Estuaries are bodies of water with one or more open connections to the sea (Leuven et al., 2016) which develop at the land-sea interface due to delivery of sediment from both rivers and the coast (Nicholls et al., 2020). Estuaries that have developed naturally tend to have a converging planform shape, often with mutually evasive ebb and flood channels which create a multi-channel system (Jeuken & Wang, 2010;van Dijk et al., 2021;Weisscher et al., 2022). They have several intertidal shoals and bars, particularly at their widest points (Leuven et al., 2016), and extensive floodplains (van Veen et al., 2005). However, many estuaries globally are now changing to a new enforced equilibrium as they are increasingly altered by human activities. The plains surrounding estuaries and deltas are rapidly urbanizing, leading to a variety of economic, environmental and ecological questions and concerns regarding long-term sustainability and management of these human-influenced systems (Loucks, 2019). Width is dramatically reduced as floodplains are embanked and intertidal areas are reclaimed, to be used for housing, ports, harbors and development of urban centers (Cox et al., 2022). Meanwhile, flood protection structures such as dikes, groynes and flood barriers are often implemented, redirecting flow and altering sediment transport regimes (O'Dell et al., 2021;Ten Brinke et al., 2004). Estuary depth is also commonly increased by dredging at a variety of scales.Estuaries are commonly identified as hotspots for climate risk (Hill et al., 2020) because they are uniquely threatened by both sea-level rise (SLR) and river basin-wide climate changes (e.g., glacial melt, temperature
<p>Estuaries are often dredged for navigational access to ports and&#160;harbours.&#160;Dredging alters the natural&#160;dynamics and&#160;morphology of estuaries, tending to create deep channels and high intertidal bars and shoals. Estuaries will face many challenges in the future including sea-level rise&#160;(SLR)&#160;which will influence both estuarine hydrodynamics and morphology.&#160;Whilst the&#160;effects of&#160;SLR&#160;have been studied through numerical modelling and have been inferred from empirical relations&#160;for natural estuaries, little is known about whether dredged systems will&#160;react&#160;differently to&#160;SLR&#160;compared to natural estuary systems.&#160;Our objective is to&#160;quantify&#160;effects of dredging,&#160;SLR and the interaction of both processes&#160;on&#160;estuarine&#160;morphology.&#160;</p><p>&#160;</p><p>We performed scale experiments&#160;in a tilting tidal flume (The Metronome)to indicate the&#160;proposed&#160;effects of&#160;SLR&#160;on&#160;estuarine&#160;morphology. Previously, dredging was induced in the flume and the response of morphology compared well with real-world examples such as the Western Scheldt.&#160;In the current study,&#160;simulated&#160;SLR&#160;was induced in both a sandy natural (undredged) estuary and a dredged estuary&#160;in otherwise the same conditions. These were compared with control experiments without&#160;SLR&#160;to isolate the effects of&#160;SLR&#160;in both types of system.&#160;&#160;</p><p>&#160;</p><p>Overall,&#160;both&#160;maintenance&#160;and capital&#160;dredging&#160;volumes increase with SLR and dredging locations tend to shift upstream.&#160;The experiments&#160;indicate&#160;that channels under&#160;SLR&#160;tend towards&#160;a&#160;new&#160;equilibrium&#160;by changing their&#160;hypsometry&#160;(width and depth)&#160;in response to&#160;excess water. This has the overall effect of&#160;increasing&#160;channel mobility&#160;and channel migration speeds. In dredged systems,&#160;banks become unstable and collapse into channels&#160;which is compounded&#160;with rapid erosion of intertidal bars and shoals.&#160;&#160;</p><p>&#160;</p><p>Intertidal areas in non-dredged&#160;systems&#160;tend to&#160;maintain their elevation and extent under&#160;SLR, though their locations&#160;shift&#160;in the upstream direction.&#160;&#160;In&#160;contrast,&#160;dredged systems&#160;typically show&#160;a decrease in&#160;total intertidal&#160;area,&#160;which means the loss of valuable&#160;intertidal habitat area and reduction of&#160;flood storage.&#160;</p><p>&#160;</p><p>In the long-term, dredged systems&#160;without&#160;SLR&#160;have an almost&#160;fixed morphology, such that the dredged channel persists&#160;even after dredging&#160;ends.&#160;But&#160;SLR&#160;overwhelms&#160;this fixation&#160;and&#160;remobilizes&#160;sediment, enhances&#160;channel&#160;meandering&#160;and&#160;migration&#160;and&#160;induces&#160;lateral expansion. If&#160;estuaries&#160;are constrained by dikes, bank&#160;protection or other flood measures,&#160;this&#160;excess&#160;energy,&#160;which&#160;is not all&#160;dissipated in&#160;meandering&#160;and migration,&#160;may&#160;have negative effects on infrastructure.&#160;In&#160;undredged&#160;systems,&#160;shallower channels&#160;have&#160;more space&#160;to&#160;deepen and widen, reducing the braiding index and&#160;providing&#160;more adaptation&#160;capacity. Infrastructure&#160;along&#160;urban&#160;dredged systems (e.g.&#160;flood protection measures, dikes) will be at higher risk under SLR than systems&#160;with floodplains and intertidal areas&#160;which have space to adapt.&#160;&#160;</p><p>&#160;</p>
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