Ocean Sci., 13, 719–734, 2017 https://doi.org/10.5194/os-13-719-2017 © Author(s) 2017. This work is distributed under the Creative Commons Attribution 3.0 License. Medium-term dynamics of a middle Adriatic barred beach Matteo Postacchini, Luciano Soldini, Carlo Lorenzoni, and Alessandro Mancinelli Department of Civil and Building Engineering, and Architecture (DICEA), Università Politecnica delle Marche, 60131 Ancona, Italy Correspondence to: Matteo Postacchini ([email protected]) Received: 25 December 2016 – Discussion started: 9 January 2017 Revised: 8 July 2017 – Accepted: 10 August 2017 – Published: 18 September 2017 Abstract. In recent years, attention has been paid to beach 1 Introduction protection by means of soft and hard defenses. Along the Italian coast of the Adriatic Sea, sandy beaches are the most common landscape feature and around 70 % of the Marche Our communities are experiencing a series of problems and region’s coast (central Adriatic) is protected by defense struc- difficulties related to the inundation risk in coastal areas, the tures. The longest free-from-obstacle nearshore area in the protection of nearshore regions, and the use of beaches for region includes the beach of Senigallia, frequently monitored tourist and recreational activities. In the few last decades, in the last decades and characterized by a multiple bar sys- increasing attention has been paid to short- and long-term tem, which represents a natural beach defense. The bathyme- predictions associated with climate change effects, which tries surveyed in 2006, 2010, 2011, 2012 and 2013 show will significantly impact on beaches and coastal areas (e.g., long-term stability, confirmed by a good adaptation of an an- see Houghton et al., 2010; Ranasinghe et al., 2013). In fact, alyzed stretch of the beach to the Dean-type equilibrium pro- such predictions are associated with both the mean sea-level file, though a strong short- to medium-term variability of the rise and the more frequent sea storms, occurring during both wave climate has been observed during the monitored peri- summer and winter. The understanding of the main physical ods. The medium-term dynamics of the beach, which deal processes driven by such changes is fundamental for (i) the with the evolution of submerged bars and are of the order modeling of the nearshore dynamics, including rapid mor- of years or seasons, have been related to the wave climate phological changes to the beach (Postacchini et al., 2016b), collected, during the analyzed temporal windows, by a wave (ii) the correct prediction of coastal flooding (Villatoro et al., buoy located about 40 km off Senigallia. An overall inter- 2014), (iii) the proper design of protection solutions (Loren- pretation of the hydrodynamics, sediment characteristics and zoni et al., 2016) and (iv) the correct analysis of future sce- seabed morphology suggests that the wave climate is fun- narios in the coastal area (see Benetazzo et al., 2012; Lionello damental for the morphodynamic changes of the beach in et al., 2012). the medium term. These medium-term time ranges during Several studies (e.g., Benavente et al., 2006; Walton and which waves mainly come from NNE/ESE are characterized Dean, 2007) showed that a proper representation of the lo- by a larger/smaller steepness and by a larger/smaller relative cal bathymetry is fundamental both to correctly predict the wave height, and seem to induce seaward/shoreward bar mi- seabed changes induced by wave/current forcing and to de- gration as well as bar smoothing/steepening. Moving south- sign efficient solutions for the coastal protection. Hence, typ- eastward, the bar dimension increases, while the equilibrium ical bedforms of unprotected sandy beaches should be taken profile shape suggests the adaptation to a decreasing sedi- into account. In particular, submerged subtidal bars usually ment size in the submerged beach. This is probably due to form on seabeds with slopes within 0:005–0:03 and their the presence of both the harbor jetty and river mouth north of height ranges from some centimeters to meters(Leont’ev, the investigated area. 2011). 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 Published by Copernicus Publications on behalf of the European Geosciences Union. 720 M. Postacchini et al.: Medium-term dynamics of a barred beach submerged bars can be ascribed to three different mecha- With the purpose to characterize the sandbar migration, nisms, i.e., wave breaking, infragravity waves and self ar- an important parameter has been recently introduced. This rangement (Wijnberg and Kroon, 2002), while the bar mi- is the local relative wave height, i.e., the ratio between local gration depends on several coastal processes and has been wave height H and water depth over the bar crest hcr. Values investigated in the field (e.g., Ruessink et al., 1998) numeri- smaller than ∼ 0:3 promote landward migration, while val- cally (Dubarbier et al., 2015) and through laboratory exper- ues larger than 0:6 promote seaward migration (Houser and iments (Alsina et al., 2016). It has been observed that the Greenwood, 2005). swash-zone slope, grain size and wave characteristics play an In particular, along the Dutch coast (Ruessink et al., important role. The influence of the slope on the bar dynam- 1998; Ruessink and Terwindt, 2000), a relative wave height ics has only been observed during laboratory experiments, Hs=hcr D 0:33 represented the onset of breaking, with Hs after an ad hoc manual reshaping of the swash zone (Baldock being the local significant height. Hence, Hs=hcr > 0:33 re- et al., 2007; Alsina et al., 2012). On the other hand, field ob- ferred to breaking intensification and undertow increase, servations confirmed that the grain size could be important leading to seaward bar migration. While Hs=hcr < 0:33 in- in bar migration rates due to the larger sediment transport in- dicated dominance of short waves and wave skewness, lead- duced by finer sands (Goulart and Calliari, 2013), while the ing to shoreward bar migration. The analysis of the veloc- wave characteristics are fundamental for the bar migration ity moments and sediment transport confirmed the correla- direction. In particular, wave breaking over the bars leads tion between medium-term wave conditions and short-term to the generation of a deep return flux, known as undertow, sediment transport measurements (Ruessink and Terwindt, which promotes a seaward motion. As an example, Gallagher 2000). et al.(1998) observed, near Duck (North Carolina), an inten- From a physical point of view, the increase of both Hs=hcr sified wave breaking occurring over the bar during storms, and breaking intensification produces an increase of the inducing a large undertow inshore of the bar that pushed breaking wave celerity (Postacchini and Brocchini, 2014), it seaward. Conversely, a shoreward bar migration was ob- leading to an intensification of the shoreward volume flux, served under small waves, during less energetic states (see hence to a wave setup (Soldini et al., 2009) and to the follow- also Goulart and Calliari, 2013). ing increase of the undertow velocity (Kuriyama and Nakat- While numerical simulations well reproduced the offshore sukasa, 2000). migration during severe conditions, some difficulties arose Only a few literature studies have been carried out to in- when reproducing the onshore bar motion during mild wave vestigate the seasonal and annual scale of the beach dynam- conditions (Gallagher et al., 1998; Plant et al., 2004), sug- ics (e.g., Ruggiero et al., 2009). Some field observations gesting that not all processes involved in the bar migration confirmed a cyclic behavior of multiple bars (Ruessink and were clearly understood and correctly simulated, e.g., lower- Terwindt, 2000; Goulart and Calliari, 2013), mainly charac- frequency waves. Further, Ruessink et al.(1998), who ana- terized by three stages, i.e., initial generation, seaward mi- lyzed the cross-shore sediment transport and morphological gration and final degradation. Conversely, other authors ob- changes occurring in the nearshore area of Terschelling (the served a continuous landward motion, until bar–shore weld- Netherlands), stated that the role of the infragravity waves ing, even during storm events (Aagaard et al., 2004). While has not been completely understood. In particular, it was the offshore migration is promoted by the undertow domi- clear that during energetic conditions the suspended load nance in the net transport balance, as already stated, the on- dominated over the bedload and the morphodynamics were shore migration is probably enhanced by storm surge: this controlled by undertow and, probably, infragravity waves. increases both skewness and phase coupling and reduces the The infragravity contribution, more important during break- undertow contribution. ing than during calm conditions, mobilizes large amounts of The present study describes the seabed evolution of a nat- sediment, which are advected offshore by the undertow. ural unprotected beach stretch of Senigallia (Marche region, The importance of infragravity waves is confirmed by Italy), a touristic town of the Italian middle Adriatic. The other authors and a detailed study about their influence on available bathymetries, covering the last decade, and the the bar dynamics was undertaken by (Aagaard et al., 1994) wave climate, enable us to analyze the medium-term mor- using field data collected at Stanhope Lane Beach (Canada). phological evolution of the beach, which is of the order of They stated that the sediment transport induced by infra- years or seasons and includes the geometry and migration of gravity waves may be either shoreward or seaward, and sus- the submerged bars, as a function of the wave forcing. To the pended sediments are mainly transported towards antinodes authors’ knowledge, this is the first study on the medium- in the water surface elevation.