A new hazard zonation methodology applied to residentially developed sea-cliffs with very low erosion rates, east coast bays, Auckland, New Zealand

• Published in: Natural hazards (Dordrecht) Vol. 40; no. 1; pp. 223 - 244
• Main Authors: JONGENS, R, GIBB, J, ALLOWAY, B. V
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer 01.01.2007; Springer Nature B.V
• Subjects: Bays; Cliffs; Coastal erosion; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Erosion rates; Exact sciences and technology; Geographic information systems; Geological hazards; Landslides; Lithology; Natural hazards: prediction, damages, etc; Remote sensing; Soil erosion; Zonation; New Zealand; North Island; slumping; risk assessment; cliffs; shorelines; SVI· Sea-cliffs; Australasia; ground water; surveys; geographic information systems; dip; Auckland· Coastal hazards; faults; rupture; bedding; aquifers; safety; zoning; Landslide Hazard zonation CLHZ; spatial variations; stability; direction; orientation; seepage; erosion rates; GIS; Waitemata Group; landslides; fault planes; methodology
• ISSN: 0921-030X; 1573-0840
• DOI: 10.1007/s11069-006-0019-5

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^sup -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.[PUBLICATION ABSTRACT]