Alongshore runup variability across contrasting beach states: Insights from field observations

Abstract

Most beaches exhibit alongshore morphological variability, which is often overlooked in predictive empirical parameterizations of the runup. This study examines alongshore runup variations in relation to intertidal and subtidal morphology on an intermediate beach in the NW Pacific of Baja California. UAV imagery, combined with topographic and bathymetric measurements, was used to determine the runup elevation along a 500-m stretch of beach with diverse morphological characteristics: Terraces and Cross-shore Channels (TCC), Transverse Bar and Rip (TBR), Reflective conditions (R), and Reflective beaches with Multiple Channels (RMC). The largest runup variations were observed under intermediate conditions (TBR), where rip channels and transverse bars significantly influenced the runup. Distinct signatures associated with different morphologies were evident in the spectral shape of the swash, with well-developed rip channels promoting more incident swash unless wave breaking occurred at bars at the head of the rip. Swash showed alongshore variability ranging by a factor between 1.4 and 2.7, depending on the beach state. This factor can be much larger when evaluated over the incident and infragravity swash components. Such variability could not be explained by changes in beach slope alone. The setup, as the lower component of the runup, exhibited minimal alongshore change, with a coefficient of variation of <23 %. Wave runup, setup and swash observations were compared against various existing parameterizations. The results showed that the variability of incident swash is well captured by parameterizations based on foreshore slope and deep-water wave parameters. While predictions of infragravity swash improve when accounting for beach morphology, most of the alongshore variability remains unrepresented, particularly for milder slopes. This research highlights the influence of alongshore morphological variations on runup and underscores the necessity of incorporating these variations into parameterizations to enhance accuracy.