Some important issues concerning the present society deal with the inundation risk in the coastal areas, the protection of nearshore regions, the use of beaches for tourist and recreational activities. In the last decade, such topics have been of great importance due to short- and long-term predictions associated with the climate change, which is representing a significant threat for what concerns the flooding issue in the present and future years e.g., see. These changes are associated with both the mean sea-level rise and the more frequent sea storms, also occurring during the summer time. The understanding of the main physical processes driven by such changes is fundamental for (i) the modeling of the nearshore dynamics (e.g., in terms of rapid morphological changes of the beach), (ii) the correct prediction of coastal flooding, and (iii) the proper design of protection solutions e.g., see.

Several studies e.g., showed that a proper representation of the local bathymetry is fundamental to both correctly predict the seabed changes induced by wave/current forcing and support studies aimed at designing efficient solutions for the coastal protection. It has been demonstrated that the use of the equilibrium beach profile e.g., properly describes the long-term equilibrium of a natural beach, as demonstrated (i) by , who investigated the main features of the equilibrium beach profile at several cross-shore locations of a nourished beach and after a sea storm, and (ii) by , who numerically confirmed the feasibility of using an equilibrium beach profile to represent the coastal inundation/flooding of a natural barred beach. The equilibrium profile of a beach represents the balance between erosive and accretive forcing, and is represented by the law: h=Ayn, where h is the water depth and y the distance to shoreline. The features of the equilibrium profile depend on a dimensional shape parameter A and on the exponent, which is usually n=2/3 . proposed an empirical correlation between the parameter A, the median grain diameter d50 and the sediment fall velocity wf, while stated that an easier correlation between A and d50 could be properly used.

Recently, further models have been developed in order to account for: (i) the beach slope at the swash zone , (ii) the sediment size variation along the cross-shore profile , (iii) the seasonal changes of the profiles , (iv) the generation of submerged bars due to wave breaking . However, the application of such models, which better represent the real equilibrium profile of a beach, is often very difficult, because they require several field data, which are not always available. On the other hand, the simple model (Eq. ) can be easily used for prediction/design purposes and well represents the long-term natural beach profile . The estimate of A can be accomplished by applying, on both natural and theoretical profiles, either a least-square method or an integration of Eq. () up to a reference depth.

Submerged subtidal bars are typical of unprotected sandy beaches. They usually generate on bottom slopes within 0.005–0.03 and their height ranges between some centimeters to meters . In semi-protected and open coasts, two-dimensional longshore bars are quite common and have been extensively studied, though the complex mechanisms of generation and migration are not yet completely understood. Generation of submerged bars can be ascribed to three different mechanisms, i.e. wave breaking, infragravity waves and self arrangement . Further, the bar migration mainly depends on the ratio between wave height H and water depth over the bar crest hcr, with values smaller than ∼0.3 or larger than 0.6 promoting, respectively, landward or seaward migration . Field observations confirmed a cyclic behavior of multiple bars, mainly characterized by three stages, i.e. initial generation, seaward migration and final degradation . Conversely, other authors observed a continuous landward motion, until bar-shore welding, even during storm events . The offshore migration is probably promoted by the undertow dominance in the net transport balance (e.g., in the Northern Dutch coast), while the onshore migration is enhanced by storm surges, these both (i) increasing skewness and phase coupling, and (ii) reducing the undertow contribution, which depends on the relative wave height H/hcr .

The novelty of the paper is represented by the detailed analysis of the seabed evolution of a natural unprotected beach of the Italian Middle Adriatic, i.e. that of the touristic town of Senigallia (Marche Region, Italy). The available bathymetries, covering the last decade, and the wave climate, enable us to analyze in detail the medium-term morphological evolution of the beach, including the geometry and migration of the submerged bars, as function of the wave forcing. To the authors' knowledge, this is the first study which analyzes the medium-term beach evolution and bar migration occurring in the sandy beaches of the Adriatic Sea.

The manuscript is divided as follows. Section  illustrates the available data, while Sect.  describes the investigated site. Results are presented in Sect.  and discussed in Sect. .

The natural beach South of the harbor of Senigallia was characterized by a number of bathymetric surveys since the 80s. More recently, due to a specific requirement of the Marche Region, a detailed survey of the nearshore region of Senigallia was undertaken in June 2006, both North and South of the harbor, such areas being respectively characterized by a protected and an unprotected beach. The surveys interested the nearshore region up to a depth of 6m and a total length of 4.3km, most of which (∼3.9km) in the Southern natural unprotected beach (the final DTM is illustrated in Fig. a).

Between 2010 and 2013, after the modification of the harbor entrance, annual bathymetric surveys up to a depth of 6m were carried out by the municipality of Senigallia on a 2.5km-long area covering part of the protected and part of the unprotected beaches (example of cross-shore profiles are illustrated in Fig. c).

The analysis of all surveys enabled us to extract 18 cross-shore profiles which characterize the unprotected beach for about 1km. This is used for the analysis of the morphological changes induced by the wave climate throughout years, in terms of both bar migration and geometry.

From the analysis of both surveys and satellite data, the submerged bars remain for a stretch of ∼12km. Further, moving southeastward, the sediment size changes, with a transition from sand to gravel occurring ∼6km South of the harbor . Hence, the initially two-dimensional longshore bars of the investigated area get closer to the shoreline, thus switching to three-dimensional (Fig. d). However, the ∼1km-long area South of the harbor can be taken as representative of the sandy beaches characterizing the Middle Adriatic Sea and will be analyzed in the next sections.

The studied coast is part of the longest unprotected beach of the Marche Region, which extends from the Senigallia harbor to ∼3.5km North of the Esino River estuary, hence for a total length of ∼12km (Fig. d). The investigated site is characterized by a swash zone with slope in the range 1:30–1:40, an array of submerged bars in a water depth between 0 and 3m, and a mild slope of about 1:200 for depths larger than 3m. The emerged beach is characterized by fine (d50=0.125–0.25mm) and medium (d50=0.25–0.5mm) sands, while fine sand was found in the submerged part.

The wave climate in the investigated area was obtained from a waverider of the Italian wave measurement network (RON), located ∼23nm East-North-East of Senigallia. It worked between March 1999 and March 2006 and between December 2009 and November 2013, the data between 2006 and 2010 surveys thus missing. During the 11 years recordings, the waves mainly came from ESE and NNE (Fig. b), the main events hence induced by Bora (coming from NNE) and Levante-Scirocco (from ESE) winds. The wave frequency (empty area) is better distributed throughout the directions, with smooth peaks in correspondence of the main directions ESE and NNE, with respect to the wave energy (full area), characterized by sharper peaks.

The analysis of the beach morphology using the concept of the equilibrium beach profile leads to the estimate of both a fitting depth e.g., and the shape parameter A, which strictly depends on the median grain diameter d50. Using either the least-square approach or the continuity of volume, i.e. integration of Eq. (), the results are similar. From the DTM of Fig. a, referring to June 2006, 66 profiles have been extracted. It is important to notice that A, and similarly d50, decreases moving southward. The largest values occur close to the Senigallia harbor (profile 1 of Fig. ), i.e. A≅0.069 and, following , d50≅0.15mm, while the smallest occur ∼3.9km South of the harbor (profile 66), where A≅0.060 and d50≅0.13mm. Such values are in agreement with the fine sand characterizing the submerged beach . A good adaptation to the equilibrium beach profiles, with similar values of A, has also been found throughout the coast for the 2010–2013 surveys, from which 18 profiles have been extracted. Further, the fitting depth increases North and decreases South of the “Rotonda”, i.e. the pile-mounted permeable structure within profiles 11 and 12, this suggesting a sediment motion occurring at larger depths in correspondence of such a structure.

The following sections illustrate the analysis of the seabed variation using the available bathymetric surveys and wave climate. Both migration and geometry of the submerged bars are also discussed.

The bathymetric surveys of the area South of the Senigallia harbor enabled us to analyze a multiple-bar array typical of the sandy beaches of the Middle Adriatic. Such a part of the basin is subject to sea storms mainly due to NNE (Bora) and ESE (Levante-Scirocco) winds, which are characterized by significantly different surges.

The seabed-depth variation and the wave climate between contiguous surveys, as well as the bar features (height, width, location) analyzed for each survey, enabled us to couple the beach/bar dynamics with the wave forcing.

In the studied area the tidal excursion (∼40cm) is small and only subtidal bars exist. Since the analyzed beach slope ranges between 1:35∼0.03 (swash zone) and 1:200∼0.005 (offshore area), such bars fall into the group of two-dimensional longshore bars . Further, the wave energy in such a microtidal environment is quite high.

From the results illustrated in Sect. , the bar dynamics in this area may be influenced by either a breakpoint-related or an infragravity wave-related mechanism, while the self-organisational mechanism is negligible, as it can be observed from the partial bar destruction in 2011–2012 and the following re-generation in 2012–2013 (see Figs. c and c and d). Such a destructive nature of some storms in the short/medium term suggests that the bar dynamics characterizing the Adriatic sandy beaches are mainly governed by breakpoint mechanisms.

In the analyzed region and during the investigated time periods, the beach experienced many sea storms that enabled us to give an overall interpretation to the bar migration process as a function of the wave climate. During the time periods dominated by ESE forcing, waves are characterized by a reduced steepness Hm0/Lp0=0.213 (exactly the same in 2010–2011 and 2012–2013), while this is about 1/3 larger during the NNE-forcing-dominated period (Hm0/Lp0=0.316). Such a behavior is also confirmed if we do not account for the most energetic waves (see Sect. ), but directly estimate the most frequent combination (Hm0, Tp). Hence, an increase of the bar steepness Hbar/Wbar is associated to a decrease of Hm0/Lp0.

Further, steep NNE waves are associated with reduced storm surges, while gentle ESE waves are characterized by large surges, due to the larger fetch generating in the Adriatic Sea. As an example, two consecutive intense storms occurred in December 2010, the former coming from ESE, the latter from NNE, were characterized by maximum surges of, respectively, 80 and 43cm, measured at the Ancona harbor (data from Rete Mareografica Nazionale, ISPRA, http://www.mareografico.it). This leads to larger water depths over the crest hcr during ESE than during NNE waves. Hence, the relative wave height H/hcr is smaller during ESE waves and larger during NNE forcing, this probably promoting a shoreward migration of the bars in the former case, and a seaward migration in the latter case, which also causes a partial bar destruction. Further, waves coming from ESE are characterized by a significant longshore component, due to the large angle between the coast and the approaching wave fronts. Differently, waves coming from NNE reach the shore with an almost perpendicular incidence, this improving the intense smoothing of the bars.

While the correlation between bar width and height is clear only for some cases, the former increasing with the latter, the cross-shore bar area clearly increases moving southward, especially from the Senigallia harbor to the “Rotonda”, this disturbing the middle bar growth. A similar trend has also been found from inspection of the A parameter of the equilibrium profile, which suggests a slight decrease of the sand size moving southward, this being in agreement with the sediment-size distribution observed in 1989 and 1990 by . A physical explanation of such a sediment distribution is the presence of the harbor jetty, which induces a complex flow field, i.e. a mix of refraction, diffraction and reflection, that generates a cross sea e.g., see, especially during sea storms coming from ESE (Fig. a). Such a dynamics promotes a mobilization of finer sands and their transport far from the jetty.

Finally, the present work illustrates a preliminary analysis of the medium-term dynamics of an unprotected sandy barred beach of the Middle Adriatic Sea. A more detailed analysis can be achieved through use of either data collected by another waverider (e.g., that of Cesenatico, FC, which is ∼80km North of Senigallia) or a reconstructed climate e.g., to characterize the wave forcing in the period 2006–2010. Further, the dynamics of the nearshore area before, during and after storm events could also be inspected by means of novel devices like: Lagrangian drifters, able at measuring both three-dimensional hydrodynamics and seabed depth , or video-monitoring of the nearshore area, available since July–August 2015 in the framework of the EsCoSed Project .