River restoration from a river manager’s point of view.
Van Breen, L. E.; Jesse, P.; Havinga, H.
A number of river restoration projects and flood alleviation projects, which also aim to restore more natural river flood plain processes, have been implemented along the Dutch Rhine branches in the past decade. This has provided some information on their effect on the river's morphology and hydraulics. Incorporating this experience into the design of new schemes is one of the main aims of today's river management. Development of vegetation in the floodplains leads to an increase in hydraulic roughness, especially in the first few years, when species such as Willow develop. Secondary channels and small structures such as bridges in the floodplains may have a considerable morphologic effect. Some projects caused bank erosion which may jeopardize river dikes, plus local erosion around structures like groynes. This points out that insufficient attention had been given to their engineering aspects in the design. Monitoring of both the morphologic effects and the development of vegetation is required in order to know when to take action, to prevent loss of function or to secure flood protection levels. To be able to intervene as early as possible, rules of thumb need to be constructed to determine the hydraulic roughness based on knowledge of the vegetation. By proper interpretation of observed changes in morphology and vegetation, translation of experience into design practice is possible. In this way the river manager gains experience to maximize the restoration potential of flood protection schemes. This paper cannot be seen as a scientific study. Visually observed changes in morphology and vegetation are discussed, as well as practical issues related to the impact of vegetation on flood levels and management of river restoration.