Beach morphologies induced by breakwaters with different orientations

Geomorphology 239 (2015) 48–57

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Beach morphologies induced by breakwaters with different orientations Nancy L. Jackson a,⁎, Mitchell D. Harley b, Clara Armaroli b, Karl F. Nordstrom c a b c

Department of Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, NJ 07102, USA Dipartimento di Fisica e Scienze della Terra, Università di Ferrara, 44122 Ferrara, Italy Department of Marine and Coastal Science, Rutgers University, New Brunswick, NJ 07102, USA

a r t i c l e

i n f o

Article history: Received 6 November 2014 Received in revised form 9 March 2015 Accepted 10 March 2015 Available online 18 March 2015 Keywords: Gap orientation Morphology Northern Adriatic Sea Offshore breakwaters Salient

a b s t r a c t A desired outcome in the construction of a detached emerged breakwater is the formation of an accretionary salient in its lee to augment the beach, improve beach amenity and provide an additional buffer from storm waves. The extent to which this salient forms and its morphology are strongly controlled by the breakwater geometry with respect to the original shoreline, sediment availability, and local wave climate. The purpose of this paper is to identify how breakwater geometry and orientation of gaps between individual breakwaters alter the direction of waves entering the gaps and change the asymmetry of the salients. Four distinct breakwater sites along the Emilia-Romagna coastline in Northern Italy were chosen for a detailed field and desktop study comprising three-dimensional topographic and bathymetric surveys, sediment sampling, LiDAR flights and historical shoreline mapping. The orientations of the shorelines at these four sites range over 43°, resulting in different exposures to the dominant waves. The oblique orientations of the gaps between individual breakwater segments at three of the four sites effectively create a “gap window” between breakwaters favoring the exposure of short-period waves from the north and diminishing the effect of longer waves from the dominant east. Salients can be symmetrical despite an acute angle of approach of the dominant deep water waves where refraction is enhanced by offshore topography and breakwaters are parallel to the shore. Waves approaching normal to the gap window undergo less diffraction due to their shorter length relative to the gap window width and undergo less attenuation by breaking and bottom friction if they are locally generated and have short periods. Greater breaking-wave energy on the gap-facing slope of the salient can create shoreline and morphological asymmetry. The implication is that breakwater orientations can be designed or altered to selectively dampen or facilitate wave energy to enhance sediment transport in a desired direction, provided that breakwaters are not too far offshore and sediment availability is not restricted to affect salient formation. Adjusting exposure via gap orientation can create morphologies that cannot be inferred from process-dominant conditions. © 2015 Elsevier B.V. All rights reserved.

1. 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;

⁎ Corresponding author.

http://dx.doi.org/10.1016/j.geomorph.2015.03.010 0169-555X/© 2015 Elsevier B.V. All rights reserved.

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

N.L. Jackson et al. / Geomorphology 239 (2015) 48–57

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 Sylvester, 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

49

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 EmiliaRomagna, 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. 2. 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

Gap window

Shoreline orientation Breakwater Sand sample

Gap width and exposure zone through gap window Gap width and exposure zone normal to shore

Fig. 1. Differences in widths and locations of zones of direct exposure to waves passing between breakwaters during shore-parallel and gap-orthogonal wave approach. Locations of sediment samples gathered for this study are also depicted.

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N.L. Jackson et al. / Geomorphology 239 (2015) 48–57

B. Site locations

A. Regional setting

Po R. 0

100 km

Po Delta N

N Map to right

20 km

0

Italy

Adriatic Sea

Adriatic Sea Ravenna

Tyrrhenian Sea

Lido di Classe Lido di Savio Cesenatico wave buoy

Rivabella Misano Adriatico

C. Wave height

D. Wave period

Fig. 2. Regional setting (A) and study site locations (B). Wave heights (C) and periods (D) are at 10 m water depth from the Cesenatico waverider buoy for the time period 2007–2014.

by groins, breakwaters, and seawalls (Armaroli et al., 2009). Breakwaters in Emilia-Romagna were built from 1933 to 1980; beach nourishment began in the 1980s (Liberatore, 1992; Preti et al., 2009). Emerged and submerged breakwaters extend at distances from the shoreline ranging from less than 10 m to over 300 m. The emerged breakwaters typically are rubble-mound structures, with elevations of 0.5–1.5 m above mean sea level and crest widths of approximately 4–8 m. The breakwaters are typically about 100 m long with gap widths of 30 m. The breakwaters are aligned parallel or at an angle to the shoreline trend (Liberatore, 1992). Tides are microtidal with a neap range of 0.3–0.4 m and mean spring range of 0.8–0.9 m. Significant wave heights less than 0.51 m occur 41% of the time but most storms can generate waves with significant wave heights of 2.5 m (Armaroli et al., 2012). The 1-yr storm return interval wave height is 3.3 m (Armaroli et al., 2009). The coastline is vulnerable to high surge levels associated with southeast storms (Armaroli et al., 2013). Surge events with even a relatively high probability of occurrence (1-in-10 year return period) can reach elevations around 1 m above mean sea level (Masina and Ciavola, 2011), enough to cause widespread inundation of the lowlying coastal hinterland and increase beach erosion caused by storm waves (Perini et al., 2011). The major axis of the Adriatic Sea is oriented northwest/southeast with a length of 800 km and a mean width of approximately 180 km.

The four study sites are located in the part of the Adriatic Sea (Fig. 2) where water depths are relatively shallow, with average depths of about 30 m (Russo and Artegiani, 1996). Waves from north to east directions are fetch-limited, while the southeast direction has a longer fetch. Waves from the north are generated across the shortest fetch because of sheltering by the Po River Delta (Fig. 1), and they have low mean heights (Hs = 0.29 m) and short mean periods (2.9 s), resulting in little modification by shoaling before breaking on the foreshore. Waves from the southeast are generally low, but winds from this direction generate significant surge levels in the northern part of the Adriatic Sea. Dominant wave approach is from the east (Fig. 2), with more frequent northeasterly waves occurring during the winter and easterly waves during the fall (Ciavola et al., 2007). The approach of these waves is approximately shore-normal at Lido di Classe and Lido di Savio, but is acute to the shoreline at Rivabella and Misano Adriatico. Refraction alters wave approach to a more shore-parallel configuration, especially the long-period waves. Net longshore sediment transport is to the north along most of the region, with some local reversals south of the mouths of streams and major shore-perpendicular protection structures (Preti et al., 2009). Lido di Classe (Fig. 3a) is immediately to the north of the Savio River. The breakwater system has ten structures that were built in the 1970s along 1.4 km of shore. The northern end of the emerged breakwater

N.L. Jackson et al. / Geomorphology 239 (2015) 48–57

A. Lido di Classe

C. Misano Adriatico

51

B. Lido di Savio

D. Rivabella

Fig. 3. Shoreline characteristics of study sites, showing shoreline change (1976–1978, 1982 and 2005) and relationships of the shoreline to breakwaters. Aerial photographs are from 1982. Dominant waves are from the east. Dashed lines depict locations of cross-shore profiles in Fig. 7.

system is bounded by a short groin and submerged breakwater. The southern end is bounded by a long jetty at the Savio River that extends seaward of the southernmost breakwater. Planform amplitude of the salients in the lee of the breakwater system is low relative to Misano Adriatico and Rivabella (Fig. 3c, d). Sediment deposition had occurred in the lee of the structures by 1982 (Fig. 3a). Lido di Savio, immediately to the south of the Savio River, has fifteen breakwaters that were built in the 1970s and extend for 2.1 km alongshore (Fig. 3b). Breakwater extensions were built in 1982 on the southern ends of each structure and oriented southeast, increasing their lengths and decreasing the widths of the gaps between them. The northern and southern ends of the breakwater system are bounded by jetties that do not extend as far seaward as the breakwaters. Salients are smaller than at Lido di Classe. The breakwater system at Misano Adriatico is 0.70 km long and comprises seven breakwaters built between 1978 and 1982 (Fig. 3c). The three northernmost segments were notched after 1982. No other structures bound the north end of the breakwater system. The southern

end of the breakwater system is bounded by a groin. The planforms of the salients have maintained their orientation and increased in size between 1982 and 2005. The breakwater system at Rivabella (Fig. 3d) is 1.15 km long and contains nine structures built between 1954 and 1978 (Fig. 3d). The northern and southern ends of these breakwaters are bounded by other breakwaters that were built with different orientations. The salients at Rivabella are offset a greater distance alongshore from the center of each structure than occurs at other sites. Sediment was added to three of the sites in recent beach nourishment operations, including 4700 m3 emplaced at Lido di Savio in 2010; 20,500 m3 emplaced at Lido di Classe in the northern area in 2002; and 14,000 m3 emplaced at Misano Adriatico (in the area protected by breakwaters) in 2009. Rivabella was used as borrow area (28,400 m3 mostly in 2011) to nourish other coastal tracts (L. Perini, Regional Geological Survey, personal communication). No data are available on where these sediments were placed (location alongshore and subaerial or subaqueous nourishment) but usually the sand is placed

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N.L. Jackson et al. / Geomorphology 239 (2015) 48–57

from the subaerial beach down to the intertidal area. These volumes are small relative to the quantity of sediment on the intertidal profile of each of the study sites.

thus only apply to the upper beach, but they do provide insight to salient development over a longer time interval. All LiDAR data are in WGS84, UTM33; the elevation reference level is the mean sea level in Genoa.

3. Methods 4. Results 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 different top elevations that allowed for evaluation of the effects of breakwater height on salient characteristics. The fourth and fifth breakwaters at Misano Adriatico were selected because they were not notched, unlike the ones just north of them. Topographic surveys were undertaken on the subaerial beach and at wading depths (extending down to approximately 1.6 m below mean sea level) on one day at each site (10–11 April 2014 at Classe and Savio and 15–16 May 2014 at Misano Adriatico and Rivabella) using two Trimble PPK-GPS receivers operating in tandem, each (following post-processing) with a vertical accuracy of approximately +/− 0.03 m. Readings were taken by walking contours in continuous-sampling mode. These post-processed data points were then used to form a digital terrain model in order to determine morphology and orientations of salients and bays and slopes of foreshores and shallow subtidal zones. Representative sediment samples were taken on the upper foreshore in the middle of the bays and salients (Fig. 1). All sediment samples were taken to 30 mm depth on the upper foreshore 2 m seaward of the upper swash limit of the previous high tide. Sediments were sieved mechanically at 0.5 ϕ intervals. Mean and sorting were calculated using inclusive graphic measures (Folk, 1974). The orientations and dimensions of breakwaters and gaps were measured from September 2011 images using the measuring tool in Google Earth. Shoreline orientation was defined by a line through the average position of the apexes of all embayments within the boundaries of the breakwater system at each site (Fig. 1). The orientations of most breakwaters did not correspond to shoreline orientation, measured along mean high water at the apexes of the bays. Accordingly, gap width was measured along the orientation of the gap as well as along the orientation of the shoreline (Fig. 1). Exposure ratios based on the definition by Chasten et al. (1993) were calculated perpendicular to the shoreline and used to compare the potential for wave reworking landward of structures built to different length and spacing. Data on distance from the initial shoreline (1976–1978) to the breakwater and distance from the salient apex to the breakwater were measured from 1982, 1991, 2000 and 2005 images available from the regional government. These data were used to assess the likelihood for salient formation based on Hsu and Silvester (1990). Cross-shore bathymetric profiles gathered by the regional government using a multibeam echosounder in 2012 were used to evaluate the effects of offshore bathymetry in diminishing wave energy reaching the breakwaters. These data have a spatial resolution of 2 m and a vertical resolution of b 4 cm. LiDAR data gathered by the regional government on 10 March 2009 and immediately following a major storm event on 9–10 March 2010 at Lido di Classe were used to characterize three-dimensional morphology behind the two breakwater segments. These data have a spatial resolution of 1 m and a vertical resolution of 20 cm. Changes in morphology were assessed by comparing these two LiDAR datasets with the more recent topographic surveys gathered in April and May 2014. The LiDAR data are confined to elevations above mean sea level and

4.1. 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 the orientation of the shoreline and the breakwaters, with the breakwaters orientated to waves from the east. The greatest departure in orientation of the breakwaters from shoreline orientation, and therefore the greatest width on the azimuth of the gap, is at Rivabella (Table 1). Breakwaters are shortest at Misano Adriatico and exposure ratios are relatively large. Elevations vary between sites and individual breakwaters, with the highest and lowest breakwaters at Rivabella. The orientations of the gaps between all breakwater segments except Misano Adriatico result in a wider opening perpendicular to the gap azimuth than normal to the shoreline (Table 1). Thus, the gap window at the three northern sites enhances the effect of waves from the north and diminishes the effect of waves from the dominant direction (Fig. 2). The combined effect of breakwater angle and gap angle at Rivabella accentuates exposure to the north. The seaward extensions on the southern ends of the breakwaters at Lido di Savio (Fig. 3) diminished the width of the opening to the north over that which occurred when the breakwaters were originally built. 4.2. Shoreline characteristics The shorelines in 1982 and 2005 (Fig. 3) have salients at the high water elevation at all four sites. The shoreline behind the breakwaters at Lido di Classe has accreted within the mid-section of the breakwater field, and this region is where the best formed salients are located (Fig. 3). Lido di Savio has the lowest variability in shoreline planform through time and the lowest planform amplitude of the salients. The breakwater extensions that decreased gap width in 1982 do not appear to have contributed to salient growth on the upper foreshore, although subaqueous platforms occur in the lee of the structures. At Rivabella, salient development was initiated at the south end in 1982, and additional salients formed landward of breakwaters to the north between the 1990s and 2005 (Fig. 3). Permanent tombolos developed relatively quickly at Misano Adriatico and have maintained their nearsymmetrical shape. Fig. 4 presents data on the ratio of the distance from the initial shoreline (S) and the distance from the apex of the salient to the breakwater (X) for the two salients at each site. The data are normalized by the breakwater length (B) for the time periods 1982, 1991, 2000 and 2005 and the model of Hsu and Silvester (1990) for estimating the position of the apex relative to the breakwater. The data reveal the progressive growth of the salients over time at Lido di Classe, Misano Adriatico and Rivabella but not at Lido di Savio. The model of Hsu and Silvester (1990) produces values for distance of the apex from the breakwater (X) that are larger than field observations, except Lido di Savio where salient growth is less than predicted. Topographic surveys from 2014 at all sites (Fig. 5) reveal that the best developed and highest salients are at Misano Adriatico, where the breakwaters were built closest to the initial shoreline and are most parallel to the shoreline. The southern breakwater at Rivabella is lower than the northern breakwater, and the salient is less well developed there.

N.L. Jackson et al. / Geomorphology 239 (2015) 48–57

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Table 1 Breakwater characteristics measured from Google Earth image 13 September 2011. Breakwater elevations were obtained from LiDAR data (2010). Breakwater number

Lido di Classe 4 Lido di Classe 5 Lido di Savio 6 Lido di Savio 7 Rivabella 4 Rivabella 5 Misano Adriatico 4 Misano Adriatico 5 a b c

Shore az.

Breakwater

Deg.

Az. deg.

164 164 168 168 131 131 125 125

169 169 183 183 152 152 126 126

Elev. ma

Length m 100 100 120 118 101 101 75 81

0.9 0.8 1.1 0.9 1.1 0.4 0.9 0.8

Gap azimuth

Gap width on azimuth

Gap width normal

Exposure ratio shore normal on azimuth

Nb deg. 156 159 142 152 90 98 119 114

Nb m 29 21 26 29 33 27 26 26

Nb

Sc

Nb

Sc

Nb

Sc

27 20 20 27 24 24 26 26

20 22 27 23 24 30 26 23

0.22 0.18 0.15 0.19 0.20 0.20 0.26 0.24

0.17 0.18 0.19 0.16 0.20 0.24 0.26 0.22

0.23 0.17 0.21 0.21 0.38 0.30 0.26 0.24

0.18 0.19 0.21 0.21 0.30 0.41 0.26 0.22

Sc deg. 159 150 152 150 98 90 114 124

Sc m 21 23 29 26 27 42 26 23

Elevation (freeboard) relative to mean sea level. North of breakwater. South side of breakwater.

Fig. 6 indicates the alongshore variability (in the north-south direction) in bed elevation at each site, derived from a transect between the shoreline and breakwaters. This transect is defined for each site as being a third of the distance between the shoreline and the structure and parallel to the orientation of each breakwater series (Fig. 5, dashed lines). In terms of bed elevation, the figure indicates that the maxima in bed elevation at Misano Adriatico coincide precisely with the center of each breakwater segment, meaning that they are symmetrical despite the acute angle of approach of the dominant waves. The least symmetrical salients are at Rivabella, where the gap orientation creates maximum exposure to the north, opposite the direction of dominant waves. Here the maxima in bed elevation are offset to the south by up to 20 m, away from the widest openings to the sea. The alongshore bed elevation at Rivabella also indicates swash bars on the more exposed northern sides of the salients (Fig. 6), while the southern sides are steeper and lacking swash bar morphology. The breakwaters and gaps at Lido di Classe are nearly parallel to the shoreline trend (Table 1) and the widest openings are more closely aligned with the approach of the dominant waves. The southern breakwater at Lido di Classe has a slight skew to the south (Fig. 6), perhaps because shoreline orientation results in a greater influence of the dominant easterly 1.6

1.2

Lido di Classe

X/B

Lido di Savio 0.8

1982 Rivabella

0.4

1991

waves compared to the less frequent northerly waves. At Rivabella the easterly waves are almost entirely blocked by the breakwaters. Cross-shore beach profiles taken between the breakwater gaps at the four sites in 2012 (Fig. 7) reveal that slope values offshore of the breakwaters differ by less than a degree. The elevations of the bars at Rivabella and Misano Adriatico are higher than at the other two sites. The higher bars may provide greater potential for dissipating wave energies, especially for longer period waves from the dominant directions. 4.3. Storm effects The morphology of the subaerial portion of the shore at Lido di Classe in 2010 (Fig. 8) reveals the effects of the 9–10 March 2010 storm. Significant wave heights reached a maximum of 3.9 m and significant period of 10 s and approached from east-northeast; water levels peaked at 0.93 m (Armaroli et al., 2013). Storm waves appear to have eroded the beach above mean sea level and smoothed the shapes of the salients at that elevation, while offshore deposition contributed to seaward growth of the salient platform (see the 0 m contour line indicated in Fig. 8). The salients grew between 2010 and 2014 and became more clearly developed on the upper foreshore. The salients are higher in elevation in 2014 and somewhat more skewed to the north. The skew is subdued relative to the skew at Rivabella, presumably reflecting the reduced exposure to the north because of the narrower gap width along the widest azimuth (Table 1) and increased exposure to the dominant easterly waves. At Rivabella, exposure to the easterly waves is minimized and exposure to the northerly waves is dominant. Sediment samples differ little within each breakwater system (Table 2). Sediments from Lido di Savio are finer than at the other breakwater systems, even Lido di Classe, which is nearby. The differences in these two systems could be due to inlet effects or beach fill practices. In any case, grain size differences do not appear to be a useful diagnostic in differentiating the foreshores in bays from salients or protected beaches from nearby unprotected beaches.

2000 Misano Adriatico

0 0

5. Discussion

2005

0.5

1

1.5

2

S/B Lido di Classe Lido di Savio Rivabella Misano Adriatico

X (mean)observed 119 123 39 9

X (Hsu and Silvester, 1990)

127 86 63 23

Fig. 4. Relationship between breakwater length (B), distance from initial shoreline to the breakwater (S), and distance from the salient apex to the breakwater (X) at the study sites for the time periods 1982, 1991, 2000 and 2005.

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 subaqueous platform is likely initially formed by the circulation cells that arise from wave diffraction processes due to the presence of the breakwater, but storms also play a role in their development as seen in the growth of the platform at Lido di Classe after the 2010 storm. Fairley et al. (2008) documented a similar response under

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N.L. Jackson et al. / Geomorphology 239 (2015) 48–57

Fig. 5. Contour plots (at 10 cm increments) from topographic surveys of the study sites taken in April/May 2014. All surveys are referenced to a local datum corresponding to the shoreline orientation. Alongshore transects parallel to breakwater series (dashed lines) represent location of data depicted in Fig. 6.

shore-normal storm waves, with erosion of the upper intertidal zone and accretion in the sheltered region behind the breakwaters. The Emilia-Romagna coastline is microtidal with low profile elevations above mean sea level such that even a small increase in water levels during storms results in considerable beach change. The data from Lido di Classe also reveal the reformation and slight increase in the size of the salient four years after a large storm. Breakwater length relative to distance from the initial shoreline, which defines surf zone width, is a key factor in determining the existence of a salient and distance of the apex of a salient from the structure (Rosen and Vajda, 1982; Ahrens and Cox, 1990; Hsu and Silvester, 1990). Apexes can be closer to the structure if: (1) the breakwaters are long; (2) they are built closer to the shore initially; (3) the beach landward of the breakwaters is nourished; (4) sediment moves into the compartment from alongshore; or (5) erosion of the upper beach contributes to salient platform construction. Mean salient apex position at all sites was closer to breakwaters than predicted (Hsu and Silvester, 1990) except at Lido di Savio where sediment availability was too limited to compensate for the greater distance of the structures offshore.

Beach nourishment, commonly used with protection structures, would likely hasten salient growth (Dolphin et al., 2012). Black and Andrews (2001a, 2001b) found salient size on natural shorelines to be underestimated at their sites in the lee of islands and reef systems and concluded that wave angle distribution played a key role in salient formation. The relation of Hsu and Silvester (1990) represents an equilibrium condition under unidirectional wave approach, but in natural systems these relationships change through time as wave conditions and sediment volumes within the system change. Even so, the predicted values for X fall within the range of values depicted in Fig. 4. Of interest for this study is the insight Rivabella and Misano Adriatico provide of the way breakwaters built in configurations other than shore-parallel can alter the effect of waves passing through the gaps and influence salient morphology. Salients become more asymmetrical when: (1) the angle between each breakwater and the shoreline departs from parallel; or (2) one end of a breakwater is adjusted to create a wider window for wave entry. It is also likely that salients landward of shore-parallel breakwaters will be asymmetrical if most wave energy comes from a single direction at an angle to the shore

Fig. 6. Alongshore variability in bed elevation for each site derived from the April/May 2014 topographic surveys. Transect locations are depicted in Fig. 5. Solid circles indicate the midpoint location of each breakwater segment.

N.L. Jackson et al. / Geomorphology 239 (2015) 48–57

Elevation (m)

Lido di Classe

55

Lido di Savio

1

1

-1

-1

-3

-3

-5

-5 1

101 201 301 401 501 601 701

1

101 201 301 401 501 601 701 801

Rivabella

Misano Adriatico 1

1

-1

-1

-3

-3

-5

-5 1

101

201

301

401

501

601

1

101

201

301

401

501

601

701

Distance (m) Fig. 7. Cross-shore profiles within one embayment at each breakwater system (locations depicted by dashed lines in Fig. 3).

and the gap is large enough to allow for passage of waves with little directional modification. Changing the wave-energy window to alter direction of transport within a breakwater field can be used to adjust the sediment budget, even as it changes the symmetry of the salient and bays. The asymmetry of salients and bays, in turn, provides alternative recreational opportunities by creating more varied conditions on the upper beach and nearshore and changing the esthetics and use of the beach. The gentle foreshore slopes and flatter terraces on the more exposed side of the salient may be preferable for wading by small children, whereas taller people may prefer the deeper sheltered side. The asymmetry of depths in the bays may cause safety to be somewhat more of an issue, because of deeper water closer to the shore on the sheltered side. The implication is that breakwater systems can be designed to create salient morphologies that are skewed opposite to the net longshore

transport direction. Wave direction is critical to the skew of a salient and the orientation of the apex. When waves approach oblique to the shoreline, the apex will shift downdrift (Dally and Pope, 1986). The embayments between the salients at Misano Adriatico are nearly parabolic despite the oblique angle of approach of offshore waves. The symmetry of the salients suggests that refraction of waves prior to reaching the breakwaters makes the approach angle less oblique, and dissipation of waves on the nearshore bar (which is lower at Lido di Classe and nonexistent at Lido di Savio) reduces their energy prior to entry through the gaps. The offshore slope and bar height at Rivabella are similar to Misano Adriatico and should be as effective in orientating approach of dominant waves to more shore parallel and reducing wave energy, but the apexes of the salients at Rivabella are positioned to the south of the breakwater midpoints, demonstrating the increased importance of waves from the

Fig. 8. Topographic changes at Lido di Classe through time taken from 2009 and 2010 LiDAR data from the region and 2014 topographic survey (in Fig. 5). All surveys are referenced to a local datum corresponding to the shoreline orientation. The feature low on the foreshore on the southern salient in 2014 is caused by a drift log.

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N.L. Jackson et al. / Geomorphology 239 (2015) 48–57

Acknowledgments

Table 2 Sediment characteristics on the beachface. Lido di Classe

Breakwater 4

Breakwater 5

Bay N

Horn

Bay S

Horn

Bay S

0.20 0.56

0.22 0.81

0.25 0.84

0.18 0.46

0.23 0.92

Bay N

Horn

Bay S

Horn

Bay S

Mz mm σΙ ϕ

0.16 0.52

0.15 0.33

0.16 0.38

0.15 0.44

0.17 0.69

Rivabella

Breakwater 4

Mz mm σΙ ϕ Lido di Savio

Mz mm σΙ ϕ Misano

Mz mm σΙ ϕ

Breakwater 6

Breakwater 7

Breakwater 5

Bay N

Horn

Bay S

Horn

Bay S

0.21 0.52

0.21 0.55

0.18 0.46

0.21 0.52

0.19 0.37

Bay N

Horn

Bay S

Horn

Bay S

0.18 0.38

0.18 0.46

0.21 0.46

0.16 0.42

0.23 0.37

Breakwater 4

Breakwater 5

north. Locally-generated waves from the north are lower in height and shorter in period than the dominant waves from the east. Waves approaching normal to the gap window undergo less diffraction due to their shorter length relative to the gap window width (Pope and Dean, 1986). Northerly waves at Rivabella would also undergo less attenuation by wave breaking and bottom friction due to their short wave periods. These factors would result in greater breaking-wave energy on the north-facing slope of the salients at Rivabella and morphology such as swash bars that are prevalent on more-exposed natural coastlines. The significance of the orientation of the gap window on shoreline change in the lee of breakwaters supports the observation of Dally and Pope (1986) that breakwater orientations can be used to selectively dampen or facilitate wave energy to enhance sediment transport in a desired direction. Our results suggest that there is likely a threshold on the gap azimuth that ultimately negates the effects of the dominant waves, thus increasing the dominance of waves normal to the gap window. Presumably, altering existing breakwaters to change the exposure through the gap window could be done as an adaptive measure postconstruction. The breakwater extensions on the southern end of structures at Lido di Savio indicate how the gap window can be adjusted this way, but a lack of sediment in the beach system prevents any significant effect on salient morphology.

6. 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 window width and they can undergo less attenuation by breaking and bottom friction if they are locally generated and have short periods. Greater breaking-wave energy on the gap-facing slope of the salient can create a shoreline asymmetry. The implication is that breakwater orientations can be designed or altered to selectively dampen or facilitate wave energy to enhance sediment transport in a desired direction, provided that they are not too far offshore to affect salient formation. Adjusting exposure via gap orientation can create morphologies that cannot be inferred from process-dominant conditions.

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