Dynamics of a reflective beach with a low tide terrace

摘要

Many beaches are characterised by having steep upper slopes, flat low-tide terraces, and mixed or bimodal sediment distributions. This study compares the surf zone hydrodynamic and sediment transport processes of these two dynamically different regimes on the beach at Teignmouth (UK) as part of the COAST3D project. In-situ measurements were made using pressure transducers, electromagnetic current meters and optical backscatter sensors simultaneously on the steep part of the beach (slope 1/10, D-50 1.1 mm) and on the terrace (slope 1/100, D-50 0.25 mm) during a range of incident wave conditions (H-b 0.25-0.75 m).The steep upper beach was characterised by: plunging breakers, high wave height/water depth (gamma) values, large gravity band cross-shore velocity variance, a well-developed subharmonic, strong undertow, high suspended sediment concentrations, and sediment transport dominated by a strong mean offshore component.The shallow shelving terrace was characterised by spilling breakers, low gamma values, weak undertow, gravity velocity variance decreasing shoreward, infragravity cross-shore velocity variance increasing shoreward, and stronger longshore currents than undertow. Mean suspended sediment concentrations on the terrace were an order of magnitude less than on the steep beach. The oscillatory cross-shore components of transport were onshore in the incident wave band but increasingly dominated by the infragravity band in progressively shallower water. Longshore sediment transport rates on the terrace were large compared to cross-shore transport rates.The measurements suggest that the dynamics of the terrace/steep beach exhibit characteristics of separate dissipative/reflective sites. However, interaction between the different regions does occur. In particular, the low tide terrace acts to reduce the wave height that is capable of reaching the steep beach, thereby protecting it to some degree from erosion. (C) 2004 Elsevier Ltd. All rights reserved.