The Dynamic Coastline

While maps typically delineate the transitions between land and sea with clear lines, in reality the coastline is much less well-defined. The position of the coastline through time is highly variable, from the daily fluctuations of the water's edge at beaches due to tides to the shoreline advance or retreat over hundreds of kilometers with geologic-scale or climatic influenced sea level fluctuations. The coastline position does not just depend on sea level; instead, the supply of sediments to the coast and the processes responsible for their redistribution within a coastal cell are both responsible for coastal evolution. Coastal erosion removes material from the shoreline and redistributes it to other parts of the coastal cell. While erosion is usually most drastic during extreme events with associated high surges and waves, shoreline erosion can also occur gradually over a longer time period due to sediment deficits.

In modern times (across engineering- or decadal- timescales), many of the world's coastlines are characterized by erosion (approximately 70% of the world's sandy coastlines are retreating). This widespread erosion is due to a variety of factors, most notably:

• Global eustatic sea level rise that has occurred over the past century
• Global reductions in the supply of sediment reaching the coast (due to sediment impoundment behind dams, urbanization, etc.)
• Human activities at the coast that restrict sediment movement (harbors, seawalls, groins)
  

• Uses of Coastal Zone