Deciphering the dominant drivers of contemporary relative sea-level change: Analysing sediment deposition and subsidence in a vulnerable mega-delta

The world's major river deltas are facing a major sustainability crisis. This is because they are under threat from being 'drowned' by rising sea levels, with potentially severe consequences for the 500 million people who live and work there. At a qualitative level we have a relatively well developed understanding of the processes that are driving these rising sea levels. Changes in delta surface elevation occur when the summed rates of eustatic sea level rise and ground-surface subsidence are not balanced by gains in surface elevation, the latter being caused by the deposition of sediments supplied from river catchments upstream. Ongoing and major environmental changes are seemingly driving greater imbalances in these factors: eustatic sea levels are rising as a consequence of anthropogenic climate change while ground-surface subsidence, which occurs naturally in deltas as a result of sediment compaction, is in many cases being significantly accelerated by groundwater and/or hydrocarbon extraction. As a result, the only factor that could potentially offset these losses in delta surface elevation is sediment deposition on the delta surface. Unfortunately, many deltas are also being starved of their supply of river sediments as a result of anthropogenic activities, such as sand mining and damming, in the feeder catchments upstream. Estimating precise values of eustatic sea-level rise, sediment supply rate, surface deposition and ground-surface subsidence, is a significant challenge. In the near term the most significant factors in this balance are sediment deposition and subsidence (in the longer term eustatic changes will become relatively more significant). However, a particular issue in estimating sediment supply is that previous studies have focused on the sediment loads at the apices of deltas, with an almost complete absence of reliable data within the delta distributary channel network downstream of the apex. Moreover, the diversity of relevant disciplinary expertise involved in determining the other drivers contributing to relative sea-level rise has thus far conspired to inhibit the integrated synthesis that is really necessary to tackle the problem systematically.

The world's third largest delta, the Mekong is SE Asia's rice basket and home to 20 million people, but it is being exposed to environmental risks as a result of rapid economic development, most notably through upstream damming and anthropogenic subsidence. The Mekong is therefore not only representative of many of the issues facing the world's deltas, but reliable data are urgently needed to help inform the sustainable management plans required to provide a safe operating space for the delta's inhabitants. In our NERC funded work we have developed new methods to estimate recent historical and future trends in the river sediments supplied to the apex of the delta. However, it is the flows of sediment within delta distributary networks, downstream of the delta apices, that are most critical in controlling local rates of delta surface deposition. In this proposal we will collaborate with Can Tho University and the Vietnamese Hydrological agency to access archived sediment transport measurements. Using novel methods developed in our existing work in the catchment upstream we will 'unlock' and translate these data into the very first estimates of sediment loads within and across the delta distributary network itself. Meanwhile, we will also work with other international groups who have been developing novel models to simulate rates of delta surface deposition (Potsdam) and ground-surface subsidence (Utrecht). Working together we will draw these data together to build the first integrated assessment of the factors driving near-term relative sea-level rise in a globally significant, iconic, delta, providing a template for similar analyses in other vulnerable deltas worldwide.

This proposal will result in the establishment and development of a novel international collaboration that will bring together a range of experts with complementary expertise to address the challenge of understanding the controls on relative sea-level rise in a highly vulnerable mega-delta. The work will provide improved understanding of suspended sediment transport dynamics in the Mekong delta. We anticipate that this will benefit the following groups:

1. Academic and commercial modellers (including engineering consultancies) will benefit from the improved knowledge of sediment transport rates, which will enable improved validation of existing models, meaning that projections of future sediment dynamics into the future can be made with greater confidence. Such projections are essential for the longer-term planning of delta protection from rising sea levels, and will thereby inform economic and social planning (see below).
2. The Mekong River Commission (MRC), the inter-governmental body responsible for promoting sustainable management and development within the Mekong basin, considers that models of the delta evolution are urgently required for economic and social planning.
3. Various organisations with whom we already have well-developed links through our prior work, such as the United Nations Food and Agriculture Organisation (FAO) and WWF-Greater Mekong, would use the outputs to inform them of likely future impacts on the natural environment within the Mekong delta and the coastal zone.
4. The Mekong delta is home to twenty million people. Their security in terms of space, water supply and political stability are conditional on the integrity of the delta in the next 100 years and beyond.

As noted above some of the benefits from the mapping and modelling advances will be of value within the next 10 years whereas the impacts on health, economic growth, social cohesion and political stability extend over time-scales of decades to one hundred years but these are the appropriate planning time horizons. The UK will also benefit from improved forward planning and targeting of overseas aid to delta regions and provision of engineering solutions to delta management.