Effect of bed roughness formulations on morphology in the Western Scheldt estuary, The Netherlands

Estuaries and tidal lagoons are collectively defined as tidal basins. They interrupt a large part of the world's shoreline with their associated coastal inlets. In fact, numerous economicactivities are supported in estuary and lagoon related industries. Some of them are navigation, recreation, fishing, aquaculture etc. On the other hand, unique and highly valuable ecosystems are often found. In the modern society, human demands on these industries are ever increasing. In order to meet them, knowing the behaviour of estuaries andlagoons will provide a better water management strategy. The Western Scheldt estuary is situated in the South West of The Netherlands. It receives water from Scheldt River, manmade canals and sea water from North Sea.There are three major functions identified in the Western Scheldt estuary viz. accessibility of ports, safety against flooding and ecological status. Human interventions have alarge impact on the physical characteristics. Especially, sand mining, dredging/dumping anddeepening the access channel lead to the morphological changes in the channel and shallowareas. These changes can be classified as either gains or losses in terms of morphology.Having a better understanding of these processes, optimum solutions can be selectedtowards the better water management strategy.This requires a sound knowledge in physical as well as in ecological processes. In order toachieve these goals, Directorate of Zeeland of Public Work and Water Management(Rijkswaterstaat) has initiated three projects dealing with hydrodynamics, morphology andecology. The present research builds on the results of the project called ZEEKENNIS, which enhances the knowledge of morphology and ecology in the estuary. It is a part of the project called ""LTV - O & M"". In this study, we focus on the effect of bed roughness formulations on the estuary morphology, in both short term and long term time scales. The objective of the research is to identify a suitable bed roughness formula for the future morphological study. In general,the research improves the knowledge of multiple channel behaviour and prediction capacityon morphology with different bed roughness formulations. The results and conclusions drawn from the detailed models beginning with 2001 bathymetry are used in the final model setups.The final series of simulations is made beginning with a smooth bathymetry without channels. The model behaviour towards equilibrium is investigated in the detailed model as well as the Western Scheldt model for long term morphological changes. Moreover, the Western Scheldt model is simulated with different transverse bed gradient factors andsmoothing steps.In conclusion, the present research has resulted in increased insight into the effect of bedroughness formulations on the morphological behaviour in the Western Scheldt estuary.Further, the research has increased the understanding of the behaviour of roughnessformulation with respect to the water depth. The modelling approach is an innovative steptowards a suitable roughness formulation for the estuary. The results show that Manning'scoefficient creates deep narrow channel pattern. On the other hand, wider shallow channelsare observed in the results of constant Chézy and Van Rijn formulas. Generally, constantChézy and Van Rijn formulas present similar patterns in terms of the morphologicalchanges. In contrast, Manning's coefficient deviates from other two roughness formulations.The final series of simulations with smooth initial bathymetry shows that initial bathymetryis important for the final bed level configurations.Recommendations for the further research are to include wave effects and humanintervention on morphology as well as extending the detailed model area to minimize theboundary effect. Finally, Van Rijn 2004 roughness formulas are recommended to use forfurther research on morphology in the Western Scheldt estuar