Environmental Engineering Reference
In-Depth Information
Fig. 3.11 Examples of artificial flow control structures: (a) a broad-crested weir in a stream in
northern Manitoba, Canada, (b) a thin-plate weir in a stream in Banff, Canada, (c) a Parshall flume.
The streamflow direction is from the left to the right in all photographs (Photos a and b by Masaki
Hayashi; photo c by Donald Rosenberry)
Ideally, artificial controls should have structural stability, their crest should be as
high as practical to eliminate the effects of variable downstream conditions, and the
stage should be sensitive to discharge. As a general rule, a weir is more advanta-
geous than a flume because it is less expensive, can be designed to have greater
sensitivity, and its rating curve can be extrapolated beyond the normal operation
range without serious errors (Rantz
1982
:18). Flumes are more advantageous in
streams carrying heavy sediment load or when backwater created by a weir is
undesirable. Specific design and characteristics of different types of weirs and
flumes can be found in Rantz (
1982
:294-326).
3.6.3 Stage Measurement
The first step in measuring stream stage is to establish a permanent datum (refer-
ence elevation) and a staff gage (Fig.
3.2
). The zero for a stream-stage datum should
be below the elevation of zero flow on a natural control, and usually at the elevation
of zero flow in an artificial control such as a weir. Changing the datum during a
