Laboratory Study on the Effect of Tidal Stream Turbines on Hydrodynamics and Sediment Dynamics

Tidal stream turbines (TST) are one potential technology for harnessing tidal energy, and the measurement and characterisation of their wakes is important both for environmental and development reasons. Indeed, wake recovery length is an important parameter for appropriate design of arrays, and wakes may result in altered dynamics both in the water column and at the seabed. We will report on laboratory scale experiments over a mobile sediment bed, which aim to quantify the detailed wake structure and its impact on sediment transport dynamics. A 0.2 m diameter model turbine was installed in a large-scale flume (16 m long, 1.6 m wide, 0.6 m deep) at the University of Hull’s Total Environment Simulator and a steady current was driven over an artificial sediment bed using recirculating pumps. A high-resolution pulse-coherent acoustic Doppler profiler (Nortek Aquadopp HR) was used to measure vertical profiles of the three-dimensional mean current at different locations downstream of the model turbine. A three-dimensional Acoustic Ripple Profiler was used to map the bed and its evolution during the experiments. Acoustic backscatter systems were also deployed in two-dimensional arrays both along the flume and across the flume. These measurements revealed that the presence of the model turbine resulted in an expected reduction of the mean current and in changes in the vertical shear profiles. The bed mapping highlighted a horseshoe-shaped scour near the model turbine, and sediment deposition in the far wake region. The model turbine significantly influenced the suspension patterns, and generated significant asymmetry in the process, which was also evident from the other measurements (flow and sediment bed). These results highlight the effects induced by TSTs on near-bed hydrodynamics, suspension dynamics, and geomorphology, which may all have to be considered prior to large-scale deployments of arrays of TSTs in shelf seas.