The Sediment Transport Model and its Applications - Sediment Transport in Irrigation Canals: A New Approach - page 191

7.5 CASE 3 CONTROLLED SEDIMENT DEPOSITION
Uncontrolled sediment transport and deposition in irrigation canals may
result after some time in clogging (obstruction) of tertiary turnouts, reduc-
tion of the canal conveyance capacity, large variation in water levels,
unpredictability of the relationship between water depth and discharge
for regulating structures, and high costs for canal desilting works. Once
the sediment enters a canal network it can be conveyed by the canals to
the farm plots or it can be removed from the water, for example, either
by deposition along the entire canal or by deposition in a sediment trap
within some of the canal reaches where it can be periodically removed
at minimum cost. Controlled deposition can be obtained by the design of
a sediment trap in the network, for example by deepening or widening
one or more canal reaches. This solution can be an attractive alternative
for controlling the deposition, but a careful selection of the location of
the sediment trap during the design phase needs careful consideration,
which needs a detailed elaboration in view of the future operational and
maintenance costs.
Case 3 will present two scenarios to control the deposition of sediment
and will show how a major part of the sediment load can be deposited
in some adapted canal reaches. A change in a canal cross section may
reduce the transport capacity (equilibrium concentration) in such a way
that it becomes smaller than the actual sediment load and part of the sed-
iment will be deposited there. In Case 3 the canal from Case 1 receives
a sediment load of 150 ppm at the headworks and is not able to trans-
port the sediments to the fields during the irrigation season. Therefore,
when no special measures are taken to control the deposition, the latter
will occur along the entire canal length. Case 3 will analyse two scenar-
ios to control the sediment deposition in the head reach by reducing the
transport capacity in that reach only.
The head of the irrigation canal will be converted into a settling basin
that can be described as:
1. Scenario 1 : deepening of the canal by 0.50 m for the first 1 000 m;
2. Scenario 2 : widening of the bottom width from 10 m to 14 m for the
first 1 000 m.
No other alternatives will be evaluated for a further optimization of
costs and/or sediment deposition. The previously mentioned data (see
Case 1) will also be used for the two scenarios in Case 3. For Scenario 1 the
original wetted area (35.13 m 2 ) will increase by 4.5 m 2 and for Scenario 2
the increase will be 9.52 m 2 , both for the same water level in that reach.
The total amount of excavation required for Scenario 2 will be larger than
9.52 m 2 and depends on the freeboard, which might be 0.5 times the design
water depth or more.