Formation of Shore-Normal Grooves (Gutters) in Sandstone by Wave Action.

Abstract

Regularly spaced incised shore-normal grooves (gutters) on plane consolidated surfaces in littoral and sublittoral zones are widely observed in the marine geological record. Despite their common occurrence there are few investigations into their origins in contemporary marine environments. While their formation is often attributed to wave-induced currents related to wave swash and backwash on the beach-face, no conceptual model has been advanced to explain the presence of gutters, their morphology or their quasi-regular alongshore spacing. The paper examines gutters cut in soft sandstone at Medmerry near Selsey, UK and argues that their formation is related to wave breaking and swash zone processes during an unprecedented sequence of storms in the winter of 2013/14. During this period exceptionally high near-shore waves (Hs around 6m) were recorded for the south coast beaches and these storm conditions persisted periodically through to mid-February 2014. The consequence was extensive beach erosion and the exposure of underlying substrates. In this study gutter morphology was quantified using terrestrial lidar and a wave-resolving numerical model was used to defined the nearshore wave conditions and swash characteristics. Three of the largest storm events during the winter of 2013/14 were modelled: (a) moderate waves coincident with an exceptionally high tide; (b) exceptionally high waves occurring during neap tides; and (c) high waves occurring during spring tides. The model results showed swash zone shear stress is a dome-shaped function of distance across the beach-face thereby controlling gutter depth. Further, high-speed sheet flows characterised by periodic, shore-normal, high and low speed streaks alongshore are thought to be implicated in the mechanism controlling gutter spacing. However, in any situation, the specific spacing of gutters is moderated by both the local sheet flow characteristics and the larger-scale morphological forcing. Together these factors indicate that gutter spacing is an emergent property which makes spacing unpredictable.