Abstract
The Vietnamese Mekong Delta was formed by rapid transgression during the second half of the Holocene by deposition of mainly unconsolidated, fine-grained (clayey) sediments undergoing high compaction rates. The natural subsidence can seriously impact the already vulnerable delta plain as its low elevation exposes the delta to global sea level rise, flooding, salinization. Human activities such as groundwater pumping, infrastructural loading, sand mining and dam construction have exacerbated the effects of natural consolidation. Here we present a novel modeling study that has allowed to reproduce the formation and evolution of the Mekong delta over the past 4000 years. Using an adaptive finite-element mesh, the model properly simulates accretion and natural consolidation characterizing the delta evolution. Large soil grain motion and the delayed dissipation of pore-water overpressure are accounted for. We find that natural compaction of Holocene deposits following delta evolution exceeds predicted values of absolute sea level rise. The unprecedented high rates (up to ~20 mm/yr) threaten the lower delta plain with permanent inundation and inevitably reduce the designed service life of flood defense structures along the coast. Total subsidence and sediment delivery to the delta plain will determine its future elevation and vulnerability to relative sea level rise.
Highlights
The populous Vietnamese Mekong Delta (VMD) hosts a thriving agricultural and industrial economy[1]
The factors that determine the rate of natural compaction, and possibly land subsidence, are sediment type and the specific depositional history that has resulted in the present stratigraphy
In VMD, very high compaction rates of Holocene strata between 25 and 41 mm per year are measured at three locations in coastal mangrove areas by surface elevation tables (SET) combined with marker horizons that register accretion at the surface[22]
Summary
The populous Vietnamese Mekong Delta (VMD) hosts a thriving agricultural and industrial economy[1]. Sediment accretion for these locations exceeds compaction rate, which results in a net elevation gain of the surface, these numbers reveal the potential of the Holocene deltaic sediments to contribute to VMD subsidence through natural consolidation. This holds especially in cases where sediment accretion is reduced by natural causes, dyke development or following cultivation[23]. We do not incorporate chemical or biological processes as they represent secondary factors contributing to compaction because of general waterlogged conditions[25,26] This approach may serve as a model to investigate natural compaction in other deltas and prograding coastal environments elsewhere in the world
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