Effects of Sea‐Level Rise on Dredging and Dredged Estuary Morphology

Estuaries and deltas worldwide are facing land loss and drowning due to sea‐level rise (SLR). Commonly home to ports, their channels are dredged and deepened for navigation. However, little is known about how such sediment management will interact with changing sediment transport patterns due to SLR. Using scale experiments, empirical relations and real world examples from global estuaries and deltas, we identify that dredging and SLR combined enhance bend migration whereas SLR alone leads to decentralizing of channels and drowning of intertidal area. In estuaries where channels are fixed, excess energy due to increasing tidal prism will manifest as bed and bank erosion, placing flood safety measures like dikes at risk. SLR increases dredging volumes in upstream reaches due to the rapid collapse of shoals and river banks along the whole estuary. Channel deepenings are ineffective under SLR conditions due to sediment import induced by increasingly flood‐dominant tides. Non‐dredged systems which have more regular and level elevations will lose intertidal area more quickly than dredged systems that have disconnected higher intertidal flats and a single deep channel. Mid‐size dredged European systems are more likely to drown due to dredging in the present century than from SLR. Effects can be avoided by pursuing sediment management strategies that help restore the morphology disrupted by dredging. Plain Language Summary River channels are often dredged to allow access for large ships to seaports. Sea‐level rise (SLR) will put coastal areas under pressure, as river channels deal with higher water levels and flood safety concerns, but arguably SLR could also lead to less intensive dredging efforts. We study the effect of SLR on natural versus dredged channels using scale experiments of river mouths to determine how SLR will affect dredging practice and the effects it will have on the channels (will they erode?) and surrounding areas (will they drown?). We have found that dredged channels show more erosion and bank collapse than non‐dredged systems. Dredging volumes will also increase due to SLR so that dredging will have to take place more frequently and will cost more in the future. Many European estuaries are presently more likely to drown due to the effects of dredging than because of SLR. These adverse effects can partly be mitigated by sediment management that accounts for the natural sand transport processes and shapes of channels and bars. Key Points Sea‐