ETL 1110-2-540
30 Sep 96
Hardware
Physical equipment used in a flood warning - preparedness
program. "Field" hardware refers to raingages, stream gages,
repeaters, weather stations, etc. "Base station" hardware refers
to computers, printers, modems, power supplies, etc.
Hydrograph
A graph or chart showing the time variation of stream
elevation or flow at a specific location.
IFLOWS
Integrated Flood Observation and Warning System, a
federally funded automated flood-threat recognition system
serving the Appalachian mountain area.
Line-of-Sight Radio Communication
Radio communications in the frequency ranges commonly used
for flood warning - preparedness programs can be blocked by
obstacles such as buildings, hills, and mountains. Also,
communications in these frequencies generally cannot go
beyond the horizon due to the curvature of the earth. Literally,
if an observer can see the destination, radio communications can
be established. Typically, line-of-sight communications over
flat terrain are limited to about 32 km (20 miles) - less if
obstacles are present and more if transmitting from hilltop to
hilltop.
N-Year Flood
A flood event of a specific magnitude is often referred to an
"N-year" flood event. In other words, the event occurs one or
more times in an N-year period on the average. For example,
an event that occurs, on an average, once in 100 years, once in
25 years, once in 10 years, etc., is known as the 100-year
event, or the 25-year event, or the 10-year event, etc. The
probability of an N-year event occurring in any given year is
1/N. A 100-year event has a 1/100 or a 1-percent chance of
occurring in any given year.
Potential Warning Time
The maximum time available to warn floodplain occupants of
an impending flood (Figure 3-1).
Quantitative Precipitation Forecast (QPF)
A specific amount of forecasted precipitation for specified
period of time. (e.g., 76.2 mm (3 in.) of rain is forecast
between 6 p.m. and 12 a.m. today)
Figure B-1. Typical ALERT/IFLOWS Raingage
Raingage
A device used to detect and measure rain. The most common
gages measure rain by volume or weight. The most popular
Recurrence Interval
rain measurement device for automated measurement and
The average interval of time (e.g., N-years) in which an annual
detection systems used for flood warning is the tipping bucket
maximum flood of a given magnitude is equaled or exceeded.
gage sized to measure rain in 1-mm (0.04-in.) increments
A flood with a recurrence interval of 100 years has a 1 percent
(Figure B-1).
B-2
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