A new hazard zonation methodology applied to residentially developed sea-cliffs with very low erosion rates,East Coast Bays,Auckland, New Zealand |
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| Authors: | R Jongens J Gibb B V Alloway |
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| Institution: | (1) Institute of Geological and Nuclear Sciences Ltd (GNS Science), Private Bag 1930, Dunedin, New Zealand;(2) Coastal Management Consultancy Ltd, 555 Esdaile Rd, RD6 Tauranga, New Zealand;(3) GNS Science, Gracefield Research Centre, PO Box 30-368, Lower Hutt, New Zealand |
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| Abstract: | A new hazard zonation methodology is applied to the East Coast Bays area of North Shore City, one of the most residentially
developed cliffed shorelines in New Zealand. It is based on a series of geotechnical cliff profiles from three pilot study
areas (George Gair Lookout, Rahopara Reserve–Kennedy Memorial Park, and Mairangi Bay–Rothesay Bay) which detail many of the
variables that influence overall cliff stability.
The methodology requires calculation of a Coastal Landslide Hazard Zone (CLHZ) width for each geotechnical profile and is
derived by quantifying three factors: the rate of long-term sea-cliff retreat; the amount of horizontal retreat expected from
either joint block fall, fault plane failure, or bedding plane failure, coupled with the amount of horizontal retreat resulting
from slumping of the top weathered layer; and a safety factor. The rate of long-term sea-cliff retreat is multiplied by a
hazard assessment period of 100 years, which is then added with the two other factors to derive a CLHZ width. Finally, the
widths are entered into a Geographic Information System (GIS) to delineate a hazard zone.
Owing to the very low rates (< 0.1 m a−1) of sea-cliff retreat in the East Coast Bays area, the long-term rate of sea-cliff retreat at each profile location could
not be quantified by conventional survey techniques. Instead, a Sea-cliff Vulnerability Index (SVI) was employed to quantify
the long-term rate. Weighted variables considered in the SVI include the bedding dip direction, the occurrence of faults and
their orientation, sea-cliff aspect, cliff-toe and cliff-face lithology, cliff-top height, and the presence of groundwater
seepage.
Calculated CLHZ widths along East Coast Bays range between 13 m and 34 m inland of a reference cliff-line in response to spatial
variations of the sea-cliff geology and morphology. The widths reflect the estimated degree of risk over the next 100 years
from coastal erosion and landslips. |
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| Keywords: | Landslide Hazard zonation CLHZ GIS SVI Sea-cliffs Waitemata Group Auckland Coastal hazards |
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