Major storms influence on the morphological evolution of a hybrid spit-tidal flat system in Argentina

Sandy coastal systems are very dynamic systems in which morphological changes occur over different time scales that ranges from hours to decades. However, it has been widely reported that major storms are the main responsible of the most significant changes in short to medium time scales. Major storms have been defined using a variety of environmental variables, but they are normally associated with high values of four main parameters: 1) Wave heights (Hs); 2) Duration (Du); or 3) Frequency (Fq); 4) Orientation (Or).

In this study we aim to characterise types of major storms and to categorize morphological impacts over a hybrid coastal system. The study site, known as Punta Rasa, is located in the Samborombón bay in the outer part of the Río de La Plata estuary (Argentina) and corresponds to a zone of interaction between a large sandy spit and a backwash tidal flat system. Methods used combine statistical analysis of wave climate time-series, analysis of wave energy patterns through nearshore numerical modelling (SWAN) and comparison of pre- and post-storm morphological changes using satellite images derived indexes (e.g. NDWI).

Results allowed to characterise four types of major storms impacting the study area: High-Energy Storms (HES), defined by an average storm Hs below the 1% exceedance (> 2.6 m), Long-Lived Storms (LLS) represented by an exceedance of the 1% of Du (> 60 hours), Storm Groups (SG) in which storm frequency is less than 6 days and Northeastern moderate storms (NMS) defined by their eastern, onshore oriented direction. Under HES and NSM storms erosional areas are dominant over depositional (62.34%), which most of the system showing shoreline retreat and a growth of the end spit area. For LLS and SG storms the morphological impact varies alongshore with a general flattens of the end spit (showing a ‘rounded-shape’ morphology) and erosional hotspots over the southeastern coastal section.