Here we take the Waal as an example. In order to keep the Waal more navigable, this river will be widened and deepened in various places. This allows ships to be loaded deeper and wider, so that much more cargo can be transported.
For example, navigability is increased by:
- Laying of bottom groynes
- Bottom screens
- Application of a fixed level
- Crib lowering
Laying of bottom groynes
Bottom groynes are dams of about 200 metres long that lie under water almost across the stream. The bottom groynes must ensure that the spiralling flow of the water is interrupted. This spiral current causes inner bends to clog up and outer bends to wear out.
Bottom screens are bulkheads that stand at a small angle to the mainstream in the outer bend of a river. These screens should ensure that the
spiral-shaped flow of water is interrupted. As mentioned before, this spiral current causes inner bends to silt up and outer bends to wear out.
Application of a fixed level
The application of a fixed level is also one of the measures to widen the river, especially in the bends. A solid level of stones at the bottom of the river causes the water and sand in the river to rub out the inner bend. The fixed level has been laid in the outer bend and the soil is now fixed there. Because the same amount of water has to cross the bend, the river adapts to the new situation and then sands out the inner bend.
Cribs keep the river’s summer bed at depth. They’re on the sides of the river, perpendicular to the water. As the cribs constrict the main channel, the water in the middle of the river flows harder. But at high tide, the cribs hinder rapid outlet. By making the groynes lower, this problem is largely solved. It does have a relatively high price tag. And much lower is not possible, because then shipping is severely hampered.
Large quantities of sand will be moved by means of special dredging works.
During the 1998 dredging season, the ‘test morphology’ was carried out. A total of 25 kilometres of channel were brought to the desired depth and width in various sections of the Waal between Nijmegen and Tiel. The main goal was to see how the river would react to the dredging work. Would the dredged and dumped sand rewind to the same shallow water a week later? Or wouldn’t it be all the way back even after a flood period? The truth turned out to be in between. At low drains, the soil remained fairly level, varying from a few weeks to a few months. But the floods of 1998/1999 destroyed most of the dredging work.
It also turned out that in some places the sand had returned much earlier, for example just after a bend. Rijkswaterstaat wants to respond to this by dredging a little less in some places and a little more in others. In order to understand how the soil reacts to dredging, measurements were taken, and again and again. The computer then calculates how the river reacts on the basis of these data.