Flooding is the most frequently occurring natural hazards in New Zealand and one of the most costly. Hundreds of thousands of people and billions of dollars worth of property are at risk from flooded rivers and rising seas. Areas prone to flooding can be predicted by mapping historic flood inundation zones and by undertaking hydraulic model studies. Ambiental’s new flood products provide an important tool for understanding these risks.
Most New Zealanders reside in coastal regions, and many communities are situated on active floodplains. Research indicates that in any given year there is a 50% chance of one or more 1 in 150 year return period flood events within populated New Zealand catchments. Floods are usually caused by heavy or prolonged rainfall but can also occur due to landslides triggered by heavy rainfall or earthquakes, failure of dams or hydraulic structures, or high sea levels at river mouths.
With a population of around 4.5 million it is estimated that almost 700,000 people are at risk of flooding. 411,516 buildings worth $135 billion are presently exposed to river flooding in the event of extreme weather events. Also exposed is 19,098 km of roads, 1574km of railways and 20 airports. Probabilistic risk modelling has indicated that the Average Annual Loss (AAL) to be expected from flooding is 400 Million US$, with storm surge accounting for a further 323 Million US$. This AAL far exceeds that of other hazards, accounting for 87% of the total. This information can be used to plan and prioritize investments and strategies for managing disaster risk.
The North Island town of Edgecumbe in New Zealand, Thursday, April 6, 2017. About 2,000 residents needed rescuing after the river burst through a concrete levee, flooding hundreds of homes and businesses. Photo: AP
Understanding the local environment of New Zealand
Climate variability affects the likelihood and location of flooding. During El Nino conditions floods are more likely to occur in the South and West of New Zealand. Reports released by the National Institute of Water and Atmospheric Research (NIWA) have highlighted the need for national flood risk maps which provide accurate and comprehensive information on the costs and impacts of flooding today and under climate change scenarios.
Ambiental’s New Zealand FloodMap provides full country coverage of fluvial, pluvial and tidal flood sources at unprecedented levels of precision and at a full range of return periods. Flood model accuracy is driven by two key data inputs: hydrology and topography. Hydrology is the scientific study of the movement, distribution and management of water.
Visualisation of the New Zealand FloodMap 100yr Fluvial (Riverine) layer at Paekakariki on the North island
For New Zealand we have extensively researched localised catchment characteristics and applied regional best practice techniques. Topography describes the shape and features of land surfaces. We have sourced the most up to date datasets available, including high precision LiDAR in modelled at 5m grid resolution in urban areas which covers over 50% of the population. In more rural areas we have modelled at 8m grid resolution using digital elevation models sourced from NIWA.
The frequent and growing problem of flooding
In recent years flood emergencies have occurred every year in New Zealand. This year has already seen significant flooding as heavy rain combined with a high tide caused the Cleddau River to break its banks. MetService New Zealand recorded over 1,000mm of rain falling in a 60 hour period at Milford Sound leading up to the 04th February 2020. A state of emergency was declared for the Southland Region, including Gore, due to the extensive floods. In 2019 notable flooding occurred in the Canterbury region during December, at Coromandel Peninsula in September and in the Westland district in March. Historically flooding has killed hundreds of people throughout the country and had major impacts on society and critical infrastructure.
Ballarat Street, Queenstown, NZ, flooded 1878, Queenstown, by William Hart, Hart, Campbell & Co. Te Papa (C.014174)
Ambiental’s flood products are calibrated and validated against historic events. When new flooding occurs our team of specialists will check the floodmaps to ensure that they have accurately predicted the flood. Our validation studies are shared with customers to provide situational awareness and to help understand impacts and financial losses. If any model improvements are required the affected areas will be rapidly remodelled to ensure that they keep pace with the latest science and land use changes.
Detailed modelling of risk to properties
Ambiental’s New Zealand FloodScore database uses FloodMap input data to quantify the likelihood of individual properties being flooded due to rainfall, overflowing rivers or tidal surges. To provide simplicity each property has a single overall 'combined' flood risk rating along with a breakdown for each flood source, providing predicted water depths at a range of return periods. It has been designed to support insurers and brokers in making quicker, more accurate decisions around pricing flood risk.
Ambiental’s New Zealand Fluvial FloodMap and FloodScore building risk data at the Hutt River as it flows through Alicetown, North Island New Zealand.
Our New Zealand catastrophe model product in development is built from the same high precision FloodMap and FloodScore data describe above, which provides a consistent view of risk across the product range. New Zealand FloodCat is a property-level precision probabilistic model for the entire country. Currently in testing phase this model will provide an unprecedented level of precision and represent the most advance view of risk available to the reinsurance industry for New Zealand. The model will be available to our customers through the ModEx and ELEMENTS platforms.
Preparing for the floods of the future
According to the Ministry for the Environment the greatest effect of climate change is likely impact New Zealand's water resources, with higher rainfall in the west and less in the east. Extreme climate events such as droughts could become more frequent in eastern areas, with increased flooding after major downpours. It is also expected that the sea level will rise 20-30 cm by 2040. By the year 2100, depending on whether global greenhouse gas emissions are reduced, the sea could rise by between 0.5 to 1.1 m, which could add an additional 116,000 people exposed to extreme coastal storm flooding.
To tackle the growing flood risk problem Ambiental is developing solutions which can identify the impacts of flooding under climate change. FloodFutures uses global climate models to predict how flood hazard will change under a range of future emissions and sea level rise scenarios. This is useful for informing long term land use and flood mitigation decisions. Ambiental is also developing FloodWatch flood forecasting systems which are able to provide advanced warning of flood events so that actions can be taken to prepare and respond to floods more effectively. Deploying these technologies in New Zealand would ensure that the nation is prepared for future flooding emergencies.
To discover more about our flood risk technology for New Zealand contact us.
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About the Author
Paul Drury is a Product Manager and Solutions Consultant at Ambiental. His role includes the steering of new product development and reporting back to stakeholders. He also works with our customers to design and develop world class data solutions to solve different global environmental challenges. Paul is an expert in GIS and data analysis with a strong understanding of the environmental data industry and underlying technical concepts. He has a BSc (hons) in Environmental Sciences from the University of Brighton.