Equilibrium configurations of sandy-muddy transitional beaches on South China coasts: Role of waves in formation of sand-mud transition boundary

Sandy-muddy transitional beaches (SMT-Beaches) consisting of the upper sandy beach and lower mudflat can be found on coasts with weak wave conditions and sufficient sediment supply. Currently, such SMT-Beaches receive limited attention, and their morphological characteristics as well as the mechanisms of the formation of sandmud transition (SMT) boundary are still not clear. Based on field surveys on SMT-Beaches at South China Coasts, traditional equilibrium functions are assessed on SMT-Beaches, and an equilibrium profile function for SMT-Beaches is parameterized with only inputs of the offshore distance of the SMT boundary, tidal ranges and offshore wave heights. The present function exhibits good performance on both mesotidal and macrotidal SMTBeaches. It improves the accuracy of profile curvature of the upper beachface compared with the traditional power function and reduces uncertainties in determining coefficients. Sediment characteristics differ between both sides of the SMT boundary. Shoreward of the SMT boundary, the probability distribution of grain size is generally unimodal with a peak located at sand grains and turns to be bimodal or trimodal with a significant increase at clay-to-silt grains seaward of the SMT boundary. Based on the exceeded Shields parameter, the critical position for the deposition of mud generally corresponds well with the SMT boundary, indicating that waverelated oscillatory motions play a dominant role in the formation of the SMT boundary.

Automated summary

Sandy-muddy transitional beaches (SMT-Beaches) are a type of coastal formation consisting of upper sandy beach and lower mudflat. This study examined the morphological characteristics of SMT-Beaches and the mechanisms of the formation of sandmud transition (SMT) boundary. Field surveys were conducted on SMT-Beaches in South China Coasts and a new equilibrium profile function for SMT-Beaches was developed. The function demonstrated good performance and improved accuracy compared to traditional methods. It was also found that sediment characteristics differ on both sides of the SMT boundary, with clay-to-silt grains increasing seaward.