Hidden Depths: modelling the mining impact on sand-bed rivers

Sand is a critical natural resource used in the construction sector, in land reclamation and coastal protection schemes. Global consumption of aggregates is estimated to be c. 40 billion tonnes a year (Peduzzi, 2014) a large proportion of which is derived from fluvial sediment sources. This figure exceeds the mass of sediment delivered annually to the global ocean (Milliman and Syvitski, 1992). Moreover, with urban populations projected to rise substantially over the next 20 years (UN, 2019), unsustainable extraction of alluvial sand represents a critical threat to the morphological and ecological integrity of rivers.Despite growing awareness amongst the scientific community and policymakers of the deleterious effect that uncontrolled extraction can have on the landscape and local populations, there remains a lack of quantitative understanding concerning the diverse potential impacts of fluvial sand extraction, and the degree to which any level of extraction may be deemed sustainable. Similarly, differences between the impacts of alternative mechanisms of extraction are poorly understood, as are the rates at which these impacts may propagate beyond the immediate extraction zone. These knowledge gaps make effective mitigation and regulation of sand extraction practices extremely challenging.This study seeks to better understand the impact of mining within large sand-bed rivers using numerical modelling. Two modes of sand extraction were considered: (1) bar-top skimming from the floodplain and mid-channel bars; and (2) wet mining by dredging of the channel thalweg. We carry out 2D physically-based morphodynamic model simulations over spatial and temporal scales of 90 km and 150 years in order to quantify the evolution of river morphology, hydraulics and sediment transport during both the period of sand extraction and an extended post-extraction period. Model simulations were designed to quantify both the fluvial responses within the immediate sand extraction zone, and the downstream propagation of the mining disturbances. Results indicate that there is a clear impact of sand extraction in all the analysed hydromorphic metrics (e.g., braid intensity, variations in the river width-depth, and in the flow patterns) and that there is a different river-evolution style and impact when considering different types of sand mining (dry mining from exposed bars at low-flow conditions or wet mining only from the channel thalweg). For example: (1) for wet mining scenarios, the system shows a very significant deepening of the channel thalweg and a consequent reduction in the mobility of the system, decreasing the inundation period on the bars; (2) in dry mining scenarios, the system develops shallower channels (when compared to wet mining), but experiences an increase in avulsion, with the rapid activation and deactivation of secondary channels and unvegetated bars (in the mining zone), enhancing bank erosion and consequent further river widening. Model results demonstrate that recovery of river systems in the absence of mining is a process that can require decades to centuries. Moreover, the influence and consequences of mining directly within the extraction zone are propagated downstream rapidly, although the contrasting response associated with different mining styles becomes less marked outside the immediate area of extraction.