Mechanisms of Coastal Cliff Retreat and Hazard Zone Delineation in Soft Flysch Deposits

Abstract

The Auckland coastline is characterised by steep cliffs formed in soft, alternating sandstone and siltstone flysch deposits. A rapidly increasing population of the greater Auckland urban area has meant increased development on the cliff tops, with little consideration given to cliff erosion rates. Three mechanisms of cliff instability are recognised: gravitational collapse of jointed sandstone blocks following removal of underlying frittered siltstone beds; planar failure along steeply dipping fault planes; and planar failure along gently dipping bedding planes. Erosion of overlying soils to maintain a stable slope configuration occurs contemporaneously with erosion of the rock faces. Average rates of cliff retreat of 2-6m per century derived from dated structures are attributed to the joint block fall mechanism. Planar failure along fault planes produces up to 5 m of retreat in a single event, and one event may he expected in any ten year period along an individual fault plane. A limit of approximately 10 m maximum retreat exists for this mechanism. Bedding plane failures are very large, but infrequent events. Planar failures along fault planes represent the greatest risk to structures over their expected lifetime. Thus, a minimum coastal hazard zone width of 16 m can be derived, based on 10 m of erosion through fault plane failure, and 6 m set-back due to the stable angle of the overlying soils. Allowing for a factor of safety increases this hazard zone to 23 m. More refined hazard zones may be derived by mapping of cliff-face structure, hence recognising zones of increased susceptibility to failure. Risks may also be minimised by careful stormwater disposal, which presently often discharges directly onto unstable cliff faces, increasing the instability.