Environmental Engineering Reference
In-Depth Information
A typical practice is to obtain a series of direct measurements of lows and stages (water surface
elevations) at a selected location. The direct measurement of lows is time consuming and expensive
as compared to the measurement of water surface elevations or stages. A common practice is to use
a series of direct measurements to obtain a relationship between lows and stages, which is known
as a stage-discharge relationship or a rating curve (see Figure 4.4). Once established, the stage is
measured and the rating curve is used to indirectly estimate the lows.
For a speciic location to be usable for such an analysis, there must be a unique relationship
between the stage and the low. In many (if not most) areas of natural channels, such as in pools,
behind dams (backwater areas), in tidal zones, and elsewhere, there is no unique relationship
between water surface elevations and lows. Locations where there is such a unique relationship
are known as control points, and the factors governing the relationship are known as controls. The
control could be, for example, an artiicial structure (artiicial control), a change in channel shape
such as downstream constriction (section control), or channel geometry and roughness, or other
factors (channel control). The speciic controls may also vary with the stage (see Rantz et al. [1982]
and Kennedy [1984] for a more detailed explanation). Since in many (if not most) areas in rivers and
streams there is no control, the selection of the location is critical.
According to Rantz et al. (1982), the ideal location is one that satisies the following criteria:
•
The general course of the stream is straight for about 300 ft. (approximately 100 m)
upstream and downstream from the gage site.
•
The total low is conined to one channel at all stages, and no low bypasses the site as
subsurface low.
•
The streambed is not subject to scour and ills and it is free of aquatic growth.
•
Banks are permanent, high enough to contain loods, and are free of brush.
•
Unchanging natural controls are present in the form of a bedrock outcrop or other stable
rifle for low lows and a channel constriction for high lows or a falls or cascade that is
unsubmerged at all stages.
•
A pool is present upstream from the control at extremely low stages to ensure a recording
of the stage at extremely low lows, and to avoid high velocities at the streamward end of
gauging-station intakes during periods of high lows.
Peachtree Creek rating curve (July 2003)
22
20
18
16
14
12
10
8
6
4
2
0
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
9,000
10,000
11,000
Streamflow (ft.
3
s
-1
)
FIGURE 4.4
An example of a rating curve. (From USGS, Available at http://ga2.er.usgs.gov/peachtree/
streamlow.cfm.)
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