Building and raising land: mud and vegetation effects in infilling estuaries

Many Holocene estuaries were infilled to form convergent, single-channel systems, while others remained partially or wholly unfilled. This difference in the degree of infilling depends partly on the balance between fluvial and coastal sediment input and the hydrodynamics that can export sediment. However, it remains unclear to what degree this balance is tipped by mud supply and eco-engineering vegetation, and by what planform patterns the infilling proceeds. This study aims to explore experimentally how mud and vegetation change the degree and process of infilling, elevate and merge bars above intertidal levels and affect the planform of estuaries. To this end, three experiments were conducted in the Metronome, a flume that tilts periodically to create tidal currents, wherein forced tidal asymmetry resulted in net importing estuaries. In the second and third experiments, mud was supplied and in the third experiment seedlings were released of three vegetation species with eco-engineering traits at a laboratory scale. With only sand, the estuary fills sufficiently to form a multi-channel pattern with intertidal bars. Both mud and vegetation settle on intertidal bars and on the fluvial bay-head delta, thereby contributing to bar stabilization and further estuary infilling, pointing at effective strategies to keep up with future sea-level rise. This reduces channel mobility and effectively narrows the summed subtidal channel width toward an ideally converging funnel shape. This seems especially effective where vegetation stabilizes the mud. The experiments suggest that a range of steady states exists between the end-members of an unfilled and a completely infilled, ideal estuary.