Research papersDistribution of natural disturbance due to wave and tidal bed currents around the UK

- POLCOMS and WAM were run for a UK continental shelf configuration, generating a 10 year dataset covering 1999-2008.
- Wave height, current speed, and bottom velocities were validated against in situ data and found to capture observed variability well.
- Climatologies of wave and tidally generated bottom stress were produced for the UK continental shelf.
- The GEV method was used to make predictions of extreme wave events and their associated bottom disturbances.

The UK continental shelf experiences large tidal ranges and winter storm events, which can both generate strong near-bed currents. The regular tidal bottom currents from tides plus wind driven 'benthic storms' (dominated by wave-driven oscillatory currents in shallow water) are a major source of disturbance to benthic communities, particularly in shallow waters. We aim to identify and map the relative impact of the tides and storm events on the shallower parts of the North West European continental shelf.A 10-year simulation of waves, tides and surges on the continental shelf was performed. The shelf model was validated against current meter observations and the Centre for Environmental, Fisheries and Aquaculture Science (CEFAS) network of SmartBuoys. Next, the model performance was assessed against seabed lander data from two sites in the Southern North Sea; one in deep water and another shallow water site at Sea Palling, and a third in Liverpool Bay. Both waves and currents are well simulated at the offshore Southern North Sea site. A large storm event was also well captured, though the model tends to underpredict bottom orbital velocity. Poorer results were achieved at the Sea Palling site, thought to be due to an overly deep model water depth, and missing wave-current interactions. In Liverpool Bay tides were well modelled and good correlations (average R2=0.89) are observed for significant wave height, with acceptable values (average R2=0.79) for bottom orbital velocity.Using the full 10-year dataset, return periods can be calculated for extreme waves and currents. Mapping these return periods presents a spatial picture of extreme bed disturbance, highlighting the importance of rare wave disturbances (e.g. with a return period of 1 in 10 years). Annual maximum currents change little in their magnitude and distribution from year to year, with mean speeds around 0.04 m s−1, and maximums exceeding 3 m s−1. Wave conditions however are widely variable throughout the year, depending largely on storm events. Typical significant wave heights (Hs) lie between 0.5 and 2 m, but storm events in shallow water can bring with them large waves of 5 m and above and up to 18 m in North West Approaches/North West Scotland (Sterl and Caires, 2005).The benthic disturbance generated by waves and currents is then estimated by calculating the combined force on an idealised object at the bed. The patterns of this disturbance reflect both regular tidal disturbance and rare wave events. Mean forces are typically 0.05-0.1 N, and are seen largely in areas of fast currents (>1ms−1). The pattern of maximum force however is more dependent on water depth and exposure to long-fetches (>1000km) suggesting that it is dominated by wave events.