The effect of sea-level rise on estuary filling in scaled landscape experiments

When sea-level rise slowed down in the middle Holocene, fluvial and coastal sediments filled the newly created accommodation, whilst others remained largely unfilled because of limited sediment supply. In view of current and future rapid sea-level rise, the question arises how estuarine systems will adapt and whether the land-level rise may keep up. Besides geological data and conceptual models of large-scale and long-term estuary filling, little is known about the filling process during sea-level rise on the decadal-to-centennial time scale that is relevant for society. This study focusses on how sea-level rise affects the morphological and hydrodynamic development of filling estuaries. To this end, scaled laboratory experiments were conducted in a tilting flume facility that creates bidirectional tidal currents and develops entire estuaries. A net importing estuary with sand, mud and vegetation was formed that was subjected to linear sea-level rise. Findings show less of the imported sand was deposited landward following sea-level rise than in an experiment without sea-level rise. The bay-head delta and the flood-tidal delta retained nearly enough sediment to keep up with sea-level rise, whilst the tidal embayment in between drowned except for the highest vegetated bars. Sea-level rise also reduced vegetation survival and sprouting potential, as prolonged inundation increased mortality, negating the potential eco-engineering effect. This resulted in lower vegetation coverage with sea-level rise than under constant sea level. These findings suggest that sea-level rise may cause natural systems to drown even if nearly sufficient sediment is available to fill the newly created accommodation, particularly in areas further away from the fluvial and marine sediment sources. Finally, depending on the sea-level rise rate, the flood-tidal delta may show back-stepping like fluvial deltas, but in the reverse direction towards the sea.