2.9 STREAM GAGING
The hydrology of the stream at a bridge crossing is determined from records obtained by State
and Federal agencies. The most extensive records of streamflow are by the U.S. Geological
Survey. Their records of stream flow and analysis can be obtained for any State at their District
offices or from their Web Site. The USGS has been collecting stream gaging records since 1888
(Corbet 1962) and maintain over 4,000 sites in the United States in cooperation with other
Federal, State, Counties, and Cities. Often a gaging station will be located near the site of a
bridge crossing. If not, regional or other hydrologic tools can be used to analyze the stations in
the region to obtain the hydrology for the bridge site.
The quality of the hydrologic records at a stream gaging site depends on obtaining an accurate
gage height record and stage-discharge relation. These depend on stream characteristics (bed
material, cross-section, and control) and, in many cases, the magnitude of the sediment
transported by the stream. These terms are defined in the glossary and will be defined later in
this section. For example, the hydrology used for the analysis of the 1987 I-90 bridge failure in
upstate New York was excellent. There was a gaging station located 22.5 km (14 mi) upstream
of the bridge. The stream at the gaging site was in bedrock with an excellent control. Thus, the
stage-discharge relation was well defined with a single curve established with actual discharge
measurements made over many years and large flows. The gage height record was excellent
with few time gaps. The stage record for the 1987 storm was excellent. The discharge was
routed to the bridge from the gage site using the U.S. Corps of Engineers HEC-1 model to obtain
the hydrology of the flow at the bridge.
In contrast, the hydrology for the 1997 I-5 bridge failure in California was not well defined. The
streambed was sand and there was no bed rock control, only channel control. The gage height
record was good, but the stage discharge relation was poorly defined with no consistent curve.
The peak discharge for the flood had to be determined using indirect methods with results that
ranged from to 420 to 1,140 m3/s (14,800 to 40,300 cfs). The range in discharge for the slope
area measurement resulted from assumptions on Manning's n and the amount of degradation.
The discharge used in the final analysis 773 m3/s (27,300 cfs) was determined from the slope-
area measurement, study of the rainfall records and discharge records of other gages in the
A program to obtain a systematic record of the stream flow consist of (1) establishing and
constructing a streamflow measurement station, (2) operating and maintaining the station, and
(3) computing, compiling and publishing the stream flow data. In addition, analyses of the long-
term stream flow data are made (flood frequency, low flow analysis, trends). The methods of
obtaining water discharge records are described in publications of the USGS (Corbet 1962,
Carter and Davidian 1968, Buchanan and Somers 1968a,b, and Kennedy 1983). In the following
sections a typical gaging station, discharge measurement, stage discharge relation and the
determination of the daily discharge will briefly be explained.
2.9.2 Gaging Station
A gaging station consists of structures and equipment to measure and record the stage and
discharge as a function of time at a given site on a stream. Stage or gage height is the height of
the water surface above a chosen arbitrary datum corresponding to the zero of the gage. The
zero of the gage is related to sea-level elevation. An accurate record of stage is essential for
computing the discharge for any period of time. Discharge is the rate of flow in m3/s or cfs.