Receiving Basin Substrate Controls on Delta Morphodynamics

Johnson, Joshua

Abstract

Deltas are inhabited by hundreds of millions of people and are critical for food security, coastal defence, carbon sequestration and ecological diversity. These intrinsically vulnerable systems are threatened by an array of anthropogenic pressures, such as accelerated sea level rise, enhanced subsidence due to sub-surface fluid extraction, retention of sediment within upstream reservoirs and levee construction isolating the delta floodplain from sediment deposition. This study investigates how delta morphodynamics are influenced by the resistance of the sediment making up the receiving basin substrate, and how these effects evolve in the face of anthropogenic forcing of relative sea level and sediment supply loss.
Numerical modelling using Delft3d_flow found bed sediment erodibility w to be a strong driver of delta land area, elevation, distributary channel geometry and channel mobility, with the potential to overcome or modulate the effects of fluvial sediment. Substrate fine sediment content was found to be dominant in setting channel mobility, but the cohesive strength of the fine sediment was dominant in setting channel depth, width:depth ratio and subaerial land area. Fieldwork in the Wax Lake Delta, Louisiana, a sediment limited delta underlain by erosion resistant fine sediment, utilised multibeam echosounder and acoustic doppler current profiler to collect high resolution bathymetric and velocimetric data. This highlighted that substrate type and sediment supply could cause delta bifurcations to be stable under flow conditions different to those dictated to produce stable bifurcations by previous studies.
This work highlights that the effects of substrate sediment on delta morphodynamics cannot be neglected if their reaction to current and future anthropogenic forcing is to be accurately predicted and demonstrates the desperate need for further work constraining the effects of channel bed erodibility on bifurcation dynamics, as well as field studies to quantify the properties of sediment underlying modern and ancient deltas.

Citation

Johnson, J. Receiving Basin Substrate Controls on Delta Morphodynamics. (Thesis). University of Hull. https://hull-repository.worktribe.com/output/4634781

• Thesis Type: Thesis
• Deposit Date: Apr 29, 2024
• Publicly Available Date: May 1, 2024
• Keywords: Physical geography
• Public URL: https://hull-repository.worktribe.com/output/4634781
• Additional Information: Energy & Environment Institute; University of Hull
• Award Date: Apr 9, 2024