Aim of the research
In this paper we present a new tool to monitor the floodplain vegetation in the Dutch part of the trained rivers Rhine and Meuse. The aim of this tool is to provide an easy screening method that helps focusing this effort, using classification of readily available satellite images in dominant vegetation types. The tool is the first-time satellite data is used for rivers in daily operations of the river manager. To develop a reliable tool, the following questions were asked:
How can the floodplain vegetation management process be aided by a monitoring tool?
What type of information and visualization will present results in a simple and effective way?
How to achieve robust and accurate classifications using up-to-date remote sensing data?

Methods

At the Dutch National Water Authority ‘Rijkswaterstaat’ (RWS) a dedicated workflow is followed to map and monitor the vegetation in the floodplains. Every 6 years the river system including the vegetation is mapped using aerial photographs and classified manually using a GIS-based approach . The manual photo-interpretation map was further combined with information on the flood frequency and inundation depths, hydrodynamic forcing categories and information on the grazing intensity of specific parcels of land to yield an ‘ecotope’ map. An ecotope is a spatial unit relatively homogeneous in vegetation structure, succession stage and the main abiotic site factors that are relevant for plant growth . The ecotope map contains different classes of vegetation each assigned a dedicated hydraulic roughness value and is used in river flood management and associated hydraulic model calculations including the planning of vegetation management .
To further aid vegetation management, communication and enforcement, a dedicated legal map of the allowable vegetation (“Legger”) was created containing aggregated photointerpretation-based vegetation classes . This map is used to communicate with the approx. 12.000 landowners in the floodplain area (Figure 2) and depicts the legal limit for allowable vegetation (as a proxy for the allowable hydraulic roughness). The river authority’s internal regulations stipulate that the floodplain vegetation should be in line with the legal vegetation map every October . The authority must report the status to the government before the winter, i.e. flood season, starts. Up to 2018 no good operational tool was available to objectively monitor the state of the vegetation in the floodplains on a more frequent basis than every 6 years. It was then when the need for an appropriate monitoring tool was defined that would allow a dedicated team at RWS to prioritize where in the floodplains field visits were needed to check if the current state of the vegetation is still in line with the legal vegetation map.