Repeat topographic surveys are increasingly becoming more affordable, and possible at higher spatial resolutions and over greater spatial extents. Digital elevation models (DEMs) built from such surveys can be used to produce DEM of Difference (DoD) maps and estimate the net change in storage terms for morphological sediment budgets. While these products are extremely useful for monitoring and geomorphic interpretation, data and model uncertainties render them prone to misinterpretation. Two new methods are presented, which allow for more robust and spatially variable estimation of DEM uncertainties and propagate these forward to evaluate the consequences for estimates of geomorphic change. The fi rst relies on a fuzzy inference system to estimate the spatial variability of elevation uncertainty in individual DEMs while the second approach modifi es this estimate on the basis of the spatial coherence of erosion and deposition units. Both techniques allow for probabilistic representation of uncertainty on a cell-by-cell basis and thresholding of the sediment budget at a user-specifi ed confi dence interval. The application of these new techniques is illustrated with 5 years of high resolution survey data from a 1 km long braided reach of the River Feshie in the Highlands of Scotland. The reach was found to be consistently degradational, with between 570 and 1970 m 3 of net erosion per annum, despite the fact that spatially, deposition covered more surface area than erosion. In the two wetter periods with extensive braid-plain inundation, the uncertainty analysis thresholded at a 95% confi dence interval resulted in a larger percentage (57% for 2004-2005 and 59% for 2006-2007) of volumetric change being excluded from the budget than the drier years (24% for 2003-2004 and 31% for 2005-2006). For these data, the new uncertainty analysis is generally more conservative volumetrically than a standard spatially-uniform minimum level of detection analysis, but also produces more plausible and physically meaningful results. The tools are packaged in a wizard-driven Matlab software application available for download with this paper, and can be calibrated and extended for application to any topographic point cloud (x,y,z).

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Traditional policies for managing river bank erosion are currently being reconsidered as a result of increased awareness regarding the unsustainable nature of some forms of bank protection, and the role played by bank erosion in providing ecosystem services and supporting geomorphological functions. River managers are therefore increasingly seeking to preserve bank erosion within a defined erodible corridor. This paper provides an overview of the erodible corridor concept, focusing on the provision of guidelines for applying the concept in practice. We argue that a nested approach is required to address management objectives across a range of scales (network scale, reach scale, local scale) and review the different geomorphic tools that are available to help managers define the extent and inner sensitivity of the erodible corridor. These tools include simple rules of thumb such as evaluation of the equilibrium meander amplitude, historical approaches based on overlays of historical channel position, and simulation modelling. The advantages and limitations of each of these tools are discussed.

The world's rivers deliver 19 billion tonnes of sediment to the coastal zone annually 1 , 17 with a significant fraction being sequestered in large deltas, home to over 500 million 18 people. Most (>70%) large deltas are under threat from a combination of rising sea 19 levels, ground surface subsidence and anthropogenic sediment trapping 2,3 , and a 20 sustainable supply of fluvial sediment is therefore critical in preventing deltas being 21 'drowned' by rising relative sea levels 2,3,4 . Here, we combine suspended sediment 22 load data from the Mekong River with hydrological model simulations to isolate the 23 role of tropical cyclones (TCs) in transmitting suspended sediment to one of the 24 world's great deltas. We demonstrate that spatial variations in the Mekong's 25 suspended sediment load are correlated (r = 0.765, p < 0.1) with observed variations 26 in TC climatology, and that a significant portion (32%) of the suspended sediment 27 load reaching the delta is delivered by runoff generated by TC-associated rainfall. 28Furthermore, we estimate that the suspended load to the delta has declined by 52.6 ± 29 explaining past 5,6,7 , and anticipating future 8,9 , declines in suspended sediment loads 34 reaching the world's major deltas. However, our study shows that changes in TC 35 climatology affect trends in fluvial suspended sediment loads and thus are also key to 36 fully assessing the risk posed to vulnerable coastal systems. 37 Mt over recent years (1981-2005The world's largest rivers contribute a disproportionately large fraction (Extended 38Data Table 1) of the terrestrial sediment flux, which has both created, and is critical in 39 sustaining, their great deltas. Moreover, river borne sediments are a key vector for carbon 40 and nutrients, thereby playing a vital role in global biogeochemical cycles 10,11 . However, a 41 significant majority (>70%) of large deltas are now recognized as being under severe 42 threat from rising relative sea levels 2,3 , in part due to reported anthropogenically-driven 43 reductions in sediment loads 5,6,7 . Many large rivers are located in tropical regions 44 (Extended Data Figure 1) that exhibit highly seasonal flow regimes affected by tropical 45 cyclones (TCs). The potential destructive or constructive impacts of tropical cyclones that 46 directly strike deltas are well established 12,13 . However, when they strike further upstream 47TCs deliver much higher than normal levels of rainfall, effectively triggering landslides 48 and mobilizing sediments into the river network, thereby generating very high 49 instantaneous sediment loads 14,15,16 . Such high sediment loads could compensate for the 50 potential destructive effects of TCs striking deltas proper but, notwithstanding some prior 51 studies in smaller drainage basins 17,18 , the role of TCs in driving sediment delivery to the 52 lowlands and coast remains unclear. As noted, this is particularly the case for large rivers 53 that carry much of the terrestrial sediment flux because these rivers are, in t...

[1] The erosion of sediment from riverbanks affects a range of physical and ecological issues. Bank retreat often involves combinations of fluvial erosion and mass wasting, and in recent years, bank retreat models have been developed that combine hydraulic erosion and limit equilibrium stability models. In related work, finite element seepage analyses have also been used to account for the influence of pore water pressure in controlling the onset of mass wasting. This paper builds on these previous studies by developing a simulation modeling approach in which the hydraulic erosion, finite element seepage, and limit equilibrium stability models are, for the first time, fully coupled. Application of the model is demonstrated by undertaking simulations of a single flow event at a single study site for scenarios where (1) there is no fluvial erosion and the bank geometry profile remains constant throughout, (2) there is no fluvial erosion but the bank profile is deformed by simulated mass wasting, and (3) the bank profile is allowed to freely deform in response to both simulated fluvial erosion and mass wasting. The results are limited in scope to the specific conditions encountered at the study site, but they nevertheless demonstrate the significant role that fluvial erosion plays in steepening the bank profile or creating overhangs, thereby triggering mass wasting. However, feedbacks between the various processes also lead to unexpected outcomes. Specifically, fluvial erosion also affects bank stability indirectly, as deformation of the bank profile alters the hydraulic gradients driving infiltration into the bank, thereby modulating the evolution of the pore water pressure field. Consequently, the frequency, magnitude, and mode of bank erosion events in the fully coupled scenario differ from the two scenarios in which not all the relevant bank process interactions are included.

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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

[1] We present an integrated analysis of bank erosion in a high-curvature bend of the gravel bed Cecina River (central Italy). Our analysis combines a model of fluvial bank erosion with groundwater flow and bank stability analyses to account for the influence of hydraulic erosion on mass failure processes, the key novel aspect being that the fluvial erosion model is parameterized using outputs from detailed hydrodynamic simulations. The results identify two mechanisms that explain how most bank retreat usually occurs after, rather than during, flood peaks. First, in the high curvature bend investigated here the maximum flow velocity core migrates away from the outer bank as flow discharge increases, reducing sidewall boundary shear stress and fluvial erosion at peak flow stages. Second, bank failure episodes are triggered by combinations of pore water and hydrostatic confining pressures induced in the period between the drawdown and rising phases of multipeaked flow events.