Science - May 18, 2011

Nature reserves along IJssel can raise water to dangerous levels

Water levels would become dangerously high if the water meadows along the Ijssel River were planted full with natural vegetation, based on calculations by Alterra. We will have to make choices, says Bart Makaske.

The banks of the Ijssel River would be a long stretch of nature reserves in time to come. Numerous plans to this effect have been finalized and some are already being carried out. A pretty picture, it would seem to be. But so much nature also has a bad side:  it raises the water level. The coarser the vegetation, the more it obstructs the water flowing towards Lake IJssel. This will result in higher water levels.
Forty centimetres
Makaske and his colleagues are the first to produce an accurate forecast related to this effect. Their work involves looking way into the future (up to a hundred years), and also takes the development of the vegetation into account. The results are alarming. If nothing is done, the water level along a large stretch of the Ijssel would be forty centimetres higher at high tide in ten to thirty years' time. In certain areas, this would even reach sixty centimetres. Regulations would then be breached way and above the safety limits.
Makaske stresses, however, that there is absolutely no need for the inhabitants in the surrounding areas to fear for their safety in the short run. 'Our results are based on the extreme scenario in which the nature plans are being carried out all at once. In practice, this will of course take place in stages.' But these calculations are alarming for the planners, he says. 'It is sad that all kinds of plans drawn up for nature development would never be carried out because they would cause the water level to rise too much. The Dutch water authority would not let that happen.'
According to Makaske, choices have to be made. For example, in the way the new nature is managed, so that the water flow faces less obstruction. Or the objectives need to be redefined. Makaske: 'What type of nature do we really want and how can we achieve that?  Carrying out all those nature plans a little at a time should not be allowed. This is not an anti-nature issue. But it's wrong to give the idea that there is enough space. Our calculations show that clearly.'
More room
A third alternative is to make more room for the river, such as shifting dikes or digging more subsidiary water channels. The river also needs more space to accommodate climate change effects. Currently, a maximum water drainage rate of 16,000 cubic metres per second is the standard. Climate change could raise this to 18,000 cubic metres. 'In that case, even more water has to flow through.'