Location and height of intertidal bars on macrotidal ridge and runnel beaches

Previous studies devoted to the morphology and hydrodynamics of ridge and r unnel beaches highlight characteristics that deviate from those initially p ostulated by King and Williams (Geographical Journal, 1949, vol. 113, 70-85 ) and King (Beaches and Coasts, 1972, Edward Arnold). Disagreements on the morphodynamics of these macrotidal beaches include the position of the ridg es relative to the mean neap and spring tide levels, the variation in the h eight of the ridges across the intertidal profile and, most importantly, wh ether the ridges are formed by swash or surf zone processes. The morphologi cal characteristics of ridge and runnel beaches from three locations with v arying wave, tidal and geomorphic settings were investigated to address the se disagreements. Beach profiles from each site were analysed together with water-level data collected from neighbouring ports. It was found that the ridges occur over the entire intertidal zone. On one site (north Lincolnshi re, east England), the ridges are uniformly distributed over the intertidal beach, whereas on the two other sites (Blackpool beach, northwest England, and Leffrinckoucke beach, north France) there is some indication that the ridges appear to occur at preferential locations. Most significantly, the l ocations of the ridge crests were found to be unrelated to the positions on the intertidal profile where the water level is stationary for the longest time. It was further found that the highest ridges generally occur just ab ove mid-tide level where tidal non-stationarity is greatest. These findings argue against the hypothesis that the ridges are formed by swash processes acting at stationary tide levels. It is tentatively suggested that the rid ges are the result of a combination of swash and surf zone processes acting across the intertidal zone. Elucidation of the morphodynamic roles of thes e two types of processes, and other processes such as strong current flows in the runnels, requires further comprehensive field measurements complemen ted by numerical modelling. Copyright (C) 2001 John Wiley & Sons, Ltd.