Environmental Engineering Reference
In-Depth Information
where
b
(m) is the width of the weir through which water is flowing, and
C
(m
1/2
/s)
is a coefficient dependent on the geometry of the weir (Rantz
1982
:296-297).
A V-notch weir has a rating curve in the form of:
52
=
Q
¼
C
tan
ðÞ
θ
2
ð
h
h
0
Þ
(3.28)
90
for the standard V-notch weir), and
C
(m
1/2
/s)
is a weir coefficient, which has a value of 1.38 in the ideal condition where the weir
plate is perfectly made and vertical and water in the pool upstream of the weir has
negligible velocity approaching the weir (Rantz
1982
:304). The formulas for several
types of broad-crested weirs and flumes, including the Parshall flume, are described
by Rantz (
1982
:306-326).
Stage-discharge rating formulas for artificial controls were developed based on
laboratory and modeling studies conducted under ideal conditions. To achieve
maximum accuracy in the field, data from these instruments should be compared
with separate measurements of discharge conducted at the weir or flume installation
(Rantz
1982
:295) and correction coefficients applied as necessary. The velocity-
area method (Fig.
3.11a
) or volumetric method (Fig.
3.11b
) is most often used for
this purpose.
The stage-discharge relation is sensitive to changing conditions of the flow
control and the channel reach in the vicinity of the gaging station. Therefore,
changes in the channel (e.g., boulder and gravel movement, streambed scouring
or filling, bank erosion, vegetation growth) may cause a shift in the rating curve,
which can only be detected through periodic stage-discharge measurements. When
an observer measures discharge at an existing gaging station, it is a good practice to
calculate discharge before leaving the site and plot the new measurement on the
rating chart. If the new point deviates noticeably from the established rating curve, a
second discharge measurement is carried out to confirm a shift (Rantz
1982
:346).
If the second measurement confirms a shift, a note should be made to pay particular
attention to the site conditions during the next visit. If several consecutive
measurements show a consistent shift, the rating curve will need to be adjusted to
account for the new channel conditions.
where
θ
is the angle of notch (
θ ¼
3.6.6 Flow Estimation by Indirect Methods
In low-gradient wetland settings, surface-water flow can be very slow and occur
over a broad area without a well-defined channel, often covered with extensive
emergent vegetation, making it impossible to measure surface-water flow using any
of the methods above. In this case, surface-water flow may have to be treated as the
residual of a water-budget equation. Surface-water flow can still be estimated,
although often with a large degree of uncertainty, using one of several hydraulic
equations with estimated parameters. If the depth of water is sufficiently large
