The effects of sea-level rise on the future morphological functioning of estuaries are largely unknown because tidal amplitudes will change due to combined deepening of the estuary mouth and shifting amphidromic points at sea. Fluvial sediment supply is also globally decreasing, which hampers infilling necessary to maintain elevation relative to sea level. Here we model 36 estuaries worldwide with varying sizes, shapes and hydrodynamic characteristics, and find that small shallow estuaries and large deep estuaries respond in opposite ways to sea-level rise. Large estuaries are threatened by sediment starvation and therefore loss of intertidal area, particularly if tidal amplitude decreases at the mouth. In contrast, small estuaries face enhanced flood risks and are more sensitive to tidal amplification on sea-level-rise-induced deepening. Estuary widening can partly mitigate adverse effects. In large estuaries, expanded intertidal areas increase tidal prism and available erodible sediment for adaptation, whereas it slightly reduces tidal amplification in small estuaries.
The long‐term morphodynamic evolution of estuaries depends on a combination of antecedent topography and boundary conditions, including fluvial input, sea‐level change and regional‐landscape interactions. Identifying effects of such boundary conditions on estuary evolution is important to anticipate future changes in specific boundary conditions and for hindcasting with numerical and physical models. A comprehensive synthesis of the evolution of the former Old Rhine estuary is presented here, together with its boundary conditions over its full lifespan from 6,500 to 1,000 cal. yr bp. This system formed during a period of sea‐level high stand, during which the estuary served as the main River Rhine outlet. The estuary went through three stages of evolution: a maturation phase in a wide infilling back‐barrier basin, a stable mature phase and an abandoning phase, both in a laterally confined setting. The Old Rhine River formed by a river avulsion around 6,500 cal. yr bp that connected to a tidal channel within a large back‐barrier basin. Decelerating sea‐level rise caused the back‐barrier basin to silt up around 5,700 cal. yr bp, resulting in shoreline progradation by beach‐barrier formation until ∼2,000 cal. yr bp. Beach‐barrier formation along the coast and natural levee formation along the river triggered peat formation in the coastal plain, laterally constraining the estuary and limiting overbank deposition, which caused most sediment to accumulate offshore. The abandoning phase started around 2,200 cal. yr bp when a series of upstream avulsions led to a substantial reduction in fluvial input. This induced a period of enhanced estuarine overbank clay deposition that continued into near‐complete silting up and estuary closure around 1200 ad. These findings exemplify how tidal systems, formed in wide coastal plains during sea‐level high stand, depend on antecedent conditions, and how they respond to connection and disconnection of a large river over long, millennial timescales.
Holocene drift-sand activity in the northwest European sand belt is commonly
directly linked to population pressure (agricultural activity) or to climate
change (e.g. storminess). In the Pleistocene sand areas of the Netherlands,
small-scale Holocene drift-sand activity began in the Mesolithic, whereas
large-scale sand drifting started during the Middle Ages. This last phase not
only coincides with the intensification of farming and demographic pressure but
also is commonly associated with a colder climate and enhanced storminess. This
raises the question to what extent drift-sand activity can be attributed to
either human activities or natural forcing factors. In this study, we compare
the spatial and temporal patterns of drift-sand occurrence for the four
characteristic Pleistocene sand regions in the Netherlands for the period
between 1000 BC and AD 1700. To this end, we compiled a new supra-regional
overview of drift-sand activity based on age estimates (14C,
optically stimulated luminescence (OSL), archaeological and historical ages).
The occurrence of sand drifting was then compared in time and space with
historical-route networks, relative vegetation openness and climate. Results
indicate a constant but low drift-sand activity between 1000 BC and AD 1000,
interrupted by a remarkable decrease in activity around the BC/AD transition. It
is evident that human pressure on the landscape was most influential on
initiating sand drifting: this is supported by more frequent occurrences close
to routes and the uninterrupted increase of drift-sand activity from AD 900
onwards, a period of high population density and large-scale deforestation. Once
triggered by human activities, this drift-sand development was probably further
intensified several centuries later during the cold and stormier ‘Little Ice
Age’ (LIA; AD 1570–1850).
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.