Chapter 16 - Natural Hazards 16.0 Natural Hazards 16.1 Introduction The Marlborough Sounds landform reflects the results of global influences on its formation. The Pelorus-Rai catchment drains into Pelorus Sound. The Sounds are a partly drowned valley system resulting from successive erosional and depositional phases. It is a geologically dynamic, natural system. Generally, the topography is very steep with some areas mountainous. There are some areas of rolling and flat land. Coastal cliffs are precipitous. The valley ridge system tends NE - SW and this influences local climate, weathering of rocks, soil formation and vegetation. 16.2 Issue The variability of natural hazards location, frequency, severity, and potential to reduce the safety of the community or cause damage to property, infrastructure or the environment. The natural hazards which occur in the Marlborough Sounds area include: • Seismic/earthquake; • Slope instability; • Coastal inundation, tsunami and potential sea level rise; • Flooding; • Fire; and • Climatic. The Marlborough Sounds lie within the most seismically active part of the country near the southern limit of the Pacific plate, with the Wairau (alpine) fault to the south and the Waimea-Whangamoa fault to the west. The area is subject to frequent deep earthquakes and numerous shallow earthquakes. Earthquake events causing serious structural damage (Modified Mercalli Intensity of VIII) can be expected every 55 - 60years. Earthquakes cause ground deformation and shaking resulting in structural damage, weakening of foundation materials, and social disruption. Climate, topography and geology all influence the occurrence of slope instability. In the Marlborough Sounds there are numerous fault zones and rock types that are inherently unstable. Unstable rock types include: • Patuki Melange; • Croisilles Melange and • Grade IIIB and IV schists. The most significant effects of slope instability are slope failures and ground subsidence. Coastal processes range from oceanic to enclosed waters, with low wave energy and strong tidal currents. These processes include erosion of beaches, cliff and shoreline retreat, changes to river mouths and inundation of estuarine areas. 16 - 1