ETL 1110-2-540
30 Sep 96
developing flash flood occurs during a time of very light
Table 4-1
meteor activity, communications may not be adequate.
Communication System Selection
Watershed Response
Communication System
(4) VHF/UHF radio. Radio systems using VHF
frequencies (near 170 MHz) and UHF frequencies (near
0-3 hr
Radio
450 MHz) are often employed where communication
3-6 hr
Radio
distances are relatively short. VHF/UHF systems are often
Satellite
called "line-of-sight" systems. Literally, if one can see it, one
can communicate with it. To communicate over the horizon
6-12 hr
Radio
Satellite
or around obstacles such as mountains or tall buildings, radio
Meteorburst
repeaters are required.
>24 hr
Radio
(a) VHF/UHF systems are generally under total
Satellite
Meteorburst
control of the local user. Event reporting techniques can be
Telephone
used easily. Interrogations in two-way systems can be done
anytime. Therefore, radio systems are in frequent use moni-
toring local events in a highly responsive mode. Flash flood
microcomputer with the appropriate telephone, satellite,
warning systems for individual communities commonly use
meteorburst, or radio communication equipment. Additional
radio-based systems.
peripheral equipment required includes printers, modems, and
standby power sources.
(b) Event reporting technologies are commonly used in
automated community flood warning systems. Data report-
(2) Data acquisition, data display, application program-
ing rates determined by the intensity of the monitored storm
ming, and dissemination are not the only functions per-
conditions provide precise information about rapidly chang-
formed by automated flood warning computers. Rapidly
ing field conditions at exactly the right moment. Peak rain-
advancing microcomputer technology makes the number and
fall intensities generate peak rainfall data reporting rates. Peak
type of information management functions virtually limitless.
rates-of-rise of water surfaces also generate peak water level
Nevertheless, these four functions are basic and key to the
data reporting rates. Both sets of information are critical
success of many flood warning systems.
when determining the genesis of flash flooding.
(3) Whether the data reporting system is interrogated,
(5) Communications system selection. The type of
timed, event, or mixed, the microcomputer is responsible for
communications system used in a flood warning system
coordinating data recovery. Once data have been acquired,
depends on the scale and time requirements of the local flood
its quality must be checked immediately. Quality checks can
warning - preparedness programs. Typically VHF/UHF sys-
be as simple as checking if the incoming data value is within
tems are employed for smaller scale applications where two or
the expected range of data. More sophisticated quality checks
fewer repeaters arranged in series are required to com-
use encoding and decoding algorithms to detect if
municate from remote gage location to the base station. More
transmission errors have occurred. If the data pass the quality
than two repeaters in series are too risky, since failure of one
tests, a database file is created. Otherwise, the data are
repeater causes the link to fail. Satellite and meteor-burst
discarded.
systems are employed in larger regionally or nationally based
systems.
Table 4-1 provides additional guidance for
(4) Data quality are critical. Poor quality data can be
communication selection based on watershed response times.
misleading, causing false alarms, unreliable forecasts, or no
response when one is required.
central feature of automated flood warning systems. These
(5) Filed data are available for use in data displays,
machines manage and control the crucial flow of information.
application programs, and for dissemination or other possible
Data from remote data platforms can be received, checked,
uses. Databases for automated flood warning systems need to
and stored for later use. Various displays can be used to
be large enough to hold data for several months or even
present data visually. Application programs can use the data
years. If not, some provision must be made to transfer data
to calculate the extent of flooding. Results can be sent to
to a permanent storage file. Data are essential for future
emergency response officials.
analysis to understand the hydrologic response of watersheds
and refine hydrologic forecast techniques.
(1) The microcomputer-based information management
center, often referred to as the "base station," consists of a
4-9
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