What characteristics define a coastline as advancing or retreating?

An advancing coastline is characterised by sediment deposition, while a retreating coastline is marked by erosion and land loss.

An advancing coastline, also known as a prograding coastline, is one that is growing outwards, towards the sea. This is typically due to the deposition of sediment, which can be brought to the coastline by rivers, ocean currents, or wind. The sediment builds up over time, creating new land. This process is often slow and gradual, but can be accelerated by human activities such as the construction of breakwaters or groynes, which trap sediment and prevent it from being washed away by the sea. The type of sediment deposited can vary, ranging from fine sand to larger pebbles and rocks, and this can influence the shape and nature of the coastline. For example, sandy coastlines tend to be flatter and smoother, while rocky coastlines can be more jagged and irregular.

On the other hand, a retreating coastline, also known as an eroding coastline, is one that is losing land to the sea. This is typically due to the action of waves, tides, and currents, which wear away the land over time. The process of erosion can be natural, resulting from the constant battering of the coastline by the sea, or it can be exacerbated by human activities such as the removal of coastal vegetation, which can destabilise the land and make it more susceptible to erosion. The rate of retreat can vary, depending on factors such as the strength of the waves, the type of land (e.g. soft sand vs hard rock), and the presence of protective structures like sea walls. In some cases, the coastline can retreat rapidly, leading to significant land loss and the displacement of human communities.

In summary, the key characteristics that define a coastline as advancing or retreating are the processes of sediment deposition and erosion, respectively. These processes are influenced by a range of natural and human factors, and can result in significant changes to the shape and size of the coastline over time.