Seasonal morphodynamics of the subaerial and subtidal sections of an intermediate and mesotidal beach

Abstract This study provides a detailed insight into the seasonal morphological evolution of the subaerial and subtidal sections of an intermediate and mesotidal beach in relation to the nearshore wave conditions. For this purpose, fortnightly − to − monthly topographic and bathymetric measurements were collected on Ensenada Beach (Baja California, Mexico) from November 2012 to February 2016. Wave data were concurrently measured at 20 m water depth, enabling the correlation of the observed morphological changes to the hydrodynamic forcing. The seasonal morphological variability of the beach comprised ± 200,000 m 3 (± 70 m 3  m −  1 ) and resulted primarily from the cross-shore exchange of sediment. The volumetric fluctuations alongshore were up to ± 100,000 m 3 and event-driven, and played an inportant role in the overall sediment balance. In association with the incoming wave conditions, the subaerial and subtidal beach sections experienced a strong seasonal variability. The subaerial beach reached the maximum volumes in the end of summer (September − October) and minimum in winter (January − February) while the opposite response was obtained for the subtidal beach, with the largest and smallest volumes occurring during the same months in winter and summer, respectively. The morphological fluctuations of the upper and lower beach sections were associated with the preceding monthly averaged wave conditions. The subtidal volume variations were highly and positively correlated to the wave height and period, while the subaerial section was highly but negatively correlated to the same variables. The morphological beach response to energetic waves was more rapid than to low-energy conditions. The largest subaerial erosion and subtidal accretion happened in three months (October − January) while the subaerial accretion and subtidal erosion occurred smoothly over nine months (January − October). The total beach volume (subaerial and subtidal) lacked of seasonal variability, and an annual loss of 50,000 m 3 (17 m 3  m −  1 ) occurred from August 2014 to August 2015, indicating that the beach remained fairly stable over that period of time. The largest volumetric variations of 300,000 m 3 (105 m 3  m −  1 ) occurred during the beginning of the 2015–2016 El Nino winter.