Waterplanten als natuurlijke stuwen (KB-14-003-046)

Aquatic plants form an essential part of lowland streams. Besides their ecological function they have a significant impact on geomorphology, water quality and water quantity of lowland streams. From a historical perspective the main focus in the Netherlands has been on the removal of aquatic plants from watercourses to maintain their primary function: water transport. The high maintenance costs associated with the removal of vegetation from watercourses in combination with the growing attention for the positive ecological effects of aquatic plants due to the implementation of the Water Framework Directive, and the issues with low of groundwater tables in summer, have led to the idea to explore the possibility to remove weirs from watercourses and to refrain from mowing aquatic plants so the plants can take over the function of the weir.

The goal of this study is to provide an overview of current knowledge on resistance to water flow in relation to aquatic plants and to determine the manner in which the properties of aquatic plants have an effect on the flow resistance in a watercourse.

The present study suggests that the omission of mowing watercourses need not always lead to flooding, especially, when it is accompanied by a decrease in water level due to the removal of weirs. At peak discharge, the peak discharge itself is not the problem, but the additional increase in water level due to the obstruction of the watercourse caused by the vegetation. In addition, submerged aquatic plants at peak discharges will bend/deflect so that the flow resistance is reduced compared to base flow.

In order to make reliable statements about the possibility of removing dams and omitting mowing in watercourses, however, it remains necessary to take into account the local situation. This can be done in several ways: ranging from a "worst case scenario" in which, for example, deflection (bending of the vegetation) is not taken into account, to the application of elaborate models in order to determine the flow resistance. There is a great diversity of methods / models in order to describe the flow resistance in water courses caused by vegetation. Except for floodplains, ​​there are no studies in which different models are compared on the basis of field measurements and/or experimental measurements. Therefore, such an exercise for watercourses would be very valuable.