Abstract Collocated detailed measurements of near-bed turbulent and intrawave flow are important for studying sediment transport processes and seabed evolution. Existing commercially available triple-axis profiling instruments do not provide collocated velocity measurements. To improve the capability to make such measurements a triple-axis coherent Doppler velocity profiler (CDVP) has been developed and tested in the marine environment. The instrument was designed to measure orthogonal velocity profiles within a narrow column of water at 16 Hz within 1 m of the bed with a vertical spatial resolution of 0.05 m. This paper describes the first deployment of the instrument, in a tidal inlet in Portugal during a multidisciplinary study, when CDVP flow velocity measurements were compared with data from other instrumentation. A pragmatic approach was adopted to assess the capability of the triple-axis CDVP, using both an evaluation of internal consistency and an assessment against two commercially available acoustic Doppler velocimeters (ADVs). Measurements of the mean and fluctuating velocity profiles were collected with the triple-axis CDVP, and these have been shown to be internally consistent and to be in good agreement with measurements obtained with the ADVs.
X-band radar remote sensing of the ocean surface using ground-based installations and radar intensity imagery is a mature technology for the determination of ocean wave properties. In order to transition the analytical methods to data collected from a moving vessel the accurate geo-registration of radar images must be performed. However, finite offsets in azimuth, range and time are generated by the physical installation of equipment aboard a vessel that may not be measurable. This paper details a simple, yet robust method to “calibrate”data recorded by an arbitrary equipment installation to allow accurate geo-registration of the radar imagery. Time-integrated radar images are generated using a set of time-stamped radar intensity images and high-frequency, high-accuracy vessel heading and position information (from global satellite navigation systems and inertial navigation systems). The time-integrated image sharpness is found to be maximal for correctly determined angular, range, and time offsets. The requirements to form an operational system from the proposed method are also discussed.
The application of multifrequency acoustic backscattering to the measurement of nearbed suspended sediment dynamics is a technique that is increasingly becoming accepted by sedimentologists. These systems provide detailed information on the suspension, but only limited data on the bed. Since it is the interactions between the flow dynamics and the bed forms that lead to the entrainment of the sediments into suspension, measurements of the bed features are an essential component in developing a framework for understanding suspension processes. Most theoretical predictions of suspended sediment profiles require a knowledge of the sea bed form, for example is the bed plane or rippled, and if the latter, what is the amplitude and wavelength of the ripples. To date the input of this parameter has been somewhat problematic because of the difficulty of obtaining such measurements. In the present work, results are reported on the development and deployment of a high frequency, 2 MHz, scanning system used to profile sea bed microtopography. The ability of such systems to provide the required subcentimetre resolution has been examined, and results from laboratory and marine studies are reported.
Maps of the sea bed in the region of a tidal inlet have been produced using a novel analysis of image sequences of waves recorded from a land based marine radar. The sea bed
changes determined using these maps and occurring over a four year period, have shown how such inlets interrupt the transport of sediment by longshore drift, directly contributing to the erosion of the downdrift coastline.
Parameterizations of near‐bed sediment processes are commonly associated with the poor predictive skill of coastal sediment transport models. We implement a two‐dimensional Reynolds‐averaged Navier‐Stokes model to directly assess these parameterizations by reproducing measurements obtained in large‐scale wave flume experiments. A sediment transport model has been coupled to wave hydrodynamics and turbulence, and numerical experiments provide temporal and spatial variations of free surface, flow velocity, sediment concentration, and turbulence quantities. Model‐data comparisons enable the direct assessment of how key suspension processes are represented and of the inherent variability of the sediment transport model. We focus on the different processes occurring above rippled beds versus dynamically flat beds. Numerical results show that increasing roughness alone is not sufficient to have good predictive capability above steep ripples. Some parameterization of the vortex entrainment process is necessary and a simple modification, which leads to constant sediment diffusivity above steep‐rippled beds, is sufficient to obtain good predictions of wave‐averaged suspended concentrations. Model‐data comparisons for the turbulent kinetic energy are also presented and highlight the need to account for the effect of vortex entrainment on near‐bed turbulence and transfer of momentum.
A marine radar was deployed on a remote clifftop overlooking a 4.8km radius area of the Inner Sound of Stroma in the Pentland Firth for 3 months during spring 2013. The area viewed by the radar includes The Crown Estate lease areas for MeyGen Ltd (Inner Sound of Stroma) and Scottish Power Renewables (Ness of Duncansby), although the data analysis has focussed solely on the MeyGen area. Data were post processed to extract current vector maps based on determining the Doppler shift of sea surface waves by the tidal current. Comparisons between current time series from two Acoustic Doppler Current Profiler (ADCP) surveys and the radar derived data are presented and show excellent correlation. The quality of the data has enabled tidal analyses to be performed and spatial variations in tidal current constituents to be mapped.
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