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Swash, in geography, is known as a turbulent layer of water that washes up on the beach after an incoming wave has broken. The swash action can move beach materials up and down the beach, which results in the cross-shore sediment exchange.〔Whittow, J.B. 2000, The penguin dictionary for physical geography. Penguin Books, London〕 The time-scale of swash motion varies from seconds to minutes depending on the type of beach (see Figure 1 for beach types). Greater swash generally occurs on flatter beaches.〔Komar, P.D. 1998, Beach processes and sedimentation. Prentice-Hall, Englewood Cliffs〕 The swash motion plays the primary role in the formation of morphological features and their changes in the swash zone. The swash action also plays an important role as one of the instantaneous processes in wider coastal morphodynamics. There are two approaches that describe swash motions: (1) swash resulting from the collapse of high-frequency bores (''f''>0.05 Hz) on the beachface; and (2) swash characterised by standing, low-frequency (''f''<0.05 Hz) 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 εb (Guza & Inman 1975) : Where Hb is the breaker height, g is gravity, T is the incident-wave period and tan β is the beach gradient. Values εb>20 indicate dissipative conditions where swash is characterised by standing long-wave motion. Values εb<2.5 indicate reflective conditions where swash is dominated by wave bores.〔Wright, L.D. and Short, A.D. 1984, "Morphodynamic variability of surf zones and beaches: A synthesis". Marine Geology, 56, pp.93-118〕 ==Uprush and backwash== Swash consists of two phases: uprush (onshore flow) and backwash (offshore flow). Generally uprush velocities are greater but of shorter duration compared to the 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.〔〔Masselink, G. and Puleo, J.A. 2006, "Swash-zone morphodynamics". Continental Shelf Research, 26, pp.661-680〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「swash」の詳細全文を読む スポンサード リンク
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