Planning on a retreating coastline : Oamaru, North Otago, New Zealand

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Forsyth, P.J. 2009 Planning on a retreating coastline : Oamaru, North Otago, New Zealand. Lower Hutt, N.Z.: GNS Science. GNS Science report 2009/25 53 p.

Abstract: The natural phenomenon of coastal erosion beconmes a hazard when it poses a threat to property and/or life. Erosion of the coastline in and near the township of Oamaru (North Otago) threatens property, assets and infrastructure close to the coast. Both short-term erosion events (particularly in 2007), and long-term retreat (over thousands of years) are observed. Coastlines are naturally dynamic, but human actions have also had measurable effects, even during the relatively short period of European settlement in New Zealand (~ 170 years). Future climatic warming, with associated sea level rise, is likely to exacerbate coastal erosion on cliffed coastlines such as those at Oamaru. International approaches to dealing with coastal erosion, including the concept of “managed retreat”, have an application in New Zealand although our coastal planning framework (principally under the Resource Management Act 1991 and the New Zealand Coastal Policy Statement) is unique to this country. Under the Resource Management Act, regional councils and territorial authorities (city and district councils) are required to have regard to the effects of climate change. The geographic and geological setting of Oamaru explains some of the observed features of coastal retreat. Most of the town is built on a coastal terrace, which consists predominantly of unconsolidated deposits and terminates in a sea cliff. Wave climate, longshore drift and the nature of beach sediment contribute to a sediment deficit that allows storm waves to break against the foot of the cliff at times, causing erosion. Parts of the coast have been modified, for example by reclamation, rock armouring and breakwaters. However, in the long term, some of the coastal terrace on which the town is built will continue to be eroded away. This study describes recent erosion effects at several sites within Oamaru township and further south along the coast. Council responses to the erosion hazard include zoning and setbacks, control of subdivisions and new developments, and hard coastal defences. These are part of the spectrum of possible statutory responses used by councils around New Zealand and elsewhere. Related (non-council) research includes analysis of coastal sediment budgets and rates of shoreline retreat, and community awareness of coastal erosion in this area. Some suggestions for further study are given, such as monitoring district and regional plan outcomes and effectiveness in this area, researching the role of insurance in modifying actions of property owners, and evaluating risk acceptance in the Oamaru/Waitaki community. (auth)