Wetenschap - 15 mei 2014

What makes a stream meander?

tekst:
Roelof Kleis

Meandering streams are iconic for the sandy ground of the Netherlands. Joris Eekhout received his PhD on the dynamics of meanders.

The Netherlands is a land of streams. Many tens of them wind across the sandy ground of the south and east of the country. Many of these streams were straightened in the first half of the twentieth century because water needed to run off farmland as fast as possible. But much of ecological value disappeared with the bend of the streams.
So in recent decades many streams have been partially restored to their original states. An expensive job. And not certain of success, as our understanding of the morphological processes in lowland streams is still limited. Joris Eekhout’s PhD research on the dynamics of meanders should change that.
The meandering of streams is a complex process, it appears from a nice field test done by Eekhout. The Aa and Maas waterways board and Wageningen University straightened out 600 metres of the Hooge Raam stream in Brabant and then let nature take its course for three years. Within eight months there was a regularly alternating bank pattern. But that disappeared just as fast when the bed slope decreased so that the water flowed more slowly. It takes more than that to get permanent meanders. Local disturbances such as spring water, for instance, which soften the bank and make it vulnerable to erosion. Eekhout draws this conclusion from a study of the Gelders-Niers canal in Limburg, where a spring caused meandering.

For a stable stream you need to make a good local analysis
Joris Eekhout

So there is a lot more to stream restoration than just studying the historical course of the stream on an old map. That became clear during the restoration of part of the Lunterse stream on the Veluwe. Within three months, the stream had straightened out a carefully laid meander. In retrospect that could be explained by interventions (damming and the deposition of sediment) elsewhere along the stream. ‘For a stable stream you need to make a good local analysis,’ says Eekhout. ‘You need to find out where the weak spots are in the vicinity.’ A section of meandering stream cannot be understood in isolation from its surroundings.

In fact, Eekhout thinks, you should only restore a stream if you have access to its full course. ‘But I can understand that waterways boards often can’t manage that. I do wonder, though, whether restoring one kilometer of stream has much effect on the ecology of the whole stream. You can only restore a stream successfully if you tackle the runoff dynamics at the catchment scale.’


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