Rapid monitoring of river hydrodynamics and morphology using acoustic holography

Accurate flow measurement in rivers is vital to build well calibrated, reliable simulation models able to predict accurately the timing and extent of floods, and also to provide the data needed for effective management of water resources in a river catchment. This project will develop a new method of acoustic wave holography to measure remotely the velocity, flow depth and bed characteristics within river channels. The proposed holography method records the pattern of reflected acoustic waves (the hologram) above a dynamic flow surface and uses this pattern to reconstruct the water surface wave field throughout a three-dimensional region of space. The project will use recent advances in computational fluid mechanics and turbulence theory. The underpinning concept is that the free surface of turbulent river flows is never flat and is always dynamically rough. There is overwhelming evidence that the 3-dimensional pattern of the free surface of a river flow is caused by the turbulence structures within the flow. These structures are generated at the river bed and rise to the free surface and express themselves in the form of a pattern of surface waves which propagate at a particular velocity which does not necessarily coincide with the mean surface water velocity. Therefore, the free surface wave pattern carries comprehensive information about the underlying hydrodynamic processes in the flow, including the flow velocity, depth, turbulence scale and intensity and bed roughness characteristics. This process is very complex and it has not been sufficiently studied in the past because of a lack of accurate and robust instruments and accurate fluid dynamics models to relate the free surface wave pattern to the flow structure beneath. Thus, there is now an opportunity to develop a clear understanding how the pattern observed on the free surface of a river flow and the underlying turbulence structures and bed surface roughness in fluvial environments interact. This new knowledge in the hydrodynamics of turbulent river flows combined with new acoustic holographic measurement capabilities will provide a paradigm shift in the accuracy, spatial resolution and speed of deployment of flow monitoring in rivers. In this respect, the proposed work has a very high degree of novelty in comparison to the broader research context of this area internationally.
The proposal is timely because it will contribute significantly to the need for us to better understand our natural environment especially under extreme conditions and in the development of Robotics and Autonomous Sensor technologies. These technologies were outlined in a report by David Willetts as one of the "Eight Great Technologies" that should be promoted and developed by the UK. The Willetts' report also states a clear need for real time forecasting of rivers, better water resource management and autonomous surveillance vehicles which require accurate on-board sensing. Our project takes an important step towards providing technology to address these requirements. The new sensor technology will also enable new theoretical foundations to be developed in the areas of wave propagation, inverse problems, holography, signal processing and computational fluid dynamics.

This project will result in new acoustic wave holography technology with the capability to rapidly survey the hydrodynamics of river channels non-invasively with an unprecedented degree of spatial and temporal resolution and spatial scale. This capability is important for assessing flood risk, channel morphology change, pollution risk from intermittent discharges, and the flow hydrodynamics controlling the quality of the physical habitat in rivers. The following groups will benefit from this work: (i) short-term (duration of project) - flow survey instrument and sensor developers and manufacturers; (ii) medium-term (up to five years) - environmental consultants and government statutory bodies; and (iii) long-term (five to ten years) - governmental regulators, non-governmental organisations (NGOs) and general public.
1. A sensor manufacturer is involved (see LoS from Gesellschaft für Akustikforschung Dresden mbH). This organisation will benefit in terms of a much better physical and mathematical understanding of the link between the hydraulic flow characteristics and the free surface, acoustic field properties scattered by the free surface and ability of the signal processing system to infer more accurately the true characteristics of the water surface from the recorded acoustic data. Other flow monitoring companies will be engaged via the project workshops. The better understanding of the physics and underpinning mathematics will enable the design more accurate, lower cost flow measurement instruments which will be commercially viable and have high market value. The flexible deployment of the sensor technology can be achieved by working with providers of field drone sensor deployment platforms.
2. The project benefits from the involvement of the EA, NRW, USEPA, ICHARM and other commercial project partners (HR Wallingford, JBA) (see LoSs). Their active involvement in the steering of the work and provision of data and testing sites will ensure that the outcomes of our work will be of high importance to government and statutory bodies and consultants engaged in water resource and river management. For example, the EA/NRW have a statutory duty under the Floods Directive to assess and manage flood risk. This is accomplished by mapping flood extent and devising adequate measures to reduce flood risk. This uses calibrated river models which are calibrated using sparse data collected at low flow conditions. The monitoring technology to be developed will allow more widespread and higher quality measurement at all flow conditions. The Water Framework Directive (WFD) requires the EA/NRW to develop programs of measures to ensure rivers have good ecological status. Accurate monitoring of the spatial and temporal hydraulic changes in rivers is of paramount importance for assessing ecological status because it controls biota-flow feedbacks and potential contaminant transport. Thus our developed technology is likely to be a core tool in NRW/EA's work by providing spatial hydrodynamic data to assess the ecological status of river reaches. Both flood risk assessment and WFD assessment modelling is performed by consultancies. The detailed data obtained from holographic sensing will provide new understanding of river hydraulics for consultancies and government statutory bodies involved in the management of water resources, prediction of flood risk, sediment movement and consequent morphological changes.
3. The ability to collect low cost flow data at multiple sites within a river catchment will over the longer term provide better assessment of the hydrological capacity of a river basin. This will lead to the improved management by governmental bodies of water resources. NGOs and the general public will benefit in terms of improved flood risk management and better ecological status achieved through provision of better, more plentiful temporal and spatial hydraulic data, which will also raise public awareness of potential ecological impacts.