A morphological investigation of marine transgression in estuaries

Abstract

The landscape setting of estuaries varies widely and is an important aspect of determining how they evolve. This paper focusses on alluvial estuaries in river valleys and how they respond to sea level rise. We examine the implications of marine transgression, as a response to sea level rise, where the estuary moves upwards and landwards to maintain its position in the tidal frame (so-called stratigraphic rollover). Here we encapsulate such kinematic movement of the estuary morphology using a 'morphokinematic' model, to assess the potential response to sea level rise and sediment supply. The model of the estuary form includes a single convergent channel, intertidal and surrounding floodplains (the valley) and allows the relative importance of the space available for deposition of sediments, the accommodation space, to be investigated as a function of rates of sea level rise and sediment supply. The transgression of the system is determined using a sediment mass balance, taking account of any supply from the river and marine environment. Model results confirm that the transgression distance, measured as the distance the entity moves landward, varies in proportion to the change in accommodation space, which mainly depends on the floodplain area. As the size of the floodplain reduces, the transgression distance is less and the system becomes much more sensitive to changes in the rate of sea level rise or changes in sediment supply. The greater demand for sediment when a floodplain is present results in greater cannibalization of the estuary form (i.e. greater landward movement) to meet the sediment demand. When the floodplain is disconnected from the estuary, the synergistic relationship is lost and the accommodation space increases. The capacity for restoration will depend on the availability of sediment and the prevailing rate of sea level rise.

Reclamation of Tidal Flats Within Tidal Basins Alters Centennial Morphodynamic Adaptation to Sea‐Level Rise

“…Embanked tidal landscapes might have a higher flood vulnerability and lower flood resilience compared with the natural landscape, because the embanked regions are deprived of sedimentation from overwash events during high tide, whereas compaction of sedimentation and subsidence reduces their elevation (Auerbach et al., 2015; Hoitink et al., 2020). In addition, embankment of low‐lying floodplains prevents marine transgression of the tidal basin and the associated lateral expansion, thus constraining the system to adapt to external changes (Townend et al., 2021). The outsized influence of tidal flat embankment on tides and morphodynamic evolution suggests that it may be feasible to reverse the undesirable outcomes, for example, managed retreat (Townend & Pethick, 2002), thereby mitigating some of the future effects of climate change.…”

“…The global coastal wetland loss rate was estimated to be 30% in the coming century, if wetlands can migrate landward and under the assumption that there is sufficient sediment supply (Schuerch et al., 2018). The fate of a tidal basin, to be raised or drowned, then depends on its ability to accrete vertically at rates equal to or larger than SLR, and/or to migrate inland at rates faster than shoreline erosion (Townend et al., 2021).…”

Modeling the Morphodynamic Response of Estuarine Intertidal Shoals to Sea‐Level Rise

“…Figures S1, S2, and S3 in the Supporting information S1 show examples of constrained systems in the Netherlands (Wadden Sea basins), Australia (Venus Bay), and New Zealand (Otago Harbour). In natural unconstrained systems, marine transgression (lateral expansion or landward migration) may also occur as a SLR adaptation mechanism (Allen,1990; Beets & van der Spek, 2000; Guo et al., 2021; Townend et al., 2021). In addition to the active shoal build‐up, marine transgression will lead to “passive shoal formation” due to the drowning of the coastal plain causing dry land to shift from the supratidal realm into the intertidal realm.…”

“…The projected 21st century accelerating SLR rate increases the risk of intertidal areas drowning especially in systems with low sediment supply and systems constrained by sea defenses or geological outcrops, with no room for lateral expansion or landward migration. In natural unconstrained systems, the intertidal areas (vegetated or unvegetated) could be maintained if SLR causes marine transgression which depends on the available accommodation space and sediment supply (Allen, 1990;Beets & van der Spek, 2000;Townend et al, 2021). Thus far, most modeling studies have focused on muddy environments, whereas limited attention was paid to sandy shoals.…”