The sedimentology of fluvial megascours

A common feature of rivers is that at bends, or where two channels join, they scour out their beds to depths that greatly exceed the average depth along their course. In the World's largest rivers such features are truly 'megascours' reaching depths of 50 m. However, despite their widespread occurrence, they are a relatively unquantified phenomenon due to issues associated with obtaining measurements from such locations. This lack of understanding matters because knowledge of megascours has a number of key societal (destruction of infrastructure) and economic (hydrocarbon exploration) applications. To tackle this pressing issue this proposal presents a unique methodology based on application of state-of-the-art high resolution marine seismic technologies together with a programme of coring and numerical modelling that will allow long-standing controversies relating to megascours to be resolved.

The proposal brings together a novel collaboration between marine scientists, river sedimentologist and numerical modellers in one of the World's largest rivers, the Jamuna, Bangladesh. Marine seismic systems have been used extensively to provide high resolution images of the subsurface in offshore environments but have never been applied in a river. However, by taking advantage of new technologies developed at the National Oceanography Centre, Southampton our team is now in a position to generate the first quantitative datasets of river megascour morphology and associated deposits. Our focus will be scours near the confluence of the Ganges and Jamuna rivers and at sites further downstream. Over these sites an understanding of the sub-riverbed will be achieved by the collection of seismic reflection data (informed by a programme of coring and some additional ground penetrating radar data), while the nature and processes acting on the riverbed will be determined from multi-beam echosounder data. This will allow morphology and the associated fill of scours to be quantified and any downstream changes between sites to be fully assessed. We will use this data to evaluate a numerical model of megascour sedimentology that can then be used, for the first time, to test contrasting conceptual models, largely developed on inference in the absence of data, of how megascours function and what they look like in the rock record. A key output of the project will thus be the first fully evaluated generic numerical model of scour zone stratigraphy that will be widely applicable to a broad range of large rivers. We will also have developed protocols for undertaking seismic surveys within rivers, so opening up the technology for others. This has the potential to have a transformative impact upon the discipline allowing new datasets to be collected that will allow advances in fundamental river science.

The generic understanding revealed by our work will also have a number of important applications that will be explored as part of our Pathways to Impact. Thus we will inform policy-makers and local communities and collaborate with specialists in the hydrocarbon industry.

The fundamental science advances that will be realised on completion of this project also have direct relevance in economic, policy and societal terms. The new understanding that will be generated on megascour sedimentology is directly applicable within sequence stratigraphic concepts. This was a framework developed within the hydrocarbon industry by the company Exxon Mobil to aid in the exploration of oil and gas reservoirs. By continuing a series of previous and ongoing consultations with this company the full economic impact of our research will be realised. The UK government recognises its commitment to improving the quality of life of the inhabitants of Bangladesh via programmes such as UKCDS (UK Collaboration on Development Sciences - partly NERC funded). By direct collaboration with UKCDS we will facilitate a series of meetings with UK and Bangladeshi stakeholders to inform policy with respect to planning for investment decisions on where to build infrastructure; megascours are notorious from an engineering perspective for destruction of roads, bridge crossings and pipelines. Finally, in terms of society the key threat to the resilience of local populations are potential changes in flow regime that could lead to increased rates of scour hole and channel bank migration, and floodplain inundation frequency. In collaboration with the Centre for Environmental & Geographic Information Services (CEGIS-part of the Ministry of Water Resources) additional numerical simulations will be carried out to explore these issues and findings communicated to local populations.