Chapter 17 - Natural Hazards 17.0 Natural Hazards 17.1 Introduction The Wairau/ Awatere is potentially subject to a wide range of naturally occurring hazards. Earthquakes, tsunamis, erosion, landslip, sea level rise, floods, sedimentation, wind, drought, fire, rain, hail and snowfalls can result in damage to private and community assets. In some cases human life will be at risk. Of those hazards, flooding has historically been given the most attention even though the effects of other hazards can be equal to or more severe than flooding. Natural hazards are commonly analysed in terms of magnitude-frequency concepts, whereby events of differing size (= magnitude) are assigned estimated average exceedence intervals (= frequency). As an example the 100 year flood level will be exceeded on average once in every 100 years. Because this is a statistical average only, more than one such event can occur in any 100 year interval. Note also that because the flood frequency relates to the so called 100 year flood being exceeded, such an event can be much larger and still be in accordance with statistical expectations. The 100 year frequency is often used in the assessment of risks when dealing with natural hazards. Below is a simplified hazard evaluation for the Wairau/ Awatere. Simplified Hazard Evaluation for the Wairau/Awatere Likely Exceedence Interval Hazard Category Less Than 100 Years Greater Than 100 Years Meteorological Climate change. Long-term climatic change. Extreme storm event (rain or wind). Major drought in part of area. Snow. Flooding Severe flooding in any river system. Breach in Wairau stopbanks Stopbank failure in many river (flooding in Blenheim, Spring systems. Creek, Grovetown, Tua Marina). Local flooding due to intense Stopbank failure in other river rainfall in low-lying areas. systems. Coastal Severe storm erosion of beaches. Major tsunami damage along Continuing cliff and shoreline coast. retreat. Long-term sea level and storm River-mouth/ spit tip changes. pattern changes. Possible sea level rise due to “Greenhouse Effect”. Landslide Severe rainstorm-generated Large earthquake-generated landslides in any catchment. rock avalanches. Localised earthquake-generated Earthquake-generated rockfalls slope failures and/ or ground upland and liquefaction in subsidence. lower Wairau, Rarangi. 17 - 1