Abstract
Alderney Race, located between the Alderney Island (UK) and Cotentin Peninsular (France), is a site with high tidal-stream energy potential. Circulation through Alderney Race is complex, largely dominated by tides with current speed exceeding 5 m/s at spring tide. Current velocity measurements from two bottom-mounted ADCP are used to assess the variability of velocity and turbulence in the water column. Using the velocity profiles recorded by ADCPs at 2 Hz, the time variability of the flow and turbulence was quantified with respect to tidal conditions. Turbulence metrics such as the dissipation rate (ε) of turbulence kinetic energy and the integral length scale (L) were estimated through the spectral approach and used in analysis of temporal variability of turbulent motions. Turbulence intensity (I) was also estimated and compared to scaling properties of turbulence. The mooring locations match areas with very different turbulence regime. This difference is found to be related to large scale bathymetry features which modify the velocity profile shape and turbulence. All turbulence parameters were found larger at the northern ADCP site where current velocity is larger. This high level of turbulence is assumed to be caused by the current interaction with irregularities of bathymetry and enhanced friction. At the southern ADCP location, over relatively smooth bathymetry, the variability of turbulence was found much smaller and to be depended on the velocity shear. These results highlight large spatial variability in turbulence characteristics occurring at rather short distance within the prospective site. The results could provide advanced information to turbine developers.