Original paper

The development of subhorizontal shore platforms by waves and weathering in microtidal environments

Trenhaile, Alan S.


A mathematical wave and weathering model was used to study the formation of subhorizontal shore platforms in microtidal environments. Backwearing (horizontal erosion) rates by wave erosion were calculated for different tidal levels using basic wave equations. Downwearing (vertical erosion) rates by weathering and debris removal by waves at the same tidal levels were derived from long-term laboratory experiments and from field data. Model runs suggested that wave erosion was more important in the past, although weathering and debris removal is often dominant today. Wave erosion can produce subhorizontal platforms with low tide cliffs, although downwearing helps to reduce platform gradient and increase platform width. Weathering and debris removal may be able to produce narrow, gently sloping or possibly subhorizontal, platforms, up to several tens of metres in width, in argillaceous rocks that are particularly susceptible to tidal wetting and drying. Weathering along joints and other discontinuities can also promote wave quarrying, and this combination created the widest and most gently sloping platforms in the model runs, although the effect was moderated by correspondingly wider surf zones, which reduced the erosive ability of the waves.