Realistic Sedimentary Bedform Prediction: Incorporating Physical and Biological Cohesion (COHBED)

The United Kingdom is a coastal nation with the majority of the population living within a few miles of an estuary or the sea. The nature of the coastline depends on the local conditions of geology and water flow. Rocky coastlines are found where the energy of the sea is high, while mud and sand are found where the energy is lower and these sediments can be deposited. These low energy muddy and sandy (depositional) habitats, are very important for the ecology and economy of the UK. They provide food for many species of birds and fish, but also protect the coastline from the erosive forces of the sea. In addition, they act as a "filter", where pollutants from the rivers are captured and eventually degraded. Because of the importance of these systems, their natural behaviour and stability is of increasing concern as sea levels rise and storm events increase in frequency with climate change. The movement of sediment around the coast of Britain has vast economic and ecological consequences, but surprisingly we have very little scientific information that helps us to predict how natural mudflats and beaches will respond to the changing forces of the tides, wind and waves. When water flows over the sea bottom, the energy of the flow shapes the sediment into wavy features called bedforms (such as ripples). These bedforms help control the erosion and transport of sand, mud, nutrients and pollutants. Information allowing us to predict the shape, size and movement of bedforms is essential for environmental management, hydraulic engineering, benthic habitat biology, computer modelling of particle transport, sedimentary geology, and many other scientific disciplines. However, there is an almost complete lack of knowledge concerning bedforms consisting of mixtures of sand and mud. Sandy sediments are known to be "non-cohesive", because the sand particles do not stick together, whereas muds are made up of smaller particles that do stick together and so are called "cohesive" sediments.

This project, COHBED, will take advantage of the latest developments in measurement technologies to produce information about the growth, movement and stability of bedforms that consist of natural mixtures of sands and muds, a natural condition that is very common but has rarely been studied before. In a new departure, this work includes a multidisciplinary team to combine the physics, mathematics, sedimentology, and biology of these systems, since we recognise that the organisms (from bacteria to sea grasses) that inhabit natural systems also change the erosional characteristics and bedform behaviour. This is why COHBED will include laboratory experiments and field surveys. A series of experiments in laboratory flow channels will investigate key factors that control the behaviour and properties of bedforms, such as:
- System energy: effects of flow velocity, bed friction and flow depth
- Bed properties: particle size, proportion of mud and sand, and biological effects
- Time: the speed of bedform growth and rate of change as flow energy changes
- Particle erosion: changes in the bedforms as smaller particles are eroded away

The results of the laboratory studies will be compared with the behaviour of natural systems. Field surveys will be conducted to validate the predictions derived from the laboratory studies, using new techniques that for the first time allow essential simultaneous measurements of flow, sediment and bedform properties. The COHBED project will maintain the UK at the forefront of this research area and will help us to manage our coasts in the face of climate change.

The main beneficiaries of the timely knowledge on mixed cohesive bedform dynamics arising from COHBED are all users who depend on more accurate models for the transport of particulate and dissolved matter in aquatic environments. There is a unique opportunity to implement the results of COHBED into improved predictive tools for bedform development, equilibrium morphology and migration, as well as for bed roughness parameters and suspended load concentration profiles that are closely related to bedform properties. These models will be directly applicable, while also forming the basis for future regional sediment transport modelling efforts in complex natural environments.

Scientists will benefit as this project will spark collaborative research in the largely unexplored field of biology- and physics-controlled cohesive mixed sediment dynamics and the project outcomes will provide a vehicle for facilitating more reliable palaeo-environmental reconstructions and improving numerical models of sediment transport. A Numerical Modelling Advisory Committee will be an integral part of COHBED. This committee of independent experts will provide guidance how to ensure that the research outcomes of the laboratory and field experiments and, notably data requirements and data formats, are best obtained for future numerical modelling.

Engineers will benefit from the predictive outputs of modified bedform dynamics from cohesive forces in flows and bed surface sediments. These will help guarantee engineering success when extracting sediment or building structures in and on the sea bed (e.g. pipelines, wind turbines, tidal barrages, oil platforms).

Planning authorities and environmental advisers/managers will benefit from improved Environmental Impact Assessments (EIAs) to which the research outputs will contribute significantly, notably because the likely impacts of new sea bed developments need to be fully understood for engineering safety and habitat preservation. We will engage with these end users, and with engineers, through regular Progress Meetings and a Knowledge Exchange Workshop near the end of the project.

The hydrocarbon industry will benefit from improved architectural models of sedimentary facies and from more confidence in reconstructing and predicting the porosity and permeability of clastic sedimentary rocks in cores and outcrops. The results from this project will be disseminated through the industry-funded Turbidite Research Group (TRG), an internationally recognised leader in the study of deep marine clastic sedimentary systems, of which PI Baas is the Associate for Wales, and PI Parsons and Co-I Peakall are integral members.

Through national and international conference visits, a project website, popular science media and events, publications in international peer-reviewed journals, and a novel Travelling Exhibition "Shifting Seas, Shifting Sands" we will not only engage with our primary users, but we will also disseminate our project outcomes and their implications to the wider public.

The Centre for Catchment to Coastal research (CCCR), SEACAMS and the Society for Underwater Technology (SUT) will be used as vehicles to interact with academic partners, representatives of governmental bodies, and industry through regular seminars/workshops and small group discussion meetings. Our strong links with various branches of the offshore engineering industry will serve to refine experimental methodologies and identify optimal dissemination products. New knowledge will be exchanged with petroleum geologists at the bi-annular sponsor meetings of TRG, and potential users (including the members of the Advisory Committee) will be invited to attend regular Progress Meetings and the Knowledge Exchange Workshop.