Geoscience Reference
In-Depth Information
0.6
0.595
0.59
0.585
0.58
0.575
0.57
0
20
40
60
80
100
120
V-notch angle (degrees)
Figure 5.10 Coefficient of discharge for V-notch weirs
(ISO 1438).
Figure 5.11 A V-notch weir. The water level in the
pond behind the weir is recorded continuously.
through is determined by the flow regime of the
river and the requirements for the streamflow data.
A common shape used is based on the V-structure
(see Figure 5.11). The reason for this is that when
river levels are low, a small change in river flow
will correspond to a significant change in stage
(measured in the stilling well). This sensitivity
to low flows makes data from this type of flume or
weir particularly suitable for studying low flow
hydrology. It is important that under high flow con-
ditions the river does not overtop the flume or
weir structure. The V shape is convenient for this
also because as discharge increases the cross-sectional
area flowed through increases in a non-linear fash-
ion. The angle of the V-notch will vary depending
on the size of stream being measured and the
sensitivity required (90° and 120° V-notch weirs are
both commonly used).
One of the difficulties in maintaining gauging
structures is that by slowing the river down in a
stilling pond any sediment being carried by the
water may be deposited (see Hjulstrom curve in
Chapter 7), which in time will fill the stilling pond
and lessen its usefulness. Because of this the stilling
pond needs to be dredged regularly, particularly in
a high energy environment such as mountain
streams. To overcome this difficulty there is a design
of trapezoidal flume that speeds the stream up rather
than slows it down (see Figure 5.12). The stream is
forced to go down a steep section immediately prior
to the gauging structure. In this way any sediment
is flushed out of the weir, removing the need for
Figure 5.12 A trapezoidal flume. The stream passes
through the flume and the water level at the base of the
flume is recorded continuously.
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