FIELD OBSERVATIONS OF RIPPLE AND MEGARIPPLE DYNAMICS IN THE NEARSHORE

Field measurements of megaripples and wave ripples were made in the surf and shoaling zones of a sandy macrotidal dissipative beach at Perranporth, Englan d, in water depths up to 6 m, and wave heights up t o 1.1 m. Megaripple heights were largest when orbital veloci ties were in the range 0.25 to 0.4 m/s, and reduced in height with increases in mean current speed. Wave ripples co-existed with megaripples and contributed up to 12% of the seabed elevation variance. Wave ripples height s increased with decreasing orbital velocity but re duced in height when mean currents were strong. Wave ripple and megaripple migration was generally onshore-directed in deeper water, and increased with incident wave s kewness in shallower water. Onshore migration was reduced to zero when the offshore-directed mean flow was gr eater than 0.15 m/s.