The Netherlands, Germany, and Belgium were hit by heavy and prolonged precipitation in July 2021. As time passed, weather warnings escalated, leading to evacuations due to predicted floods, including in the Rur catchment. It was difficult to forecast the flooding of the Rur, raising the question of which elements are crucial in a flood predictionmodel for the Rur river. This question is addressed by addressing both a hindcast of the 2021 flood event and creating forecasts based on the weather forecast of July 13, 2021.

The Rur river basin is characterised by topographic and geological variations, with the steep Eifel responding differently than the flat lowlands, and human intervention in the form of reservoirs and lignite mines. A hydrological Wflow_SBM model has been derived for the Rur river basin, encompassing these characteristics, along with a hydrodynamic ProMaIDes model for the downstream reach of the Rur. These models were compared to investigate various aspects: river routing, floodplain flow, tirbutary interactions, the influence of reservoirs, and the impact of reduced groundwater levels.

The results of the 2021 floods indicate that modelling flows in floodplains is crucial to shaping the flood wave, both in tributaries and the Rur itself. Additionally, the reservoir played a significant role in attenuating the flood wave, with the increase in the outflow of the reservoir primarily affecting the tail of the wave. The reduced groundwater level was simulated by adding a leakage termto the saturated subsurface zone, whose indirect effect is significantly greater than the leakage termitself. Moreover, the tributaries Worm and Inde, particularly, are influential in the Rur’s discharge. These characteristics are also evident in the simulated forecasts, although the spatial and temporal resolution is significantly lower for these meteorological predictions.

Finally, the characteristic response of the Rur demonstrates that not everymodel type is equally practical for flood forecasting. The dominant flow from the reservoirs is highly regulated and is unlikely to induce inundations downstream. Complex flow patterns in floodplains only become relevant in the Dutch Rur, which makes two-dimensional modelling particularly valuable here. Therefore, it is recommended to use a one-dimensional discharge model, incorporating delay effects from winter bed flows. When predicted discharges at the Stah station are exceeded, two-dimensional simulations may provide a solution, the model area reduced to the Dutch Rur, focussing on predictions where a critical value related to floodplain capacity (Qlimit = 300 m^3/s) is exceeded.

During the calibration of SWAN for shallow water lakes in the Netherlands, problems have been encountered. Especially the emergence of an unnatural, secondary peak in the low frequency part of the energy density spectrum poses problems. Therefore, this investigation aims to address the nonlinear shallow water source terms in SWAN that may introduce this artefact.

Firstly, the basics of linear wave theory and spectral modelling are introduced. Hereafter, these basics are extended to the modelling on nonlinear wave behaviour, as these phenomena form the basis of the relevant shallow water source terms.

The shallow water lake problem is reduced to a stationary, one dimensional problem, for which several model results are produced using varying parameters. Three main subjects can be identified: 1) the influence of depth-induced wave breaking using the BJ78-model or the $\beta-kd$-model, 2) the proportionality factor of the triad source term using the CCA-formulation, and 3) numerical properties such as frequency resolution and the magnitude of the flux limiter.

The results demonstrate that, for strong winds ($U_{10} = 40$ m/s) in shallow water ($d = 2.5$ m), source term interactions become complex as they are all of similar order of magnitude. Especially the interactions between triad wave-wave interactions and depth-induced wave breaking, and triads and quadruplets are important for determining the resulting wave spectrum. Increased wave breaking reduces the secondary peak but underestimates significant wave heights and dampens the spectrum. A more elaborate wave breaking formulation (the $\beta-kd$-model) showed improved performance, especially when it was set to allow younger wind waves (high $kd$) to develop.

Lowering the effect of the triad source term also removes the secondary peak, but may not be desirable as it is by definition a relevant shallow water source term. Reducing source term interactions by means of a stricter action limiter resulted in instabilities for the quadruplet source term and thus no convergence of the results.

A possible solution may be to investigate the possibility of reducing the effect of quadruplets for shallow water, as this term (together with the triad source term) contributes to the formation of the secondary peak. However, this has not been included in the scope of this investigation. Furthermore, improving the frequency resolution resulted in smoother spectra and less wiggles in the sum of the triad and quadruplet source terms. However, this may affect the performance of the quadruplet source term as this term has been calibrated for a fixed relative frequency resolution.