Summary

Research is in progress to determine the nature and pattern of the degradation processes in an actively eroding stretch of the Barton Clay cliffs in the Naish Farm area of Highcliffe, Hampshire. In response to loss of beach material the rate of recession has accelerated and is currently averaging 1.9 m/year, resulting in steepening of the overall cliff angle from an average of 13° in 1947 to a maximum recorded average of 19° in 1976. The presence of three preferred bedding plane shear surfaces at various elevations within the cliff produce a benched type of cliff profile, comparable with that seen in the Gault and Lias Clay controlled cliffs of Fairy Dell, Dorset but in contrast with the published descriptions of London Clay cliffs.

The degradation processes include scarp slumping, spalling (including toppling and soil falls), bench sliding (involving movement of colluvium over a preferred bedding plane), debris sliding (including movement of screes over clay scarps), mud sliding (i.e., the traditional lobate ‘mudflows’), mud runs (true flows), stream (or gully) erosion and man-related processes. Regular surveying and monitoring using a variety of techniques and having to surmount difficult field conditions, has begun to elucidate the characteristics, rates and inter-relationships of these processes. A flow chart representing the systematic transfer of soil from the in situ state via various colluvial modes (including the three bench levels) en route to the sea and an accompanying colluvial soil budget for the year July 1981 to July 1982 has been drawn up. It is shown that bench sliding is by far the most significant process in terms of volume of colluvium moved through the under-cliff, accounting for 930f the total volume of colluvium contained in the area. Mud and debris sliding, although important processes, are of relatively minor significance.

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