Bed roughness over vegetated beds: sonar imaging techniques and effect on unidirectional currents

Abstract

Small scale roughness of the seafloor is of direct relevance to a range of interests, including boundary layer hydrodynamics, sediment transport and high-frequency acoustic scattering. Despite its importance, only few studies have quantitatively resolved seafloor height at the relevant scales. In particular, characterisation of roughness over vegetated beds is needed to better understand hydrodynamics and sediment transport in the coastal zone. A new Benthic Roughness Acoustic Device (BRAD) has been developed to define microtopographical roughness through high-resolution imagery of the seabed. BRAD, composedof a profiling sonar – the Sediment Imager Sonar (SIS) – and a motor, both mounted on a frame, enables measurements of the seabed elevation over an area of 1.7 m². A thresholdmethod was established to detect the seabed from the SIS raw data. Laboratorydeployments were carried out in order to assess the system accuracy over known targetsand its ability to discriminate sediment sizes. Field deployments at 6 sites enabled the imaging of a variety of seabed types; in particular bioturbated fine sand and mud, seagrass canopies, gravelly sand and ripple fields. Spectral analysis applied on the seabed elevations was used to characterise roughness type. Seagrasses are flowering plants that have adapted to the submerged marine environment.They develop extensive underwater meadows in coastal areas around the world, forming complex, highly productive ecosystems. The SIS was used together with a towed videocamera system to survey a seagrass (Zostera marina) bed in Calshot, UK. A method was developed to assess Z. marina presence from the SIS data and its results were tested against video data. The SIS proved to be a useful tool for seagrass surveying and the use of the SIS and the video yielded a preliminary map of the seagrass bed. Seagrass canopies exert strong effects on water flow inside and around them. The influenceof Zostera marina canopies on flow, turbulence, roughness and sediment movement was evaluated through laboratory experiments. Numerous runs were carried out in an annular and a straight, recirculating flume using live Z. marina and a mobile sand layer. Flow was greatly decelerated inside the canopy while turbulence was increased. The Turbulent Kinetic Energy was observed to be maximal at the canopy/water interface. This was hypothesised to be related to the canopy ‘wavy’ motion. Sediment movement was observed within the canopy as scour around the stems. Ripples formed downstream of the canopy at velocities lower than the sediment threshold of motion. Intermittent turbulence associated with the burst phenomenon is thought to be responsible for this.

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