Relationships between nearshore water circulation and sandy beach geomorphology at Durras, New South Wales

Abstract

Daily field surveys of the nearshore water circulation and inshore-foreshore morphology, along a 2.25 kilometre bay head beach at Durras on the South Coast of New South Wales, are reported. Three main elements of the nearshore water circulation pattern are discerned; primary and secondary rip catchments and rip catchment divides. Associations between nearshore water circulation and inshore-foreshore morphology are established by pairing the primary elements of circulation with inshore bar and foreshore characteristics. Relationships between the primary circulation and morphology generally are in accord with the observations of McKenzie (1958a), Sonu, et a l . 3 (1966), and Sonu (1972). Catchment divides most frequently occur over breaker zone bars, inshore shoals, low water shoreline and shoal edge projections. On the other hand, primary rips flow seaward from shoreline and shoal edge embayments, through channels breaching the breaker zone bar, and discharge beyond the breaker line. Secondary rips are sometimes associated with promontory-embayment morphology, in a manner similar to primary rips but at a smaller scale. This suggests that secondary cells may influence swash intensities and other beach face processes, as suggested for primary rips. However, secondary rips are seen here as accessory features, developing either as ephemeral cells within higher-energy primary catchments, or characteristically developing under declining energy conditions. The association between primary rips and shoreline embayments raises interesting problems. Several cases are recognised, which apparently form in different ways: first , by progradation of the embayment "horns", during low to moderate wave conditions; and second,by beach back-cutting within the embayments. Back-cutting may result from severe backwash erosion, or slope fai lu re. Since observations at Durras show slope failure to be more widespread than previously considered, the role of slope fai lure on sandy beaches is qualitatively explored.