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.

Comments are not for extended discussion; this conversation has been moved to chat. — James, Nov 30 '17 at 19:44
Um. you DO realize that this amount of water is about 25% more water then the whole USA uses? Combined agricultural, industrial, residential, private and public together. The works! — , Nov 13 '20 at 12:35
@MarvinKitfox Yes. It's the same as the average out flow of the Mississippi or about 8% of the outflow of the Amazon. As in our world most of the river water flows out to the sea. In the latest version of my story the water will be divided into five separate channels from the main head water canal described, which begs a number of other questions... I feel another question coming on! — Slarty, Nov 13 '20 at 12:58

score 50 · Answer 1 · edited Jun 16 '20 at 11:03

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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.

edited Jun 16 '20 at 11:03

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answered Nov 27 '17 at 12:02

Separatrix

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Seems true of canals in general that some dredging will be needed, however I'm not sure the ancient Egyptians ever had to dredge the Nile to ensure it made it to the sea. ;o) Perhaps I just need to adjust the flow rate / gradient? – Slarty Nov 27 '17 at 14:53
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@Slarty, you might think that, but Leonardo da Vinci designed a dredger. As soon as there's something, like a sunken boat or a large rock, that your flow rate can't clear, it'll start silting up. Maintenance is required and it's better to have a culture of maintaining the canal than having to work out how to fix it later. – Separatrix Nov 27 '17 at 15:09
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@Slarty The big difference is that the Nile is a natural river, not a canal. Silt will be deposited either along the way in bends and sharp turns or is forced out when the river periodically floods the Nile Delta, which deposits valuable silt and sludge there. This is why egypt could grow so large in early human history. – Valthek Nov 27 '17 at 15:32
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@Slarty if the ancient Egyptians wanted to prevent a particular distributary channel in the delta from silting up, they would have had to dredge to achive that. See https://en.wikipedia.org/wiki/Nile_Delta#Ancient_branches_of_the_Nile – Vince Bowdren Nov 27 '17 at 16:25
@VinceBowdren yes true but if they didn't the water would still reach the sea. Also the thousand odd miles between Aswan and Cairo didn't have to be regularly dredged to keep the Nile flowing. – Slarty Nov 27 '17 at 19:15
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@Slarty, that's an unmanaged natural waterway though. You're not bothered by which channel it's using or where it flows. In the Port of London it's important where the channel runs and that deep water is available at docks. If you're happy to have periodic flooding and a wandering deep water channel then yes, you can leave it be, some water will always get through. Ancient Egypt lived around the flooding cycle after all. – Separatrix Nov 27 '17 at 19:35
@Separatrix I'm thinking that what needs to be built are large enbankments to keep the waterway in. Periodic flooding was a requirement in the question. – Slarty Nov 27 '17 at 19:40
@Slarty, I assumed you meant periodic controlled flooding, not wherever it felt like happening. Building embankments doesn't save you from a silted up stream, yes you want slack in your channel depth but there's no getting away from dredging in the long term. – Separatrix Nov 27 '17 at 19:51
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This is the way the Brandon Sanderson in the Mistborn trilogy handles it. I mean it's practically just mentioned in passing, because while the canals are important they're not exactly central to the plot. Otherwise they'd fill up with ash and no longer work. – Wayne Werner Nov 27 '17 at 20:08
@Separatrix I'm thinking that the gradient should be steeper perhaps 1/10,000 rather than 1/1000,000. That way it should be able to wash itself clear of silt like the Nile river does (that's been running for thousands of years and doesn't need regular dreging to keep the water flowing). – Slarty Nov 27 '17 at 20:45
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@Slarty this is what the Nile looks like further up, it's not all safe and tame like the lower stretches. A bit of silt won't stop that, but it will stop anyone using the waterways – Separatrix Nov 27 '17 at 21:28
@separatrix I was thinking of the 1000 odd km downstream from Aswan all of which is navigable at roughly 1/10,000 gradient and doesn't seem to suffer from any insurmountable silting problems. – Slarty Nov 27 '17 at 22:00
@Slarty, the modern Nile is dredged – Separatrix Nov 27 '17 at 22:02
@separatrix, yes but to ensure there is sufficent depth for shipping to get through. Not to ensure the water can reach the sea. I'm sure the Nile would find its way to the sea without dredging of any sort. – Slarty Nov 27 '17 at 22:15
You also need to be careful when dredging, depending on the system used: https://www.worksopguardian.co.uk/news/memories-of-pulling-the-plug-on-canal-1-624620 – SeanC Nov 27 '17 at 22:50
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@Slarty: The modern nile would silt up without dredging. It's the embankments that cause silt build up. Rivers WITHOUT embankments are what make non-dredging work because silt build up will not clog the river instead it would cause a kink in water flow which would erode the opposite bank. It's this erosion that prevent rivers from clogging - any build up of silt will wash away an equivalent amount of silt on the opposite bank. And since it's practically impossible to perfectly drop silt in the exact middle of a river there's always an "opposite" bank. – slebetman Nov 28 '17 at 07:39
@slebetman Yes too much water has been extracted from the Nile for irrigation and its causing problems. I must ensure there is sufficient water to allow run off to carry silt away. I think you are partly right about the embankments, but it depends how far away the embankments are. Rivers meander inside their valleys but the route they take generaly doesn't break out of the valley. So very wide embankment would produce a situation more like a river valley. Very narrow embankments would cause a problem. – Slarty Nov 28 '17 at 09:19
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@Slarty, Rivers don't break out of the valley because the river has spent millions of years creating the valley. Sometimes the reason for the river is the valley and sometimes the reason for the valley is the river, but the two are tied. Yours is a canal not a river though, its geography is artificial – Separatrix Nov 28 '17 at 09:22
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"Ultimately the people will have to dredge to keep the channels clear." Depends on how advanced (and forward-looking) the civilization that built the channels was. It needs to be dredged, but perhaps it has self-dredging mechanisms, or self-cleaning silt traps or something like that. That kind of engineering isn't my specialty, but at least with that much water moving there's a permanent source of renewable energy to power the mechanisms. – GrandOpener Nov 29 '17 at 09:54
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Long story short: if you want a river or canal to stay where they are, you have to dredge them. If you don't they silt-up and then move somewhere else. Note: Canals that move somewhere else are called "rivers". – RBarryYoung Nov 29 '17 at 19:27
Perhaps they flood the banks and create fertile fields if they are NOT dredged. The designers would have made the system so it does not break if left unatended due to temporary causes. – KalleMP Dec 05 '17 at 20:57

score 20 · Answer 2 · answered Nov 27 '17 at 12:24

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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.

answered Nov 27 '17 at 12:24

Rissiepit

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these slugs will probably sometimes die in canals. And where would they poop? 2. Wouldn't they need maintenance? 3. Good one.

Mołot

Nov 27 '17 at 12:40

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@Mołot they poop on land (that's the whole point, right?) and obviously they eat any dead silt slugs too... – Nov 27 '17 at 15:21

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@dan1111 Attack of the cannibalistic silt slugs! – BrettFromLA Nov 27 '17 at 21:29

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because nothing could go wrong with bio-engineered cannibalistic anything :) +1 for that. – Paul TIKI Nov 27 '17 at 21:57

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Ah, but if you use the silt slugs, you also have to deal with the hive queen. Might need to ask Ender for an appointment. – BlackThorn Nov 27 '17 at 23:13

Along the lines of #3 if at the bottom of the drop there was a drain that was normally closed a person could periodically open the drain and flush the debris out. The bottom would be funnel shaped so all the debris would go down the center to the drain. – cybernard Nov 28 '17 at 01:12

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I considered adding in slug farmers. Yes, as dan1111 says, they poop on land. And if the silt is nutrient rich then the slug poop would be a valuable fertilizer. Marvelous and useful "cottage" industry. – Rissiepit Nov 28 '17 at 08:10

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An interesting detail is that options one and two are not mutually exclusive. Self-repairing machines are a form of life, even subject to natural selection. – Censored to protect the guilty Nov 30 '17 at 12:23

score 9 · Answer 3 · answered Nov 27 '17 at 13:05

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.

I'm thinking there will have to be a very powerful and sizable preisthood based acrosss the network with the responsibility to maintain it. But I like the idea of the semi automted set up with decanters and pressurised cleaning — Slarty, Nov 27 '17 at 14:35
Perhaps they could find some very large glacial melt water lakes to tap — Slarty, Nov 27 '17 at 14:46
@Slarty: note the "Arctic Ocean" may be mostly frozen, but should be swarming with life. Also hot climate plus water plus slow current plus dirt on bottom equals huge algae vegetation. Such large channels will also be hosting peculiar aquatic fauna. — ZioByte, Nov 27 '17 at 15:40
I am wondering why a canal must silt up when natural waterways seem to flow for thousands of years without dredging? The Nile seems to flow smoothly at a roughly 1/10,000 gradient between Aswan and Cairo without getting blocked by vegetation so why can't we just engineer an equivalent set up? — Slarty, Nov 27 '17 at 22:10
@Slarty: natural waterways do sift up, if there's not enough pendence. When the original riverbed is too clogged the river simply finds another lower level bed. Problem nowadays (and in your setup) is we do not want the flow to "find another way to sea"; in modern times because there's no "free land" we can relinquish to the river in your world because there'd no sea to point to (generic assumption here all the land will be somewhat above sea level. This is, however, how all alluvial plains formed: deposit some dirt along the riverbed, when too clogged go fill another place, repeat. — ZioByte, Nov 27 '17 at 22:44
you’re right when the gradient / water velocity are low as in a delta where the sediment is deposited and distributaries start to form, but as we can set the gradient this need not happen. If the gradient is set to say 1/10,000 the river should not significantly silt up. The Nile has been flowing in the Nile valley for thousands of years. — Slarty, Nov 27 '17 at 23:18

score 9 · Answer 4 · answered Nov 27 '17 at 14:51

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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.

answered Nov 27 '17 at 14:51

matt

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Just from the volume of water being transported, you can tell this is a major irrigation work. 3000 tons/yr would cause havoc on a small field, but won't even be noticeable when spread out over the Sahara. – Mark Nov 27 '17 at 19:26
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Salinity would be a major issue, but I propose the water drains down into an old aquifer several hundred meters below the surface and then out towards the remnants of the old seas. This would be augmented by drains where needed as in Egypt today where the land is being desalinized despite 250ppm of salts in the water before irrigation. At around 1 person per hectare 50,000,000 people require around 500,000 square km of land so that 3000 tons of dissolved granite would end up spread at the rate of 6kg per square km per year assuming that none is washed away in the drains. – Slarty Nov 27 '17 at 20:11
Although if they are melting it from ice directly and putting it right in lined channels the ability of the water to dissolve anything will be minimal, very little surface area to act on, dissolved granite is actually a good thing as it is basically fertilizer. – John Nov 28 '17 at 04:48
If the canal was lined, it would not pick up any dissolved minerals except those it started with, and those that managed to blow in. If it had an apron, like a pool deck, and the sides were elevated, there would be very little accumulation of minerals in the water. You end up with only what you started, – Justin Thyme Nov 28 '17 at 15:28

score 7 · Answer 5 · edited Jun 16 '20 at 11:03

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.

I like that idea, although I'm thinking of increasing the gradient from 1/1000,000 to 1/10,000 so the flowrate should increase and the need for dredging or flushing would be diminished although not removed entirely. — Slarty, Nov 27 '17 at 20:41
@Slarty even with increased flow rates, I'd still build in a method for flushing the canals. Even if buildup takes 10 years instead of 1, you'll still want to flush. — Green, Nov 27 '17 at 21:00
yes I think it would be a wise precaution although it should work without in the same way the river Nile does. — Slarty, Nov 27 '17 at 21:56
Remember that the flow rate only means something when the water has somewhere to go. If the canals are a closed system (they do not lead to an ocean) the only flow would be due to the water withdrawal due to irrigation. If no water is withdrawn, where does it go? Do they pump it back into the polar regions? With no exit basin, this canal is essentially stagnant when no water is withdrawn, By the way, where DO the flood waters go? Or do you have a huge subterranean reservoir, or an ocean? In which case, what is the water cycle? — Justin Thyme, Nov 28 '17 at 15:13

Justin Thyme · Answer 6 · 2017-11-28T18:55:32.970

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.

Raised sides, drift fences and canal bed corrugations all sound like good ideas. And a curved bottom should also make dredging easier. This is a world similar to Earth but much drier, so there would be winds and storms although not a lot of rain. The civilization that built the canals had a level of technology similar to ours and as that civilization was in danger of extinction due to lack of water, the resource available would have been massive. — Slarty, Nov 27 '17 at 20:28
And if you placed a solid plasticrete apron on either side for, say half a kilometer wide, they could keep the edges swept up. It would be labor intensive, but low-tech. — Justin Thyme, Nov 28 '17 at 00:33
The culvert-river is about three km. wide. On each side of it, build a flat apron (or deck, like one around a pool) that is about half a km. wide. It could be kept clear of debris, like a swimming pool deck, to minimize any debris getting into he stream. If the yearly flooding followed the water way, the flow of flood water would also keep it clean. It has the added advantage that the waterway would not erode and change directions. If you then placed tall pillars on the apron, they would disrupt the wind speed and cause the wind blown grit to fall behind them. — Justin Thyme, Nov 28 '17 at 01:05
Properly designed and placed, they would crate a high pressure near the ground and force the wind high over the waterway, like the wind tunnels in the center of a high-rise city. The effect would be augmented if the apron sloped up towards the water edge on each side, causing the air to compress over the river. — Justin Thyme, Nov 28 '17 at 01:06
If you have looked at wind-blown snow patterns over a snow drift, just behind the drift on the leeward side the snow is clear. That is the effect you are after. You don't have to micro-design or engineer it, just assume the correct shape. — Justin Thyme, Nov 28 '17 at 01:10
The canal is far too wide for wind-breaks to do much to keep dust from blowing in. The heaviest particles in a light breeze which never get more than a couple meters off the ground would be stopped, but 3km is so astoundingly wide this is hardly worth the effort. So much would be blown above the wind-breaks and drift down onto the water that the wind-breaks hardly seem worth the continual maintenance costs. — pluckedkiwi, Nov 28 '17 at 15:54
@pluckedkiwi 'Effect of topography on wind behaviour in a complex terrain' and see for instance figure 6 for a graph of wind disturbances that is in the vicinity of the distances of this scenario. — Justin Thyme, Nov 28 '17 at 18:08
ctd The artificial wind breaks could be any height. Say, 320m high pillars. Ten times the height gives the required 'shadow' of 3200m. The average wind turbine is about 100m high and a tight row of them can affect wind for a km or more leeward of them. — Justin Thyme, Nov 28 '17 at 19:14

ohwilleke · Answer 7 · 2017-11-27T22:39:26.513

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.

score 3 · Answer 8 · edited Nov 28 '17 at 14:11

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/

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score 3 · Answer 9 · edited Jun 16 '20 at 11:03

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.

Shelterbelts - yes I like it. Good where the desert might cause dust problems. My world is an Earthlike world so I can't do too much, but there might be a little wriggle room. — Slarty, Nov 28 '17 at 17:28

KalleMP · Answer 10 · 2017-11-30T22:30:45.553

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.

Very true, I was planning to have the remaining silt laden slightly salty water in the canal system overflow on to spillways at some of the lowest points in the network and run down another 10-100 km or more out into the old ocean beds where it could evaporate in a giant salt pan. They would also have an old aquifer deep under a lot of the land into which water could flow, supplemented by some artificial drainage in places. — Slarty, Nov 30 '17 at 09:02
I just realised what the biggest danger would be over time. Plant life taking root at the banks and eventually blocking the channel. Quite a challenge. — KalleMP, Jan 08 '18 at 19:19

score 2 · Answer 11 · answered Nov 13 '20 at 16:00

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.

score 2 · Answer 12 · answered Nov 28 '17 at 17:21

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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!

answered Nov 28 '17 at 17:21

RRH

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Sweeping the canals with the tides is imaginative, although it would require a moon in a ploar orbit or at least a highly inclined orbit to be effective. – Slarty Nov 28 '17 at 17:33

score 1 · Answer 13 · answered Nov 28 '17 at 18:28

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.