Bed roughness over vegetated beds: sonar imaging techniques and effect on unidirectional currents.
University of Southampton, Faculty of Engineering Science and Mathematics, School of Ocean and Earth Science,
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
A new Benthic Roughness Acoustic Device (BRAD) has been developed to define microtopographical
roughness through high-resolution imagery of the seabed. BRAD, composed
of 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 threshold
method was established to detect the seabed from the SIS raw data. Laboratory
deployments were carried out in order to assess the system accuracy over known targets
and 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 video
camera 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 influence
of 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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