The nature and causes of coastal landsliding on the Maungatapu Peninsula, Tauranga, New Zealand

Lengths of cliffed coastline in Quaternary sediments around Tauranga Harbour, and particularly on the Maungatapu Peninsula, have been subjected to major landsliding episodes during and after rainstorms in recent years. This study is intended to broaden the database concerning coastal landsliding by documenting a descriptive and analytical study of the landslides, and to provide information useful for those attempting to control the effects of the landslides. A field mapping project was undertaken, associating patterns of landslide scars, marine erosion and groundwater seepage from cliff faces. More intensive studies at three sample sites included surveying, soil strength and physical index testing, hydraulic conductivity determinations and groundwater table observations. A number of working hypotheses, concerning the nature and causes of the landslide events, are erected and tested using the results of these descriptive studies. The hypotheses are further evaluated in factor sensitivity studies, using a simplified Janbu slope stability analysis program, which was written as part of the study. It has been found that the landslide events occur as parts of one of two cliff evolutionary sequences observed on the Maungatapu Peninsula. Where the rate of basal erosion by tidal currents and wind driven waves exceeds the rate of cliff crest retreat due to various cliff face modifying processes, the cliff segment gradually steepens. Failures in the material above an impermeable clay bed in the Pahoia Tuffs are triggered by high pore water pressures. Critical pore water pressures occur when long wet periods, culminating in medium to long return period storms, produce high seepage flows through the sandy material above the impermeable clay bed. The failures initially occur in a rotational manner, apparently causing the liquefaction of a bed of low bulk density silty sand lenses within a denser sandy mud matrix. Subsequent movements are therefore rapid and translatory, producing elongate annular debris lobes extending from the cliff base. The results of the study are then applied in formulating coastal management strategies, and in suggesting appropriate site stabilisation techniques