Multiple intertidal bars on beaches: A review

Ridge and runnel features were originally described by King and WiIliams (1949) from observations at Blackpool beach (U.K.) and laboratory experiments. They were characterised as intertidal, shore-parallel sandbars (ridges), commonly 2–6 bars in total, and disconnected from each other by troughs (runnels). The nomenclature ‘ridge and runnel’ was, however, also used by Hayes (1967) to describe multiple-barred beaches but referring to subtidal bars. The more specific term ‘Multiple Inter-Tidal Bars’ (MITB) was subsequently adopted for intertidal beaches exhibiting successive shore-parallel sandbars. To date, a detailed understanding of the formation of MITB has remained elusive and their precise definition is still unclear. It has been suggested that MITB features are the result of both swash and surf zone processes acting on the intertidal beach profile. These processes are involved in the formation, the long-term persistence, and behaviour of MITB. Despite the long-term persistence of MITB systems they are dynamic at short timescales. Ridge crest positions are regularly modified over each tidal cycle by successive surf and swash processes. At seasonal scales, ridges may undergo erosion and cross-shore migration under high energy conditions (winter) while ridges are well developed during summers. Via a meta-analysis of 93 separate published works at 67 sites globally, we define MITB, characterise their morphodynamics and assess their global distribution. Our study shows that the distribution of MITB is a function of thresholds in beach slope (<0.02), tidal range (3–10 m), and wave period (3–8 s). They are developed at sites with sufficient sediment supply, limited wind and wave fetch, meso- to macrotidal (>3 m) and on low gradient (wide) intertidal beach slopes.