Beach morphologies induced by breakwaters with different orientations
Introduction
Coastal landforms tend to orientate themselves according to the dominant processes acting on them. Aeolian dunes have well defined stoss and lee slopes based on dominant wind directions. Beaches on irregularly shaped shorelines, where longshore transport is impeded, align themselves perpendicular to the dominant direction of wave approach (Woodroffe, 2003, Cooper, 2013). Human actions can alter morphology, size and location via direct manipulation of landforms. Engineering structures can shelter or expose a shoreline to waves from specific directions, alter processes and sediment interaction, and produce landforms that would not evolve in the absence of the structure.
Many countries continue to rely on engineering structures (groins, seawalls, revetments and breakwaters) for shore protection, either alone or in conjunction with beach nourishment (van Rijn, 2011, Nordstrom, 2014). The success of these structures is generally evaluated by the volume of sediment retained on the beach profile or the planform configuration of the shoreline. Emerged detached rubble mound breakwater systems are a common form of shore protection in Europe (Lamberti et al., 2005, Anfuso et al., 2011, Dolphin et al., 2012, Araujo et al., 2014), Japan (Uda, 1988, Sane et al., 2007) and are also found in the USA (Chasten et al., 1993, Hardaway and Gunn, 2010). The low elevation of many of these breakwaters relative to mean sea level makes them a preferred alternative for shorelines where tourism demand is high and the development of shoreline salients in their lee increases the width of the recreational platform (Saengsupavanch, 2013).
Detached breakwater systems are designed to reflect, dissipate, refract and diffract waves, resulting in lower energy conditions in the lee of the structures, thereby restricting storm damage and long-term erosion and increasing the longevity of beach fills. How a beach responds in the lee of a detached breakwater system is a function of design and placement. Length of the structure, gap width, and the orientation between segments determine the direction and magnitude of wave energy entering the gaps (Dally and Pope, 1986). Permeability and elevation of the structure determine wave transmission through and over the structure (Dean et al., 1997, Cappietti et al., 2013). Distance of the structure from the shoreline determines the amount of open water within the lee over which wave energy is transmitted (Suh and Dalrymple, 1987). Incident wave angle relative to structure orientation determines wave refraction and diffraction around the structure, and sediment availability determines the ability of the shore to achieve a stable equilibrium condition (Chasten et al., 1993). The general shape of the shoreline landward of breakwaters is highly dependent on the directional nature of the wave climate. Salients that form landward of breakwaters tend to align themselves to the dominant wave direction, but if the dominant waves are oblique to the shore, the apex of the salient will shift in the downdrift direction (CERC 1984). The preceding studies indicate that changes in the shape or orientation of the breakwaters can alter the distribution of wave energy. The focus of our investigation is to explore how adjusting the orientation and width of the gap between breakwaters can alter wave exposure and the resultant morphology in the lee of the structure, specifically how adjusting exposure via gap orientation can create morphologies that cannot be inferred from dominant wave conditions.
The classic shoreline response to a breakwater is development of an accreting salient in the lee of the structure, becoming a tombolo when the salient reaches the breakwater. The tombolo may be termed tidal if the salient is not attached at high tide. Tombolos may develop in the lee of some but not all breakwaters. Because of the temporal nature of these features and the possible confusion in use of terms to compare different breakwater systems, we elect to use the term salient for general discussion of all shoreline protrusions caused by breakwaters. Empirical parameters derived from laboratory and field data from low wave energy environments have been useful in the design of breakwaters and anticipating shoreline morphology in the lee of structures that are parallel to the shore and perpendicular to wave approach (Rosen and Vajda, 1982, Dally and Pope, 1986, Hsu and Silvester, 1990, Ming and Chiew, 2000). The length of the breakwater (B) and the distance from the structure to the shoreline (S) are often considered the most important parameters that determine whether a salient forms. As the length of the structure increases relative to the distance from the initial shoreline, the apex of the salient (X) will move closer to the breakwater. Hsu and Sylvester (1990) quantified this relation (X/B = 0.6748(B/S)1.2148) based on both field and laboratory data. Where more than one breakwater is built, the gap width relative to the wavelength will influence the level of wave energy dissipation in the lee of the structures (Pope and Dean, 1986), with decreasing gap width resulting in increasing sediment volume accumulation (Harris and Herbich, 1986, Bowman and Pranzini, 2003).
Case study assessments of breakwater systems using models based on wave conditions, breakwater design and placement (i.e. Dally and Pope, 1986, Suh and Dalrymple, 1987, Ahrens and Cox, 1990) reveal that not all breakwaters have a predicted depositional salient in their lee. This departure is attributed, in part, to local scale conditions such as a pronounced longshore sediment transport rate (Bowman and Pranzini, 2003), reduction in longshore drift by growth of updrift tombolos (Thomalla and Vincent, 2004, Sane et al., 2007, Dolphin et al., 2012), broad wave directional spectrum and large tidal range (Thomalla and Vincent, 2003).
The ability to design breakwaters as non-parallel (offset) structures to selectively dampen or facilitate wave energy from one direction to enhance sediment transport in another direction has practical application in the distribution of sediment resources along the shore (Chasten et al., 1993). Salients tend to align with the predominant shallow water wave direction, and their shape is further controlled by wave diffraction patterns. Many breakwater systems identified in the literature are aligned parallel to the initial shoreline or to the predominant wave approach (Dally and Pope, 1986, Bricio et al., 2008). Where wave approach is normal to the initial shoreline, salient formation is near symmetrical, and the apex of the salient is generally near the center of the breakwater (Chasten et al., 1993). Highly oblique waves can alter the morphology of the salient and skew the beach planform and apex downdrift (Chasten et al., 1993) or cause salients to migrate downdrift during storms (Dolphin et al., 2005) resulting in erosion on the updrift side and deposition on the downdrift side (Fairley et al., 2009). Shorelines can be exposed to waves from more than one direction that may shift on a seasonal basis. It is possible to align breakwaters and gaps to specific wave directions to influence the direction of sediment transport landward of them and enhance or suppress salient development (Dally and Pope, 1986). A non-skewed salient results when waves approach parallel to the shoreline and the breakwater, resulting in a diffraction pattern that creates a shoreline with salients and bays that are everywhere parallel to diffracted wave crests (Chasten et al., 1993). The orientation of the gap can alter this relationship. A wider gap relative to breakwater length can increase the length of exposed shoreline. Chasten et al. (1993) quantify this phenomenon using an exposure ratio, which is the width of the gap divided by the sum of the gap width and breakwater length. This exposure ratio can be increased in one alongshore direction and decreased in the opposite direction if breakwaters are constructed en echelon, with the long axis of each structure at an angle to the shoreline but with the breakwater series parallel to the shoreline trend (Fig. 1). The departure in orientation from shore-normal can create a “gap window” favoring exposure to waves coming from one side of the breakwater series. An issue is how the morphology of the shore landward of the breakwaters is influenced by the dominant direction of wave approach or the orientation of the gap window that provides a selective filter on waves passing it.
We identify how gap orientation affects salient asymmetry using data from four sites (Lido di Classe, Lido di Savio, Rivabella and Misano Adriatico) on the shoreline of the Adriatic Sea in the Region of Emilia-Romagna, Italy (Fig. 2). The sites were selected because they contain breakwaters with different orientations, elevations, gap widths, and distances from the shoreline but within a region where wave processes are similar. The data indicate how the morphology and orientations of the salients are not necessarily determined by the direction of dominant waves within the region but by the way waves from different directions are modified by the orientation of the structures and gaps between them.
Section snippets
Study area
The Emilia-Romagna coastline is 130 km long and consists of low sandy beaches fronting developed shorefront resorts or undeveloped parklands. Much of the shoreline is eroding because of reduced sediment discharge from the Po River and its tributaries, stabilization of the river mouths with groins and jetties, and regional subsidence, which is reported as a maximum of 1.7 cm per year at the coast near Ravenna (Preti et al., 2009). Fifty-seven percent of the coastline is modified by groins,
Methods
Two salients and their adjacent bays were selected near the middle of each breakwater system for field investigation. The fourth and fifth breakwaters (from the north end) at Lido di Classe were selected because they had the best developed salients in the series and were far from end effects. The sixth and seventh breakwaters (from the north end) at Lido di Savio were selected because they were readily accessible. The fourth and fifth breakwaters at Rivabella were selected because they had
Characteristics of breakwaters
The orientations of the shorelines at the four sites (Table 1) range over 43°, resulting in different exposures to the dominant waves (Fig. 2). The individual breakwaters are most nearly parallel to the shore at Misano Adriatico but are most acute to the dominant wave direction from the east. Breakwater and shoreline orientations at Lido di Classe are similar to each other and orientated perpendicular to waves form the east-northeast. At Lido di Savio there is a small difference (15°) between
Discussion
The development of salients in the lee of breakwaters has been described in terms of stages, including (1) initial development of circulation cells; (2) a transitional deposition stage when salients form; and (3) a stable equilibrium stage with temporary adjustments to changes in wave energy (Rosen and Vajda, 1982). Shoaling can occur behind the breakwater and result in the development of an accretionary platform that is distinct from salient formation (Dally and Pope, 1986). The apex of the
Conclusions
Salients can be symmetrical despite an acute angle of approach of the dominant deep-water wave approach where diffraction is enhanced by offshore topography and breakwaters are parallel to the shore. Departure in orientation of breakwaters from shore-normal can create a “gap window” between breakwaters favoring exposure to waves coming from one side of the breakwater series. Waves approaching normal to the gap window undergo less diffraction due to their shorter length relative to the gap
Acknowledgments
This research was supported by the National Geographic Society, Research and Exploration Program under Grant No.9372-13 and the Italian–American Fulbright Commission. The authors would like to thank Gabriela Cabral da Rocha Weiss and Edoardo Grottoli for their assistance with the GPS surveys and Dott.ssa Luisa Perini of the Geological Service of the Emilia-Romagna Region for the information provided.
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