Paul S. Bell

Shallow water bathymetry derived from an analysis of X-band marine radar images of waves

Image sequences of surface gravity waves are readily produced by standard marine X-band radars. These image sequences contain a great deal of information regarding wave parameters and their variation over an area. By approaching the analysis of such data, using image analysis techniques, it is possible to map the variations in wave behaviour in a shallow water area. Wave celerity and direction can be mapped, and inversion algorithms based on linear wave theory used to infer the underlying bathymetry. Knowledge of the period of the waves is necessary for the analysis, and this can also be calculated from the radar data, although there is some discrepancy between the figures derived from radar data and from an offshore S4DW instrument. These discrepancies are investigated, and the results presented. Three days of data from the Holderness II deployment during 1995 have been analysed, and estimates of the bathymetry produced. The calculated depths show a mean water level comparable to that read off an Admiralty chart for the area, and a tidal signal is also clearly superimposed on the data, giving confidence in the water depths derived using this technique.

Observations and analysis of sediment diffusivity profiles over sandy rippled beds under waves

Acoustic measurements of near-bed sediment diffusivity profiles are reported. The observations were made over two sandy rippled beds, classified as ‘medium’ and ‘fine’ in terms of sand grain size, under slightly asymmetric regular waves. For the medium sand, the ripples that formed had relatively steep slopes, while for the fine sand, the slopes were roughly half that of the medium sand. In the medium sand case, the form of the sediment diffusivity profiles was found to be constant with height above the bed, to a height equal approximately to the equivalent roughness of the bed, ks, while above this the sediment diffusivity increased linearly with height. For the case of the fine sand there was no constant region; the sediment diffusivity simply increased linearly with height from the bed. To understand the difference between the respective diffusivity profiles, advantage has been taken of the high temporal-spatial resolution available with acoustic systems. Using intra-wave ensemble averaging, detailed images have been built up of the variation in concentration with both the phase of the wave and also height above the bed. These intra-wave observations, combined with measurements of the bed forms and concepts of convective and diffusive entrainment, have been used to elucidate the mixing mechanisms that underlie the form of the diffusivity profiles observed over the two rippled beds. These mechanisms centre on coherent vortex shedding in the case of steeply rippled beds and random turbulent processes above ripples of lower steepness.

Quality Assessment of Surface Current Fields From TerraSAR-X and TanDEM-X Along-Track Interferometry and Doppler Centroid Analysis

All existing examples of current measurements by spaceborne synthetic aperture radar (SAR) along-track (AT) interferometry (ATI) have suffered from short baselines and corresponding low sensitivities. Theoretically, the best data quality at X-band is expected at effective baselines on the order of 30 m, i.e., 30 times as long as the baselines of the divided-antenna modes of TerraSAR-X. In early 2012, we had a first opportunity to obtain data at near-optimum baselines from the TanDEM-X satellite formation. In this paper, we analyze two TanDEM-X interferograms acquired over the Pentland Firth (Scotland) with effective AT baselines of 25 and 40 m. For comparison, we consider a TerraSAR-X dual-receive-antenna (DRA)-mode interferogram with an effective baseline of 1.15 m, as well as velocity fields obtained by Doppler centroid analysis (DCA) of single-antenna data from the same three scenes. We show that currents derived from the TanDEM-X interferograms have a residual noise level of 0.1 m/s at an effective resolution of about 33 m × 33 m, while DRA-mode data must be averaged over 1000 m × 1000 m to reach the same level of accuracy. A comparison with reference currents from a 1-km resolution numerical tide computation system shows good agreement in all three cases. The DCA-based currents are found to be less accurate than the ATI-based ones but close to short-baseline ATI results in quality. We conclude that DCA is a considerable alternative to divided-antenna mode ATI, while our TanDEM-X results demonstrate the true potential of the ATI technique at near-optimum baselines.

A Self-Contained Subsea Platform for Acoustic Monitoring of the Environment Around Marine Renewable Energy Devices–Field Deployments at Wave and Tidal Energy Sites in Orkney, Scotland

The drive towards sustainable energy has seen rapid development of marine renewable energy devices (MREDs). The NERC/Defra collaboration FLOw, Water column and Benthic ECology 4-D (FLOWBEC-4D) is investigating the environmental and ecological effects of installing and operating wave and tidal energy devices. The FLOWBEC sonar platform combines several instruments to record information at a range of physical and multitrophic levels for durations of two weeks to capture an entire spring-neap tidal cycle. An upward-facing multifrequency Simrad EK60 echosounder is synchronized with an upward-facing Imagenex Delta T multibeam sonar. An acoustic Doppler velocimeter (ADV) provides local current measurements and a fluorometer measures chlorophyll (as a proxy for phytoplankton) and turbidity. The platform is self-contained, facilitating rapid deployment and recovery in high-energy sites and flexibility in gathering baseline data. Five 2-week deployments were completed in 2012 and 2013 at wave and tidal energy sites, both in the presence and absence of renewable energy structures at the European Marine Energy Centre (EMEC), Orkney, U.K. Algorithms for target tracking have been designed and compared with concurrent, shore-based seabird observations used to ground truth the acoustic data. The depth preference and interactions of birds, fish schools and marine mammals with MREDs can be tracked to assess whether individual animals face collision risks with tidal stream turbines, and how animals generally interact with MREDs. These results can be used to guide marine spatial planning, device design, licensing and operation, as different device types are tested, as individual devices are scaled up to arrays, and as new sites are considered.

Nautical Radar Measurements in Europe: Applications of WaMos II as a Sensor for Sea State, Current and Bathymetry

This paper presents the remote sensing techniques of measuring sea states, currents and bathymetry by using an X-band nautical radar. It briefly describes the fundamental methods to infer sea state information (e.g. ocean wave and current parameters) from nautical radar imagery. In addition, this work describes in detail the performance of the Wave Monitoring System WaMoS II (a commercial system for real-time monitoring of wave fields based on nautical radar technology). Two examples of nautical radar applications are presented: the first application is an example of the standard WaMoS II installation for sea state measurements, and the second application shows results of a research project aiming at the determination of shallow water bathymetry by means of nautical radar imagery.

Measurements of sea bed ripple evolution in an estuarine environment using a high resolution acoustic sand ripple profiling system

To predict suspended sediment dynamics a knowledge of sea bed roughness is generally required. This measurement has traditionally been difficult to obtain when significant amounts of suspended sediments are being transported. However it is now becoming possible to characterise the form of the sea bed, and its evolution, in-situ using high resolution acoustic techniques. In the present work, time series of two dimensional backscatter slices taken vertically through the water column have been produced using a high resolution 2 MHz rotary scanning sonar. The data were recorded over a number of flood tides in an estuarine environment, and signal processing techniques were subsequently used to pick out the position of the bed from the resulting images. Analyses of the sand ripple profiles are given over time and show the evolution of the sand ripples in terms of wavelength, amplitude and migration rate.

Determining currents from marine radar data in an extreme current environment at a tidal energy test site

A marine X-band radar has been deployed at a coastal location overlooking a region of extreme tidal currents used for the testing of tidal stream energy turbines. Near-surface tidal currents are known to reach 3.8m/s in water depths of the order of 50m. Preliminary results from the analysis of radar data collected at this site will be presented with particular reference to the currents determined using an inversion of the wave spectrum. Comparisons will be made with current profile data collected using an Acoustic Doppler Current Profiler (ADCP) deployed within the field of view of the radar.

Acoustic instrumentation for measuring near-bed sediment processes and hydrodynamics

Improved understanding and modelling of interactions between biological and physical processes has required progress in the understanding of sediment processes and their relationship to bedforms, hydrodynamics and benthic biology. Recently, simultaneous, co-located in situ measurements of suspended sediments, near-bed velocities and bed morphology have been made possible using new acoustic instruments, including a triple frequency acoustic backscatter system, (ABS); a uniaxial, and triple axis, coherent Doppler velocity profiler, (CDVP); a sand ripple imager (SRI); and a sand ripple profiler (SRP). Together these instruments measure the relevant hydrodynamic and sediment parameters necessary to investigate sediment processes in the bottom metre above the bed, with centimetric resolution. The range of data from such instruments is required to fully understand the related processes occurring near the seabed. The outcome of such measurements should improve our understanding of benthic processes in general.

High resolution current & bathymetry determined by nautical X-Band radar in shallow waters

The wave and current monitoring system WaMoS II is a remote sensing system based on a nautical X-Band radar generally used for navigation and ship traffic control. It has been used in recent years to monitor sea state information from moored platforms, coastal sites and moving vessels. A nautical radar can scan the sea surface over a large area (∼ 10km2 ) with a high spatial (∼7.5m) and temporal resolution (∼2s). Directional wave spectra and standard sea state parameters such as significant wave height, peak wave period and direction can be derived by analyzing the sea surface image sequences. Using the temporal and spatial evolution of the sea surface wave images it is also possible to determine high resolution current and bathymetry information. In the paper a brief introduction into the measuring principle of WaMoS II is given and results of a high resolution current and bathymetric mapping technique for shallow water areas (≪20m) are presented. For validation these results are compared with model data and in-situ measurements.

Assessment of Acoustic Coherent Doppler and Cross-Correlation Techniques for Measuring Near-Bed Velocity and Suspended Sediment Profiles in the Marine Environment

Abstract The simultaneous measurement of current flow and suspended sediment concentration in the marine environment is central to the study of sediment transport processes. In view of this, two acoustic approaches for measuring flow were tested in a tidal estuary to assess their capabilities in this environment. A coherent Doppler velocity profiler and a cross-correlation velocity profiler were assessed using conventional current meters and a commercially available acoustic Doppler velocimeter. Mean velocity profiles were obtained up to a range of 1.47 m in 0.046-m range bins over a number of flood tides. The measurements compared well with the reference instruments and regression analysis produced gradients close to unity. Turbulent velocities measured with the coherent Doppler profiler were comparable with turbulent fluctuations measured with the acoustic Doppler velocimeter. The cross-correlation velocity profiler was shown to be unable to measure turbulent velocities. The backscattered signals receiv...