Northland Civil Defence Emergency Management Plan, 2010

RIVER FLOODING ARISING FROM LOCALISED HEAVY RAINFALLOverviewRiver flooding as a result of sustained or short duration, high intensity rainfall (typicallythunderstorms) is the most frequent and widespread hazard throughout the region.Thunderstorms generally have their worst impact in a localised area less than 100sqkm.They are less predictable than larger weather fronts.The high flood risk in Northland arises from exposure to intense weather systems and atopography which sees rapid run-off from a steep terrain leading into flat areas whereflood waters recede slowly, exacerbated by tidal lower reaches. Flooding damage is oftenworsened by large amounts of silt and debris in the floodwaters and land slips are also afrequent consequence of rain in Northland.HazardLikelihood(A/B)HazardConsequence(3)How do wemanage therisk?What moreshould we bedoing?A storm with 12 hourly rainfalls in excess of 150 mm in a local area has an Annual ReturnPeriod of 100 years or more. Shorter duration, higher intensity rainfall can also causemajor flooding, such as Pungaru in 1999. Regionally the likelihood is higher as there is acumulative risk of storms in different areas.Social: Distress due to displacement of people (in a localised area, but possibly for extendedperiods) and loss of possessions. Extreme events may result in injury or loss of life. Psychological problems for victims reliving or recalling events. Public health risk from water/sewage contamination. Loss of irreplaceable, significant Maori land holdings, taonga and waahi tapu sites.Built: Significant loss of infrastructure, particularly roads and bridges due to landslips,flooding and bridge pier scouring. Power, water supplies and communication systemscan also be affected if landslips or flooding impact on critical facilities, but generallyare affected less than the road network unless the rain is accompanied by high winds.Economic: Loss of crops (floodplains are a major contributor to primary production). Cost of restoration (flood insurance increasingly difficult to obtain in some areas).Natural: Alterations to river channels; bank erosion, channel scour/build-up/diversion. Sediment-debris deposition, and dispersal of chemicals, effluents (dairy farm ponds,septic tanks, town sewage networks) and rubbish (from tips) across floodplains.River Management Policy and agreements between the four councils.Land planning/building standards.River flood mitigation schemes and Whangarei and Kaitaia City Flood Control Schemes.Flood forecasting systems for Kemp House, Kerikeri (Historic Places Trust) and Kaitaia.NIWA weather models and rain forecasting.Evacuation / community plans in place for some high-risk areas.Development of flood management plans for high priority catchments.Development of community response plans for at-risk communities.Improvements to rain and flood forecasting/warning systems.Improved linkages with planning departments to prevent building in at-risk areas.What is thefuture risk?Expected to be increasing high intensity rainfall events with climate change.Community exposure may decrease as land planning improves, but also increasingpopulation density may counteract that.Northland Civil Defence Emergency Management Plan, 2010-2015 Page 73