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Think of China’s Sorrow, the Huang Ho (Yellow River). Like the world’s other major rivers, it carries a heavy load of silt. As the river flows to the sea some of that silt settles onto the river’s bed, thereby raising the level of the bed. Periodically the river floods: the land around the source of the river receives too much rain and the extra water overfills the channel. The raised riverbed makes the flooding worse.
Floods, to say the least, interfere with settled societies. People typically respond to the threat of floods by building levees along the river to contain the extra water and thus protect the land. In Northern China, for centuries, people have responded to the silting up of the Yellow River by raising their levees ever higher. Now, in some parts of the country, the bottom of the river actually lies above the level of the surrounding countryside. As a major consequence the breaking of the levees cause floods that kill millions of people. Clearly we want to devise some means of preventing such dire events from occurring.
Instead of raising levees ever higher, we need to lower the riverbed. By re-deepening the channel we can recreate the original river and thereby prevent floods. But traditional dredging is expensive, so we need something else. The river itself makes that something else possible.
Consider the device called a harrow. The word harrow refers to a farm implement that consists of a heavy frame carrying an array of sharp steel teeth or upright disks. Dragging it behind a tractor, a farmer will use it to break up and even off plowed ground. If we were to drag one of these things across a riverbed, it would tear up the deposited silt and throw it up into the current.
We could, of course, drag the harrow behind a boat, but we have a more convenient tractor at hand. We need only fit the harrow with a sail and drop it into the river. Weights on the bottom of the sail keep it close to the bottom of the river and floats on the top of the sail keep the sail deployed across the current. Thus the power of the river drives the harrowing of the riverbed. In addition, small turbines on the harrow can harness some of the current to generate electricity to operate onboard computers and small winches that control the deployment of the sail and thereby guide the harrow.
Now imagine how this system will work. Pick a silted up river, such as the Yellow River, and prepare to dredge it. To begin the process of autodredging the river, drop your harrows into the river near its mouth. At first the harrows will keep their sails partly furled, keeping them deep enough under the surface of the river to avoid interfering with other traffic on the river. When the harrow reaches the mouth of the river and begins descending into deeper water boats with grappling hooks will retrieve it and take it back upstream. A sonar transponder on each harrow will enable the dredging team to track the device.
As the river deepens, drop the retrieved harrows progressively further upstream. Where the river has deepened the harrows will unfurl their sails to harness more of the current and thus speed up their gouging of the river bottom. Mile by mile the harrows will improve the river’s flow until they reach a point where the river no longer presents a flooding hazard.
We can expect that the harrows will tend to follow the center of the channel and gouge out a trench there. For that reason the harrows will need active control to ensure that they scour out the entire river bottom. As each squadron of harrows proceeds downstream the lead harrow with scour the center of the channel and the following harrows with scour parts of the riverbed progressively closer to the riverbanks. Thus we will get a uniform deepening of the river.
Every now and then a harrow will get hung up on a snag. We will have to sent divers down to free the harrow and to remove the obstacle. Certainly this will make the first cleaning of the river slow and expensive, but once the snags have been removed subsequent dredgings of the river will proceed more smoothly.
This process applies to more than the Yellow River. Any slow-moving, silty river that has a tendency to flood stands ready for a fleet of harrows to keep its channel properly deepened. As obvious examples, we have the Ganges, the Nile, the Amazon and Orinoco, and the Mississippi. Anywhere that the flow of silt-laden water threatens human settlement, a simple farming implement can mitigate the threat.
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