Morphological evolution of wave-influenced deltas: Empirical and model analyses

Abstract

River deltas are dynamic geomorphological features known for their immense natural resource potentials; they are hotspots of population concentration and form some of the most biodiverse environments when in pristine form. However, they also comprise some of the most vulnerable coastal environments, heavily threatened by anthropogenic activity and climatic changes. In response to current threats, some deltas are experiencing large-scale coastline retreat, flooding, saline intrusion, and depletion of the wetland ecosystems. Dealing with these threats requires an improved understanding of the dynamic interplay between multiple processes that govern delta evolution as well as deltas’ response to external disturbances. In this thesis, I aimed to deepen insight into the dynamic changes in coastal delta morphology in response to changes in key boundary conditions, specifically fluvial discharge, and waves. The thesis has focused on four research questions, using the Niger deltas as a case study: (i) What are the contemporary trends in the natural and anthropogenic factors governing river delta dynamics? (ii) What changes can we observe in the shoreline mobility of the Niger delta in recent decades? (iii) What has been the trends in recent fluvial discharge and wave regimes along the Niger delta, and might these trends be associated with the observed morphological changes? (iv) Is it possible to identify the relative efficacy of river flow vs waves in dictating delta evolution under changing climatic conditions with numerical simulation? The review of the dynamics and sustainability of river deltas established the importance of sediment supply and distribution as well as the impact of contemporary anthropogenic-induced processes and ongoing climatic changes on deltaic morphological evolution. The study also revealed that recent decadal shoreline dynamics along the Niger delta lacked any entrenched spatial and temporal patterns, leading to the suggestion that current anthropogenic impact on the delta is moderate. The effect of natural process dynamics on deltaic shoreline mobility could not be directly inferred notwithstanding the recent increasing trends in both fluvial inputs and wave energy intensity into the Niger delta. The novel numerical model developed in this thesis elucidated how the dynamic coupling between ocean waves and fluvial processes informs the longshore sediment flux, progradation, aggradation, and patterns of channel migration in coastal delta settings. Overall, this thesis has made significant advances in the understanding of morphological evolution of deltas significantly influenced by fluvial and marine processes at yearly to decadal timescales.

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Identifiers

ORCID for Steve Darby: orcid.org/0000-0001-8778-4394

ORCID for Eli Lazarus: orcid.org/0000-0003-2404-9661

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