Design Criteria for Irrigation Canals - Sediment Transport in Irrigation Canals: A New Approach - page 87

The same general equation is also valid for an undershot structure
and the variables are given in Table 4.10; the value of h here is the gate
opening. The discharge coefficient is included in Table 4.9.
A number of the above mentioned structures are included in the
mathematical model and they are discussed in Chapter 6.

4.4.4 General remarks

• flow control needs extra care in networks with sediment transport.

The hydraulic conditions will continuously change due to erosion or
deposition in the upstream or downstream reaches. Even for the
same flow and after some time, free flow conditions may switch to
submerged flow conditions or vice versa. Any obstruction of the flow
will change the hydraulic conditions and has a significant impact on the
sediment transport capacity;

• the deposition of sediment in the upstream bed will change the bottom

slope and increase both the flow velocity and the sediment transport
capacity of the section. Depending upon the incoming sediment load,
deposition will continue until the velocity is large enough everywhere
to convey the total incoming sediment load downstream of the structure.
The deposition will change the flow hydraulics and the head discharge
relationship;

• a broad or a sharp-crested weir with a sill may induce sediment depo-

sition in the upstream reach and after some time the structure may
change into a sudden drop. The discharge coefficient as well as the dis-
charge equation is not valid anymore, resulting in higher water levels,
lower velocities and sediment deposition. A flume as measuring device
has more advantages than a broad-crested weir in terms of sediment
transport efficiency.

• if the design of the set point is low and deposition in the down-

stream reach occurs, then a condition may be reached, where it is
not possible to maintain the flow for that set point. A proper head
discharge equation requires a minimum head loss over the structure.
If, due to deposition, the downstream water level rises and the avail-
able head decreases than the design discharge cannot be attained. This
aspect needs special attention when designing a network with sediment
transport;

• to prevent deposition upstream of a structure the water level (set point)

should be more than the sum of the downstream water level and the total
head loss ( h ). A set point that is too low may result in sedimentation
when large discharge fluctuations occur during the irrigation season;

• to minimize sediment problems the upstream water level of a siphon

should be high enough above the invert to prevent a gravity flow in the
siphon;