Shore-parallel breakwaters in meso-tidal conditions: tidal controls on sediment transport and their longer term, regional impacts at Sea Palling, UK

Shore-parallel breakwater systems built in meso-tidal conditions display a more complicated morphological response than predicted by existing models and their impact on coastlines in the longer term (10-20 years) is unclear. The shore-parallel breakwaters at Sea Palling, UK, built to protect low-lying land from inundation, comprises four surface-piercing and five overtopping breakwaters, spanning a 4.0km length of coast. Earlier studies of the impacts of these breakwaters considered the impacts of waves but neglected the influence of the strong asymmetric tidal currents. TELEMAC was used to simulate tidal currents and elevations around the breakwaters, focussing on sediment transport over the tidal tombolos behind the four, northern-most (updrift) breakwaters; a small amount of wave-stirring (Hs=0.5m) was also included. The model suggests that the asymmetric tidal currents transport ~40,000 m3 of sand through the system annually, around 20% of the prebreakwater longshore transport rate. Measurements of waves, near-bed currents, suspended sediment concentrations and bedform structures enabled verification of modelled sand transport rates (within 50%). RTK-GPS surveys provided high resolution topography and bathymetry of the beach, tombolo and nearshore seafloor; the rate and direction of tombolo movement from the surveys was also consistent with the modelled transport rates. In addition to the tidally-driven transport, storms drive sand through the breakwater system but erosion of beaches down-drift indicates that the sand supply here is much reduced. Beach and bathymetric surveys over the 10 years since the breakwaters’ construction show sand continuing to accumulate at the northern end of the breakwaters but that sand in the longshore transport is also bypassing and accumulating offshore of the breakwaters