Distinct sandbar behavior on a gently sloping shoreface sea-breeze dominated beach

Sandbars are common features in sandy nearshore environments that readily migrate in response to changing hydrodynamic conditions and, when in shallow water, can provide coastal protection by inducing wave breaking and through sediment feeding to the beach. Prior process-based studies related the off- (on-) shore sandbar migration with the increase of the undertow current (wave acceleration skewness) associated to storm (mild) conditions. However, onshore sandbar migration has, under some circumstances, also been reported to occur during storms at mild sloping shorefaces. A comprehensive 9-year data set of (200) weekly to bi-weekly surveys of the beach and shallow nearshore, undertaken on an accretive micro-tidal sea-breeze dominated beach along the southeastern coast of Mexico, are presented here that shed new insights into hydrodynamic drivers of inner surf zone sandbar and shoreline dynamics. During spring-summer, short period waves, mainly associated to local sea-breezes, drive offshore sandbar migration. On the other hand, winter storms, occurring during fall-winter months, generate more energetic swell waves that induce onshore sandbar migration. In general, seasonal changes of shoreline and inner sandbar position are coupled, with on- (off-) shore sandbar migration being synchronous to seasonal shoreline advance (retreat), suggesting a gradual feeding of sediment from the bar system onto the beach. Analysis of the data, in conjunction with comparison to equilibrium model results, are used to explore the physical drivers of sub-seasonal sandbar evolution at the site. The sandbar dynamics in the study area, showing an opposite behavior to conventional expectation of storm-induced offshore transport, are well correlated to seasonal changes of waves properties. This distinct sandbar behavior might be present at other gently sloping shoreface sea-breeze dominated sandy beaches. • A comprehensive dataset including 200 repeat beach surveys of a barred beach system is presented. • Sandbar migrates onshore during winter storms and migrates offshore during milder sea-breeze conditions. • Field observations suggest gradual feeding of sediment from the bar system onto the beach. • A simple data-driven model suggests a high correlation between wave conditions and seasonal shoreline/sandbar changes. • A distinct sandbar behavior might be found at other mild shoreface sea-breeze dominated sandy beaches.