Non-equilibrium turbulent stresses and sediment transport in the benthic boundary layer of a shallow shelf environment influenced by flow obstruction

The shallow continental shelf is increasingly used to site infrastructure for marine energy conversion and aquaculture. In this shallow typically energetic environment, tides and waves cause significant sediment fluxes, which interact with and are modified by emplaced infrastructure. This contribution presents observational field data to quantify non-equilibrium turbulent stresses caused by an obstruction in a tidal flow and its impact on suspended sediment transport.

Observations of the turbulent properties of the benthic boundary layer (BBL) in an energetic nearshore environment were made over a 4-month period in Cemaes Bay, Anglesey, UK. The area experiences a high energy semi-diurnal tidal regime with a maximum range of 7.5 m. Tidal current velocities were a maximum of 1.1 m s⁻¹ during springs tides and the strength of the tides ensures that the water column was vertically well mixed. An instrumented lander deployed in 13 m depth on a region of flat sand-sheet sampled the turbulent flows in the BBL using a pulse coherent Nortek Aquadopp and a Vector ADV. An Acoustic Backscatter System was mounted coincidently to sample suspended sediment concentrations.

Vertical profiles of mean flow show that during the flood tide an obstruction upstream of the sampling region modified the BBL causing the breakdown of the constant stress layer and a reduction in velocity shear compared to the opposing ebb tide currents. The turbulent dissipation rate computed using the inertial dissipation and structure functions methods illustrate an order of magnitude increase in dissipation and identify a strongly non-equilibrium relationship between turbulent dissipation and production during flood tides, which varies with elevation above the seabed. The non-equilibrium turbulence effects the suspension and transport of seabed sediments by modifying the vertical profile of sediment diffusivity. These effects are quantified and impacts discussed.