Recurrence interval of riverbed sand mining hotspots in the Mekong delta: Potential indications of unsustainable replenishment rates.

Rampant and illegal river sand mining in the Vietnamese Mekong Delta (VMD) has led to substantial sediment losses and bank erosion. However, regulation of this issue remains a significant challenge due to insufficient monitoring and enforcement efforts, partly attributed to limited data and technology. To support an improved monitoring system in the VMD, this study investigates the spatiotemporal changes in sand mining hotspots and their underlying drivers. The recurrence intervals of sand mining boats were assessed from 2014 to 2020 using Sentinel-1A, and its association with riverbed incisions were examined from bathymetry field surveys between 2017 and 2020. Our results attest to sand mining intensification from 2015 to 2020, particularly in the upstream section of the VMD, where 70% of the activity was recorded. Not only was there an apparent increase in sand mining hotspots by 32.4%, but most hotspots recorded a recurrence interval of 2 years. This potentially indicates a minimal timeframe of sediment replenishment before the same locations become economically viable for further sand extraction. Additionally, a correlation was detected between sand mining hotspots and significant riverbed incisions, although the lack of spatial overlaps in some regions suggests other upstream influences like bank collapse and river damming. Our study, through the meticulous assessment of sand mining boat movement and river bathymetry data, ultimately sheds light on the potentially unsustainable scale of sand mining activities in the VMD. It aims to support informed decision-making and effective regulations that tackle excessive sand extraction amid the dynamic environmental challenges we face, while offering valuable insights to similar deltas worldwide.