Final answer:
Coarse-grained beaches have a steeper equilibrium slope because coarse grains accumulate more steeply and are less easily moved by waves compared to fine grains, which create gentler sloping beaches.
Step-by-step explanation:
Beach Equilibrium Slope and Sediment Size
The equilibrium slope of a beach refers to the angle at which the beach naturally inclines and remains relatively constant over time, balancing the processes of deposition and erosion by waves. Coarse-grained beaches typically have a steeper slope compared to fine-grained beaches, and there are several reasons for this difference. First, the size of the sediment grains affects the beach profile; coarse-grained sediments such as pebbles and gravels are less easily moved by waves than fine-grained sediments like sand. Thus, they accumulate more steeply on the shore.
Waves transport sediment in two primary ways: up the beach (swash) and down the beach (backwash). Coarse-grained beaches experience a stronger swash that carries the larger grains up the slope, whereas the backwash is often weaker due to the permeability of the coarse material, allowing water to percolate down rather than dragging sediment back to sea. Therefore, sediment accumulates more readily, contributing to a steeper slope.
On the other hand, fine-grained sand beaches have a less steep slope since the smaller, lighter grains can be easily moved both landwards and seawards by the action of waves, leading to a gentler sloping beach profile. The graded bedding phenomenon provides insights into depositional environments, illustrating how sediment size varies vertically within a deposit, while cross-bedding reveals wind or water direction based on the sediment deposition patterns of ripples and dunes.