STABILITY OF MEGARIPPLES IN THE DUTCH COASTAL AREA, SOUTHERN NORTH SEA, UNDER STORM AND FAIR-WEATHER FLOW CONDITIONS

This paper discusses the stability of megaripples (λ = 1-40 m) in the North Sea through periods of fair-weather conditions and multiple seasonal storm events. The purpose of this study is twofold. First, sustainable development of densely populated coastal areas requires knowledge of the morphodynamics of the seabed in e.g., navigation, the construction of pipelines and cables on the sea floor, or ecological marine habitat studies. Second, relating bedforms to flow conditions provides geologists with a modern reference to interpret ancient deposits. Process studies in flow ducts and in the field concentrate on small-scale bedforms (< 5 m) due to the size limitations of the measuring set-up. However, larger compound bedforms (> 15 m wavelength), consisting internally of smaller bedforms have been observed in outcrop studies. The presence of ripples, megaripples and hummocks in shallow shelf seas was observed previously, but their mutual relations remain poorly understood. Therefore, a periodic monitoring study was carried out as a complimentary approach to process studies to provide data on the behavior of bedforms at larger temporal and spatial scales. Multibeam and side-scan sonar data were collected off the Dutch coast to monitor bedforms through multiple storm and fair-weather events. Boxcores were used to observe the vertical bed structure and grain-size (D₅₀ = 0.25-0.35 mm). Bedform stability diagrams were constructed to relate the bedforms to flow velocities and dimensionless (Shields) shear stress parameters. In the Dutch coastal area, 2D megaripples (λ = 1-15 m) are the dominant bedforms in current-dominated (> 0.4 m/s) tidal flow regimes with oscillatory flows < 0.15 m/s. At slightly higher energy conditions 3D megaripples (λ = 5-15 m) begin to form. After seasonal storms, at oscillatory flows > 0.4 m/s undulating bed topography of mound-like 3D bedforms (λ = 20-40 m) is observed on the shoreface. These bedforms start to form around sheet-flow conditions and are related to sets of low-angle converging laminae in boxcores interpreted as hummocky cross-stratification (HCS). These field observations considerably extend the knowledge on the stability of large-scale bedforms in shallow clastic seas beyond the limits of small-scale process studies.