Bedform geometry on a barred sandy shore

An instrumented sled was used to repeatedly measure nearshore profiles and bedform height (η), length (λ) and asymmetry (α) on a double-barred beach at Mangawhai (Northland, New Zealand). The 6-week experiment measured cross-shore profiles at 30-m intervals to 12-m water depth and 500 m alongshore. Bedform response to changing wave conditions, that included both fairweather swell and storm waves, was sampled. Bedform geometry is closely tied to large-scale morphology with alternating bands of flat bed and lunate megaripples (LMR, η ≤ 0.3 m, λ ≤ 2.9 m) on the longshore bars. A transition in bedform morphology occurs on the seaward face of the outer bar, with symmetrical vortex ripples (η ≤ 0.04 m, λ ≤ 0.7 m) replaced by LMR. Within the transition zone, η increases relative to λ, so that η/λ increases onshore with a shift to increasingly 3-dimensional forms. Bedforms reconfigured to the fairweather swell 2 weeks after a storm at the start of the experiment. This was shown by a substantial reduction in longshore and cross-shore variability in average bedform size and steepness. Wave measurements made on the ripple field seaward of the longshore bars at − 5.2 and − 6.9 m mean sea level (MSL) indicate that sand mobilisation was episodic under fairweather swell (skin-friction Shields parameter (θw′) ≤ 0.35). By comparison, θw′ ∼ 2 under storm waves indicates that the entire nearshore bed was active, at least to − 6.9 m MSL. This is consistent with the presence of LMR on storm deposits to − 6 m MSL. Lunate megaripples dominate bed roughness when parameterised by η2/λ. They persist seaward of the longshore bars for weeks after storms because substantial sand transport is required to reconfigure the bed to fairweather conditions. The natural variability in bedform geometry will need to be accounted for if realistic morphodynamic models are to be realised.