Active coastal zone
Definition of Active coastal zone:
The active coastal zone (sometimes also called active coastal profile) is the beach zone over which sand is exchanged in crossshore direction by natural processes. The seaward limit corresponds to the closure depth and the landward limit to a hard boundary (seawall, cliff, ..). In the case of a dune coast the active zone comprises part of the front dune that can be eroded by storm waves.
This is the common definition for Active coastal zone, other definitions can be discussed in the article

For sections of a sandy coast where no losses (or gains) of sand occur caused by gradients in littoral drift, by barrier overwash or by landward aeolian transport, the volume of sand in the active coastal zone is constant in time. However, the sand distribution within the active zone can change, see Fig. 1.
The seaward limit of the active coastal zone is the most landward depth seaward of which there is no significant change in bottom elevation and no significant net sediment transport between the nearshore and the offshore, for a given or characteristic time interval. This depth is generally called closure depth. The extent of the active coastal zone thus depends on the time scale at which changes in beach profile are considered. When considering beach evolution at seasonal time scale, the closure depth (generally called inner closure depth, [math]h_{in}[/math]) can be estimated from the yearly mean significant wave height [math]H_s[/math] by the approximate formula (for micro/mesotidal beaches): [math]h_{in} \approx 9 H_s[/math].
When considering beach evolution at larger time scales, the seaward limit of the active zone corresponds to a larger closure depth, often designated by [math]h_{out}[/math]. Estimates for [math]h_{out}[/math] are given in the literature. It is more careful, however, to derive [math]h_{in}[/math] and [math]h_{out}[/math] from field data because the closure depth depends on local characteristics of wave climate, tide, sediment and bathymetry.
Related articles
 Definitions of coastal terms
 Closure depth
 Shoreface profile
 Types and background of coastal erosion
 Bruun rule
 Natural causes of coastal erosion
 Human causes of coastal erosion
 Accretion and erosion for different coastal types
 Typical examples of structural erosion
 Dune erosion
 Classification of coastlines
 Characteristics of sedimentary shores
 Coastal Hydrodynamics And Transport Processes
 Littoral drift and shoreline modelling
 Swash zone dynamics
 Beach Cusps
 Shallowwater wave theory
 Rhythmic shoreline features
 Data analysis and statistical approaches
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