The hydrodynamics of microbial landscapes

The way in which water flows across a surface is one of the most complex phenomena to model and predict accurately in the environment. Understanding complex flows moving into and from pebbles and gravels on river beds is especially challenging An additional level of complexity that has been largely overlooked in this environment is the effect that microorganisms such as algae, (collectively known as biofilms), attached to surfaces have on the flow processes. Given that biofilms occur in all natural environments and many engineered contexts such as wastewater systems this represents a significant knowledge gap. But why do we need to understand flow-biofilm interactions? Firstly, stream ecologists recognise that the bed of the river is an important habitat for a diverse range of species. The way flow from above the bed makes its way into the subsurface largely dictates how much oxygen and nutrients are supplied to this habitat. Secondly, fisheries managers have long understood that the probability of salmon eggs hatching in river beds will be dependent on a continuous supply of oxygenated water to the gravelly sediments in which they are laid. Thirdly, knowledge of how biofilms can affect the conveyance of flow within wastewater systems or lead to discolouration of potable waters is an important consideration for water managers. There are thus a broad range of highly important environmental and engineered contexts that require detailed predictions of how water moves, yet there is no way of measuring or modelling this accurately which takes into account the effect that biofilms may have to influence these processes.

The overall aim of this proposal is to develop a quantitative numerical representation of micro-scale hydraulic response to biofilm forcing. This will be achieved by using pioneering new experimental and numerical approaches to meet this challenge. The first task is to accurately measure flow both right at the bed and within the biofilms and pore spaces of the bed themselves. This significant problem will be overcome by using laboratory PIV (particle imaging velocimetry) techniques in a range of small channels containing intact biofilm cultures. The technique works by seeding the flow with tiny reflective particles, and providing high intensity illumination from a laser, a camera then records how they move within the flow around the biofilms and within the pore spaces of the experimental channel. Using a special processor, these digital images can be turned into numerical data that accurately records how flow moves across and then into the river bed. Such measurements have never been possible before. The second phase of the project is to use the new understanding made possible by this unique dataset to develop and test a 3-D numerical model that can be used to further understand and explore the influence of biofilms on the flow processes at and within the bed over a much broader range of environmental and engineered contexts. This will be achieved using a specially modified computational fluid dynamics (CFD) model which will be developed so that it can account for the dynamic nature of the biofilms (i.e. the fact that they move with the flow) that live on the more stable channel surface.

The advances in measurement and modelling approach that will be used in this project represent real breakthroughs that will unlock the inherent problem of gaining useful data from one of the most challenging of environments. Meanwhile, the development of a numerical model that can be widely used will ensure that this new understanding can be applied and adapted to meet a variety of real world environmental challenges as well as being of relevance to areas such as the wastewater industry.

A significant deliverable of our work is the new macroscopic models for the drag force that accounts for the presence of biofilms growing on a range of permeable and impermeable bed types. This will be of direct benefit to end-users within the wastewater, potable water and agricultural industries. Our research is relevant to these industries because the new understanding generated from the project can help to improve the design of wastewater distribution channels, improve predictions of discolouration events and make the use of fertiliser uptake within water-bed based crop production more efficient. There are thus clear societal, health and economic benefits that may arise from the research. Specific details of engagement with these three end-user groups are more fully articulated in the Pathways to Impact document.

Within the broader natural environment, our research is of direct application to those involved with finding solutions for the management of gravel-bed rivers, both in the public (e.g. Environment Agency, Centre for Ecology and Hydrology) and private sectors (e.g. consultancies). For example, for managers who seek to assess how the quality of our rivers is controlled by the interrelationships operating between physical and ecological processes, which ultimately affect the pathways of nutrients and sediment. Likewise environmental engineers who need to develop management strategies for sustainability and safety of engineering structures such as compensation release flushing flows required to clean gravels downstream of dams for the improvement of the physical habitat of a range of riverine species.

Knowledge of turbulence in porous media also has application beyond the environments that are the focus of the research proposal and is also of interest to many other industries. Specifically, the nuclear industry has a range of porous media, in the form of nuclear reactor rod bundles and pebble bed reactors, which are also subject to turbulent flows. The enhanced process understanding and numerical simulation capability that will result from our project will be of direct relevance to improving functionality within this industrial context.

Over longer timescales this work will have broader impact as the project team are responsible for training the next generation of environmental managers. For example at Durham University, via its well regarded MSc programme in Risk and Environmental Hazards and at the University of Birmingham via MSc programmes in River Environmental Management and Water Resources Technology and Management. Knowledge from this project will be made available to students taking these courses via the direct involvement of the project team.