Swash
Swash, or forewash in
There are two approaches that describe swash motions: (1) swash resulting from the collapse of high-frequency bores () on the beachface; and (2) swash characterised by standing, low-frequency () motions. Which type of swash motion prevails is dependent on the wave conditions and the beach morphology and this can be predicted by calculating the surf similarity parameter (Guza & Inman 1975):
in which is the breaker height, is gravity, is the incident-wave period and is the beach gradient. Values indicate dissipative conditions where swash is characterised by standing long-wave motion. Values indicate reflective conditions where swash is dominated by wave bores.[3]
Uprush and backwash
Swash consists of two phases: uprush (onshore flow) and backwash (offshore flow). Generally, uprush has higher velocity and shorter duration than backwash. Onshore velocities are at greatest at the start of the uprush and then decrease, whereas offshore velocities increase towards the end of the backwash. The direction of the uprush varies with the prevailing wind, whereas the backwash is always perpendicular to the coastline. This asymmetrical motion of swash can cause longshore drift as well as cross-shore sediment transport.[4][5]
Swash morphology
The swash zone is the upper part of the beach between backbeach and
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.[4][5]
Berm
The berm is the relatively planar[clarification needed] 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.[4] The berm height can be predicted using the equation by Takeda and Sunamura (1982)
where is the breaker height, is gravity and is the wave period.[clarification needed]
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
where is the sediment fall velocity. Step height increases with increasing wave (breaker) height (), wave period () and sediment size.[4]
Beach cusps
![](http://upload.wikimedia.org/wikipedia/commons/thumb/8/88/Undertow_in_Nantucket.jpg/280px-Undertow_in_Nantucket.jpg)
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.[4]
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
in which is incident wave period and 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.[4]
Self-organization model
The
where the constant of proportionality f is c. 1.5.
Sediment transport
Cross-shore sediment transport
The cross-shore sediment exchange, between the subaerial and sub-aqueous zones of the beach, is primarily provided by the swash motion.[6] The transport rates in the swash zone are much higher compared to the surf zone and suspended sediment concentrations can exceed 100 kg/m3 close to the bed.[4] The onshore and offshore sediment transport by swash thus plays a significant role in accretion and erosion of the beach.
There are fundamental differences in sediment transport between the uprush and backwash of the swash flow. The uprush, which is mainly dominated by bore turbulence, especially on steep beaches, generally suspend sediments to transport. Flow velocities, suspended sediment concentrations and suspended fluxes are at greatest at the start of the uprush when the turbulence is maximum. Then the turbulence dissipates towards the end of the onshore flow, settling the suspended sediment to the bed. In contrast, the backwash is dominated by the sheet flow and bedload sediment transport. The flow velocity increases towards the end of the backwash causing more bed-generated turbulence, which results in sediment transport near the bed. The direction of the net sediment transport (onshore or offshore) is largely governed by the beachface gradient.[5]
Longshore drift
Management
The swash zone is highly dynamic, accessible and susceptible to human activities. This zone can be very close to developed properties. It is said that at least 100 million people on the globe live within one meter of
Construction of
Understanding the sediment transport system in the swash zone is also vital for beach nourishment projects. Swash plays a significant role in transportation and distribution of the sand that is added to the beach. There have been failures in the past due to inadequate understanding.[9] Understanding and prediction of the sediment movements, both in the swash and surf zone, is vital for the nourishment project to succeed.
Example
The coastal management at Black Rock, on the north-east coast of Phillip Bay, Australia, provides a good example of a structural response to beach erosion which resulted in morphological changes in the swash zone. In the 1930s, a
Research
It is said that conduct of morphology research and field measurements in the swash zone is challenging since it is a shallow and aerated environment with rapid and unsteady swash flows.
See also
- Beach cusp
- Beach nourishment
- Coastal management
- Longshore drift
- Sea wall
- Sediment transport
References
Notes
- ^ Whittow, J. B. (2000). The Penguin Dictionary for Physical Geography. London: Penguin Books.
- ^ Prentice-Hall.
- .
- ^ a b c d e f g h i Masselink, G. and Hughes M.G. 2003, Introduction to coastal processes and geomorphology, Hodder Arnold, London
- ^ a b c d e f Masselink, G. and Puleo, J.A. 2006, "Swash-zone morphodynamics". Continental Shelf Research, 26, pp.661-680
- ^ Masselink, G. and Hughes, M. 1998, "Field investigation of sediment transport in the swash zone". Continental Shelf Research 18, pp.1179-1199
- ^ Zhang, K., Douglas, B.C. and Leatherman, S.P. 2004, "Global warming and coastal erosion". Climatic Change, 64, pp.41-58
- ^ Rae, E. 2010, "Coastal Erosion and Deposition" in Encyclopedia of Geography. Sage publications, 21 March 2011, <"Coastal Erosion and Deposition : SAGE Knowledge". Archived from the original on 2013-02-01. Retrieved 2011-05-04.>
- ^ a b c Bird, E.C.F. 1996, Beach management. John Wiley & Sons, Chichester
- ^ Blenkinsopp, C.E., Turner, I.L., Masselink, G., Russell, P.E. 2011, "Swash zone sediment fluxes: Field observations". Coastal Engineering, 58, pp.28-44
Other
- Guza, R.T. and Inman, D. 1975, "Edge waves and beach cusps". Journal of Geophysical Research, 80, pp. 2997–3012
- Hughes, M.G. and Cowell, P.J. 1987, "Adjustment of reflective beaches to waves". Journal of Coastal Research, 3, pp. 153–167
- Takeda, I. and Sunamura, T. 1982, "Formation and height of berms". Transactions, Japanese Geomorphological Union, 3, pp. 145–157
- Werner, B.T. and Fink, T.M. 1993. "Beach cusps as self-organized patterns". Science, 260, pp. 968–971