Observations of sand waves, megaripples, and hummocks in the Dutch coastal area and their relation to currents and combined flow conditions

Abstract

This paper aims to investigate the distribution and stability of large‐scale bed forms in response to storm and fair‐weather conditions in a shallow marine environment. Multibeam and side‐scan sonar data off the Dutch coast (median grain size 0.25–0.35 mm) were collected to monitor sand waves (λ = 100–800 m) and superimposed megaripples (λ = 1–40 m) through multiple storm and fair‐weather events. Box cores were used to observe the vertical bed structure and grain size. In the Dutch coastal area, two‐dimensional (2‐D) megaripples (λ = 1–15 m) are the dominant bed forms in current‐dominated (>0.4 m/s) tidal flow regimes with oscillatory flows <0.15 m/s. Effects of trapping of fine material under these conditions by tube building Lanice conchilega colonies on bed form development are suspected but need further study. At slightly higher energy conditions, 3‐D megaripples (λ = 5–15 m) begin to form on the shoreface. After seasonal storms, at oscillatory flows >0.4 m/s, undulating bed topography of mound‐like 3‐D bed forms (λ = 20–40 m) is observed. Immediately after storms, these bed forms are covered by smaller 3‐D megaripples, which are related to sets of low‐angle converging laminae in box cores, interpreted as hummocky cross stratification (HCS). The sand waves form compound bed forms of sets of 2‐D and 3‐D megaripples. The morphology of the sand waves is a function of the general wind‐wave climate of the marine environmental setting, with flat‐topped 3‐D sand waves occurring in shallow wave‐dominated settings and 2‐D sand waves occurring in the tide‐dominated environment farther offshore.