The world's 33 largest deltas are being drowned by relative sea level rise and are, as a result, rapidly losing land. This process is also driving an exacerbation of flood risk in these environments, which is placing many large cities, key infrastructure and over 0.5 billion people at risk globally. These issues are most acute for deltas across Southern and Southeast Asia, where an estimated 20% of land will be lost by 2100. These risks are significant. For example, floods during the 2011 Asian monsoon killed an estimated 2000 people and caused ~US$45 billion in economic damage across SE Asia. Moreover, these deltas, and their ecosystem services, underpin regional food security for rapidly growing populations. There is therefore an urgent need to evolve an improved generic understanding of the processes behind the relative sea level rise and flood risk dynamics in these deltaic environments into the future. Significant recent advances have been made in our understanding of many aspects of delta morphodynamics and evolution. This has included work on distributary channel flow processes, bifurcation stability and bar dynamics, and the profound influence of tidal backwater effects on longer-term channel hydro- and sediment dynamics. However, despite this progress there are significant uncertainties around the influence of: i) upstream migrating backwater effects, forced by sea-level rise, on delta bifurcation stability; ii) declining sediment delivery and increased hydrological variability on distributary channel stability; iii) connectivity between the channels and the delta surface on the routing, dispersal and trapping of sediment. Each of these uncertainties are key knowledge gaps that must be addressed for effective delta management, flood risk mitigation and maintenance of ecosystem services. Our project will investigate flow and sediment routing through the Mekong delta across the annual monsoon flood and develop a new generic understanding of the impact of relative sea-level rise and sediment routing processes through distributary channels and key bifurcation sites on the delta. This will be achieved through collection of new state-of-the-art field datasets, development and application of morphodynamic numerical modelling and utilization of system dynamics modelling to guide aquaculture and agriculture adaptations to changes. We will leverage a range of existing links we have to engage with, and communicate the outcomes of the work, to agencies and policy makers in the region and inform water resource planning and mitigation/adaptation strategies in the context of climate change.

The world's largest rivers transport ~19 billion tonnes of sediment each year, with a significant fraction being sequestered in the large deltas that are home to 14% of the world's population. Most (>70%) of these large deltas are under threat from rising sea levels, ground surface subsidence & declining riverine sediment supply required for delta construction. However, while measurements & projections of sea level rise & subsidence exist for many deltas, data quantifying historic changes in fluvial sediment supply are sparse, limiting our understanding of how delta building is related to climatic fluctuations. This situation reflects the complexity of controls on river sediment loads, which include the influence of climate & land use change in upland areas, dam construction, & flood driven storage & remobilisation of sediment within the extensive floodplains that characterise the lowland reaches ("sediment transfer zones") of the world's major rivers. This project will provide the first comprehensive quantification of these controls on riverine sediment fluxes for one of the world's largest rivers (the Mekong), leading to new generic understanding of the relationships between climatic variability, fluvial processes & sediment flux to deltaic zones & the ocean. To meet this aim we will develop a new generic simulation model that will, for the very first time, quantify the effects of climatic & morphological controls on all individual components, & at sub-annual resolution, of the alluvial sediment transfer budget of a large river. The approach is to use a hydrological model to predict sediment supplied from the catchment to the head of the river's sediment transfer reach (the part of a river that links sediment source areas upstream with sediment sinks downstream). Within the transfer reach the model will account for the key morphodynamic processes of river bed & bank erosion, & floodplain sedimentation, which either supply material to the transfer reach, or store the material for later release. The model will be parameterised & validated using targeted field data that we will collect in this proposal. We will run the model to explore historical trends of within-reach sediment fluxes over a multi-decadal period encompassing the last 50+ yrs. The data derived from our simulation model will be unique: the very first annually resolved mega-river sediment budget encompassing a multi-decadal period. These data will enable us to explore a series of specific research questions: What is the net effect on the Mekong sediment load of sediment exchanges within the alluvial transfer reach? Do sediment fluxes associated with floodplain storage & bank erosion promote a net increase or reduction in efflux from the transfer zone? How large is this modulating effect in both absolute & relative terms? How strong is the interannual variability in this modulation, & what factors drive this? In fact, we expect interannual variability to reflect the net effect of changes in the various components of the budget linked to specific climate indices that control each component. This will be explored by testing specific hypotheses concerning (i) the role of specific modes of climate variability (Indian Ocean Dipole & the El-Niño Southern Oscillation) in modulating sediment transfer, and; (ii) the ways in which extreme events (associated with tropical cyclones) control river bank erosion & floodplain deposition. Predicting fluvial sediment transfer through one of the world's great rivers is a scientific challenge that is novel, timely & significant. Addressing this challenge will improve our ability to predict sediment transfer from 'source-to-sink' thereby aiding (i) interpretations of floodplain sedimentary records, (ii) understanding of how sediment, nutrient & carbon fluxes respond to climate, (iii) assessment of changes in flood risk within deltas, & (iv) the physical processes by which ecosystem services within large rivers are sustained.