| Popis: |
River deltas are home for ~300 million people worldwide and are hotspots for biodiversity. In recent decades, river deltas have been facing growing stressors due to dams and reservoirs, sand mining and sea-level rise, putting the human and natural systems that rely on river deltas at considerable risk to land subsidence, inland and coastal flooding, and other socially impactful geomorphological changes. The geomorphic response of river deltas to these growing stressors depends on the geomorphic controls on delta morphology and their scaling relationships. However, our current understanding of delta’s geomorphic response to these forcing is limited to local cases, fragmented physical and numerical delta experiments, and simplified global models. In this thesis, I explore the possibility of adopting scaling relationships originally developed for quantitative watershed analysis of fluvial systems to river delta systems to understand deltas’ morphodynamic response towards these stressors. Chapter 1 introduces the challenges faced by modern and ancient river deltas along with the scaling relationship theory. Chapter 2 details the methods used in this thesis, including modern delta observation, numerical modelling and characterisation of ancient delta deposits. Chapter 3 introduces a novel globally consistent scaling from 114 modern river deltas using satellite imageries observation, solving previously debated avulsion scaling. I found that slope break and avulsion location scale consistently, opens potential insights into how delta naturally respond to the growing stressors and how the avulsion mobility is closely tied with slope break, that are imperative for understanding delta flood risk. Chapter 4 investigates the role of the novel scaling found in Chapter 3 in controlling avulsion-bifurcation timescale and interaction in river deltas using numerical model. Avulsion is primarily controlled by delta topset slope, in which it occurs simultaneously with bifurcation process. In Chapter 5, novel palaeodischarge estimation models are proposed by correlating water discharge with delta channel widths and catchment areas from modern global deltas by adopting hydraulic geometry concept. These simple and rock-record focused correlations produce palaeodischarge estimates within the same order of magnitude as the palaeodischarge derived from existing, more complex approaches. In Chapter 6, I reclassified palaeodischarge models built in Chapter 5 based on the marine influence that affects the hydraulic geometry assumption used in earlier models. By establishing more detailed scaling relationships, I found that estimating discharge using hydraulic geometry concept is only applicable for river- and wave-dominated deltas, but not for tide-dominated deltas. Finally, Chapter 7 synthesises the previous chapters and proposes further works. |
