Characterising the relaxation distance of nearshore submarine morphology: A southern Baltic Sea case study

Abstract

A method for defining the length of coast with alongshore uniformity of nearshore submarine morphology is required in many shore-normal, nearshore profile modelling approaches. Here, we introduce a new variable, the ‘relaxation distance’, to describe the alongshore spacing between two adjacent shore-normal profiles characterised by distinct morphologies. Within this distance, alongshore morphological changes are too small to distinguish, so the nearshore submarine morphology is regarded as uniform along the coast. We apply a semi-variogram approach, which expresses decreased spatial correlation of the nearshore submarine morphology with alongshore distance, to quantify relaxation distances. We apply this morphological variable to analyse 225 measured shore-normal submarine profiles at 500 m intervals in three distinct sites of the tideless, wave-dominated southern Baltic coast: Swina, Łeba, and Hel. The relaxation distances of these three sites are approximately 4500 m, 3000 m, and 2200 m. Of the three sites, Swina has the smoothest nearshore submarine morphology and the weakest wave energy. The relaxation distances of different quasi-straight segments at each site appear similar but those at different sites decrease with enlarged morphological perturbations, which are residuals resulting from detrending the shore-normal profiles. The standard deviations of these morphological perturbations are correlated with the incidence wave angles and wave energy. We also found that large perturbations (such as longshore bars and channels) have a significant impact on the estimation of underwater volume using shore-normal profile trends. Therefore, relaxation distance can indicate morphological smoothness, the length of shore-parallel uniform nearshore submarine morphology, and the impact of local processes on submarine morphology.