Climatic and Autogenic Controls on the Morphodynamics of Mega-Rivers: Modelling Sediment Flux in the Alluvial Transfer Zone

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.

Hydrological & morphodynamic processes on the Lower Mekong River (LMR) sustain a wide range of ecosystem services that impact on the lives of the region's inhabitants, many of whom are poor & thus vulnerable to environmental change. Of the many environmental problems extant within the Mekong Basin, arguably the most pressing concerns the sustainability of its delta. Home to more than 17M people, & SE Asia's rice basket, changes in fluvial sediment delivery to the Mekong delta have fundamental implications for flood risk, food security & migration because: (a) fluvial sediment delivery is required for delta building & thus controls the extent of flooding as sea-levels rise, (b) reductions in fluvial sediment supply accelerate coastal erosion, which in turn can displace large numbers of people, and; (c) fine-grained sediment deposition delivers nutrients that sustain agricultural productivity. The most important policy issue concerns evaluating, & responding to, the impacts of future changes in flow & sediment regimes associated with (i) climate change & (ii) planned construction of hydropower dams on the Mekong & its tributaries. Indeed, these issues are significant enough to have been the focus of widespread recent media attention (e.g. Science, 12th August 2011; BBC Earth Reporters, June 2011; The Guardian, 21st August 2011; Nature, October 2011).

In focusing our impact activities on the issue of future sediment delivery to the Mekong delta, our impact plan is deliberately discrete in both time & space from the 'blue sky' science that is the focus of the main research programme (since the latter quantifies the historical sediment budget of the Mekong's sediment transfer reach). Rather, our impact pathway should be understood as literally & figuratively following the flow of the project outputs: Since the downstream limit of our study reach is adjacent to the delta's apex, net efflux from the Mekong's sediment transfer reach is equal to the fluvial sediment flux delivered to its delta. Furthermore, to address future impacts, we can readily implement the modelling tools developed in our proposal to undertake a tailored (i.e., developed in conjunction with key end-users) set of simulations for a suite of climate change & hydropower development scenarios. This requires only modest resource since the relevant data needed to run the models for these climate change & hydropower scenarios are already available (via collaboration with a project partner, Dr Kummu). We will promote incorporation of the results of these projections into practice by engaging with key agencies (e.g. Mekong River Commission, the inter-governmental body responsible for sustainable management of the Mekong; Cambodian agencies responsible for river management & fisheries, Laos agencies involved in the design & regulation of proposed dams, & Vietnamese agencies responsible for flood & erosion hazard management) at a policy formulation workshop. Engagement with these governmental agencies will be ensured by partnering with regional partners (WWF Greater Mekong & Vietnam's Southern Institute for Water Resources Research, SIWRR) who will act as workshop facilitators. Importantly, we have a strong track record of working with WWF-GM & SIWRR, & both enjoy extant links with the relevant end-users.

We also expect our project to generate significant scientific impact since the findings will enable (i) improved interpretations of the stratigraphic record in floodplains & receiving basins, (ii) improved quantification of carbon budgets of large rivers, and; (iii) improved understanding of flood & erosion hazards in large rivers. We are therefore planning activities to disseminate findings to the scholarly community through (i) presentations at academic conferences & publishing papers in learned journals & (ii) a bespoke Knowledge Exchange workshop. Finally, we are also planning outreach activities to engage with the general public at home & overseas.