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Critical bed shear stress and threshold of motion of maërl biogenic sediments

Authors
Publisher
PANGAEA
Publication Date
Identifiers
DOI: 10.1594/pangaea.825359
Keywords
  • Average
  • Comment
  • Resultant Velocity
  • Resultant Velocity
  • Error
  • Res Vel
  • Res Vel E
  • Reynolds Stress
  • Reynolds Stress
  • Error
  • Rs
  • Rs E
  • Run
  • Run Number
  • Run Purpose
  • Subset 1 (1 Minute)
  • Subset 2
  • Subset 2 (1 Minute)
  • Subset 3
  • Subset 3 (1 Minute)
  • Tke
  • Tke E
  • Tke Shear
  • Tke Shear E
  • Turb Int
  • Turb Int E
  • Turbulence Intensity
  • Turbulence Intensity
  • Error
  • Turbulence Kinetic Energy
  • Turbulence Kinetic Energy
  • Error
  • Turbulent Kinetic Energy Shear Stress
  • Turbulent Kinetic Energy Shear Stress
  • Error
  • Velocity
  • Shear Wave
  • Velocity
  • Shear Wave
  • Error
  • Vs
  • Vs E
Disciplines
  • Biology

Abstract

Critical bed shear stress for incipient motion has been determined for rare biogenic free-living coralline algae known as maërl. Maërl from three different sedimentary environments (beach, intertidal, subtidal beds) in Galway Bay, west of Ireland were analysed in a rotating annular flume and linear flume at Delft University of Technology (TU Delft). Velocity profile measurements of the benthic boundary layer were obtained using an Acoustic Doppler Velocimeter (ADV) at four different velocity runs. Bed shear stress was determined using three methodologies; Law of the Wall (LOTW), the Turbulent Kinetic Energy (TKE) and Reynolds Stress (RS) methodologies. The critical Shields parameter (Theta C), as a non-dimensional mobility number has been evaluated, with results compared with the Shields curve for quartz. Maërl falls below the Shields curve due to the lower specific gravity of this biogenic sediment. A related detailed study of the grain shape and the settling properties of maërl by the authors suggest that the roughness (quantified by convexity-1) is greatest for intertidal maërl beds, due to their increasingly branched nature (Joshi et.al, submitted 2013). Interlocking thalli resist entrainment and result in interparticle contacts between maërl grains forming a matrix that resists motion. The rarely-documented feature of maërl megaripples were observed in the rotating annular flume and are hypothesised to form at shear stresses above the critical threshold, where tidal currents, oscillatory flows or combined-wave current interaction results in the preferential transport of maërl. The TKE methodology gives the most consistent results, agreeing with previous comparative studies. The final part of the study will integrate the results of the flume work with coupled- hydrodynamic sediment transport modelling results to obtain spatial distribution of sediment mobility for maërl habitats, with applications in biological conservation management.

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