On the Sediment Dynamics in a Tidally Energetic Channel: The Inner Sound, Northern Scotland

Abstract

Sediment banks within a fast-flowing tidal channel, the Inner Sound in the Pentland Firth, were mapped using multi-frequency side-scan sonar. This novel technique provides a new tool for seabed sediment and benthic habitat mapping. The sonar data are supplemented by sediment grab and ROV videos. The combined data provide detailed maps of persistent sand and shell banks present in the Sound despite the high energy environment. Acoustic Doppler Current Profiler (ADCP) data and numerical model predictions were used to understand the hydrodynamics of the system. By combining the hydrodynamics and sediment distribution data, we explain the sediment dynamics in the area. Sediment particle shape and density, coupled with persistent features of the hydrodynamics, are the key factors in the distribution of sediment within the channel. Implications for tidal energy development planned for the Sound are discussed.

Numerical Simulations of the Effects of a Tidal Turbine Array on Near-Bed Velocity and Local Bed Shear Stress

“…The tidal current velocities and the bed shear stress fields calculated by McIlvenny et al [11] using a 2D hydrodynamic model were largely consistent with the observed sediment distributions, with sediment banks lying adjacent to the main flow through the Inner Sound. However, as with [8], the depth-averaged modelled velocities used by McIlvenny et al [11] cannot be considered ideal to calculate bed shear stress fields.…”

“…Bed shear stress is a key parameter in the erosion of seabed sediment, and understanding potential implications of tidal energy developments on local sediment dynamics requires first an estimate of possible changes to the bed shear stress. Building on the 2D modelling work reported by McIlvenny et al [11], we extend the modelling study to three dimensions, thereby resolving the vertical structure of the flow; we expect this to improve the estimates of near-bed velocity and therefore local bed shear stress during flood and ebb tides, and to allow better predictions of the potential changes to bed shear stress following the installation of tidal turbines in the region. In the following sections, we briefly describe the formulation of the model, including the parameterization of tidal turbines, and verify its performance against observations of sea surface height (SSH) and tidal current velocity.…”

“…Fairley et al [9] combined a variety of data to construct a map of sediment type through the Pentland Firth and surrounding area, but detail in the Inner Sound was limited. Information on local sediment distributions in the Inner Sound is gradually being accumulated through multibeam [3] and sidescan sonar surveys [11,12]. Sediment banks are known to lie to the south of the island of Stroma (Figure 1) but the local sediment dynamics, and the potential impacts of tidal energy extraction on the deposits, are only beginning to be understood.…”

“…Sediment banks are known to lie to the south of the island of Stroma (Figure 1) but the local sediment dynamics, and the potential impacts of tidal energy extraction on the deposits, are only beginning to be understood. McIlvenny et al [11] suggested that the local sediment banks in the Inner Sound ( Figure 1) have been locked in place for a significant period of time, with rates of sediment erosion and deposition expected to be low over the period. The tidal current velocities and the bed shear stress fields calculated by McIlvenny et al [11] using a 2D hydrodynamic model were largely consistent with the observed sediment distributions, with sediment banks lying adjacent to the main flow through the Inner Sound.…”

“…McIlvenny et al [11] suggested that the local sediment banks in the Inner Sound ( Figure 1) have been locked in place for a significant period of time, with rates of sediment erosion and deposition expected to be low over the period. The tidal current velocities and the bed shear stress fields calculated by McIlvenny et al [11] using a 2D hydrodynamic model were largely consistent with the observed sediment distributions, with sediment banks lying adjacent to the main flow through the Inner Sound. However, as with [8], the depth-averaged modelled velocities used by McIlvenny et al [11] cannot be considered ideal to calculate bed shear stress fields.…”