Contemporary genesis of sand ridges in a tideless erosional shoreface

Abstract

A sand ridge field located over a retreating deltaic lobe in the Ebro Delta (NW Mediterranean) is characterized using three sets of co-located multibeam bathymetric data acquired in 2004, 2013 and 2015, measurements of near-bottom currents, high-resolution seismic profiles, and aerial photographs. The aim of this study is to illustrate the processes and timing involved in the initial stages of sand ridge development. The sand ridge field extends from the 5 to 15m isobaths and the ridges have maximum heights and wavelengths of 2.5 and 400m, respectively; they have straight crests that are arranged obliquely to the shoreline and are composed of fine sand. In general, the sand ridges are symmetric, although asymmetries with the lee side of the ridge facing to the SE and to the NW are also present. The genesis of the sand ridge field is closely related to the contemporary evolution of the Ebro River mouth. The change of the main river channel in the 1940s led to progressive abandonment of the former river mouth and to severe coastal retreatment (~37m·yr−1), making large amounts of sediment available for ridge formation. The persistent Mistral winds (NW) induce near-bottom currents flowing towards the SE, which are able to rework and transport sandy sediments. The sand ridges are currently active, with mean SE migration rates of ~10m·yr−1 most likely in pulses, when high-speed currents occur. Wave-storm events induce reverse flows (E-SE), which reshape the ridges to symmetric or opposite asymmetry geometries.The bedform morphologies, the oblique arrangement with respect to the shoreline, the angle between the strongest current and the crestlines and the sediment grain size match well with those of shoreface-connected (attached) sand ridges and, particularly, with the initial stages of sand ridge development on storm-dominated continental shelves. Sediment availability, shoreline retreat, relatively strong near-bottom currents induced by winds, seafloor irregularities and relative sea-level rise on the Ebro Delta combine to provide a suitable transgressive environment for sand ridge development. Time-scales related to sand ridges are usually of hundreds/thousands of years, but here it is demonstrated that the genesis of sand ridges can take place within a few decades. The studied bedforms are unlikely to persist in the absence of rapid sea-level rise, leading to sediment scarcity and wave and current reworking.