Theoretical framework for coastal accretion-erosion analysis: material budgeting, profile morphology, shoreline change

Shoreline dynamics is often used as a criterion for coastal erosion or accretion. However, this criterion may not be valid because it does not incorporate the factors of material budget and coastal profile morphology. Based upon an analysis of the properties of sediment budget equation and the profile morphology of beach and tidal flat systems, it is argued that only by combining the material budget with the rate of shoreline retreat or the profile morphology can the status of accretion-erosion be accurately identified. The sediment budget equation contains the information on the magnitude of a sedimentary system, accretion-erosion intensity, and the growth limit of the system. The beach profile shape depends on particle size and wave energy. The minimum wave energy dissipation principle implies the existence of equilibrium morphology, while the tidal flat profile shape depends on sediment supply, particle size composition and tidal dynamics. On such a basis, the erosion of both sandy coasts (represented by beaches) and muddy coasts (represented by tidal flats) can be understood in terms its mechanisms, rate and temporal scales by taking into account the various factors such as extreme events induced by storms and sea level rise. The rate of shoreline change may vary by orders of magnitude, ranging from low values of ＜10⁰ m/a to high values of 10¹~10² m /a, with time scales for accretion-erosion processes ranging from 10⁻² a (storm events) to 10³ a (sea level changes). According to the different combinations of sediment budget and shoreline advancing/retreating patterns, the dynamic behaviour of the coastal zone associated with beaches and tidal flats has four possible situations: one of them is related to accretion, and the others are linked with erosion. The different types of erosion are each determined by the geomorphic evolution direction and the temporal scale. High intensity, long-time persisting erosion is mainly related to material supply cutoff and sea level rise, and is influenced by anthropogenic factors.