Comparison between the wintertime and summertime dynamics of the Misa River estuary

Abstract

The Misa River on the Italian Adriatic coast is typical of the rivers that drain the Apennine Mountain range. The focus of this study, conducted in the late summer of 2013 and mid-winter of 2014, was to contrast the general wintertime-summertime dynamics in the Misa River estuarine system rather than investigate specific dynamical features (e.g. offshore sediment transport, channel seiche, and flocculation mechanisms). Summertime conditions of the Misa River estuary are characterized by low freshwater discharge and net sediment deposition whereas, in the wintertime, the Misa River and estuary is characterized by high episodic freshwater discharge and net erosion and sediment export. Major observed differences between wintertime-summertime dynamics in the Misa River and estuary are a result of seasonal-scale differences in regional precipitation and forcing conditions driven largely by the duration and intensity of prevailing wind patterns that frequently change direction in summertime while keep almost constant directions for much longer periods in wintertime, thus generating major sea storms. Sediment deposition was observed in the final reach of the Misa River and estuary in the summertime. However, in the wintertime, large flood events led to sediment erosion and export in the final reach of the Misa River and estuary that, in conjunction with storm-wave-induced mud transport, led to sediment deposition at the river entrance and in the adjacent nearshore region. The seasonal cyclic pattern of erosion and deposition was confirmed with bathymetric surveys of the final reach of the estuarine region. A critical component for the balance between summertime deposition and wintertime erosion was the presence of an underlying mat of organic deposits that limited the availability of sediments for erosion in winter, when massive debris transport occurs. Further, suspended cohesive sediments flocs were subjected to smaller hydrodynamic stresses in the summertime favoring deposition within the estuary. Conversely, during wintertime storms, flocs were subjected to larger hydrodynamic stresses favoring breakup into smaller flocs and deposition outside the estuary.