In situ measurements of spring–neap variations to unsteady island wake development in the Firth of Forth, Scotland

Field data and a numerical model have been used to investigate the von Ka´rma´n vortex street generated by a 50 m wide island in the Firth of Forth for which the characteristics differ between flood and ebb. During the flood, water depth in the lee of the island is shallow, hence little vertical structure develops in the velocity due to bottom-induced mixing. In contrast, the ebbing water depth is deeper in the lee of the island, allowing vertical structure to develop in the velocity field. However, the ebb wake is complicated by the presence of a lateral flow due to a bathymetric restriction. The effect of this lateral flow is to inhibit development of the ebb wake during spring tide conditions; whereas the effect is minimal during neap (lower freestream velocity) conditions. This change in wake development was clearly visible in the in situ horizontal and vertical velocity fields. While the observed flood wake varied considerably between neap and spring, the characteristics of the ebb wake were similar for neap and spring because of this overbank flow. The overbank flow has an important role in determining the varying magnitude of the vorticity of eddies generated either side of the island.