Despite their wiranging use as cost-effective and sustainable forms of coastal de-fence (e.g., Johnson, 1987; Aminti et al., 2003), relatively little research has been directed at understanding detailed morphodynamic processes on gravel beaches in comparison to their sandy counterparts (Mason and Coates, 2001; Buscombe and Masselink, 2006), and in particular the processes that determine their morphody-namic response to storms (Poate et al., 2013). In general, however, gravel beaches are known to differ from sandy beaches in terms of dominant hydrodynamic pro-cesses, groundwater effects, sediment transport modes and morphodynamic response (Buscombe and Masselink, 2006).
Gravel beaches typically have a steep beach face, with a slope in the order of 1:20 – 1:5, and tend to fall in the reflective domain (Carter and Orford, 1984) of the morpho-logical beach classification of Wright and Short (1984). As such, these beaches are characterised by an unsaturated and narrow surf zone with plunging waves breaking close to the shoreline and hence a high spatial concentration of wave energy dis-sipation on the beach face. Compared to dissipative sandy beaches, the surf zone of a gravel beach has limited capacity for the generation of infragravity wave en-ergy through the shoaling and release of bound long waves (Longuet-Higgins and Stewart, 1962, 1964; Battjes et al., 2004), or the time-varying breakpoint mechanism (Symonds et al., 1982), and its surf and swash zones are consequently dominated by incident-band motions (e.g., wind-wave or swell-wave band motions; cf., Wright et al., 1979; Miles and Russell, 2004), although zero-order subharmonic edge-waves may be generated at the shoreline under certain conditions (e.g., Huntley and Bowen, 1975; Guza and Inman, 1975). Furthermore, whereas the swash zone on sandy beaches is almost always saturated (i.e., increasing the incident wave height does not lead to increased swash variance; cf. Brocchini and Baldock, 2008), the swash zone on gravel beaches is unsaturated during long-period wave conditions on mild-sloping beaches and during most wave conditions on steep-mild-sloping beaches (Figure
4 6 8 10 12 14 16 18
Deep water significant wave height (m)
Unsaturated swash zone regimes
Figure 2.1:Approximate regimes in which gravel beach swash zones are unsat-urated, based on the swash period relation of Brocchini and Bal-dock (2008), where swash saturation is estimated to occur when
H
gT2(tan βb)2
14
& 0.4, and H is the offshore wave height, g is the grav-itational constant, T is the offshore wave period and βb is the angle of the beach slope. The dark, medium and light grey shaded areas cor-respond to unsaturated swash zone conditions on gravel beaches with a beach slope of 1:20, 1:10 and 1:5 respectively (note that these areas overlap in the lower-right area). The parameter space outside the shaded area corresponds to saturated swash conditions. Deep water peak wave steepness values are displayed as dotted contour lines for comparative purposes.
2.1), implying that the swash zone on gravel beaches cannot necessarily be assumed to be saturated during storm events. This observation is relevant to the dynamics of gravel beaches during storms, since the degree of swash saturation determines how much energy in the swash zone will increase, or stagnate with increased offshore wave forcing during storms, and the phasing of swash-swash interactions is thought to control the morphodynamic response of the swash and surf zone (e.g., Kemp, 1960; Kirk, 1970).
Due to their grain size and large interstitial pores, gravel beaches are relatively per-meable compared to sandy beaches with hydraulic conductivity values typically in the range of 1 · 10−3 – 1 · 100 ms-1 (e.g., Bear, 1972). Infiltration and exfiltration of surface water through the beach face has long been known to affect the
morpho-dynamic response of permeable beaches through the first-order effect of reducing the backwash volume (e.g., Bagnold, 1940; Grant, 1948; Carter and Orford, 1993), as well as through second-order effects in the form of vertical pressure gradients, resulting in the modification of the effective weight and mobility of particles in the bed (e.g., Martin and Aral, 1971; Nielsen, 1992), and ventilation of the bed bound-ary layer, resulting in altered bed shear stresses (e.g., Martin, 1970; Oldenziel and Brink, 1974; Conley and Inman, 1994; Nielsen, 1997). Masselink and Li (2001) found through numerical model investigation that the first-order effect of infiltration (i.e., reduction of the backwash volume) only significantly altered the morphody-namic response of beaches with hydraulic conductivity greater than 1 · 10−1 ms-1, which they equated to a median grain diameter of 1.5 mm. Butt et al. (2001) invest-igated the second-order effects of groundwater exchange and found that the result of the two opposing processes (e.g., vertical pressure gradients and boundary layer ventilation) was net onshore-directed sediment transport for grain sizes greater than approximately 0.5 mm, and net offshore-directed sediment transport for smaller grain sizes. Field, laboratory and numerical investigations have confirmed the importance of groundwater processes on gravel beaches (e.g., Austin and Masselink, 2005; Horn and Li, 2006) and have further highlighted the importance of the thickness of the groundwater aquifer (e.g., Powell, 1990) and groundwater level fluctuations (e.g., Austin and Masselink, 2006a; Masselink and Turner, 2012) on the morphodynamics of gravel beaches.
The spatial concentration of incident wave breaking on the beach face of gravel beaches means that the critical threshold for sediment transport is almost always exceeded (cf. Buscombe and Masselink, 2006). However, in contrast to sandy beaches, sediment transport on gravel beaches is almost entirely composed of bed load and sheet flow transport and grain saltation (Carter and Orford, 1993; Isla and Bujalesky, 1993), and suspended transport is negligible1due to the high fall velocity
1Note that visual observations made at Loe Bar and Chesil Beach during this research appear to suggest that some gravel may be entrained in the water column at the location of wave breaking during energetic conditions. However, to the author’s knowledge, no measurements of such sediment
of the particles. The variation in grain size on gravel beaches is generally several orders greater than that found on sandy beaches (Buscombe and Masselink, 2006), which allows the spatial and temporal variation in the uprush and downwash transport capacity to generate differentiated patterns of grain sizes on the beach face, where larger than average grains are found at the step and berm (e.g., Austin, 2005). These sedimentary patterns can become self-regulatory through positive feedback mechan-isms related to particle interactions (e.g., particle acceptance and rejection; cf. Moss, 1962, 1963) and permeability (e.g., increased or decreased infiltration rates) and are a function not only of the grain size, but also of the shape of the grains (e.g., Bluck, 1967; Williams and Caldwell, 1988; Isla, 1993). The importance of such interactions is captured in the concept of morpho-sedimentary dynamics (cf. Carter and Orford, 1993; Buscombe and Masselink, 2006), where sediment heterogeneity is accepted as a fundamental and driving component of the morphodynamics of gravel beaches.
Gravel beaches are characterised by the presence of a step, a steep-faced, submerged feature at the base of the foreshore (e.g., Kirk, 1970). The morphology of the step is known to respond to the nearshore hydrodynamic forcing conditions by increasing in height under increased wave forcing (Hughes and Cowell, 1987) and by migrating across the cross-shore profile in step with the tide level (e.g., Masselink et al., 2010;
Austin and Masselink, 2006a; Almeida et al., 2015). The step strongly controls the location of wave breaking by presenting an abrupt change in water depth (Hughes and Cowell, 1987) and strongly influences breaker-zone and swash-zone dynamics (Masselink et al., 2010; Poate et al., 2013; Almeida et al., 2015), and is therefore often described as the equivalent of a break-point bar on sandy beaches (cf. Austin and Masselink, 2006a; Austin and Buscombe, 2008). The step is thought to help maintain the reflective state of the beach by creating a sediment convergence point close to the shore and thereby limit the potential for the formation of a dissipative and mild beach slope (cf. Hughes and Cowell, 1987; Buscombe and Masselink, 2006;
Austin and Buscombe, 2008).
concentrations have been attempted.
In a similar manner to the step, the berm is considered one of the mechanisms through which gravel beaches present their reflective nature (e.g., Austin, 2005). A berm characterises a marked change in slope on the beach from a steep, seaward-facing slope, to a flat terrace at the start of the backshore. Berms are thought to be cre-ated by asymmetric swash sediment transport, leading to sediment convergence in the upper swash (cf. Bagnold, 1940; Grant, 1948; Duncan, 1964; Carter and Orford, 1993) and therefore their morphology is determined both by the still water level and the incident waves. Gravel beaches may display multiple berms at varying eleva-tions, relating to tidal high water conditions and storm wave conditions (Jennings and Shulmeister, 2002; Austin and Masselink, 2006a; Masselink et al., 2010). On gravel beaches the interaction between surface water and groundwater (i.e., infilt-ration) plays a particularly important role in the generation of berms (Austin and Masselink, 2005, 2006b).
Due to its limitation of being a 1D cross-shore profile model, the model to be de-veloped in this research will not be able to simulate the three-dimensional cusp horns and bays common to many swash-aligned gravel beaches (e.g., Huntley and Bowen, 1975 , and many others). An analysis of the theories of the generation of cusps on gravel beaches is therefore omitted from this section.