The
swash zone is the upper part of the beach between backbeach and
surf zone, where intense erosion occurs during storms (Figure 2). The swash zone is alternately wet and dry.
Infiltration (hydrology) (above the
water table) and exfiltration (below the
water table) take place between the swash flow and the beach groundwater table. Beachface, berm, beach step and
beach cusps are the typical morphological features associated with swash motion.
Infiltration (hydrology) and
sediment transport by swash motion are important factors that govern the gradient of the beachface.
Beachface The beachface is the planar, relatively steep section of the beach profile that is subject to swash processes (Figure 2). The beachface extends from the berm to the low
tide level. The beachface is in dynamic equilibrium with swash action when the amount of
sediment transport by uprush and backwash are equal. If the beachface is flatter than the equilibrium gradient, more sediment is transported by the uprush to result in net onshore
sediment transport. If the beachface is steeper than the equilibrium gradient, the sediment transport is dominated by the backwash and this results in net offshore sediment transport. The equilibrium beachface gradient is governed by a complex interrelationship of factors such as the sediment size, permeability, and fall velocity in the swash zone as well as the wave height and the wave period. The beachface cannot be considered in isolation from the
surf zone to understand the morphological changes and equilibriums as they are strongly affected by the surf zone and shoaling wave processes as well as the swash zone processes.
Berm The berm is the relatively planar part of the swash zone where the accumulation of sediment occurs at the landward farthest of swash motion (Figure 2). The berm protects the backbeach and coastal dunes from waves but
erosion can occur under high energy conditions such as storms. The berm is more easily defined on gravel beaches and there can be multiple berms at different elevations. On sandy beaches in contrast, the gradient of backbeach, berm and beachface can be similar. The height of the berm is governed by the maximum elevation of
sediment transport during the uprush. The berm height can be predicted using the equation by Takeda and Sunamura (1982) Z_{\mathrm{berm}}=0.125H_{b}^{5/8}(gT^2)^{3/8} where H_{b} is the breaker height, g is gravity and T is the wave period.
Beach step The beach step is a submerged scarp at the base of the beachface (Figure 2). The beach steps generally comprise the coarsest material and the height can vary from several centimetres to over a metre. Beach steps form where the backwash interacts with the oncoming incident wave and generate vortex. Hughes and Cowell (1987) proposed the equation to predict the step height Z_{\mathrm{step}} Z_{\mathrm{step}}=\sqrt{H_{b}Tw_{s}}, where w_{s} is the sediment fall velocity. Step height increases with increasing wave (breaker) height (Z_{\mathrm{step}}), wave period (T) and sediment size.
Beach cusps The beach cusp is a crescent-shaped accumulation of
sand or
gravel surrounding a semicircular depression on a beach. They are formed by swash action and more common on gravel beaches than sand. The spacing of the cusps is related to the horizontal extent of the swash motion and can range from 10 cm to 50 m. Coarser sediments are found on the steep-gradient, seaward pointing ‘cusp horns’ (Figure 3). Currently there are two theories that provide an adequate explanation for the formation of the rhythmic beach cusps: standing
edge waves and
self-organization.
Standing edge wave model The standing edge wave theory, which was introduced by Guza and Inman (1975), suggests that swash is superimposed upon the motion of standing edge waves that travel alongshore. This produces a variation in swash height along the shore and consequently results in regular patterns of
erosion. The cusp embayments form at the eroding points and cusp horns occur at the edge wave nodes. The beach cusp spacing can be predicted using the sub-harmonic edge wave model \lambda = \frac{g}{\pi}T^2\tan(\beta), in which T is incident wave period and \tan{(\beta)} is beach gradient. This model only explains the initial formation of the cusps but not the continuing growth of the cusps. The amplitude of the edge wave reduces as the cusps grow, hence it is a self-limiting process.
Self-organization model The
self-organization theory was introduced by Werner and Fink (1993) and it suggests that
beach cusps form due to a combination of positive feedback that is operated by beach morphology and swash motion encouraging the topographic irregularity and negative feedback that discourages accretion or erosion on well-developed beach cusps. It is relatively recent that the computational resources and
sediment transport formulations became available to show that the stable and rhythmic morphological features can be produced by such feedback systems. The beach cusp spacing, based on the self-organization model, is proportional to the horizontal extent of the swash motion S using the equation \lambda = fS, where the constant of proportionality
f is
c. 1.5. ==Sediment transport==