are also used for flash flood
(4) Crest of flood. The eventual maximum elevation of
warnings, extreme care must be taken to eliminate confusing
flooding defines the expected area of inundation. Response
signals. The response for a tornado warning would be to go
activities can be focussed on those structures within the identified
to the basement. However, the basement is not the place to
flood area. Evacuation orders can be more specific and be
be during a flash flood. Clearly, signals for these two events
confined to areas affected by the high water.
must be immediately and unmistakably distinguishable.
(5) Time of flood crest. The time of peak flooding, along
(1) The siren example illustrates that an extremely
with the crest forecast, allows emergency officials to finetune
their efforts further. The exact timing of the arrival of the crest
content of the warning message. It must be clear and
may give vital insights on how long a particular access/egress
unambiguous. Response to a flood warning improves as the
route will be available for evacuations.
message becomes more specific.
(6) Time flooding starts. The time that flooding begins is
(2) To effect a response, the message recipient must feel
most important to the occupants of the lowest portions of the
that he or she is in jeopardy. The recipient must personally
flood plain. A crest forecast for midnight may not be valuable
feel threatened by the coming event to be motivated to take
information to flood plain occupants that experience high water
action. Table 4-3 shows an example (Barrett and Davis
hours before the crest occurs.
1988) of how a forecast message varies from general to
(7) Time reaching specific locations and elevations. The
availability of the complete hydrograph showing the time each
flood elevation is reached allows emergency officials and
floodplain occupants to choreograph their activities accordingly.
Examples of Flood Warning Specificity
Flood warning for Adams County.
Minor flooding expected in Adams County.
(8) Time of flood recession at specific locations and
Minor flooding expected in Adams County this morning.
elevations. Knowing the time flood waters will recede in each
A crest of 25.0 feet is expected on Adams Creek in
location of the floodplain enables emergency officials to
Adams County at 6:00 p.m.
efficiently reenter and secure the flooded area.
Flooding is expected between Adams Street and Eve
Road tonight between 6:00 p.m. and midnight. All
houses bounded by these streets will flood tonight.
(9) Time flooding ends. The time the flood completely
recedes signals the end of the immediate emergency and allows
Note: 7.6 m (25 ft)
officials to coordinate reentry and recovery activities.
d. There are a number of attributes of a flood forecast that,
(10) Identification of Special Hazards.
when included, enable increasingly specific and efficient
backwater effects, flow bottlenecks that cause debris dams, and
responses to the impending flood.
flows that "short-circuit" the natural channel at certain elevations
and/or velocities are all "out of the ordinary" flooding but present
(1) Location. The location of the impending flood is
a very real and often unforeseen challenge to emergency officials.
generally the first element that must be included. Without some
Early identification of these events can further prevent damage
mention of location, message recipients probably will not react
and loss of life.
until they actually see the water rising - losing valuable response
d. The attributes of a flood warning cover the full range
of sophistication possible in a local flood warning -
(2) River or stream in flood. Identification of the specific
preparedness program. At the simplest level, providing a
river or stream that will flood provides an additional level of
general location of impending flooding (e.g., Level 1 in Table 4-
information to act on. Those living near the water courses
5) requires the least sophistication - perhaps just listening for a
mentioned will likely pay more attention to developing conditions
flash flood warning on the radio. To supply a complete flood
and take action.
hydrograph and identified areas of inundation (e.g., Level 5 or
better in Table 4-5), a complete hydraulic engineering study of
the floodplain and an automated local flood warning system with
(3) Time of flooding. Mentioning the expected time of
hydrologic/hydraulic modeling capabilities may be required. An
flooding gives warning recipients a better idea of how much time
example of a detailed, site-specific flash flood warning is
they have left to prepare and what kind of activities that they have
included in Table 4-4.
time to do.