The hydrology of the degrading soil cliffs at Naish Farm, Hampshire

The purpose of the study was to increase understanding of the inter-relationship between hydrology and mass movements in an area of actively degrading soil cliffs. An undefended section of the Barton Clay cliffs of Christchurch Bay was used as the exemplar. Similar studies of such areas have been neglected in the past. It is considered that many of the difficulties encountered, techniques used, and ideas developed could usefully be employed in other similar areas.The cliffs in the study area are composed of Plateau Gravel overlying the Barton Clay. The field studies included a survey of the gravel thickness, meteorological measurements, piezometric observations in the Plateau Gravel, Barton Clay and undercliff colluvium, and soil moisture measurements using a Neutron Probe in the undercliff colluvium. The inter-relationship between these measurements and the mass movements was investigated. Estimates were made of the seepage into the undercliff colluvium and its water balance.A field monitoring programme was carried out over two years, from October 1982 to October 1984. During this time, the Plateau Gravel contributed a considerable amount of groundwater flow to the undercliff. An estimate was made of its temporal variation. It was shown that intercepting this flow would have a considerable effect on the undercliff water balance.A model was developed which related meteorological conditions to groundwater levels. The model was used to determine the relative level of groundwater levels at the time of occurrence of a number of slumps. It was found that the timing of slumps was dependent on both the groundwater level fluctuation, as a result of meteorological conditions, and the gradual loss of lateral support afforded by the undercliff colluvium. The deeper the base of the slump, the less the groundwater level fluctuation, and the greater the influence of the variation in lateral support.Groundwater flow in the undercliff colluvium is mainly via permeable tension cracks, shear surfaces and gravel seams. Thus, groundwater levels, and hence mass movement, respond rapidly to meteorological conditions. Mass movement is also considerably influenced by changes in the distribution of loading. The groundwater levels are themselves affected by mass movement due to the changing boundary conditions of the groundwater flow regime. The content of this thesis adds considerable understanding to the inter-relationship between the hydrology and mass movement of the soil cliffs at Naish Farm. The methodology of this work could usefully be used in similar studies of other areas. (D82147)