On the Dynamics of "Almost Equilibrium" Beaches in Semi-sheltered Bays Along the Southern Coast of the Gulf of Finland

Beaches along the northern coast of Estonia form an interesting class of almost equilibrium, bayhead beaches located in bays deeply cut into the mainland in an essentially non-tidal, highly compartmentalised coastal landscape, and that develop mostly under the influence of wave action. These beaches, although often suffering from a certain sediment deficit, are stabilised by the postglacial land uplift. We describe the basic features of their appearance and functioning from the viewpoint of sediment transport processes. Wave action normally impacts a relatively narrow nearshore band and additionally stabilises the beaches through littoral drift of sandy sediment and gravel towards the bayheads. Eolian transport and fluvial sediment supply have typically very modest magnitude. Such beaches, in general, evolve quite slowly and may represent an almost equilibrium stage, even when the active sand mass is very limited. The concept of the equilibrium beach profile is an adequate tool for their analysis. As an example, its parameters and longshore transport patterns are evaluated for Pirita Beach based on a granulometric survey and long-term simulation of wave climate. It is demonstrated that net sand changes for such beaches can be estimated directly from the properties of the equilibrium profile, land uplift rate, and loss or gain of the dry beach area. Another type of highly dynamic equilibrium exists owing to interplay of the effects of river flow and wave action at the mouths of large rivers such as the Narva River.