| dc.description.abstract | A crevasse is a geomorphological unit that consists of channel, bar, and splay deposits that form at a breach in the natural levee of a river, extending into the flood basin. They are common features in river delta and tidal landscapes where they form relatively highly elevated elements in otherwise low-lying flood basins. Hence, crevasses have an effect on sediment distribution within deltas and tidal landscapes.
So far, research on crevasses is generally limited to single crevasse case-studies in fluvial settings. Due to the focus on the effects of fluvial boundary conditions, there is a knowledge gap in the understanding of the effects of tidal boundary conditions. Therefore, the objective of this thesis is to assess the effect of tidal and fluvial boundary conditions, their interplay, and the initial flood basin conditions on the formation, evolution, and morphology of crevasses in the fluvial-tidal realm.
To address the objective, the Old Rhine system was used as a case study. The Old Rhine is a former main branch of the Rhine in the Western part of the Rhine-Meuse delta in the Netherlands. Multiple abandoned crevasses are well-preserved and its palaeogeographic development and changes in boundary conditions have been generally well reconstructed. The sedimentary build-up of four long crevasses was reconstructed based on borehole data of hand-augered corings. Lithogenetic cross-sections of each crevasse were constructed. Archaeological data were used in conjunction with the crevasse stratigraphy to get an indication of crevasse ages. Hence, three crevasse phases of the Old Rhine were identified. Their timing was compared to changes in boundary conditions and flood basin configuration of the Old Rhine to infer the effect of these changes on crevasse formation and evolution.
It was found that with regular tidal flushing of the crevasse throats (i.e. the initial natural levee breach) due to water level fluctuations induced by the tidal backwater effect, more crevasses can be formed by preventing crevasse healing. As such, regular tidal flushing facilitates crevasse evolution and progradation. When the tidal backwater effect decreased in the Old Rhine due to gradual closure of the estuary, the number of active crevasses decreased. This occurred because of the abandonment of existing crevasses on one hand and a decrease in the formation and establishment of new crevasses on the other as infilling of the crevasse throats was no longer or to an increasingly lesser extent prevented.
Flood events cause the initial natural levee breach which starts the crevasse formation. Subsequent flood events cause crevasse maturation and progradation, given that filling-in of the crevasse throat is prevented by regular flushing by the tidal backwater effect. Periods with a high frequency of flood events coincide with phases of increased crevasse formation and extensive progradation. Periods with a decreased frequency of flood events compared to a preceding high frequency period show a decrease in number of crevasses formed and a cease of crevasse progradation. Storm surge events probably have the same effect on crevasses as flood events, but the lack of a storm surge record for the Old Rhine estuary prevents any definitive statements about their effect.
Pre-existing channels, in the case of the Old Rhine peat drainage channels draining the raised bogs in the flood basins, can be intercepted by progradating crevasses. Hence, hybrid crevasse-peat drainage channels are formed. Such channels can still function as drainage channels of the raised bogs after a decrease or cease of morphodynamic crevasse activity due to a decrease in the tidal backwater effect, in the flood frequency or both. In the case of the Old Rhine, the three most upstream located crevasses in this study were formed as hybrid crevasse-peat drainage channels.
The outcomes presented in this thesis increase the understanding of the formation and maturation of crevasses in the fluvial-tidal realm of lowland river systems. They show that changes in both upstream as well as downstream boundary conditions affect the potential of crevasse formation and maturation. The presence of raised bogs in the flood basins and associated peat drainage channels can lead to the formation of hybrid crevasse-peat drainage channels.
These outcomes have implications for the archaeology of the Old Rhine area and future natural land-building projects in deltas. The three identified crevasse phases aid archaeologists to estimate the archaeological expectation of crevasse deposits in the Old Rhine area. Crevasses could be used in natural land-building projects to provide sediment to vulnerable flood basins to match (relative) sea level rise rates. Whether crevasse sedimentation rates would be high enough cannot be concluded based on the outcomes of this thesis. Implementation of crevasses for natural land-building projects would require that enough fluvial or marine sediment is available, sufficient vegetation cover is present to trap sediment efficiently, and flood events and storm surges are allowed to flood the flood basins. | |
