Storm-wave development of shore-normal grooves (gutters) on a steep sandstone beach face.

Abstract

Shore-normal grooves (gutters) cut into the seabed have been reported widely from the marine geological record. Grooves commonly are spaced regularly across plane, consolidated surfaces in the littoral and sub-littoral zones and may be deeply incised. Despite their common occurrence in the rock record, there are few detailed descriptions of examples from modern environments. Previously reported examples have been ascribed to erosion by wave-induced currents, especially storm-driven near-shore flows. In particular, examples from beach faces have been related to either wave swash or backwash. However, no conceptual model exists to explain the presence of grooves, their morphology or their spacing alongshore. Herein, quasi-regularly spaced grooves on a soft sandstone beach face are described and interpreted to have formed due to wave breaking and swash zone processes consequent upon exceptional storms at sea. The groove morphologies are quantified using terrestrial laser scanning. Numerical modelling of the translation from offshore waves to nearshore breaking waves provides estimates of the swash zone parameters. A consideration of swash zone processes provides an explanation for formation of the grooves. In particular, the swash zone shear stress distribution and consequent bed erosion is a dome-shaped function of distance across the beach face, and this controls the cross-shore variability in groove depths. High-speed sheet flows, such as swash and backwash, develop periodic, shore normal, high and low speed streaks alongshore. Consequent streaky erosion produces the quasi-regular alongshore groove spacings. However, on any given beach face the specific spacing of grooves is likely a property, not only of the local sheet flow attributes, but also of larger-scale morphological forcing. This outcome suggests that spacing is an emergent property of the coupled sheet flow and larger-scale forcing, and thus specific spacings on any beach face remain unpredictable.