Modelling the Effect of Human Interventions and Climate Change Impacts on the Sediment Balance and Salt Intrusion in Engineered Estuaries

Estuaries are the transition zones between rivers and seas and support dense human populations due to their navigability, freshwater availability, agricultural and ecological value. However, these environments are heavily influenced by human interventions and climate change impacts from both the river and sea. This thesis studies how human interventions, channel deepening and land reclamation/restoration, in combination with climate change effects influence the sediment balance and salt intrusion in estuaries.
Chapter 1 explores how sediment dynamics in estuaries are affected by changes in channel depth and intertidal wetlands. Findings highlight how wider wetlands reduce sedimentation by serving as sediment sink and by strengthening tidal currents, which reduce channel sedimentation and thus potentially reduce channel dredging. Chapter 2 extends the model framework of Chapter 1 to also consider upstream interventions and CC impacts. Results reveal an exponential decrease in the estuary’s annual sediment budget as peak flow velocities increase due to planform changes. Additionally, results demonstrate that a single CC scenario can have an ambiguous impact on the estuary's sediment budget depending on human interventions, highlighting the need to analyze CC-impacts in conjunction with human interventions.
Salt intrusion, affected by channel depth and wetland area, is analyzed in Chapter 3. Findings show that salt intrusion length changes depending on system stratification: in weakly stratified estuaries, increased wetland area extends salt intrusion lengths, while in strongly stratified areas, it reduces it. Intertidal areas also reduced the stratification, demonstrating the relevance of intertidal area's for sediment dynamics and ecological functioning.
Finally, Chapter 4 investigates 3D fine sediment dynamics affected by changes in salt intrusion dynamics, highlighting that intertidal areas enhance sediment trapping by estuarine circulation flow but reducing sediment trapping by (asymmetrical) tidal flow. Since the former was the dominant mechanism in this study, increasing intertidal wetland area reduced sediment trapping. On the other hand, sediment flushing was almost entirely determined by the peak river discharge and channel depth.
Overall, this thesis highlights the significance of intertidal wetlands for estuary-wide dynamics particularly demonstrating the potential of wetland restoration to reduce channel sedimentation in dredged estuaries.