How to stop a massive canal system from silting up?
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).
background
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.
This post was sourced from https://worldbuilding.stackexchange.com/q/98910. It is licensed under CC BY-SA 3.0.
1 answer
Have the very durable material canal meander slightly and have narrow side channels all along the length at the inside of the meander curves where the silt will collect. If the channel is 30m deep and erosion proof and the median water flow requires only 15 meters depth then the water will cut a channel through the silt as required. It is wasteful of channel depth but solves the silting problem. Rivers silt up because the flow rate slows down. Sewers and storm water drains do not because the flow rate is maintained and the slope is fixed by design.
Over time the side channels will silt up and reduce wasted water unless a community is located there and wants to dig out a channel to gain a water portion. This makes the silt removal a win situation for the primitive locals with free plant nutrients and reliable water flow.
The real problem here is you need an ocean sized dustbin to put all that silt over generations as you are not going to be able to just keep pushing it (by community labour or periodic floods) to the sides of the canal without it being left at the bottom of a ravine formed of silt. Your world will have a finite operating life before it will become an Okavango Delta type of situation where the water stops flowing because there is no more downhill for it to flow into.
The periodic flooding of the Nile to clear out salting and renew the silt only worked because the salty, nutrient depleted silt could be washed into the Mediterranean Ocean.
EDIT:
With an option of having the canal eventually spill into a dry ocean bed the silt could be allowed to be transported all the way to the terminus with the last of the water. Periodic flooding is no longer required as any farmers who want to keep growing will have to transfer old salty silt back into the canal by manual effort as they fetch fresh silt from the canal to replace it to maintain their fields below the water level. This process will cause the water in the canal to increase in saltiness downstream as citizens exchange salty for fresh silt. The excess saltiness may make upstream sites more valuable for the ruling castes or possibly the additional humus in the down stream silt may be a benefit to more salt tolerant vegetation which would be a win-win scenario. More land can be slowly created for those prepared to farm in a salty delta.
The canal designers would have made some method that will be feasible and intuitive to keep the canal working for millennia. Having silt at the bottom will protect the facing material for free if the design maintains a layer everywhere.
Having the lining made of local bedrock in hexagonal tapered sections would allow the new citizens to repair it if there is earthquake damage, a sink hole or extreme wear in some place due to harbour activity or such.
Making a parallel run of canals would allow one to be a backup while the other is under renovation. In Arthur C Clarke's Rama series everything was done in 3's as a redundancy feature. Having millions of citizens reliant on a single point of failure sounds a bit unkind of the canal designers.
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