The Variability of Ridge and Runnel Beach Morphology: Examples from Northern France

The morphology of three macrotidal ridge and runnel beaches in northern France was analysed from over 200 profiles in order to identify intertidal spatial and short-term (weeks) profile variability, at both the inter-site and intra-site levels. The beaches, essentially composed of medium to fine sand, are exposed to fetch-limited waves variably dissipated by nearshore sand banks. They have spring tidal ranges of 5.6 to 7.2 m, and sediment budgets ranging from equilibrium to deficient or surplus. The results show that spatial and temporal morphological variability is controlled by: (1) variations in exposure to wave action that depend on the proximity of nearshore sand banks, as well as on protection offered by artificial structures; and (2) by the state of the beach sediment budget. Where equilibrium sediment budget conditions prevail, as in the Dunkerque-Est sector, the beach exhibits a regular alternation of ridges and runnels that represent a cross-shore alternation of fluid-bed interaction domains involving surf/swash activity and channel flow conditions. Energy dissipation at the bed is spent in the construction and destruction of wave and tidal micro- and meso-scale bedforms, leaving little scope for macro-scale ridge migration or change in form, except under exceptionally high wave energy conditions. Chronic sediment losses, as in Wissant Bay, or gains, as in Calais-Hoverport, are recycled respectively alongshore and to embryo dunes and are not necessarily translated in terms of significant meso-scale (years) beach volumetric changes. The short-term beach sediment budget changes however favour active bed readjustments that explain distortion of the regular ridge and runnel form and marked profile mobility, even under low to moderate wave energy conditions.