1 CHAPTER 1 . INTRODUCTION Sydney Beaches and Indeed The
1 CHAPTER 1. INTRODUCTION Sydney Beaches and indeed the majority of beaches along the South East coast of Australia are subject to orthogonal rather than longshore wave-power and for this reason, sediment movement is predominantly onshore-offshore. Studies of wave-sediment interaction at Narrabeen (Short, in press) suggest that the beach system operates as a compartment with the seaward boundary marked by a textural discontinuity. A number of studies along the Sydney Coast, have described local sediment (Gibbons, Dee Why 1967), rip-currents (McKenzie, 1958), Beach erosion (Foster et al, Cronulla, 1963)» Bilgola, 1966, Avalon, 1967, Collaroy, 1968) and energy-sediment inter action (Cowell, Palm Beach, 1975), (Bryant, Palm Beach, 1976), (Lees, Cronulla, 1977), (Phillips, Dee Why - Long Reef, 1977). However, the submarine extent of the Sydney Beaches remains an unknown through essential factor in understanding beach s e dim en t budget s. Initially, this thesis was aimed at identifying the seaward extent of each beach system in terms of the null- point hypothesis and thus, the textural discontinuity. The null-point concept and the closely related equilibrium profile attempts to model the complex nearshore wave-sediment interaction. The seaward-fining textural gradient characteristic of the nearshore zone, (Swift et al, 1971) is a result of the relationship between average sediment particle size and the amount of wave energy penetrating the sea-bed. The null-point concept utilizes asymmetric wave ripples as the fundamental hydraulic mechanism for nearshore transport. For any given grain size, a line of no movement or null-line exists where gravity and fluid forces balance and the grains are in a state of oscillating equilibrium (Swift, 1970).
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