Mobilisation of landslide debris on a shore platform

Abstract

This thesis contributes to the knowledge of the mobilisation and transport pathways of landslide debris on a shore platform. Two experiments were undertaken at Waiake Bay on Auckland‟s North Shore to collect measurements of current flow characteristics and of tracer mobilisation. This was in order to resolve thresholds of mobilisation and transport pathways. The first experiment involved two deployments and was undertaken during low energy fair weather conditions over four days. The second also involved two deployments under higher energy storm conditions over six days. Following the second deployment the tracers were left in the system for a further 65 days to study sediment breakdown over longer time periods. The research found that low energy conditions currents were able to mobilise large clasts weighing up to 15.9 kg. Tracers were moved primarily landward toward a boulder beach. Under storm conditions the dominant transport direction was also landward, but a higher proportion of clasts moved seaward than under fair-weather conditions. During the 65-day deployment all tracers were mobilised. Three primary transportation pathways were identified. 1) Landward movement toward the boulder beach. 2) Landward movement toward the cliff toe, and 3) seaward movement onto the shore platform. This deployment also showed a high incidence of burial beneath the beach surface. Results also indicated a rapid breakdown of tracers. The results indicate that landslide debris is readily mobilised when it enters the nearshore system, even under low energy conditions. Debris is primarily moved toward the cliff toe where a boulder beach occurs. This thesis focussed on the transport of „free‟ clasts on the platform surface. However it is apparent that preferential transport of clasts toward the boulder beach might result in clast trapping, with higher current velocity thresholds then required to further mobilise individual clasts owing to the effects of inter-granular friction. Current velocities were found to be as much as 60% faster near the beach than at mid-platform. This data implies that debris transported onto the platform during storm conditions are unlikely to be subject to higher transport rates than free clasts remaining near the cliff toe. Observations of clast breakdown during transport confirm that abrasion rates of sandstone cobbles are very high (between 1% and 92% over 69 days) despite modest current velocities and small transport distances. Results from this thesis show that landslide debris is readily mobilised even under relatively benign flow conditions, and that relatively small transport distances are sufficient to result in rapid attrition of clasts. Collectively these results have implications in respect to (i) the residence time of landslide debris, which act to protect the cliff toe from wave attack, and (ii) the potential importance of cobble abrasion as a source of beach sediment.