Why are coarse-grained beaches generally steeper than fine-sand beaches?

The slope of a beach's foreshore is significantly influenced by the grain size of its constituent sediment. In general, the coarser the sediment grains, the steeper the foreshore will be.1

This relationship occurs because coarser materials, such as gravel, allow for greater permeability and different interactions with wave energy compared to fine-grained sand. When waves wash up onto a beach composed of coarse material, the water can percolate more easily into the sediment, reducing the volume of water that flows back down the beach as backwash. This reduction in backwash limits the amount of sediment transported seaward, allowing the beach to maintain a steeper profile. Conversely, fine-sand beaches tend to be less permeable, resulting in more significant backwash that carries sediment down the slope, leading to a more gentle, gradual incline.1