This paper investigates the spatial and temporal variability of three different mixed sand and gravel beaches on the Suffolk coast, U.K. The beaches consist of a highly variable mixture of medium (−3.3 to −3.47 ϕ), moderately sorted gravel (0.85 to 0.78 ϕ), and 15 to 30% coarse (0.47 ϕ) moderately sorted (0.96 ϕ) sand. Above the high water mark beach sediments are predominantly gravel, whilst the sand fraction is concentrated in a planar region of the lower foreshore.
The beaches studied exhibit some characteristics of pure gravel beaches, some of pure sand beaches and some which are unique to mixed beaches. Distinctive sedimentary features include a coarser sand fraction and finer gravel fraction, the existence of multi-modality within the gravel fraction, very limited shape sorting, and high spatial and temporal variability. Beach profile responses are similar to those of pure sand and pure gravel beaches, including those changes occurring over semi-diurnal, spring-neap, and seasonal timescales. Some storm responses were also similar, including the formation of beach scarps, large ridges, reduced lower foreshore levels and a wide planar beachface region. However, beach profiles can flatten or steepen under storm conditions depending on the presence of seawalls at the back of the beach.
A conceptual model for beach accretion is developed which involves ridge formation on the upper foreshore and subsequent onshore migration to form beach ridges under high energy conditions. Post storm recovery is seen to rely on the accretion of these beach ridges. On the lower foreshore, accretion does not involve ridge accretion and occurs under lower energy conditions. The importance of sediment supply, beach width and nearshore water depth are also discussed.