Coastal Processes, Hazards, and Society

Nearshore, Beaches, and Dunes


Nearshore, Beaches, and Dunes

How can beaches be zoned? What is the basis for this zonation: morphology or process? Why are some beaches steep and higher gradient than other beaches? What processes can drive sediment exchange between the nearshore, beaches, and dunes? How does the storm profile of a beach look compared to the fair weather profile for the same beach? If ample sediment is available, what can happen to a beach following a major loss of sediment? Can it recover? How?

For many of you, the concept of the nearshore, beach, and dunes probably conjures ideas such as swimming about in breaking waves, games of Frisbee on a sandy surface, or heavily vegetated mounds of sediment that have to be crossed in order to reach the beach. From a coastal geologist's morphological perspective, each of these has a unique definition, where the:

  1. nearshore is a broad classification defined as the region extending from the land water interface (shoreline) to a location just beyond where the waves are breaking,
  2. beach is defined as the zone of unconsolidated material that extends landward from the low water line to a place where there is a marked change in physiographic form or a line of permanent vegetation representing dunes,
  3. dunes are defined as topographically elevated ridges and or mounds that may be heavily vegetated, but are formed as sand is transported by wind and subsequently deposited.

Each of these environments is unique in form and composition and is characteristically molded by fundamentally different processes that act to modify each environment on a daily or longer timescale

See caption and list above.
Idealized cross-sectional profile of an offshore to coastal mainland profile showing the distribution of the nearshore, beach, and dune environments as a function of elevation, water depths, and morphology. MLWL, MSL, and MHWL refer to mean low water level, mean sea level, and mean high water level respectively, and refer to the range of average tidal conditions that exist on a conceptual nearshore to beach system. Keep in mind that this range in tidal elevations is also the vertical range across which waves typically interact with the beach and that, during storms, elevated sea level can result in waves breaking across higher elevations along the profile.
Credit: M. Kulp

Zonation by Tidal Elevations

Another convenient and slightly more simplistic way of dividing the nearshore through dune system is to recognize areas along such a transect as a function of where they lie relative to the tidal range for the area. In this sense, the nearshore through the dune system can be divided, on the basis of elevation relative to mean sea level, into subtidal, intertidal, and supratidal zones.
  1. Supratidal zone: is situated above the high tide elevation and only occasionally is flooded, most commonly during high spring tides and storms. It includes the uppermost part of the beach as well as the dunes, and so, the non-storm process acting to transport sediment in this area is wind (aeolian transport). In this context, this area is recognized as the backshore and dunes zone.
  2. Intertidal zone: is located between the normal low and high tide levels. This zone is therefore repeatedly inundated by water and exposed to air. This also represents the zone where waves are routinely interacting with the land, leading to daily transport of sediment. In the image above this area is recognized as the foreshore or beachface.
  3. Subtidal zone: consists of regularly submerged, relatively shallow water area seaward of the intertidal zone. Waves and tides are always acting to move sediment in this environment. In the image above, this area is recognized as the shoreface.