Measurements of tidal flow variability in Ramsey Sound, Pembrokeshire

Abstract

The nature of the flow at in-stream tidal energy sites is particularly important for predicting array and device performance, and also for operations and maintenance planning. Flow characteristics of Ramsey Sound, based on analysis of 2 weeks of Acoustic Doppler Current Profiler (ADCP) data from the site of the Deltastream device are presented here, alongside tide gauge data from the nearby Standard Port of Milford Haven. The ADCP was located approximately 300 m across the channel, at the northern end of Ramsey Sound, where the channel width was 1200 m and the mean depth was approximately 33 m. The flow dynamics were examined specifically to look at times potentially suitable for offshore operations. Results demonstrate that flow velocities exhibited clear asymmetry, with stronger flows on the northerly directed flood tide than on the ebb. Cross correlation indicated that the flow speed generally led the elevation by 20 minutes. In contrast to the expected theory, the current strength at mid-depth was stronger than at the surface on the maximum flood tide. The maximum flow speed in the tide was reasonably predictable from the tide range at Milford Haven. A threshold-based analysis of the ADCP measurements allowed the duration of slow-moving water to be identified for operation planning.