The Sediment Transport Model and its Applications - Sediment Transport in Irrigation Canals: A New Approach - page 212

concentration at the headworks. At the boundaries between canal reaches
the sediment load passing the downstream boundary is c 0 for the next
reach. The depth-averaged equilibrium concentration c e is based upon
one of the three sediment transport predictors. The predictors give the
sediment transport per unit width, and the bottom width B and the cor-
rection factor α are required to determine the total sediment transport,
which will form the basis for the equilibrium concentration. The model
simulates the total sediment load, namely the bed load and suspended load
together. If a structure has a raised crest then it might trap a part of the bed
load upstream of the structure. Only the whole suspended load part will
move to the downstream canal reach. Internal conditions such as bifurca-
tions or confluences are needed for continuity of water flow and sediment
transport to find changes either in bottom width or bottom level.
During the first time step of the simulation process the water flow
is uniform or develops a gradually varied flow. For these conditions the
sediment characteristics will be determined, especially the equilibrium
sediment concentration. When the actual sediment concentration differs
from the equilibrium concentration, the latter will try to adapt to the equi-
librium concentration by deposition of the sediment in the water or by
erosion of sediment from the bottom. Due to the sedimentation and ero-
sion the hydraulic characteristics will change and hence the water flow
variables for the next time step will change. A new gradually varied flow
will be established, resulting in a quasi-steady flow. In this step the actual
sediment concentration will again adapt to the equilibrium concentra-
tion, resulting in sedimentation or erosion, which will lead to changes in
the geometrical characteristics of the canal, which will be used for the
sediment calculations in the next time step. This process will continue
either until the actual sediment concentration is in line with equilibrium
concentration or will continue until the end of the simulation period.
A reflection of the previously presented examples and simulation
results shows that the SETRIC model is a useful tool to develop a better
understanding of the behaviour of sediment transport in irrigation canals
and to decide on improved water delivery plans in view of the operation
and maintenance concepts under different flow conditions and sediment
characteristics. It can be applied to evaluate designs of an irrigation net-
work and to analyse the alternatives, but it can also be used as a decision
support tool in the operation and maintenance of a system and/or to deter-
mine the efficiency of sediment removal facilities in an irrigation system.
In spite of that, the mathematical model still has to be calibrated and
validated with field data before the model can be fully used as a simu-
lation tool. The performance of the model has to be confirmed by field
measurements that should prove whether the physical processes are well
represented by the model or whether there are some deficiencies as a
result of the assumptions used to describe those processes. Monitoring of
the sediment deposition in an irrigation network is needed to evaluate the