The effects of sea-level rise on estuary morphology in dredged and undredged systems
Authors: Cox Jana , Lingbeek Josephien , Weisscher Steven , Kleinhans Maarten 
Published: March 2022

&lt;p&gt;Estuaries are often dredged for navigational access to ports and&amp;#160;harbours.&amp;#160;Dredging alters the natural&amp;#160;dynamics and&amp;#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&amp;#160;(SLR)&amp;#160;which will influence both estuarine hydrodynamics and morphology.&amp;#160;Whilst the&amp;#160;effects of&amp;#160;SLR&amp;#160;have been studied through numerical modelling and have been inferred from empirical relations&amp;#160;for natural estuaries, little is known about whether dredged systems will&amp;#160;react&amp;#160;differently to&amp;#160;SLR&amp;#160;compared to natural estuary systems.&amp;#160;Our objective is to&amp;#160;quantify&amp;#160;effects of dredging,&amp;#160;SLR and the interaction of both processes&amp;#160;on&amp;#160;estuarine&amp;#160;morphology.&amp;#160;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;We performed scale experiments&amp;#160;in a tilting tidal flume (The Metronome)to indicate the&amp;#160;proposed&amp;#160;effects of&amp;#160;SLR&amp;#160;on&amp;#160;estuarine&amp;#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.&amp;#160;In the current study,&amp;#160;simulated&amp;#160;SLR&amp;#160;was induced in both a sandy natural (undredged) estuary and a dredged estuary&amp;#160;in otherwise the same conditions. These were compared with control experiments without&amp;#160;SLR&amp;#160;to isolate the effects of&amp;#160;SLR&amp;#160;in both types of system.&amp;#160;&amp;#160;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Overall,&amp;#160;both&amp;#160;maintenance&amp;#160;and capital&amp;#160;dredging&amp;#160;volumes increase with SLR and dredging locations tend to shift upstream.&amp;#160;The experiments&amp;#160;indicate&amp;#160;that channels under&amp;#160;SLR&amp;#160;tend towards&amp;#160;a&amp;#160;new&amp;#160;equilibrium&amp;#160;by changing their&amp;#160;hypsometry&amp;#160;(width and depth)&amp;#160;in response to&amp;#160;excess water. This has the overall effect of&amp;#160;increasing&amp;#160;channel mobility&amp;#160;and channel migration speeds. In dredged systems,&amp;#160;banks become unstable and collapse into channels&amp;#160;which is compounded&amp;#160;with rapid erosion of intertidal bars and shoals.&amp;#160;&amp;#160;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Intertidal areas in non-dredged&amp;#160;systems&amp;#160;tend to&amp;#160;maintain their elevation and extent under&amp;#160;SLR, though their locations&amp;#160;shift&amp;#160;in the upstream direction.&amp;#160;&amp;#160;In&amp;#160;contrast,&amp;#160;dredged systems&amp;#160;typically show&amp;#160;a decrease in&amp;#160;total intertidal&amp;#160;area,&amp;#160;which means the loss of valuable&amp;#160;intertidal habitat area and reduction of&amp;#160;flood storage.&amp;#160;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;In the long-term, dredged systems&amp;#160;without&amp;#160;SLR&amp;#160;have an almost&amp;#160;fixed morphology, such that the dredged channel persists&amp;#160;even after dredging&amp;#160;ends.&amp;#160;But&amp;#160;SLR&amp;#160;overwhelms&amp;#160;this fixation&amp;#160;and&amp;#160;remobilizes&amp;#160;sediment, enhances&amp;#160;channel&amp;#160;meandering&amp;#160;and&amp;#160;migration&amp;#160;and&amp;#160;induces&amp;#160;lateral expansion. If&amp;#160;estuaries&amp;#160;are constrained by dikes, bank&amp;#160;protection or other flood measures,&amp;#160;this&amp;#160;excess&amp;#160;energy,&amp;#160;which&amp;#160;is not all&amp;#160;dissipated in&amp;#160;meandering&amp;#160;and migration,&amp;#160;may&amp;#160;have negative effects on infrastructure.&amp;#160;In&amp;#160;undredged&amp;#160;systems,&amp;#160;shallower channels&amp;#160;have&amp;#160;more space&amp;#160;to&amp;#160;deepen and widen, reducing the braiding index and&amp;#160;providing&amp;#160;more adaptation&amp;#160;capacity. Infrastructure&amp;#160;along&amp;#160;urban&amp;#160;dredged systems (e.g.&amp;#160;flood protection measures, dikes) will be at higher risk under SLR than systems&amp;#160;with floodplains and intertidal areas&amp;#160;which have space to adapt.&amp;#160;&amp;#160;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;