Implementing vegetation development into a quantitative hydromorphological model: a case study of inundated polder “Kleine Noordwaard” in the Biesbosch (the Netherlands)

Abstract

Inundated polder Kleine Noordwaard in fresh water tidal delta the Biesbosch (Netherlands) opened in 2007 and was constructed to increase the water storage capacity and conveyance of the Rhine-Meuse system, and as a nature conservation area with a wetland ecosystem. The Kleine Noordwaard is featured in a hydromorphological model (Delft3D) of ongoing research into processes of hydrodynamics and morphology in a fresh water tidal wetland and to see if this area can maintain itself in the face of predicted climate change and sea level rise. This Bachelor thesis contributes to this research by providing a strategy and data for incorporating vegetation development into the hydromorphological model. Vegetation interacts with hydrodynamics by altering the Vlow pattern and dissipating wave energy (of wind driven waves in the Kleine Noordwaard). Hydrodynamics in turn inVluences indicator values of vegetation such as Vlooding frequency and groundwater level, which determines moisture content of the soil, a basic condition for suitability of habitat for plants. A realistic representation of vegetation cover greatly improves the accuracy and validity of a model. To this end, a strategy for representation of vegetation was developed, the DoVeC-RTS model, that was applied to the Kleine Noordwaard, but also provides a useful framework for other modelling studies, because it can be adapted to the specific needs of a modelling study and the characteristics of a study area. The vegetation cover of the Kleine Noordwaard was categorised into six important vegetation types, each represented by a key-species. The characteristics of these species serve as indicative for quantifying those parameters for hydraulic roughness, ecological requirements and succession sequences, that were found to be most relevant for the specific hydrodynamic and ecological processes in the Kleine Noordwaard, resulting in the numerical knowledge rules of DoVeC-RTS, for determining a dynamic vegetation cover. The applicability of DoVeC-RTS was tested on output of the hydromorphological model for selected gridcells. For the Kleine Noordwaard, it appeared that hydrodynamics was of greater influence on vegetation than the other way around. Results of the test for the year 2015 showed that DoVeC-RTS is suitable for predicting the vegetation cover and that the vegetation in the Kleine Noordwaard is still adapting from inundation since 2007. The test for the year 2050 included KNMI climate scenario WH (KNMI, 2014), with global temperature rise, sea level rise and more precipitation in winter, of which sea level rise in particular causes a strong increase in water levels in the Kleine Noordwaard. Results of the 2050 WH test showed, that vegetation types such as helophytes and riparian (willow) forests in the Kleine Noordwaard are in serious danger of drowning if water levels should rise. The way that banks and “plas-dras gebied” (marshy areas) are constructed in the Kleine Noordwaard leaves some room for species that inhabit areas around the mean (high) water line, but migration in keeping with water level rise is blocked by steeper slopes and dikes, which poses a serious threat to the development of a long-lasting, diverse wetland ecosystem. The findings of this thesis are therefore of importance for design, construction and management of wetlands and floodplains, such as the construction of the neighbouring Grote Noordwaard.