The dredging industry has been the backbone of the Dutch economy for centuries. If canals, harbours and rivers weren't dredged out for a few years, the whole country would literally grind to a halt.
Today, dredging happens with oil powered ships. However, the Dutch waterways were dredged mostly by hand for centuries, using simple but ingenious tools. Could it be done again?
Siltation is a serious problem in the Low Countries, because of the low flow rate of the water. The Netherlands is located in the delta area of various rivers, which supply an estimated 10 million of m3 of silt and clay particles per year. The country also has many waterways and is home to the largest port in Europe, Rotterdam.
Each year, some 30 to 35 million m3 of mud are dredged out in the maintenance of the Dutch waterways. Approximately 75% comes from salty waters. In the port of Rotterdam alone, 20 million m3 of mud is collected each year.
The demand for dredging keeps increasing. Ships keep getting bigger -- both inland and seagoing vessels -- requiring ever deeper and wider waterways. A "modal shift", in which cargo transport moves from the road to the water, means more larger ships and therefore wider and deeper canals -- and thus more dredging.
Dredging by hand in Delft, the Netherlands. Source: Maritiem Digitaal.
Although most of the mud is dumped into the sea, each year 3.5 to 5 million m3 of contaminated sediments must be landfilled. Then there is the dependency on fossil fuels. A typical suction hopper dredger has a pumping power of 2,500 kW and removes 100 m3 of sediments per minute. The largest dredgers have 30,000 kW engines and 6,000 kW of pumping power. At full power, these ships consume 3,000 litres of oil per hour.
Dredging a Country by Hand
Silting is a very old problem in the Netherlands, so how did this job happen before the arrival of fossil fuel powered dredging machines and boats?
For centuries, the Netherlands were mainly dredged out by hand. Dredgers stood on a small boat and scraped mud from the bottom with their "dredge bag" ("baggerbeugel"). In an alternative configuration, the dredger stood on a wooden board that was supported by the river bank on one side, and by a floating container on the other side.
A dredging bag: Maritiem Digitaal
Dredging by hand, standing on a boat. Source.
The dredge bag, a tool that was also used for peat cutting, was a long stick (up to 6 metres long) with an annular metal scraper and a net at the end. There were different types of nets and bags, depending on the composition of the sediment. Working with the dredge bag, the handle was rested against the shoulder, so that the net could be dragged over the bottom with two hands.
The mud was pulled ashore or deposited in a flat barge. For large dredging works, such as the construction of the Northern Holland Canal in 1822-1825, thousands of workers with dredging bags were deployed. Until about 1960, contractors of dredging works employed men with dredging bags for the maintenance of shallow ditches and canals. The tool is still for sale.
Manual dredging is heavy and time-consuming work, so people designed technology that could ease and speed up the task. Furthermore, ships became ever larger. In the last quarter of the sixteenth century, the "dredge mill" was introduced. It was still based on human power, but now people were only the source of energy for a machine.
Several people worked large treadmills or capstans driving a paddle wheel that scooped the mud from the bottom and threw it into a barge that was moored across. The dredge mill was usually made up of two flat barges with the rotating wheel in between. It worked up to a depth of two metres. These machines were often operated by prisoners.
Dredge mill. Source: Beeldarchief Rijkswaterstaat.
The depth of a merchant ship in the seventeenth century, depending on the cargo, was between 3.5 and 5 metres -- too deep for the dredge mill. In 1622, the first horse-powered dredge mill was built. Three to six horses ran a pivot which set in motion a bucket chain. Because of the heavy work the horses had to be changed every hour.
In 1829, this technology could be used to dredge up to a depth of 5-7 metres. Working at a depth of 3.2 metres, with three to six horses, approximately 20 m3 of mud could be collected each hour. By comparison, the average modern suction dredger -- which removes 100 m3 of mud per minute -- is as powerful as 300 horse powered dredge mills.
Mechanical dredge bags on a pontoon. Bron.
The original dredging techniques were also improved in later centuries. Mechanical dredge bags emerged in the sixteenth century, when someone got the idea to pull the dredging bag with a rope over a winch. Mechanical dredge bags could be mounted on ships, but several dredge bags and winches could also work side by side on a pontoon.
During the first half of the nineteenth century the valve barge was invented. The bottom of this small boat could be opened without causing it to sink. In this way, time could be saved with the removal of the mud.
Valve barge with dredge bag. Maritiem Digitaal.
Dutch dredgers also took advantage of renewable energy sources to lighten the work. In particular wind and tide energy were used. From the fifteenth century onwards, the "scratcher" ("krabbelaar") was used, a scraper that could dredge out gullies if there was enough current.
With a strong current, dredging becomes easier, because the mud only needs to be loosened. The tide ensures the discharge of the material to the sea.
Hand powered scratcher. Source: Maritiem Digitaal.
In harbours with strong winds and tides, scratchers were rigged with sails. These triangular sailing ships had a broad back and a flat bottom. Attached to the bottom was a harrow with iron spikes. At mid tide, the scratcher was placed just before the lock gates of a scouring basin, which was filled during high tide.
1699 model of a scratcher rigged with sails. Credit: Maritiem Digitaal
At low tide, the sluice gates of the basin were opened and the scratcher was pushed through the harbour with great force as the iron teeth scraped across the bottom. The ship gathered extra speed through the wide back and, if the wind was good, the use of sails. Horses could also be used, pulling the ship in the absence of good winds.
Wind powered scratchers were in use at least since 1435 in the southeastern part of the Netherlands. The flat bottom of the scratcher hinged and could sink with the help of cables to improve the draft. Two revolving doors, which could make a sharp angle of about 45 degrees with the ship, increased the reach of the barge.
Nevertheless, manpower was still needed. Five to six men kept the monster in the right lane, while two to three men kept the harrow at the desired depth through pulleys and hoist blocks.
Alternatives to Dredging
Dredging was not the only answer to the siltification of large rivers. Until the nineteenth century, the choice was also made to make the banks or dikes even higher, so that the water level was allowed to rise.
In a report from 1825, the dredging of large rivers is not considered because they were too deep and too wide for the technology of those days. It was only with steam power that dredging was also done on major rivers.
Een Friese Skûtsje. Foto: Skûtsje Langwar.
The province of Friesland, in the north of the country, reveals yet another alternative to dredging. The Frisians never used dredge mills, horse mills or other tools than dredge bags. They continued to dredge by hand until the arrival of the steam engine.
However, they innovated in a different way: from 1889 to 1933, they built 1200 large cargo ships with a very limited draft -- the so-called "skûtjes". Boats with a smaller draft meant less dredging.
How many people do we need?
In conclusion, dredging was mainly a human powered activity until the arrival of fossil fuels. As part of the Human Power Plant project, we are investigating how many people would be needed if were to dredge in the Netherlands by hand again. To answer this question, we will be dredging a piece of Frisian waterway on 24 May to find out how long it takes to collect 1 m3 of mud.
Based on preliminary data, it's already clear that many people will be needed. The aforementioned pump capacity of a suction hopper dredger -- 2500 kW -- corresponds to the capacity of approximately 25,000 human dredgers. The largest dredgers have a pump capacity that is equal to 60,000 human dredgers.