Storm-driven cusp behaviour on a high energy gravel beach

Gravel and mixed sand-gravel beaches are characterised by steep reflective profiles which provide effective forms of wave absorption and therefore coastal defence to many mid-latitude regions, including northwestern Europe and North America. In the UK the combination of energetic wave conditions and large tides creates very dynamic and responsive morphology often dominated by cuspate features. Recent storm-responsive field campaigns at Loe Bar, Cornwall, UK, have captured highly energetic wave conditions (H-s = 2.5-5.8 m) using temporary video camera installations, low tide 3D topographic surveys with real time kinematic GPS, local tide level measurements and inshore directional wave data. Characterised by fine gravel (D-50 = 3 nun) and a steep reflective profile (tan beta = 0.118), the barrier at Loe Bar is exposed to an annual 10% exceedence significant wave height H-s10% of 2.4 in arriving predominantly from the southwest (Atlantic Ocean) and shore-normal to the beach. Under medium-wave conditions (H-s = 2-3 m), contrasting cusp behaviour was recorded with accretion and erosion, principally, through horn growth and decay (bed-level change Delta z = c. 1 in). During more energetic conditions (H-s = 5.8 in), the morphological response is more consistent and the waves drive erosion of the lower profile causing bed-level changes over a tide in excess of 1.5 in. Very rapid recovery to pre-storm bed levels is observed with defined cusp evolution occurring within 12 hours during the falling limb of the storm as incident wave energy decreases. The unique gravel cusp dataset suggests free behavior due to cusp morphodynamic feedback, rather than hydrodynamic forcing, plays an dominant role in cusp evolution.