The development of multibeam echosounding for the quantitative monitoring of suspended sediment dynamics in aquatic environments
Background:
This is a NERC Partnership Grant with RESON.
Successful environmental management is dependent upon an accurate, and ideally holistic, quantitative monitoring of processes operating within natural systems. Recent years have seen many advances in the successful monitoring of such processes involving techniques and methods that can provide detail and identify processes, and their changes, over relevant spatio-temporal scales; ideally these techniques should be non-intrusive and not affect the processes that are being monitored. In many aquatic environments, some of the most important physical processes involve the interactions of fluid flow and mobile sediment, with these interactions also having many implications for biochemical processes and nutrient exchanges. Such interactions between the turbulent flow field and transport of mobile particles encompass sediment transport both along the bed and in suspension. This interaction also has consequent implications for the form and nature of the bed itself, with a complex feedback existing between the bed morphology, turbulent fluid flow and sediment transport. Therefore, the effective monitoring of these processes are a prerequisite for improving understanding of morphological forms, the longer term evolution of landforms and thus development of appropriate environmental management strategies, in environments extending from rivers to estuaries to the shallow and deep marine realms.
In order to adequately characterise and monitor such interactive and complex processes, we ideally require a simultaneous three-dimensional, qualitative and quantitative picture of both the bed morphology and the associated sediment transport patterns. However, this goal is invariably difficult, if not impossible, to attain and has often been attempted using a range of at-a-point sampling techniques. However, over the past 15 years developments in multibeam echosounding (MBES) have revolutionised bathymetric surveying of all natural aquatic environments, and enhanced understanding in many disciplines, from oceanography and earth sciences to archaeological investigations. These developments have also transformed many routine survey operations required to accurately deploy or investigate infrastructure within aquatics environments, such as object location (including mines for example), wreck surveys, pipe and cable laying, harbour/port sedimentation and the dredging of navigation channels. MBES systems rapidly provide three-dimensional bathymetric information with a centimetric resolution and sub-centimetric precision in a range of conditions. This technique provides unrivalled three-dimensional bathymetric details over scales from individual sand ripples to large areas of seabed. RESON, the partner in this application, have recently developed a new series of MBES systems, called the ‘7k’ series, that are capable of collecting and recording data not only from the strongest acoustic return (normally the sea/lake/river bed), but also from the full digital returns for the complete travel of the acoustic pulse through the water column. This has many advantages and opens up huge new opportunities, since the full water column data can be interrogated used to analyse the changes in the acoustic pulse return in its complete travel through the fluid medium. Using an older model of RESON MBES without this full water column data capability through NER/B/S/2003/00243/2, we were able to demonstrate that such technique has the potential to quantify both the bathymetry and the suspended sediment concentrations within the water column and also track the three-dimensional evolution of coherent flow structures and sediment plumes. These recent developments in the development of MBES acoustics in the 7k systems, now open up a new era for quantifying environmental sediment dynamics within the whole flow field: this proposal seeks to fully develop, test and apply this technique in association with the World’s leading developer of high-resolution shallow water MBES systems. The principal aim of the project is to therefore fully develop the RESON 7k system, and associated software, so that the raw beam-formed water column data can be used to fully quantify the concentrations and densities of sediments within the water column, thus providing a means to monitor sediment dynamics in a range of aquatic environments. To achieve this aim, a series of four specific sequential themes and objectives have been identified, that will be addressed through an integrated laboratory and field experimental program using techniques and field areas that we have already proven suitable for this purpose.
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