Data-driven 3D modelling of long-term Holocene delta evolution and sediment compaction: The Mekong Delta

Abstract

The Vietnamese Mekong River Delta (VMD) is one of the largest and lowest elevated deltas on Earth, shaped over the past thousands of years following delta progradation and sediment deposition. The geologically young delta sediments have high porosity and compressibility, resulting in high natural sediment consolidation (also known as autocompaction). Autocompaction is a natural intrinsic process that governs the spatio-temporal morphological evolution and shallow compaction (i.e., land subsidence) in a delta. As a delta aggrades and progrades, the weight of accumulated sediments increases the effective stress experienced by underlying sediments, driving internal shallow compaction processes. Compaction of shallow sediments considerably contributes to land subsidence in the VMD, influencing the morphology and elevation of the delta plain and increasing the deltas exposure to natural hazards like flooding and relative sea-level rise. In this study, we introduce a novel methodology to quantify sediment accumulation and autocompaction while taking into account the depositional history and heterogeneous nature of subsurface sediments in deltas like the VMD. We derived the depositional history, spatial heterogeneity and palaeo-sedimentation rates by combining extensive datasets with lithological borelogs, sediment datings and geomechanical characterization of the delta's most representative lithologies. To simulate the spatio-temporal formation and evolution of the delta over the last 4000 years, we employ the NATSUB3D finite element model to simulate sediment deposition and consolidation over time using an adaptive three-dimensional mesh. The resulting 3D hydro-stratigraphical and geomechanical characterization provides unique insights on past Holocene spatio-temporal evolution of the VMD and current autocompaction dynamics. The model enables the prediction of shallow compaction rates under future sediment deposition and can facilitate process-based quantification of delta elevation evolution under natural and human-engineered sedimentation. This unlocks new opportunities to evaluate the effectiveness of nature-based solutions and sediment enhancing strategies aimed to prevent elevation loss and combat relative sea-level rise in the Mekong delta and similar lowly elevated coastal-deltaic landforms elsewhere.