Observations of bedforms on a dissipative macrotidal beach

Abstract

<jats:title>Abstract</jats:title> <jats:p>Field measurements of wave ripples and megaripples were made with a Sand Ripple Profiler in the surf and shoaling zones of a sandy macrotidal dissipative beach at Perranporth, UK in depths 1–6 m and significant wave heights up to 2.2 m. A frequency domain partitioning approach allowed quantification of height (<jats:italic>η</jats:italic>), length (<jats:italic>λ</jats:italic>) and migration rate of ripples and megaripples. Wave ripples with heights up to 2 cm and wavelengths ∼20 cm developed in low orbital velocity conditions (<jats:italic>u</jats:italic> <jats:sub>m</jats:sub> &lt; 0.65 m/s) with mobility number <jats:italic>ψ</jats:italic> &lt; 25. Wave ripple heights decreased with increasing orbital velocity and were flattened when mean currents were &gt;0.1 m/s. Wave ripples were superimposed on top of megaripples (<jats:italic>η</jats:italic> = 10 cm, <jats:italic>λ</jats:italic> = 1 m) and contributed up to 35 % of the total bed roughness. Large megaripples with heights up to 30 cm and lengths 1–1.8 m developed when the orbital velocity was 0.5–0.8 m/s, corresponding to mobility numbers 25–50. Megaripple heights and wavelengths increased with orbital velocity but reduced when mean current strengths were &gt;0.15 m/s. Wave ripple and megaripple migrations were generally onshore directed in the shoaling and surf zones. Onshore ripple migration rates increased with onshore-directed (+ve) incident wave skewness. The onshore migration rate reduced as offshore-directed mean flows (undertow) increased in strength and reached zero when the offshore-directed mean flow was &gt;0.15 m/s. The migration pattern was therefore linked to cross-shore position relative to the surf zone, controlled by competition between onshore-directed velocity skewness and offshore-directed mean flow.</jats:p>