Ebb and flow occur twice a day. Twice a month, the high tide is higher, and the low tide is lower. A lunar month is the time it takes the moon to pass through all its phases. When the gravitational forces of the sun and moon work together, a spring tide occurs. That is when the water reaches its highest level. If this coincides with a powerful westerly to northwesterly storm, the water can rise by another 3 metres. The combination of a spring tide with such a heavy storm is called a storm tide.
Tidal marshes are higher parts of the riverbank that only flood at strong high tides. Saline tidal marshes are mainly home to herbs and grasses that can withstand quite a lot of salt, such as sea lavender and glassworts. Brackish tidal marshes welcome a mixture of salt and fresh water and attract reeds and sea asters. Freshwater marshes form further upstream. They form a changeable landscape with shrubbery, reed beds and tidal willow forests.
The authorities in Flanders use spatial implementation plans (ruimtelijke uitvoeringsplannen) to define what is and isn't permitted on all plots in a given area. This type of plan can be prepared by a municipality, a province or the Flemish region.
A ring levee is built on the outer edge of a flood area to protect the hinterland. Such a levee is strong enough to hold back water during a flood, even during storm tides. There is usually a towpath on top of the ring levee. The towpath allows cyclists and walkers to enjoy the tidal natural environment.
A polder is a relatively low-lying piece of land that people have drained permanently. The polder water level is artificially controlled.
Flood areas can naturally absorb water, so water managers often use them for this purpose.
An overflow levee allows water to flow onto a flood area when the water level is extremely high.
In the past, the Grote Nete was surrounded by a levee in the entire valley. This is now causing local flooding. The removal of the levee and some outlet structures in certain places means that the tributaries are reconnected with the Grote Nete. This will prevent flooding and allow fish to move freely again.
Depoldering means giving land back to the river. First, we construct a new levee inland. Then we make openings in the old levee, creating additional room between the old and the new levee where the river can (over)flow. This reduces the power of the water and the water level of the river while limiting the risk of flooding in the region. The tides are given free rein again, which results in the resurgence of valuable tidal nature.
An artificial sand martin wall consists of concrete elements placed next to each other. Each element has a number of holes that sand martins can use as nesting sites.