Geological Control on Beach Form: Accommodation Space and Contemporary Dynamics

All beaches exist within a specific 3-Dimensional geological framework. This determines the boundaries within which the beach forms (accommodation space) and fluctuates in space and time as it is worked upon by dynamic forces. Those forces are themselves mediated by certain geological parameters including rock outcrop (which changes bed roughness, influences wave breaking and moderates water flow through the beach). Much Of Our thinking on beach morphodynamics is dominated by consideration of unconstrained beach environments; particularly in the profile dimension (shoreline profile of equilibrium concept and the Bruun Rule). The relationship between dynamic forcing, expressed as composite indices such as surf scaling parameter, Dean's parameter and Relative Tidal Range, has resulted in the development of a suite of conceptual models of beach morphodynamics. While the beach states identified in these Studies have achieved wide Currency, the terms `dissipative' and `reflective' being ubiquitous in beach morphodynamics, a number of studies have pointed to differences between beach state predicted by these models and beach state observed in the field. A coastal stretch along the SE coast of Northern Ireland is presented where there is an apparent strong lateral and submerged geological control. Their placement and behaviour are outlined and parameters that constrain beach behaviour are examined in the context of finite sediment Volumes and geological control. It is proposed that geological control on beach morphodynamics is actually widespread and the application of current models may be inappropriate. Indeed, geologically constrained beaches represent a new class of beach that likely operates outside the bounds of existing models.