Recent growth of the construction industry has fuelled demand for sand, with considerable volumes being extracted from the world's large rivers. Sediment transport from upstream naturally replenishes sediment stored in river beds, but the absence of sand flux data from large rivers inhibits assessment of the sustainability of ongoing sand mining. Here, we demonstrate that bedload (0.18 Mt yr-1 ± 0.07 Mt yr-1) is a small (1%) fraction of the total annual sediment load of the lower Mekong River. Even when considering suspended sand (6 Mt yr-1 ± 2 Mt), the total sand flux entering the Mekong delta (6.18 Mt yr-1 ± 2.01 Mt yr-1) is far less than current sand extraction rates (50 Mt yr-1). We show that at

The distinct turbulence dynamics and transport modulated by a common seagrass species were investigated experimentally using a flexible surrogate canopy in a refractive-index-matching environment that enabled full optical access. The surrogate seagrass replicated the dynamic behaviour and morphological properties of its natural counterpart. The flows studied were subcritical with Froude numbers $Fr<0.26$ and concerned five Reynolds numbers $Re\in [3.4\times 10^{4}, 1.1\times 10^{5}]$ and Cauchy numbers $Ca\in [120, 1200]$. Complementary rigid canopy experiments were also included to aid comparative insight. The flow was quantified in wall-normal planes in a developed region using high-frame-rate particle image velocimetry. Results show that the deflection and coordinated waving motion of the blades redistributed the Reynolds stresses above and below the canopy top. Critically, in-canopy turbulence associated with the seagrass lacked periodic stem wake vortex shedding present in the rigid canopy, yet the flexible canopy induced vortex shedding from the blade tips. Inspection of spatial and temporal characteristics of coherent flow structures using spectral proper orthogonal decomposition reveals that Kelvin–Helmholtz-type vortices are the dominant flow structures associated with the waving motion of the seagrass and that modulated the local flow exchange in both rigid and flexible canopies. A barrier-like effect produced by the blade deflections blocked large-scale turbulence transport, thereby reducing vortex penetration into the canopy. In addition, we uncovered a transition from sweep-dominated to ejection-dominated behaviour in the surrogate seagrass. We hypothesise that the vortices created during the upward blade motion period play a major role in the sweep-to-ejection-dominated transition. Conditionally averaged quadrant analysis on the downward and upward blade motion supports this contention.

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&lt;p&gt;Deltas are home to 4.5% of the global population and support a range of ecosystem services that are vital to lives and livelihoods. As low-lying regions, deltas are also amongst the most vulnerable areas to the threat climate change and relative sea-level rise, which are being exacerbated by ongoing local resource exploitation. Anthropogenic activities such as riverine sand mining, construction of flood embankments, deforestation and changes of land use and hydropower dams are disrupting the natural evolution of deltaic systems, with many of the world&amp;#8217;s large deltas now being sediment starved. This is important because changes of the sediment flux into large deltas can have implications for the evolution of the morphology of delta bifurcations and their function at routing water and sediment seaward. This can amplify flood hazard and risk for riparian communities and intensify processes such as bank erosion, presenting hazards to human lives and exacerbating land loss. The present study focuses on the Chaktomuk junction at the apex of the Mekong delta, connecting the Mekong with the Tonle Sap Lake and the downstream delta. The junction is important as it provides the connection between the Mekong and the largest freshwater lake in Southeast Asia and because of the proximity of the junction to the rapidly expanding urban centre of Phnom Penh. We present a combined 2D hydrodynamic and sediment transport model for the Chaktomuk junction, constructed and based on high-resolution bathymetric data obtained with multibeam echosounders. A series of established sediment transport equations are adopted and tested through a sensitivity analysis to identify the most appropriate sediment transport solver for the model, which is then validated against field observations. The model was forced with a series of scenario combinations including changes of water and sediment flux and rates of sand mining. Simulation runs are presented that project the future evolution of the apex of the Mekong delta, including changes in bifurcation morphology, water and sediment routing seaward through delta distributary channels and changes in water and sediment exchanges between the Mekong and the Tonle Sap. The implications of these future trajectories will be discussed in terms of the sustainability of the delta to future change.&lt;/p&gt;

&lt;p&gt;Biodiverse coastal ecosystems are vulnerable to microplastic (&lt;5 mm) pollution due to inputs from riverine and shoreline sources which pose ecological threats and have repercussions for social ecosystem services. These ecosystems may contain an aquatic canopy covering the bed, such as seagrass meadows or coral reefs that can trap particles. Despite field measurements revealing the accumulation of plastic debris in a variety of aquatic canopies, the transport and dispositional processes that drive microplastic trapping within such canopies is barely understood. Here, we investigate for the first time the prevalence of biofilmed microplastic retention by sparse and dense branching coral canopies in a hydraulic flume under unidirectional flow. Corals were replicated through 3D-printing using a scan of a staghorn coral &lt;em&gt;Acropora &lt;/em&gt;genus, a branching coral that encompasses one-fifth of extant reef-building corals, globally.&lt;/p&gt; &lt;p&gt;Trapping mechanisms by coral canopies were identified, and include: a) interception of particles with the coral acting as a barrier and microplastics and settling to the bed; b) settling of microplastics on the branches or within the structure of the coral and c) accumulation in the downstream region of individual corals. Trapping efficiency was found to depend on bulk velocity and canopy density, with up to 99% of microplastics retained across the duration of the experiments. Surprisingly, sparse reefs may be as vulnerable to microplastic trapping and contamination as denser canopies under certain flow velocities, with the latter found to retain only up to 18% more microplastics than in sparser conditions. Flow velocity profiles provide insights into the relationships between canopy hydrodynamics and microplastic trapping and distribution. The results indicate coral reefs may form areas of accumulation for microplastic pollution through their observed high trapping efficiency that may otherwise have been transported greater distances.&lt;/p&gt;