How to stop a massive canal system from silting up?

Question

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.

Answer 1

Dredging

The first thing to note when you search for dredging is that it's not Wikipedia at the top, it's an advert for a dredging contractor.

This isn't something that can be ignored, it's a matter of ongoing maintenance in any managed or artificial waterway.

When water enters, whether through the channel or as runoff, it carries suspended particles that are dropped as the energy in the water drops. The more initial energy the water has, the higher the particle load, the more the silt builds up as the flow slows. Most things that end up in the water eventually sink, adding to the build up. Plants will grow and die in the water, fish will poo and die in the water, animals will occasionally die in the water. There's no avoiding the problem on an open waterway.

Ultimately the people will have to dredge to keep the channels clear.

Answer 2

Three possible solutions spring to mind off the top of my head:

One The advanced civilization bio-engineered silt slugs to eat the silt, crawl out of the canal and deposit the silt on the banks of the canal, all done in an environmentally friendly way, of course, and nicely integrated into the ecosystem(s) surrounding the canals.

Two Automated dredgers traverse the canals and scrape any accumulated silt from the bottom and deposit them on the banks of the canal. A likely problem here would be that your present civilization does not have the skill to repair the dredgers, so they would have to be somehow self-repairing.

Three Since your canal's waters run fairly slowly, the floor of the canal is broken every few hundred meters by a sharp rise, which then proceeds to gradually decline. The silt will collect at these obstructions and can be scooped out using a simple mechanical contraption.

You might even incorporate more than one of the above options, using different means at different sections, or combining them.

Answer 3

The only thing that might lessen (not avoid) the problem is some decanter stations, especially at the start of channel, where melting ice is collected to feed the channel system and flux is maximum.

Water current is very slow, so it won't keep in suspension particles heavier than water, which will deposit.

Having at start and regular intervals very large "decanter ponds" (if you really want to have channels 3.5Km wide you must allow for real lakes at least three times that size) will help to keep channels as clean as possible.

Remember sand desert sports "sand waves" (dunes) moving across and thus "ancient builders" should have built some embankment to protect the channels from direct sand assault.

This, however, cannot protect from dust carried by wind. This is obviously worse since you insist on open sky channels.

Having decanter lakes at least twice as deep as the channels and conic shaped will lessen the problem, but it won't solve it. Sooner or later the lakes will fill and lose their cleansing effect.

Some kind of maintenance will be necessary, also because decanters can lessen, but not eliminate sediment in channels.

In order to do some "automatic" maintenance for the decanters the ancient builders, in their wisdom, coupled the water taps with decanters, building closed and airtight pipes starting from the deepest point of decanter. Another (smaller) pipe would carry pressure air to the water intake thus creating a kind of airlift which will have a double effect: pump water from the channel system to wherever is needed and dredge the bottom of decanters. Means to generate compressed air flow needed to power the system are left to learner exercise, but I suggest some kind of wind power. Pressure required would be quite high and not available with technology of current civilization (air flow don't need to be very high, just pressure needs to be at least one bar for each 10m depth).

Note: other kinds of pump would work just the same, as long as water intake is near bottom, but would need a rather constant flux, while airlift can build quite a powerful suction, able to unclog most situations.

Other useful things Builders would have done is build channel system out of some smooth non-stick material

In any event some kind of maintenance will be needed and replacing failing machinery with low-tech "equivalents" could provide many plot ideas.

Answer 4

There's probably a more fundamental issue with this... as found by the mesopotamians (see for instance http://www.waterencyclopedia.com/Hy-La/Irrigation-Systems-Ancient.html) - water is a good transport of soluble minerals that get left in the land when the water evaporates. Silt blocking the canals so they no longer feed water to the salt-poisoned lands would perhaps be a blessing... silt would certainly be one of the lesser worries in the long term.

500 cubic kilometers is 500 billion litres. Even 'insoluble' quartz at 6ppm (solubility of quartz in water at STP) is producing in the order of 3x10^^6 Kg (three thousand tonnes / tons) of quartz deposit per year at the fields... which would create chaos at the areas being irrigated. And flooding fields doesn't get rid of this... unless you can wash it into oceans.

Answer 5

Let water and gravity do the cleaning

You already have variable waterflow in the form of seasonal flooding of the fields. If you get silt accumulation at 0.366m/s, then increasing the water flow should pick up that silt again and deposit it somewhere else. The customary location for silt to go is the ocean. Ensure that your water flow helps it get there.

Assuming there are periodic locks in this canal system, it shouldn't be difficult to develop a schedule where higher waters upstream induce flows greater than 0.366m/s.

Say we have three channel segments: A, B, and C where C is closest to the ocean. When the locks AB and BC are fully open, we get the full 0.366m/s flow rate across sections A, B, and C. However, when AB is 10% open and BC is 100% open, then sections B and C will drain out, leaving plenty of water in A. Once there's a substantial difference in water height, when AB is opened 100%, there should be a substantially higher flow rate than normal. This higher flow rate should scour the canal bottom and carry the silt further down stream.

Even with higher mean flow rates, there's still going to be bigger particles that will build up over time. Higher flow rates just mean that the particulates that precipitate out will be larger.

Lock Design Building lock gates that span half the 3km canal is ridiculous. Not only are these difficult to build without modern engineering techniques and modern materials, they really don't need to be that wide anyway. Build up stone piers in a line across the canal in much the same way that bridge piers are build. The distance between the piers will be slightly less than double the maximum width of a lock gate. Construct the piers in such a way that lock gates can be attached to them and will hold the weight of the water.

Once the piers are complete, build a bridge across the tops of the piers.

Answer 6

Periodic intense flooding.

Controlled waterways like canals and dammed rivers tend to accumulate more silt than they should. One way that this is alleviated in dammed river system is to open the flood gates of the dams for a week or so, every year or two, to flush sediment that accumulated during slow flow periods out of the system.

Seasonal flooding is one of the reasons that the Nile River never clogged up.

An explanation from the High Country News can be found here. A scholarly assessment can be found here. A blog describing the High Flow Experiment (HFE) can be found here.

Of course, not all of the sediment flushed out of the sand sinks in the canal system would end up out of the system entirely. It would also create sand bars and beaches further down the system.

The amount of sediment that accumulates would also depend greatly upon the material the canal was built with and its texture. A solid lining for the canals with a "slippery" texture is going to accumulate less sediment than one with sandy earthen walls with lots of nooks and crannies for sediment to accumulate in to start sand banks and the like.

Another option, not inconsistent with this one, would be to have dead end spurs where sediment was intentionally diverted through natural water flow, to keep the main channel clear. Sediment is often good soil for crops, so it has value out of the canal.

Answer 7

That is a very wide, very deep culvert.

In fact, it is a river. A very slow-moving river, over three kilometers wide, five stories deep.

A critical factor is what it is lined with. Is it simply excavated out of the existing soil? Is it rock, or clay-like? How sticky is the surrounding regolith?

If this is all 'fair game' for manipulation, then I would suggest that the advanced civilization would have lined it with some form of very smooth, low-friction, extremely durable plasticrete. Give it sides above ground level of, say, three meters to prevent surface soil from drifting in. You make no mention of any winds or storms, or their frequency. Installing 'drift fences' on either side further out from the sides would reduce sediment from blowing regolith. Ideally, the sides would be engineered to produce wind flows that form an air curtain over the top surface, so dirt and dust is completely blown over the top, and not deposited on the surface.

A combination of reducing the sediment before it enters, and sides that prevent it from sticking, would lessen the problem.

Now, put corrugations in the bottom, parallel to the sides, and the sediment is localized into channels. This makes dredging easier. Putting corrugations perpendicular to the sides would produce sediment traps, and dredging would be further localized. Perhaps drag lines, perpendicular to the sides, in these pre-formed channels, would make dredging a routine maintenance procedure. I am thinking perhaps a curved culvert, instead of a flat bottom channel, like half of a pipe, so the silt would naturally fall to a central point along the smooth sides. This would make it much deeper, to maintain the same volume.

But the crutch is the degree of engineering, construction, and materials that you are allowing of this advanced but extinct civilization that built the infrastructure.

EDIT

A lot of answers here base the flow rate on the slope of the channel. For this scenario, this assumption is not accurate. The flow rate would be based on how much water is removed from the basin. A bathtub has a very shallow gradient, and virtually no flow, until you pull the stopper. Then, the flow rate depends on the size of the discharge drain. This system is essentially a very big and very long elongated bath tub. Apparently it does not drain into an ocean, so the water is removed only for irrigation and consumption. The flow would not be constant, but would depend on demand. The more water is removed, the faster the flow.

Methinks the greatest factor would be the volume of water the basin holds, the amount of water withdrawn, and the amount of water that can be added by the tap (the polar region ice flows). If the tap supplies less water than needed the basin drains. If the tap supplies more water than needed, the basin overflows. If the tap supplies the same volume of water that is removed, the water level in the basin stays level. The gradient is irrelevant. It is the effect of gravity on the entire body of water that matters. Like a bathtub, drain one end and the level in the other end falls with it.

Irrespective of gradient, the more water that is removed, the faster the level falls. The narrower the channel, the faster the water flows towards the drain. It is hampered by the friction with the channel sides, not the slope. Smooth sides, less friction, faster the water flows.

However, the rate of evaporation depends on the flow of the water. The more stagnant the water, the greater the evaporation rate. So a narrower but deeper channel is more advantageous than a wider, shallower channel. A 3200m deep but 45m wide channel is just as effective, and delivers the same amount of water, but much less surface area for evaporation and silt accumulation.

Answer 8

If the canals drain into an ocean and the current created by annual floods is sufficient to push the silt to the mouth of the canal or canals, as a partial solution, long silt jetties could be built at the mouth to narrow the canal and increase the current. The increased current will cut through any sand bars building up at the mouth.

The engineer James Buchanan Eads designed such a system for the Mississippi River in the 1800s. His solution is described here: https://www.hnoc.org/south-pass-jetties-mississippi.

John McPhee mentions Ead's solution in the first essay of the interesting book The Control of Nature: https://en.wikipedia.org/wiki/The_Control_of_Nature.

Silt jetties are also in use at the end of the Mitchell River in the state of Victoria in Australia: https://www.marinerscoveresort.com/around-the-lakes/things-to-do/mitchell-river-silt-jetties/

Answer 9

Solution 1: floating plants.

I suspect that duck weed will be your easiest solution. Any small plant that floats will do. Give in very fine roots that trap silt. The locals in addition to hauling water out of the canals, skim the duckweed off as fertilizer, bringing silt with it.

Solution 2: Lower atmospheric pressure.

The size particle that can be carried is dependent on the ability of moving air to transfer momentum. Thinner air = lower transport. This postpones the problem. Higher wind speed can compensate however.

Solution 3: More stable atmosphere.

The size particle that wind can pick up is very dependent on wind speed. If you can come up with a plausible reason for low wind velocity near the canals, very little silt can be carried to them.

Solution #4 Shelterbelts

Create a tree like a redwood, but without the fog dependence. Have a tradition of the the last mile to the banks of the canal are planted with mongo trees. Give your planet lower gravity, and they could be a thousand feet high. Tradition is that only windfall in the forest can be harvested, but peasants plant a row whenever a child is born to provide a dowry later. These wouldn't be the giants along the canal, but even here on Earth a 20 year old balsam poplar is a good sized tree. (60-80 feet 1-2 foot diameter)

Solution #5 Cover crops.

Have a tradition of farming with cover crops. Soil is never bare. The permaculture crowd talks endlessly about cover crops and multiple cropping systems.

Answer 10

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.

Answer 11

The canals are slightly spiralling from poles to equator. The Coriolis force helps to move the water and in additon a fast-orbiting moon of the planet sweeps the canals with the tides. Canals are lined with synthetic nano-carbon-concrete that is badass durable. Friction of moving water in the waterbed creates energy that dissolves silt. Yay!

Answer 12

Well, it's been over 20 years since I took a hydraulics class, but ...

Don't use a flat bottomed canal.

Although you'll want to do periodic flushing of the canals by flooding the whole system (either through releases from a dam or a monsoon season), you can minimize the silt buildup by using a more parabolic-like cross section. This helps to concentrate the water flow down the middle of the canal, keeping the water relatively fast moving even when there's a lower volume of water moving through it.

The sewer systems in Liverpool make use of this technique, with the sewer cross section being sort of egg-shaped, with the pointed bit at the bottom, rather than being a cylinder.

The material used in the construction of the canal also matters. The smoother the surface (Manning roughness), the less slowing that you'll have at bottom of the canal, and thus less silt depositing.

Answer 13

Elevate the canal, like some sections of the Roman aqueduct. This will reduce the chance of debris except of the avian variety. Then, you only need to ensure it is silt-free before entering, which you could do with a distillation pool.

However on a desert planet you'd probably want to avoid having it exposed because of evaporation, which would also fix your sediment issue. The Romans constructed their aqueduct system pre 400bce, with covered and uncovered parts. The only difference is yours flows more water. If covering is not an option because you need all 3200m and cover management is impractical, then just have special side channels to prevent ingress of material in the main channel. Then the side channels can be dredged much more easily.

The only other option is to up the flow rate so it can carry more in suspension.