Wetenschap - 8 februari 2018

Using ferries to measure sand transport

tekst:
Roelof Kleis

If it is up to Ton Hoitink, all ferries would soon be fitted with equipment for monitoring currents and sediment transportation in rivers. These measurements could add much to our understanding of erosion, sedimentation and salinization in the Dutch delta, says the newly appointed personal professor of Environmental Fluid Mechanics.

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If it is up to Ton Hoitink, all ferries would soon be fitted with equipment for monitoring currents and sediment transportation in rivers. These measurements could add much to our understanding of erosion, sedimentation and salinization in the Dutch delta, says the newly appointed personal professor of Environmental Fluid Mechanics.

Riverbeds change constantly as a result of erosion and the deposition of particles. Those processes are influenced by the currents in the water. The volume of water and sediment on the move determines whether erosion or its opposite, sedimentation, takes place. An additional factor closer to the coast is the intrusion of salty seawater.

An understanding of all these processes is crucial to keeping the rivers navigable and freshwater fresh. There are models for this, but to test those models against reality requires large amounts of data. And that is why measurements are so crucial, explains Hoitink: measurements such as those currently being taken on a daily basis on the ferry between Den Helder and the island of Texel. Accoustic measurements there are providing a picture of the currents and the particle transport in the Marsdiep tidal channel.

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The technology used is called an ADCP: Acoustic Doppler Current Profiler. ‘The apparatus transmits high frequency signals which are reflected back by the particles in the water on the way down to the riverbed. The doppler effect that then occurs is related to the speed of the current in the water. The strength of the reflected signal also tells us something about the number of particles in the current.’

Ton Hoitink wants to establish a salt monitor, 'like a weather radar system but for salt intrusion'

As it plies up and down, the ferry collects ‘an incredible dataset’. Hoitink would like to use this equipment at other locations too, starting with the Nieuwe Waterweg to find out more about the influx of salt water in the Zeeland delta.

All this measuring should result in a salt monitor, says the professor. ‘Like a weather radar system but for salt intrusion. A website where you can see roughly how much salt intrusion there is going to be in the next few days. A service like this could be very useful for farmers, water boards and drinking water suppliers.’


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