The Sediment Transport Model and its Applications - Sediment Transport in Irrigation Canals: A New Approach - page 180

transport capacity of the canal. The adjustment towards the equilib-
rium transport capacity is assumed to follow Galappatti's depth-integrated
model. A mass sediment balance in each canal reach will result in either net
deposition or net entrainment. When the incoming sediment load is larger
than the sediment transport capacity of the reach, deposition will occur.
When the incoming sediment load is less than the transport capacity two
possibilities can be identified. In the first case entrainment of previously
deposited sediment occurs until the sediment transport is fully adapted
to the equilibrium transport capacity. In the second case, no entrain-
ment takes place and the incoming sediment load is conveyed without
any change.
In the following examples the sediment deposition over a certain irri-
gation period will be simulated for an irrigation canal, for which the
geometrical and hydraulic design data and the incoming sediment char-
acteristics will be described in detail. The SETRIC model will be used to
evaluate the effects of the following cases on the sediment behaviour:
1. Changes in the design discharge;
2. Changes in the incoming sediment load (concentration and diameter);
3. Controlled sediment deposition by deepening or widening of a canal
section;
4. Overflow and undershot structures for upstream flow control;
5. Maintenance activities in a canal network;
6. Management activities in a canal network;
7. Effect of the design of control structures on the hydraulic behaviour
and sediment transport.

7.2 DESCRIPTION OF THE HYDRAULIC AND SEDIMENT
CHARACTERISTICS FOR THE GIVEN EXAMPLES

The examples in this chapter will start with an analysis of the effect of
changes in the incoming discharge (Case 1) and incoming sediment load
(Case 2) on the sediment deposition in an irrigation canal. Case 1 will
extensively describe the differences in sediment deposition for the three
sediment predictors that are available in SETRIC. The behaviour of the
sediment for the cases 1 to 4 will be analysed for a single, straight irrigation
canal with a trapezoidal cross section and a constant water depth at the
downstream end (set point). The main hydraulic conditions and sediment
characteristics during the simulation include:
• the discharge is constant during the simulation period;

• the water level at the downstream end of the main canal is kept at a set

point that is determined by the normal water depth for the design flow
and without sediment deposition in the canal; for other discharges the
flow will be gradually varied;

• the downstream water level is managed by an underflow structure for

cases 1, 2 and 3;