The evolution of tidal coastal landscapes and the controls on pathways to equilibrium

The evolution of tidal basins is fundamental to understanding the drivers of our vulnerable estuarine and lagoon environments, ultimately determining key ecological parameters such as intertidal area, inundation regime, connectivity and accommodation space. This thesis uses multiple methods including idealized modelling, field observation of an estuarine shoal covered with mangroves, and a calibrated and validated model with field data in order to explore how different components of tidal environments influence the short and long-term dynamics of tidal lagoons. The idealised modelling specifically focused on the roles of initial bathymetry, bed sediment and mud concentration at open boundaries on shaping equilibrium profile development of tidal basins. The results of the model showed that geomorphological development (long-term evolution) is similar in sandy environments with different initial bathymetries and without a mud supply. This was concluded because the equilibrium profiles and tidal asymmetries evolved similarly at the end of simulations. However, residence time, development of channels and energy dissipation were observed to be different. In the case of available incoming mud from open boundaries, equilibrium bathymetry was dominated by the mud supply and sediments accreted all over the domain, forming a mudflat and a deeply incised ebb-dominated channel, with several minor side channels of which their size and number were dependent on mud concentration. The response of tidal environments to sea-level rise has been previously studied often by neglecting the importance of initial configuration in the model. In such process-based modelling, different initial bathymetries are only considered to influence the time to reach a stable equilibrium condition but will always lead to identical equilibrium bathymetry. Here we demonstrated that initial profile not only affects some aspects of tidal basins such as channel formation, residence time and energy dissipation in equilibrium condition, but also impacts their geomorphological development under sea-level rise. In the northern North Island of Aotearoa New Zealand, tidal environment evolution is mediated by mangroves. The governing dynamics on a recently evolving estuarine, mangrove-covered channel-shoal system in Whitianga Estuary, Aotearoa New Zealand. Observations of water level, flow velocity, suspended sediment concentration and bed sediment characteristics were used to infer flow asymmetry and sediment transport pathways around the shoal. The impact of mangrove colonization on channel dynamics, sediment texture of upper layer and mud layer thickness was studied by a one-week field campaign. Due to changes in land use, the estuary was subjected to an increase in sediment supply, causing the tidal flats to elevate and provided suitable conditions for the expansion of mangroves in 1940s. By the use of historical images, the expansion of mangroves in Whitianga Estuary was captured and the results were combined with the measured data of water level, current and sediment dynamics in order to explore the short-term dynamics around the shoal and the role of vegetation as eco-engineers in inter-tidal environments. The shoal expanded laterally independent of mangrove colonization. However, observed mud layer thickness was larger around mangroves compared to un-vegetated parts of the shoal. The mangrove creek (which was a shoal channel before expansion of mangroves) was consistently ebb-dominated and the shoal channel located outside the forest on the edge of the shoal was flood-dominated. The results suggested that mangroves impose a control on their surrounding environment and can ultimately lead to a regime shift in channels and changes in sediment texture. The understanding provided by the field observations was confirmed with a 2-D numerical model, developed in the Delft3D modelling system, which was calibrated and validated with the data collected from the field. This model was used to study the short-term influence of mangroves and tidal creeks on tidal circulation and large-scale tidal asymmetry. The results showed that the effect of vegetation on tidal asymmetry and flow routing was larger compared that of the tidal creeks. However, the small impact of the channels may be due to the small size of the creeks compared to the size of the forest. When vegetation was removed, tidal velocity asymmetry changed substantially whereas channel infilling caused a shift in tidal asymmetry inside the creeks and around the head of the creeks mostly (from being ebb-dominated to flood-dominated). Removing vegetation led to an increase in speed all over the forest except inside the creeks and elevated parts of the forest where velocity decreased. In a scenario in which channels were filled in, currents became weaker in the location of the creeks and velocity increased around the creeks and on the edge of the forest. Furthermore, a scenario was designed in which morphology of the shoal in the 1940s prior to expansion of mangroves was simulated, aiming to quantify the hypothesis posed by field measurement with numerical models. The results of the model demonstrated that mangrove colonization caused a regime shift in the channels, showing that as mangroves colonize inter-tidal areas, they play a critical role in shaping the geomorphology. In Whitianga Estuary, expansion of mangroves around changed a flood-dominated pre-existing shoal channel to an ebb-dominated mangrove creek, suggesting that mangroves are likely to be eco-engineers of the estuarine system. The idealized modelling experiments helped identifying the parameters and elements that govern the long-term morphology and equilibrium of coastal tide-dominated environments and the way they respond to sea-level rise force. Results of this theoretical exploration showed that processes localised, and timescales for equilibrium development vary vastly across the landscape. Elucidating local changing to asymmetry became the focus of subsequent chapters, firstly with a field exploration, and then supported by numerical modelling. The fieldwork presented here elucidated the short-term hydrodynamics and suspended sediment dynamics in an estuarine shoal covered with mangroves. Moreover, observation of aerial images in a multi-decadal timescale combined with hydrodynamics inside a mangrove creek and a shoal channel highlighted the complex non-linear interactions of hydrodynamics and vegetation that lead to a regime shift of a channels in the mangrove forest in Whitianga Estuary, Aotearoa New Zealand. Finally, the results of the numerical model developed and validated by utilizing field data presented here provided insights into the detailed short-term hydrodynamics. Moreover, the scenarios designed by removing channels, vegetation and reconstructing the condition prior to mangrove expansion showed that the effect of vegetation extends beyond the footprint of the vegetation to the surrounding environment with implications for hydrodynamics, morphological evolution and channel dynamics.
The University of Waikato
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