ETL 1110-2-540
30 Sep 96
bucket" gage sized to measure rain in 1-mm (0.04-in.)
Location far enough upstream of the confluence of
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increments. This type of gage is illustrated in Appendix B,
another stream or tidal influence.
Figure B-1. The type of raingage selected is based on
availability, cost, reliability, and vender support, in addition to
In addition, many of the same items pertaining to the location
installation, operation, and maintenance considerations.
of precipitation gages, as indicated above, must be consid-
ered when determining the location of stream gages.
(2) Stream gages. Stream gages provide information
about the current state of the hydrologic response system. A
(3) Radar estimates of rainfall. The possibility of using
typical pressure transducer type stream gage is shown in
radar to estimate rainfall has intrigued hydrometeorologists for
Appendix B, Figure B-2. For flood recognition systems
many years. Raingages provide adequate measures of rain
without sophisticated forecasting, stream gages may be the
falling at a point but are less proficient at estimating areal
only gages needed. This is especially true for watersheds
rainfall. Radar, on the other hand, can observe rain falling
where the warning time is sufficient to respond based on a
over wide areas. However, radar estimated amounts of
stream gage upstream of the damage site. As forecast tools,
rainfall over specific areas are often inaccurate.
the effectiveness of stream gages varies. In larger watersheds
where downstream forecast points are dominated by routed
(a) Radar analysis of rainfall depends upon receiving
flows, upstream stream gages are important indicators of
electronic signals bounced off rain drops. Many things inter-
future flows. However, in flash flood events dominated by
fere with signal quality including obstructions such as build-
local runoff, stream gages become little more than verification
ings and mountains, mixed precipitation (snow, sleet, hail),
tools, confirming current flood forecasts rather than adding
the curvature of the earth, and anomalous signal propagation,
significant value to them.
to name a few. Despite these difficulties, new technologies
are improving abilities to estimate rainfall with radar.
(a) Stream gage alarms set to sound at certain levels or
programmed to detect rapid changes in water level can
(b) Since raingages are accuratae for measuring rainfall
recognize events that other "upstream" monitoring systems
at a point and radar is sensitive in detecting the areal extent of
could possibly miss.
rainfall, the ideal situation would be to combine the two
techniques. Real-time raingage measurements can be used to
(b) The location of stream gages in an automated flood
calibrate radar estimates to provide a "best estimate" of the
warning - preparedness program is determined to satisfy two
true volume of rainfall entering a watershed.
elements: public warning requirements and hydrologic fore-
casting and associated watershed modeling requirements.
(c) The use of radar together with a network of rain-
First, a stream gage should be located where forecast stages
gages can reduce the number of fixed gages needed to
are required. Key potential damage points or well-known
achieve the same level of accuracy in the estimate of areal
locations that the general public can relate to are important
rainfall. Crawford and Andra (1987) demonstrated that the
potential gage locations. Stage alarm gages should be located
sampling quality of a 31-raingage network in Tulsa, OK,
at key damage points and key points at sufficient distance
could be matched with radar plus just three fixed gages in the
upstream to yield enough warning time to be of value. If the
field. In addition, for the radar/raingage combination, Craw-
watershed is large enough to require the definition of
ford and Andra showed that all gages in that network above
hydrologic subwatersheds, stream gages should be located at
10 to 15 were essentially redundant; the additional gages
the subwatershed outlets to facilitate hydrologic modeling.
provide no new information.
General considerations for the location of any stream gage for
proper operation includes the following:
(4) Meteorological forecasts. Forecasts of developing
meteorological conditions are especially useful in getting an
Location subject to minimum scour and deposition.
early start in analyzing potential emergencies and in extend-
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ing current forecasts. Meteorological forecasts of severe
General course of the stream that is straight for
weather can alert forecasters to conditions that could lead to
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100 m upstream and downstream of the gage
flooding should the storm materialize. Early precautions can
location.
be taken.
Total flow confined to one channel for all stages.
(a) Current flood forecasts can be extended using
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meteorological forecasts.
Forecasts solely based on
Location within a reach satisfactory for measuring
measured rainfall when it is still raining hard are likely to
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discharge for all stages near the gage.
under estimate the actual flood. Therefore, a prudent forecast
4-5
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