Science

Alterra and the art of meandering

Today, the course run by very many of our streams is far from natural. As a measure to combat flooding, many traditionally meandering streams were straightened in the last century. Straight streams carry water away at a faster rate. But these days that is no longer what we want.
Roelof Kleis

Rapid drainage leads to drying out and straightened streams offer few opportunities for a diversity of flora and fauna. With stream restoration, we are trying to turn that tide. Bart Makaske and Gilbert Maas of Alterra have written a book about the best ways we can let streams meander.

This is trickier that it seems at first sight. According to Makaske, much more is involved than simply restoring the former meanders shown on old maps. ‘The historical situation is very often not the best aim,’ he explains. ‘The natural course of a stream is in harmony with all kinds of factors, such as hydrology and the condition of the terrain. Changes in these factors can mean that what counted as harmony in the past no longer applies. In addition, the dynamics of the stream’s forms must be accurately estimated. Will the location of bends shift due to erosion? Will sandbanks and embankments be created? Old maps can’t tell you that.’

The answers to such questions can be found in the Handbook for Geomorphic Stream Restoration. The main principle underlying the approach is revealed by the book’s title: geomorphology. The natural condition of the terrain is of primary importance. The form and dimensions of the meanders must be appropriate to their environment. ‘It’s all about using nature, letting nature do as much of the work as possible,’ says Makaske. ‘In this way you can create a natural harmony that is self-sustaining.’

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This is because how a stream finds its course and behaves can be calculated with a fair degree of accuracy. Provided you know the conditions – the drainage, the difference in height, the type of subsurface and so on. The design process used by Makaske and Maas also involves a clear-cut step-by-step plan. The idea is that if the form and dynamics of the stream are well chosen, ecological restoration will follow. ‘Rather than being about returning to the historical situation, restoration is about limiting human intervention.’

And that last point is music to the ears of water authority managers. A stream that is self-sustaining keeps management costs at a minimum. This approach is completely at odds with taking the desired ecology as the starting point for stream restoration, an error which Makaske believes has been often made in the recent past. ‘That leads to artificial meanders and a stream profile that has nothing to do with the natural forms that arise under the influence of flowing water.’

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