THE INFLUENCE OF SAND-MUD INTERACTIONS ON MIXED SEDIMENT DYNAMICS

Estuaries and near-shore coastal zones are amongst the most productive, dynamic and complex ecosystems in the world. Their high environmental, social and economic value is reflected in EU and UK government policy through legislation targeted at managing these assets in sustainable and integrated ways. This policy is tasked with balancing future economic growth from continued human activities (e.g. ports, navigation, marine renewables), socio-economic impacts from increasing coastal erosion and flood risk, and the protection and enhancement of sensitive habitats and ecosystems. Under these competing demands, there is an urgent need for coordinated sediment management plans for UK estuaries and coastal regions to ensure sustainable practices are implemented and improved resilience in future planning strategies is realised.

To achieve these challenging goals, it is vital that engineers and scientists have the fundamental understanding of physical processes controlling the dynamic behaviour of sediments in estuarine and coastal systems and their role in large-scale morphological evolution. One of the biggest challenges to this, however, is a current lack of knowledge on the behaviour of mixed bed sediments that contain both non-cohesive sands and cohesive muds. Critical shortcomings in current estuarine and coastal modelling systems typically relate to their inadequate representation of important small-scale dynamical processes associated with these sand-mud mixtures including erosion, entrainment, flocculation, settling and deposition. As a consequence, the sensitivity of many estuaries and near-shore coastal regions to future impacts from climate change and anthropogenic development is currently subject to a high degree of uncertainty.

The proposed project will address these shortcomings in a systematic way. The research programme includes fundamental experimental studies in sophisticated settling column and benthic annular flume facilities to investigate these mixed sediment processes in greater quantitative detail than previously undertaken. This experimental work will be complemented by novel analytical model developments to provide better understanding of mixed sediment flocculation and differential settling processes through more physics-based descriptions of the important sand-mud interactions involved. A key outcome from this research will be to establish, for the first time, important linkages between these sand-mud interactions within mixed sediment suspensions and the resulting structural and compositional characteristics of mixed sediment bed deposits. The combined experimental data and analytical model outputs from this project will underpin the development of advanced numerical models required for the development of more sustainable and resilient estuarine and coastal sediment management strategies in the future.

The potential beneficiaries from the research outcomes of this project will include: academic researchers (e.g. universities and research institutes); regulatory bodies and statutory consultants (e.g. SEPA, EA, Marine Scotland); environmental consultants and modellers; port authorities; marine renewables developers; and the UK and Scottish Governments.

From an economic perspective, a wide range of industry sectors are dependent on functional estuaries (e.g. ports, marine renewables, aggregate/sand extraction, aquaculture and fisheries, land reclamation, water abstraction, etc.). However, the economic success of many of these sectors depends entirely on the adoption of appropriate sediment management schemes to ensure both navigational access and minimal requirement for on-going remedial measures (e.g. maintenance dredging) within these environments. Intuitively, such anthropogenic interventions will lead to regional alterations to sediment sources (budgets), transport pathways and sinks, as well as local scour and sediment redistribution around individual marine structures. In this regard, the proposed project will provide much-needed clarity in the fundamental understanding mixed (sand-mud) sediment transport processes, typically encountered in these estuarine and near-shore environments. This will benefit future economic growth within these sectors by driving improvements in the representation of these processes within fully-coupled, integrated coastal modelling systems, required to improve current spatial planning practices and ensure more efficient and sustainable development within these environments. This is particularly crucial for the UK marine renewable energy sector to maintain its competitive advantage in the global market.

Recent UK Flood and Coastal Erosion Risk Management (FCERM) research strategy also highlights research into sediment management as being an urgent priority for political and operational drivers relating to the socio-economic impacts of coastal erosion and flooding. In the UK, over at least 6 million properties are "at risk" of flooding from a wide range of sources including rivers, estuaries, coastal waters and groundwater; while in Scotland alone, over 30,000 are known to be at direct risk from coastal flooding. This is especially pertinent given that these current risk levels are likely to increase under growing climatic and socio-ecomonic pressures. Similarly, pressures are also prevalent from an environmental perspective as estuaries and coastal zones are often made up of highly-productive habitats and ecosystems that are protected under existing EU legislation (e.g. Water Framework Directive, Birds and Habitats Directives). As such, the integration of ecological and socio-economic values is increasingly advocated within near-shore spatial planning as being essential to ensure sustainable development within these enviroments. Thus, through improved linkages between mixed sediment transport and morphological processes, the project outcomes will inform the development of integrated and adaptive management strategies that are both resilient to this increasing flooding and erosion risk, whilst ensuring protection of these important and sensitive ecosystems.

To this end, the proposed project aims to deliver research outputs focused on fundamental processes such as sand-mud erosion, flocculation, settling and deposition that will facilitate improved prediction of mixed sediment sources, transport pathways and sinks within regional-scale transport and morphological models. The critical pathway towards realising the socio-economic and environmental impacts and benefits from the proposed research will be delivered through the immediate availability of project outputs that are strategically beneficial to UK engineering consultancies, marine scientists and governmental agencies tasked with legislation and policy implementation.