Bar and channel patterns in estuaries

Abstract

Estuaries, also called river mouths, form the transitional zones between rivers and oceans. Estuaries occur all over the world at a wide range of spatial scales from tens of metres to tens of kilometres. The open connection to the ocean allows for tidal currents to periodically flow in and out of estuaries. Since estuaries are also connected to rivers, a transitional zone arises from river dominance with unidirectional flow to tidal dominance with bidirectional flow. The transition creates an along-channel gradient of flow velocity, sediment type, salinity and vegetation. Estuaries accumulate fluvial sand and finer marine mud, which are sediments available for the formation of intertidal sand bars, mudflats and salt marshes. The channels in estuaries provide access to billion-dollar-harbours (e.g. the Western Scheldt, Elbe, and Yangtze), while the intertidal bars form valuable habitat for biodiversity and the surrounding land is often densely populated. In total, 21 of the world’s 30 largest cities are located in the vicinity of estuaries. Despite the importance of estuaries, it remains largely unknown how estuaries will respond to future sea-level rise and human intervention. Moreover, little is known about the equilibrium bar and channel patterns in estuaries and the dynamics of bars and channels over time. In this thesis, I show that the bar and channel pattern of estuaries is predictable from the shape of estuaries, which is observable on aerial photography. Tidal bars and meanders scale with local estuary width. Bars predominantly occur where the estuary is wider than expected from an ideal converging shape. This allows predicting the cross-sectional hypsometry along estuaries. Moreover, the dynamics of bars and channels was studied in laboratory scale experiments, which showed the formation of mutually evasive ebb- and flood-tidal channels in an initially straight channel. On the longer term, growing forced midchannel bars are responsible for an estuary outline with quasi-periodic variations. The resulting rules from this thesis were applied to study the effects of sea-level rise on estuaries worldwide. I found that small estuaries face enhanced flood water levels of up to 1 metre on top of the 1 metre increase in mean sea level expected from climate change. However, if they receive sufficient sediment, they will adapt to their new boundary conditions and water levels only increase by the increase in mean sea level. In contrast, large estuaries face sediment starvation, especially if the tidal amplitude at the mouth decreases. Insufficient sediment supply means drowning of the estuary with the implication of loss of ecologically valuable intertidal area. However, results also show that managed realignment, i.e. allowing estuaries to increase in width, is an important measure that aids in mitigating the adverse effects of sea-level rise.