Introduction Figure 1. Definition sketch for the nearshore littoral zone swash zone width exaggerated. After [1]. The swash zone forms the land-ocean boundary at the landward edge of the surf zone, where waves runup the beach face figures 1, 2.

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Introduction Figure 1. Definition sketch for the nearshore littoral zone swash zone width exaggerated. After [1]. The swash zone forms the land-ocean boundary at the landward edge of the surf zone, where waves runup the beach face figures 1, 2.

It is perhaps the region of the ocean most actively used by recreational beach users and, being very visible, is the region of the littoral zone most associated with beach erosion and the impacts of climate change. The landward edge of the swash zone is highly variable in terms of geomorphology, and may terminate in dunes, cliffs, marshes, ephemeral estuaries and a wide variety of sand, gravel, rock or coral barriers.

This influences the exchange of sediment between the land and ocean, which ultimately forms the coastline. In terms of coastal processes and coastal protection, a large part of the littoral sediment transport occurs in the swash zone, both cross-shore and longshore, which influences beach morphology, and beach erosion and beach recovery during and after storms.

Wave runup is an important factor in the design of coastal protection and also generates hazards for beach users, and is the dominant process leading to the erosion of coastal dunes. Swash hydrodynamics also influence the ecology of the intertidal zone and groundwater levels in sub-aerial littoral beaches and low lying islands, which is often critical for freshwater water supply on islands and atolls [2]. Characteristics of the swash zone Figure 2a. Photo shows conditions after a swash rundown, with only small bores reaching the swash zone.

Figure 2b. Photo shows the inner surf zone and a bore reaching the swash zone in the background and a swash uprush reaching the top of a beach berm in the foreground. Other important distinctions are that friction becomes more important in controlling aspects of the shallow flow in the swash zone than in the surf zone, and that turbulence and sediment transport in the swash zone is generated locally in the swash zone and advected into the swash zone from the surf zone.

A key feature of both the hydrodynamics and sediment dynamics in the swash zone is intermittency, where the extent and degree of inundation of the swash zone varies over different timescales, from orders of seconds and tidal periods to years and decades.

This is a challenge for coastal scientists, both in terms of measurement and modelling of the physical processes. For the purposes of this summary, the swash zone will be considered as the region of the beach face exposed to the atmosphere over wind, swell and infragravity wave durations, i.

The characteristics of the swash zone hydrodynamics and sediment transport are governed by the inner surf zone and the underlying beach, with feedback of course between the morphology and hydrodynamic processes. The beach slope is a controlling parameter [3].

On dissipative beaches, with wide surf zones, most of the wind wave and swell energy is dissipated seaward of the swash zone. Therefore, swash processes are dominated by those due to long, or infragravity waves , which are frequently non-breaking standing waves figure 2a.

On intermediate and reflective beaches, short wave energy reaches the beach face in the form of bores or shore-breaks, which collapse at the beach, initiating a runup motion characterised by a thin sheet of water with a rapidly propagating wave tip which is analogous to a dam-break flow over a dry bed figure 2b.

This sheet of water is slowed by gravity and friction until the flow reverses and forms another shallow flow seaward, the backwash. On coarse grained sand and gravel beaches a significant volume of the uprush and some of the backwash may percolate into the beach, reducing the volume of water in the surface backwash flow.

These two distinct types of swash zone make modelling hydrodynamic processes difficult, since parametric models rely on similarity of processes, and therefore phase resolving models of the whole surf zone, or at least the inner surf zone, are required if the details of the hydrodynamics are required.

Fortunately some processes are modelled very well by parametric models, perhaps more accurately than phase-resolving models, particularly wave runup. Wave runup and overtopping Figure 3. Typical pattern of bore-driven swash oscillations vertical component. Red and green squares indicate maxima and minima of individual swash events. Wave runup is perhaps the most important aspect of swash zone flows. While the motion of the water volume as a whole may be considered as runup, conventionally wave runup refers to the landward limit of the swash motion on the beach face, usually defined vertically above the ocean level.

The runup and rundown of the shoreline is referred to as the swash excursion or oscillation. A typical plot of the shoreline motion is shown in figure 3. Remarkably, the maximum runup is still most reliably described by a simple empirical parametric formula, which is perhaps one of the oldest regularly in use, proposed by Hunt Despite numerous variations, the underlying scaling still holds over a very large range of wave conditions, both in the laboratory and field.

This is strictly only applicable for monochromatic waves and the maximum runup. However, the same formulation has been widely used to describe random wave runup, using appropriate statistical parameters to describe the wave conditions. Atkinson et al. Figure 4. Swash energy spectra from a reflective, b intermediate, and c dissipative beach-states. From Hughes et al. Swash-swash interactions occur through the overtaking of a swash uprush by the following bore or during the collision of the backwash flow with the next uprush.

The magnitude, or vertical excursion, of the swash oscillations, from rundown position to runup, is strongly influenced by interaction between wave uprush and backwash, with the period of the incident waves also controlling the period of the swash oscillations at swell and wind wave frequencies [8]. Hence, given a finite time for the uprush and backwash to occur, there is a finite magnitude for a swash oscillation at a given frequency on a given beach slope if the motion is solely controlled by gravity.

This leads to swash saturation, where an increase in incident wave height does not increase the magnitude of the swash oscillations. This can be parameterised for individual events, or through a spectral representation.

Huntley et al. Figure 5. Source: Adapted from Donnelly, Kraus, and Larson [12]. When the runup exceeds the elevation of the crest of a structure, beach berm or dunes, wave overtopping or wave overwash occurs. This process is very important in building beach berms higher, in association with the spring-neap tidal cycle, but also leads to coastal flooding and inundation of the backshore region.

The geomorphology of barrier islands and gravel barriers is strongly dependent on swash overtopping, and breaching of these systems by landward transport of sediment during the overtopping can lead to rapid and potentially catastrophic failure of protective coastal barriers figure 5.

The response of coastlines to sea level rise is also be influenced by swash overtopping and sediment overwash, which increases recession in comparison to the classical Bruun Rule [13]. A combination of parametric modelling and numerical techniques is required to model these scenario [14] Swash zone hydrodynamics Figure 6.

Forward solid and backward dashed characteristic curves a , and contours of flow velocity b , surface elevation c and depth d for swash initiated by a near uniform bore in the non-dimensional coordinates of Peregrine and Williams [15].

From [16] , with permission. Swash zone flows have several features that differ from those in the surf zone, but this is dependent on the dominant wave conditions in the inner surf zone as noted above. Many key characteristics again depend on the Iribarren number [17]. For infragravity standing long waves, the variation in flow depth and flow velocity at a point is relatively symmetrical during uprush and backwash.

Short wave bores generate more asymmetrical flows, with the maximum velocity occurring at the start of inundation, with a rapid rise to the maximum depth, with an almost linear deceleration to flow reversal, and a correspondingly similar uniform acceleration in the backwash, at least until the flow becomes very shallow, when friction retards the flow significantly.

A key aspect of these flows is diverging flow, which means the swash lens thins rapidly. The runup durations are typically shorter than the backwash duration, and the backwash depths are shallower than during the uprush, and therefore the velocity moments tend to be skewed offshore, which has important implications for the sediment dynamics [18]. The asymmetry is however affected by the mass and momentum advected into the swash zone, which depends on the flow in the inner surf zone.

Self-similar solutions for different boundary conditions are presented by Guard and Baldock [16] , following the work of Peregrine and Williams [15] , figure 6.

These indicate the fundamental nature of the hydrodynamics, which comprise of a near parabolic motion of the shoreline due to gravity being the dominant process and a saw-tooth shaped variation in velocity with time, which decreases at a near linear rate from the peak velocity, which occurs as the shoreline passes a given location.

The water surface slope dips seaward for nearly the whole swash cycle, i. This results in a key difference between the surf zone and swash zone bed boundary conditions, namely that there is generally little phase lag between velocity and the bed shear stress in the swash zone, i.

Close to the time of flow reversal, the flow near the bed does however reverse prior to the flow higher in the water column, due to the adverse pressure gradient during the uprush [19]. The boundary layer is thinnest at the seaward edge of the swash zone during uprush, and grows following the flow up the beach. The boundary layer largely vanishes at flow reversal, and again grows from the bed as the flow recedes.

Accounting for such processes in sediment transport models remains to be tackled. Figure 7. From Deng et al. There are several sources of turbulence in the swash zone. In the runup, turbulence is advected from the inner surf zone, which combines with further generation of turbulence at the bed. The boundary layer is evolving, generally increasing in thickness and may become depth limited.

During the backwash turbulence generation occurs mainly at the bed, with swash-swash interactions generating further turbulence as the next wave arrives. Overall, the high turbulence near the bed leads to high bed shear stresses and the potential for high concentrations of suspended sediment transport [21].

Friction plays a large role in controlling the shoreline motion, with friction factors based on conventional fluid mechanics principles typically in the range [math]0. Friction effects are strongest when the flow is shallowest, at the swash tip, reducing runup excursions, and late in the backwash, so the shoreline recedes more slowly. Simple models, comprising of a ballistic motion plus friction, describe the shoreline motion reasonably well, although details are missing [22] [23].

Inclusion of friction effects in the internal flow requires numerical modelling at present, or the use of integral models which can avoid the uncertainty in the treatment of friction at the shoreline [24].

However, recent results suggest that the effects of friction on the internal flow are small compared to the effects at the swash tip. For example, figure 7 shows a method of characteristics solution for the swash flow with and without friction. The numerical results indicate that the two solutions are similar when water is present, which is interesting given the significant effect of friction on the location of the shoreline. The reason is the supercritical nature of the flow, which is a particular feature of the swash zone.

This means that the large change in the shoreline position due to friction does not significantly affect the flow seaward of the shoreline until the flow becomes subcritical, which does not occur until late in the uprush.

Similarly, the supercritical nature of the backwash flow means that the changes to the shoreline position further landward cannot significantly affect flow further seaward. Thus, the supercritical nature of the swash flow means that the significant changes in shoreline position do not significantly affect the flow in the interior of the swash lens.

Sediment transport mechanics Figure 8. A turbulent bore containing entrained suspended sediment just prior to reaching the swash zone.

The sediment is then advected into the swash zone during the runup. The end of a supercritical backwash flow is visible in the right of the image. The posts are 1m apart and the orange stringlines are horizontal.

Source: Adapted from Hughes, Aagaard, and Baldock [25]. Cross-shore sediment transport in the swash zone generally occurs as a combination of bed load under sheet flow conditions, with a flat bed, plus an additional component of suspended load, generated locally and advected into the swash zone by surf zone bores.

For the bed load, the Meyer-Peter and Muller [26] formulation, or derivatives, generally perform well with calibration, i. In these models, the transport is typically a function of the velocity cubed see for a more detailed discussion the article Sand transport.


Swash zone dynamics

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Handbook of beach and shoreface morphodynamics



Handbook of Beach and Shoreface Morphodynamics


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