Circulation during Storms and Dynamics of Suspended Matter in a Sheltered Coastal Area
The Gulf of Gaeta, in the western margin of central Italy, is characterized by a coastal morphology that creates a natural sheltered area in which fine sediment settles. The new port regulatory plan provides for dock expansions and dredging works that could alter the suspended particulate matter (SPM) concentration. The present study investigates the dynamics of the Gulf of Gaeta with a focus on the dynamic processes that affect the fine particle concentration. The study was conducted through a
... conducted through a multidisciplinary approach that involves remote sensing acquisitions (satellite imagery and X-band radar), measurements in situ (water sampling, wave buoy, weather station, turbidity station, CTD profiles), and numerical modelling (SWAN and Delft3D FLOW). The X-band radar system supports the analysis of the dynamic processes of the SPM concentration providing a large dataset useful for the hydrodynamic model's validation. The analysis reveals a strong influence of nearby rivers in modulating the SPM at the regional scale. Short-term high and low fluctuations in SPM concentration within the gulf are triggered by the local effect of the main physical forces. In particular, the direction of events and bottom sediment resuspension play a key role in modulating the SPM concentration while micro-tidal regime does not appear to influence turbidity in the study area. This approach represents an important tool in improving the long-term coastal management strategy from the perspective of sustainable human activities in marine coastal ecosystems. Remote Sens. 2018, 10, 602 2 of 27 seas [5, 6] . Rapid land cover changes over wide areas into the watershed also impact water and sediment fluxes. This has caused a significant shift in ecosystem dynamics, and scientific evidence of the adverse effects of anthropogenic fluxes are demonstrated by, as an example, a continuous decline in seagrasses [7, 8] , which represents a biodiversity hotspot in the Mediterranean Sea  . The interactions between terrestrial inputs and circulation forcing was previously investigated in several coastal sites such as the South Atlantic Bight , Mobile Bay [11, 12] , the Columbia River Delta , the Patos Lagoon  and the Black Sea . Numerous studies in Mediterranean coastal areas focused on the dynamic of SPM discharged by the Rhône [16,17] and Po rivers , which are the main rivers of the Mediterranean Basin. Some studies have assessed coastal areas influenced by smaller rivers [19, 20] that, despite their low flows, can also have a high local impact because they rapidly bring terrestrial material to the sea. These studies are accomplished through field data analysis, remote sensing, and analytical and numerical model experiments. Studies conducted in the Mediterranean Sea primarily focus on the analysis of nutrients, organic matter and contaminants released; multidisciplinary approaches of the coastal processes that affect the distribution of suspended particulate matter are currently rare for the Italian coastal area. Remote sensors have enabled synoptic studies that monitor oceanic and atmospheric phenomena, such as marine pollution , algal blooms  and river plume dynamics  ; however, their application (in the case of satellite data acquired at visual or infrared spectral bands) is often limited to cloud-free days and, furthermore, to large-scale patterns due to their low spatial resolution [23, 24] . The radar systems within remote sensors allow measurements of the sea state and surface currents with accuracy and spatial resolution. The 'Remocean' system provides a surface current estimation using a normalized scalar product procedure developed by Serafino et al.  , which has given robust agreement with respect to in situ observations  and demonstrates an ability to resolve turbulent structures observed within the coastal boundary layer  . This work is the first to use an X-band radar system to analyze the hydrodynamic processes and the associated dynamics of suspended particulate matter. Sediment flux toward the sea is primarily driven by natural factors (geography, geomorphology, geology, and climate change)  and large-scale human activities such as dams, jetties, and other coastal constructions. These factors challenge natural processes and alter the sedimentary balance  . Previous studies demonstrated that the river plume dynamic is primarily controlled by the wind intensity and direction [29-32] and tidal action    . Together with terrestrial inputs, the resuspension of bottom sediments modulates the turbidity of coastal waters. In shallow, micro-tidal environments, the resuspension is primarily induced by surface gravity waves; however, in a sheltered coastal area, wind stress plays a fundamental role in bottom sediment resuspension [36, 37] . Marine phenomena, such as flooding and erosion during storm events, represent a natural source of damage to coastal systems  and human properties. In micro-tidal environments, such as the Mediterranean Sea, storm waves are the principal driver of short-term coastal erosion and flooding and are the main factor controlling the hydrodynamics that lead to the morphological structure of beaches. In the Mediterranean Sea, storms originate from the "sub-synoptic lows" triggered by the major North Atlantic synoptic systems entering the Mediterranean Sea  . The study area is located on the western margin of central Italy and focuses on a gulf with characteristic exposure to meteorological events. This morphological feature produces a naturally sheltered area that is characterized by the settling of fine sediments. Humans have long taken advantage of its natural morphology and have urbanized the surrounding land. There are many marine activities such as those associated with ports as well as fish and mussel farming. The new port regulatory plan provides for dock expansions and dredging works that could alter nearby suspended particulate matter (SPM) concentrations. This study is the first to investigate the dynamic of the Gulf of Gaeta with a focus on the dynamic processes that affect fine particle concentration within a coastal site. In the analyzed coastal area, the presence of a river delta is a natural source of siltation that stresses benthic communities  , and new coastal works may further enhance the pressure on benthic organisms.