Modelling the Effect of Human Interventions and Climate Change Impacts on the Sediment Balance and Salt Intrusion in Engineered Estuaries
Rutger Siemes is a PhD student in the Department Water Systems. (Co)Promotors are prof.dr. S.J.M.H. Hulscher, dr.ir. T.M. Duong and dr.ir. B.W. Borsje from the Faculty of Engineering Technology.
Estuaries are transitional zones between rivers and seas. Dynamic interactions between the marine and fluvial factors create unique environments with significant socio-economic importance due to their navigability, freshwater availability, and high agricultural and ecological value, supporting dense populations.
These estuaries are continuously changing. Upstream interventions like dam construction or change in land-use alter water and sediment supply. Additionally, they are deepened to improve port logistics while intertidal areas are reclaimed for human use, resulting in engineered estuaries which are artificially deep and narrow. Moreover, these regions are vulnerable to climate change (CC) impacts from both the river and the sea, e.g. changes in river discharge extremes, sea-level rise and changes in wind and wave climate. These diverse changes generate strong interest in exploring estuarine functioning in potential future states. This work focuses on two processes in particular: the sediment balance and salt intrusion in estuaries.
The sediment balance indicates if estuaries gain or lose sediment over time which is crucial for understanding future system functioning. Sea-level rise and a reduction in sediment supply can cause estuary drowning, increasing flood risk and saltwater intrusion, thereby limiting freshwater availability, while highly valuable intertidal areas may be lost. In other estuaries, human induced deepening and increased sediment supply may necessitate more dredging and increase the turbidity. Advances in morphodynamic modelling significantly improved understanding of estuarine morphodynamics but knowledge gaps remain regarding the interactions between human interventions and CC-impacts, particularly in environments significantly influenced by marine (tides and waves) and fluvial processes.
Salt intrusion into estuaries mainly occur due to tidal currents and density-driven currents whereby the denser saltwater flows landward near the bed, replacing the freshwater discharged by the river near the surface, called estuarine circulation flow. Salt intrusion limits freshwater availability during low river discharge and storm surges. Additionally, interactions between fresh- and saltwater trap marine and fluvial sediments in estuaries and influence ecosystem functioning. While the influence of channel deepening on these processes is generally well known, knowledge gaps exist regarding the impact of intertidal wetlands on salt intrusion and sediment trapping.
Hence, the aim of this research is to improve quantitative understanding of human interventions and climate change impacts on sediment dynamics and salt intrusion processes in engineered estuaries.
Chapter 2 explores how large-scale interventions (changing channel depth and intertidal wetland width and location) affect the morphology of the coupled channel-wetland systems. A schematised depth-averaged morphodynamic model was used, inspired by conditions in the Rotterdam Waterway, the Netherlands, a typical highly engineered estuary, considering a reference year (monthly averaged conditions). Findings show how channel depth increased channel sedimentation, while sedimentation in the wetland area remained mostly unchanged. Contrarily, increasing wetland width significantly reduced channel sedimentation by serving as sediment sink but more substantially by strengthening tidal flow which reduces sedimentation and promotes erosion in the channel. When varying wetland location, results showed that wetlands mostly reduce channel sedimentation locally, downstream the effect diminished, suggesting that strategic wetland restoration can reduce dredging at critical locations in navigational channels.
In Chapter 3 the model framework is extended to evaluate local interventions (channel depth and wetland width) in conjunction with upstream interventions (change in sediment supply) and CC-impacts (sea-level rise (SLR), changes in wave climate, and fluvial discharge, as well as future bathymetry adjustments due to SLR and biomass accumulation). The impacts are quantified using the system's annual sediment budget: the change in sediment volume within the system at the end of the year. A key finding is that as peak flow velocities increase—due to planform changes such as local interventions or sea level rise (SLR)—the estuary's annual sediment budget decreases exponentially.
External factors, such as changes in fluvial sediment discharge or tidal dynamics, can shift this relationship along the axis. However, despite these external changes, the exponential relationship caused by planform modifications remains.
Similar relationships are expected to exist in other estuaries based on morphodynamic equilibrium theory. Additionally, results demonstrate that a single CC scenario can have an ambiguous impact on the estuary's sediment budget. Namely, the direction and magnitude of change in sediment budget caused by a CC scenario depends on local interventions, highlighting the need to analyze CC-impacts in conjunction with human activities.
Additionally, results demonstrate that the impact that the change in sediment budget due to a single CC scenario is ambiguous, with the direction and magnitude of change depending on local interventions, which highlights the need to analyze CC-impacts in conjunction with human activities.
Chapter 4 addressed how salt intrusion is affected by local interventions (as considered previously) and SLR. A schematised 3D hydrodynamic model was developed and validated representing the Rotterdam Waterway, the Netherlands. The analysis was performed for average weather conditions, a low discharge event, and a storm surge event. Results showed how increasing wetland area increases the salt intrusion (SI) lengths in weakly stratified systems while it decreases in strongly stratified systems. The underlying mechanism: an increase in wetland area increases the tidal flow which in turn reduces stratification. This increases salt import by tidal flow but reduces salt import by estuarine circulation flow. In mixed estuaries the former is dominant, thus increasing SI-lengths, while in strongly stratified estuaries the latter is dominant, reducing SI-lengths. Even for one system configuration, this mechanism can decrease the SI-length during average discharge while increasing it during low discharge or storm surges, when stratification is substantially lower. Notably, the effect of intertidal wetland area on the SI-length was subordinate to channel depth. However, both types of interventions altered stratification to a similar degree, demonstrating the relevance of intertidal area's for sediment dynamics and ecological functioning.
Next, Chapter 5 addresses how alterations in SI processes, induced by channel depth and wetland width, affect fine sediment dynamics. The 3D hydrodynamic model from Chapter 4 is extended to include fine sediments and is subsequently calibrated and validated. Average discharge conditions are considered to evaluate sediment trapping and high discharge events (with return periods of 1, 10 and 100 years) are considered to evaluate sediment flushing. Results demonstrate that an increase in intertidal area enhanced sediment trapping by estuarine circulation flow while it reduced sediment trapping due to (asymmetrical) tidal flow, the two primary trapping mechanisms. Since estuarine circulation was the dominant mechanism in this study, increasing intertidal wetland area reduced sediment trapping. Sediment flushing was almost entirely determined by the peak river discharge and channel depth, increasing exponentially with the freshwater Froude number. Finally, contrary to some studies, no clearing of the water column was observed after high discharge events. Turbidity even increased temporarily as a secondary region of high turbidity formed further downstream, caused by the re-suspension of sediments flushed downstream before during high discharge. This mechanism can explain high SSCs observed in estuaries during periods of lower discharge.
Overall, this thesis highlights the significance of intertidal wetlands for estuary-wide dynamics, besides their increasingly well-known potential to locally improve flood safety and ecology. Additionally, the thesis demonstrates the importance of considering various system changes in conjunction, on various temporal scales, when making decisions for mitigation and adaptation for an uncertain future.
