D-FAST morphological impact : evaluation in the Rhine-Meuse estuary

Within the SIMONA suite, WAQMORF is used for estimating long-term equilibrium sedimentation in the main channel (cf. Sieben, 2010; Mosselman, 2013) for local measures with a length of at most 4 km. Jagers (2020) generated a first sixth generation version of WAQMORF rebranding it to D-FAST Morphological Impact (also D-FAST MI), a rapid assessment tool for morphological impact. Jagers and Giri (2022) generated a first approach of D-FAST MI to tidal conditions and Giri and Jagers (2022) did a first analysis for a channel based on the Rhine-Meuse estuary. According to the Rivierkundig Beoordelingskader (Rijkswaterstaat, 2023) there are different ways in which a measure can be evaluated, namely by way of expert judgement by the river manager, and if necessary supported by the correct tool, D-FAST MI or a full morphodynamic simulation using D-HYDRO (for the fifth generation this was WAQMORF or Delft3D 4). In the Rhine-Meuse estuary, there is not a clear tool available to give supporting evidence for the response of the bed level after implementation of a measure. Therefore this research was undertaken to evaluate the applicability of D-FAST MI to the Rhine-Meuse estuary. The current research describes the model setup for Delft3D 4 for the hydrodynamics and morphodynamics, as well a D-FAST MI input example for the Lek in the Rhine-Meuse estuary. The hydrodynamic convergence of the Delft3D 4 model is thoroughly tested, as this result is converted to D-HYDRO output, which is used by D-FAST MI. The Hollandsche IJssel grid was coarsened to generate a stable flow condition. Next, different simulations to trenches of 1m were included to test the sensitivity to the choice of MORFAC of the morphodynamic result with respect to reference simulation without the trench. The infilling of the trench over a year allowed to compute estimates of the timescales of adaptation per branch and for different flow conditions. Finally, different simulations were run where a 5% extraction was imposed in all the branches. For the Nieuwe Waterweg and the Spui ten years of morphodynamic development were computed by Delft3D 4. The results were compared to the equilibrium erosion and sedimentation as found by D-FAST MI. The sedimentation using Delft3D 4 at the upstream location of the extraction was lower than the equilibrium value found from D-FAST MI. Overall the sedimentation at the measure was overestimated by D-FAST MI. Possible causes for this are that fixed layers are not considered in D-FAST MI. The equilibrium erosion of D-FAST MI underestimates the erosion found using the Delft3D 4 model. Another complicating factor is that for the Delft3D 4 morphodynamic simulation, measures easily affect the discharge distribution in the Rhine-Meuse estuary, and therefore the effect of a measure cannot be isolated to a single branch. This is not unexpected, and hence forms a limitation for the application of D-FAST MI in a network of short branches such as in the Rhine-Meuse estuary. Based on the current results, the advise is to directly use a morphodynamic model if it is expected that the discharge distribution is affected. The morphodynamic computation as it stands also takes rather long, and therefore, the computation of an equilibrium effect of a measure is not yet possible. D-FAST MI could be possibly be used by considering the maximum effect. If this effect is below a certain threshold (e.g. the sedimentation is below a percentage of the water depth), it is fair to say that the impact of the measure is small, and no further research into the morphodynamic effect is required. If the effect is larger than the preset threshold, then a full morphodynamic analysis is advised. Although the tool provides a structured way to analyse this, this could also be done directly on the basis of change in the flow velocity between the measure and the reference. If this effect is smaller than, for instance a percentage of the flow velocity, this would show a similar effect. It is important to not compound effects and implement them too soon after one another, as in that case, an unwanted compound effect could result. In such a case, a combined morphodynamic analysis is advised.