Levee morphology and evolution in the fluvial-tidal realm
Summary
Levees are geomorphological units consisting of sand, silt and/or mud that form along channels and slope towards the floodplain. They are common and pronounced features in river, delta and tidal landscapes in which they form elevated areas in further low-relief plains. Levees influence the hydraulics and distribution of sediment within river, delta and tidal systems, consequently influencing their evolution. Despite the relevance of levees for delta evolution and future delta management, research on levees is sparse and commonly limited to fluvial case-studies. Which causes a lack in general understanding and implications of levee evolution and morphology on a larger scale. Thereby, the limited amount of research on levees formed in tidal environments causes a gap in knowledge on the effects of tidal boundary conditions on levees. Hence, the objective of this study is to assess the influence of boundary conditions and hydromorphological feedbacks on the evolution, morphology and sedimentology of levees in the fluvial-tidal realm.
The findings presented in this thesis add to our understanding of the evolution of river, delta and tidal systems under changing boundary conditions and hydromorphological feedback mechanisms. Furthermore, the results provide a first step towards the development of a sustainable management strategy for building new land in sinking deltas. The results of this thesis have shown that levee-crevasse
complexes become larger if more sediment is transported out of the main channel, that their width is negatively affected by large flow velocities in the floodplain and that especially a low concentration of fine sediment can limit their growth. These conclusions have implications for the implementation of natural land-building projects in delta areas. In fluvial-sediment starved deltas as the Rhine-Meuse delta in The Netherlands, the fine sediments will have to come from a marine source (tidal import), which implies allowing the sea to influence the land; a sensitive topic in the country that battled against the water for centuries.