sediment-laden streams, as deposition may take place upstream of the
weir, thus changing the approach conditions of the flow. Floating debris
and ice may also dent the crest of sharp-crested weirs.
Flumes are suitable for small streams carrying sediment-laden flows.
The contraction introduced in the channel can be designed to create criti-
cal flow conditions at the throat (contracted section). Sometimes a raised
sill (Featherstone and Nalluri, 1995) in a throatless flume may be intro-
duced to create critical flow conditions. The water level in the upstream
subcritical flow is then directly related to the discharge passing through the
flume. The length of the flume may be reduced by providing a hump at the
throat, which ensures critical flow conditions. The rise in upstream water
levels is smaller than in the case of weirs.
Different types of weirs and flumes in use are summarized in Table 8.1.
The behaviour of the flow-measuring structures largely depends on
the downstream water level (dependent on downstream channel control)
which increases with discharge. The increased level may drown the struc-
ture and the unique free-flow (modular) relationship between the stage
and discharge is lost. It is not always practicable to set the weir sill at
higher elevations to avoid submergence because of the consequent flood-
ing and waterlogging problems upstream of the structure. On the other
hand, the drop in the sill level of the Parshall flume can considerably
increase the measuring range of modular flows, and such a structure can
conveniently be used as a drop- and flow-measuring structure in irrigation
canals. However, equation (8.38) may also be used to gauge non-modular
flows with appropriate correction factors (Water Resources Board, 1970;
Bos, 1976; Ranga Raju, 1993) (Chapter 9).
The weir and flume structures are not normally suitable for measur-
ing flood flows. In such cases open-channel methods such as the slope-area
method, the constant fall method, etc. may be used. If there are no radical
changes in the cross-section of a natural control with rising stages (e.g. no
floodplains) extreme flood flows can also be predicted by special tech-
niques such as extending the available stage–discharge curves.
8.6 River flood routing
Flood routing is the process of transforming an inflow passing through a
river reach (or a reservoir) into the outflow. During this process the flood
inflow hydrograph changes its shape: its peak is usually lowered and its
base extended, i.e. the flood subsides. The usual task is to determine the
peak reduction – attenuation of the flood between inflow and outflow –
and the time lag between the peaks. Flood subsidence is controlled both
by the frictional and by the local resistance and acceleration terms in the
equation of motion and by storage.
338 RIVER ENGINEERING
