Based on the desert world from this question I need to transport around 500 cubic kilometres of water / year across a desert. Using this calculator. this can be achieved if the main feeder canal is 15m deep, 3200m wide and has a 1mm/km gradient. This gives a flow of around 0.366 m/s and provides the required amount of water.
The problem is that the main channel (and all of the smaller canals in the network) will suffer from silting up. How can I prevent this from happening?
The canals could be put into tunnels but this would be very costly and would wreck the plot so I would rather not use tunnels, but all other options would be considered. Almost any aspect of the canal can be adjusted such as the size, shape, gradient and elevation, but the silt must be prevented from entering, be removed or otherwise dealt with by the design (and the length is fixed).
The world is roughly earth like but has much less water and most of what there is, is locked up in the icecaps hence the canals that run from the poles to the temperate zones via a canal network built by an advanced civilization which has since disappeared. The canal system is currently occupied by a much more primitive civilization (pre 400CE).
The total population living on the canal network is about 50 million. They whole area is a desert similar to the Sahara but crisscrossed by a canal network 3000km across. The lands near the canals are agricultural with mixed vegetation including woodland, grassland and a variety of crops including wheat. Every year the land is flooded to prevent the build-up of salts in the soil.