Modelling the Effects of Anthropogenic Disturbances on the Evolution of a Mega- Delta

Le, Quan Quan

Abstract

River deltas provide ecosystem services that are vital to the world's population, supporting both lives and livelihoods. However, these low-lying areas face heightened vulnerability to the effects of climate change. This is intensified by local resource exploitation including sand mining and hydropower expansion that cause the lowering of riverbeds and modulate freshwater flux. These cumulative impacts, coupled with changes in input hydrological conditions and rising sea levels, have the potential to cause considerable disruptions in the flow dynamics across river deltas. Despite numerous studies into anthropogenic influences in delta evolution, a significant knowledge gap persists regarding how the combination of stressors that drive riverbed lowering influences alterations in hydraulic patterns and sediment transport capacity.
Here the Lower Mekong Basin is used as an exemplar of sediment starved lowland rivers and deltas globally. Long-term hydrological data are combined with a 1D hydrodynamic numerical model and a 2D coupled hydrodynamic – sediment transport model to examine system response to rapid riverbed lowering. Assessing the relationships between riverbed lowering, water level, tidal amplitude and sediment transport across a range of spatiotemporal scales allows the quantification of the effects of riverbed lowering during a historical 20-year period and future projection.
Historical data analysis and hydrodynamic model results suggest that for median freshwater flux conditions, the system's historical average riverbed lowering of approximately 3.06 m (𝜎 = 2.03 m) from 1998 to 2018 has led to simultaneous declines in average annual water levels of approximately 0.65 m (𝜎 = 0.75 m) and an increase in the average annual tidal range by approximately 0.19 m (𝜎 = 0.15 m). The reduction in water level is more pronounced landward, whereas the increased tidal range is more prominent seaward. Under anticipated Future scenario (to the year 2038), where the riverbed lowering is projected to average around 5.92 m (𝜎 = 2.84 m) compared to 1998, declines in mean water level of approximately 1.27 m (𝜎 = 1.5 m) are projected while, the maximum water level reduction landward reaches may reach 4.19 m. Simultaneously, the mean tidal range is expected to increase by approximately 0.46 m (𝜎 =0.27m), with the maximum rise potentially reaching more than 1 m in seaward areas. Furthermore, model results indicate that riverbed lowering significantly reduces water flux from the river to its floodplain and towards the Tonle Sap Lake, one of the world’s most productive lake-wetland systems, with wide implications for food security.
Hydrodynamic and sediment transport model results indicate that riverbed lowering diminishes sediment transport capacity. Specifically, simulated sand transport at the apex of the delta has decreased by approximately 30% over the nine-year period from 2013 to 2022. By 2022, simulated data at the apex of the delta indicates that sand transport is roughly 10 times lower than the observed total sand extraction volume across the entire Lower Mekong Basin. The significant disparity between sand transport capacity and sand extraction in the delta, coupled with the decrease in sediment supply due to upstream damming and natural reductions in sediment load from shifting tropical cyclones will further exacerbate the adverse effects of sand mining and sediment starvation caused by upstream river impoundment.

Citation

Le, Q. Q. (2025). Modelling the Effects of Anthropogenic Disturbances on the Evolution of a Mega- Delta. (Thesis). University of Hull. https://hull-repository.worktribe.com/output/5087525

• Thesis Type: Thesis
• Deposit Date: Mar 24, 2025
• Publicly Available Date: Mar 25, 2025
• Public URL: https://hull-repository.worktribe.com/output/5087525
• Additional Information: Energy & Environment Institute; University of Hull
• Award Date: Feb 5, 2025