5 NATURAL HAZARDS Contents 5.1 Introduction 5.1.1 Flooding 5.1.2 Land Instability 5.1.3 Coastal Hazards 5.1.4 Fire 5.1.5 Climate Change 5.2 Resource Management Issues 5.3 Objectives 5.4 Policies 5.5 Strategy 5.6 Implementation 5.6.1 District Plan Regulatory Methods 5.6.2 Other Regulatory Methods 5.6.3 Other Methods 5.7 Anticipated Environmental Results 5.8 Rules Auckland Council District Plan (Rodney Section) 2011 ................................................ Natural Hazards : Chapter 5 Auckland Council District Plan (Rodney Section) 2011 ................................................ Natural Hazards : Chapter 5 5 NATURAL HAZARDS 5.1 INTRODUCTION Natural hazards are natural events caused by the weather, earth, water or sea (including earthquake, erosion, volcanic and geothermal activity, landslip, subsidence, sedimentation, wind, drought, fire or flooding) the action of which adversely affects or may adversely affect human life, property or other aspects of the environment. The Rodney District experiences a range of natural hazards including flooding, land instability, coastal erosion, fire, cyclones, high winds and potential volcanic and earthquake events. Sometimes these hazards may occur independently of each other, but at other times they may be the result of each other. This chapter of the Plan deals with the four most significant natural hazards within the District: (a) Flooding; (b) Land Instability; (c) Coastal Erosion and Inundation by the Sea (d) Fire. While it is recognised that natural hazards such as volcanic activity, earthquakes, tsunamis, cyclones and high winds can have significant adverse effects, the methods of dealing with them lie outside the scope of the Plan. It should be noted that there are no rules within this chapter. The rules which apply to this chapter can be found in: Chapter 7 - Rural Chapter 8 - Residential Chapter 9 - Business Chapter 10 - Open Space and Recreation Chapter 16 - General Rules Chapter 18 - Urban Land Modification and Vegetation Protection Chapter 23 - Subdivision and Servicing Also, in dealing with hazards, regard needs to be had to the Auckland Regional Policy Statement, the Regional Plan: Coastal, and the New Zealand Coastal Policy Statement. The Auckland Regional Council has also prepared a Coastal Hazards Strategy/Coastal Erosion Management Manual. The purpose of this manual is to provide further detail on the way in which all those with an interest in the coast can achieve the outcomes sought by the RMA, Regional Policy Statement and the Regional Plan: Coastal. The manual provides detail on how to avoid coastal hazards and how to mitigate coastal hazards where they exist. 5.1.1 Flooding Flooding is the main hazard in the District and is the most common reason for the declaration of a civil emergency. It tends to occur more frequently in the larger catchments in winter, because the soils are often saturated and the catchment is near capacity. However, in summer there is a bias towards Auckland Council District Plan (Rodney Section) 2011................................................ Natural Hazards : Chapter 5 1 smaller catchments flooding after intense rainfall, because the rain does not have time to penetrate the soil. Within the District there are a number of urban areas which, usually for historical reasons, have development within the 100 year floodplain of adjacent rivers. These areas are subject to flooding and include parts of Helensville, Kumeu-Huapai, Waimauku, Parakai, Kaukapakapa, Taupaki, Warkworth and Waitoki. Rural floodplains are also subject to modification and require identification. 5.1.2 Land Instability Land instability is the second most significant natural hazard within the District. The soil formation which is well-known for land instability is the Onerahi Chaos Breccia Formation. However, virtually any slope greater than 1:5 (and 1:7 for Onerahi Chaos Breccia) in the District is potentially subject to land instability to a lesser or greater extent, and therefore, the land instability of any property with this degree of slope needs to be assessed before development can occur. Mass movement can occur at any time of the year after high rainfall. It can also be caused by the careless placement of excavation material or other fill, unsupported excavations and the removal of vegetation cover. In many cases stormwater can also contribute to land slippage by concentrating flows. 5.1.3 Coastal Hazards The coastal environment is an area of dynamic equilibrium that is constantly changing and adapting to inputs from the sea and land. The result is coastal erosion due to sea and wind action, dune building and dune removal, landslips, and flooding from the sea and estuaries. Coastal erosion is the long- term landwards translation of the coastline. The east coast beaches tend to be prone to erosion under storm conditions (eg. Omaha, Snells Beach, Algies Bay, Orewa, and most of the beaches on the Whangaparaoa Peninsula). The possibility of natural repair of the system is limited by residential development on the backshore, which has reduced the supply of sediment and changed the dynamics and equilibrium of the foreshore areas. Development of the coastal environment also leads to previously harmless shifts of the dynamic foreshore having unacceptable impact. There are a number of areas which are prone to inundation by the sea. Historically, large areas of the coast, mainly along the shores of the Kaipara Harbour, were reclaimed. These areas flood frequently, but there is a minimal threat to human life and property. In a number of cases, such as at Point Wells, Manly Beach and Orewa, urban development has occurred in very low- lying areas, which are almost at sea-level or in the case of parts of Stanmore Bay, where development has occurred below sea-level, making them prone to the possibility of inundation. Recently, technical work has highlighted the complex interplay of coastal geometry, bathymetry, tidal effects and climate change and other physical phenomena which requires Councils to take a cautious and prudent approach to new development and mitigation of existing risk. Flooding also occurs when heavy rainfall coincides with high tides, low pressure weather systems and specific winds which drive the waves inland up estuaries or rivers. The Kaipara and Hoteo Rivers, and the Orewa estuary are Auckland Council District Plan (Rodney Section) 2011................................................ Natural Hazards : Chapter 5 2 typically affected in these conditions. It is also recognised that projects carried out outside the District in the Coastal Marine Area, such as the proposed Orewa Beach Reef Project, may assist in reducing coastal hazards. Measures for the protection, enhancement and maintenance of Orewa Beach are particularly important given the historic, recreational and landscape importance of the beach to Orewa. [Amendment 101] 5.1.4 Fire Wildfires are caused by lightning strike and human carelessness. Of most concern within the District is human carelessness, especially in forested areas. In the District, there are large areas which are forested with both indigenous and exotic species. The areas of planted forests (both native and exotic) are likely to increase as landowners move away from the traditional pastoral farming into forestry. This will increase the risk of forest fires, especially in late summer and early autumn. In addition, as recreational and visitor use of the larger exotic forest areas such as Riverhead and Woodhill increases, so will the potential for fires. 5.1.5 Climate Change Over the last few decades, there has been growing concern that human activities, such as forest clearance, reclamation of wetlands, burning off of the land and industrial air omissions, have led to an increase in the atmospheric concentrations of ‘greenhouse gases’ like carbon dioxide and methane. This is considered likely to cause global warming. The major results of global warming could be a rise in sea level and climate change. The result would be an increase in frequency and probability of a number of natural hazards including: flooding; coastal inundation and erosion (because of sea-level rise); land instability (because of changes in watertable levels and rainfall intensity); drought (because of drier summers); wind hazards; and fires (because of drier summers). This would have a major impact on human activities within or adjacent to hazard areas within the District. To date, no specific scenarios on climate change and sea-level rise have been developed for the District. However, climate change and sea-level rise could have major impacts, especially for low-lying urban areas such as Omaha, Orewa, Parakai and Point Wells. Auckland Council District Plan (Rodney Section) 2011................................................ Natural Hazards : Chapter 5 3 5.2 RESOURCE MANAGEMENT ISSUES Issue 5.2.1 Natural events can pose a hazard to people, property, the environment, and the social, economic and cultural wellbeing of communities. By definition, a natural hazard does not exist unless it affects humans and their property. For example, Mangatawhiri Spit (Omaha), like all spits, is by nature unstable and prone to periodic inundation by the sea. This would not be a hazard if the area did not have residential dwellings on it. The spit would erode and accrete in a cyclical process based on meteorological and tidal conditions. However, because people have built houses on the spit and live on it, the instability and erosion of the northern spit end and the foredunes pose a risk to people’s property and potentially to human life. Flooding and instability This is the case with other natural ha zards in the District, such as flooding and land instability. These are natural processes or events, which are a significant issue only when they impact on people and property. For example, along the Kumeu River, the 100 year flood plain is relatively large. However, flooding is mainly an issue where industrial, commercial and residential land is affected in Kumeu-Huapai, Waimauku, Taupaki and Helensville. Flooding tends to be less of an issue in the rural areas because the impacts tend to be less severe, as fewer structures and people are affected.