Lucas Mackie , Paul Evans, Magnus Harrold, Tim O`Doherty, Matthew Piggott, Athanasios Angeloudis

Characterising tidal hydrodynamics in the vicinity of submerged features can be demanding given the hostility of the marine environment. Logistical challenges in the measurement of such flows has promoted research on wake studies through physical and numerical modelling. In this study, site measurements and modelled data are combined to provide an insight into the regional hydrodynamics within a macrotidal strait, namely Ramsey Sound in Pembrokeshire, Wales, UK. The site has received interest from tidal energy developers but is characterised by various steeply inclined bathymetric features, including a semi-submerged pinnacle known as Horse Rock. Understanding how prominent submerged features impact on flow conditions can be crucial for the feasibility and deployment of marine (energy) infrastructure. Observational bed- and vessel-mounted acoustic Doppler current profiler (ADCP) data are used to calibrate a coastal ocean model. The depth-averaged model is generated using Thetis, a coastal flow solver based on the finite-element engine Firedrake. Hydrodynamic characteristics are examined at different stages of the spring-neap tidal cycle, highlighting the local and regional influence of prominent bathymetric features. In particular, this study focuses on how common assumptions in tidal stream resource assessment models could hinder model calibration. A variable Manning coefficient field is utilised to represent differences in seabed roughness; this is an approach not generally undertaken in similar modelling studies where a uniform constant coefficient is typically calibrated. Also highlighted is the effect of mesh resolution on capturing certain wake structure characteristics, which points to the need to locally refine the mesh around key bathymetric features to a level that is also not typically applied in modelling studies used for tidal resource assessment.