Beach cusps and inner surf zone processes: growth or destruction? A case study of Trafalgar Beach (Cádiz, Spain)

Abstract

SUMMARY: Large beach cusps (LBC, wavelength of ~ 30 m) are intertidal features that can alternately exist in the swash and in the inner surf zone due to tidal sea level changes. They have a larger cross-shore extent (up to 50 m) than traditional cusps. This extent has been explained by a shift of the swash zone during falling tide. The cusps immerse at rising tide and previous studies indicate that surf zone processes are exclusively destructive. Here, the behaviour of large beach cusps in the inner surf zone is investigated by using a 2DH morphological numerical model applied to Trafalgar Beach (Cádiz, Spain). The model results indicate that the inner surf zone processes do not always destroy the cusps but can in fact reinforce them by considering neither the swash processes nor the tidal changes. More generally, in conditions favouring the presence of the LBC the surf zone of a beach can be unstable, leading to the formation of transverse/oblique sand bars that can have characteristics similar to the LBC. Thus, in principle, the LBC could emerge not only due to swash zone morphodynamics but also due to surf zone morphodynamics or a combination of both.Keywords: beach cusps, surf zone, sand bars, instability, beach morphology, beach features, rip currents, wave processes on beaches.RESUMEN: Formas cuspidales de playas y procesos de la zona de rompientes interna: crecimiento o destrucción? Aplicación a la playa de Trafalgar (Cádiz, España). -Las formas cuspidales de grandes dimensiones (LBC, longitudes de onda ~30 m) constituyen un sistema morfológico rítmico a lo largo de la playa que tiene una parte que se encuentra alternativamente en la zona de swash (flujo/reflujo) y en la zona de rompientes interna debido a los cambios del nivel del mar. Tienen una distancia de penetración de hasta 50 m, superior por tanto a la de las cúspides ordinarias. Esta elongación parece debido a la traslación de la zona de swash durante la marea descendente. En marea ascendente estas estructuras están sumergidas y los estudios previos consideran que los procesos de la zona de rompientes las destruyen. En este trabajo se analiza el comportamiento de estas formas en la zona de rompientes de la playa de Trafalgar (Cádiz) usando un modelo numérico morfológico 2DH. Los resultados muestran que, sin considerar ni los procesos de swash ni el cambio de marea, los procesos de la zona de rompientes no necesariamente destruyen LBC, sino que pueden reforzarlas. De forma más general, en condiciones favorables a la presencia de LBC, se pueden formar barras de arena con características similares a LBC debido a procesos de auto-organización en la zona de rompientes.Palabras clave: cúspides de playa, zona de rompientes, inestabilidad, morfología de playas, estructura de playas, corriente de retorno superficial, procesos del oleaje sobre las playas.

Habitat use by southern right whales, Eubalaena australis (Desmoulins, 1822), in their main northernmost calving area in the western South Atlantic

“…Beach cusps are regular longitudinal series of horns and embayments, resulting in an undulating shoreline (Garnier et al . ). In reflective beaches, the wave energy can be trapped by refraction.…”

Erosion/accretion patterns and multiple beach cusp systems on a meso-tidal, steeply-sloping beach

“…Longuet-Higgins and Parkin, 1962;Sallenger, 1979). Beach cusps have a direct interaction with swash (Coco et al, 2003;Dodd et al, 2008;Ortega-Sánchez et al, 2008) and surf zone processes (Ciriano et al, 2005;Garnier et al, 2010). However, despite the extensive literature on beach cusps and the several proposed mechanisms of their formation, the particular conditions under which such features appear and evolve remain uncertain.…”

UAS Identify and Monitor Unusual Small-Scale Rhythmic Features in the Bay of Cádiz (Spain)

“…In fact, the study and monitoring of cusps and other types of rhythmic bars or features has traditionally relied on remote sensing techniques, mainly video monitoring systems [22,59,60] and/or orthophotos [61]. Investigations based on the use of UAS for this purpose are still scarce in the literature, although they could be frequent in the upcoming future.…”