Calculate Flow Discharge?..(flood Water) Streamflow:

¶ … calculate flow discharge?..(flood water)

Streamflow: Why it is important to measure it

Often, during a severe rainstorm or other water event, a news announcer or spokesperson for the government will proclaim that the river is expected to crest at a peak of a particular foot interval ("How streamflow is measured," USGS, 2008). He or she will warn nearby inhabitants that an evacuation may be necessary, or for individuals to avoid a particular area because it is prone to flooding. The interval the announcer is referring to is the stream stage, and although some people might take for granted the fact that we have such knowledge about the likelihood of flooding, streamflow and stream stages are carefully calculated figures, monitored by scientists. Also, stream flow is something that must be recalculated on a frequent basis, to allow for inevitable environmental changes that can affect areas near water.

"Stream stage (also called stage or gage height) is the height of the water surface, in feet, above an established datum plane where the stage is zero. The zero level is arbitrary, but is often close to the streambed" ("How streamflow is measured," USGS, 2008). Streamflow, or water discharge, is the volume of water flowing past a fixed point in a fixed unit of time, expressed in a value of cubic feet per second (ft3/s). When plotted on a graph, stream stage and streamflow have a clear relationship. "This chart, known as a rating curve, shows that there is a relation between stream stage and streamflow. The stage-streamflow relation is used to relate water level to an associated streamflow. The rating curve for a specific stream location is developed by making successive streamflow measurements at many different stream stages to define and maintain a stage-streamflow relation. These streamflow measurements and their corresponding stages are then plotted on a graph. Continuous streamflow throughout the year can be determined from the rating curve and the record of river stage" ("How streamflow is measured," USGS, 2008). This makes it easier to anticipate likely flood areas and the severity of the flood's impact upon the area.

Multiple data series and frequent measurements are necessary to have a truly scientifically accurate result with predictive powers. "In order to accurately determine streamflow, measurements must be made of its width, depth, and speed (velocity) of the water at many horizontal and vertical points across the stream. To develop a stream-stage/streamflow relation (rating curve), streamflow must be measured at many different stages" and must be recalculated on a regular basis ("How streamflow is measured," USGS, 2008). "Rating curves frequently shift due to changes in the factors that determine the relation between stream stage and streamflow" ("How streamflow is measured," USGS. 2008). These factors include the slope of the stream which affects the water's velocity, the roughness of the channel, the area of the channel at each stream stage, the shifting backwater effects of nearby tributaries, and the natural or man-made acts of filling in or scouring out of the river banks ("How streamflow is measured," USGS, 2008).