Flooding is the most frequently occurring natural hazard 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. Fortunately, areas prone to flooding can be predicted by mapping historic flood inundation zones and by undertaking hydraulic model studies. 

That's why CoreLogic has partnered with world leading flood experts Ambiental to introduce NZ FloodMapTM and NZ FloodScoreTM to the New Zealand market. These products are valuable tools for anybody with a need to better understand potential flood impact for any property in New Zealand.


Author: Paul Drury, Product Manager and Solutions Consultant, Ambiental

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 occurring 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.

It’s estimated that almost 700,000 Kiwis are at risk of flooding, with 411,516 buildings, in turn worth $135 billion, presently exposed to river flooding from extreme weather events. Also exposed are 19,098km of roads, 1,574km of railways and 20 airports. 

Probabilistic risk modelling has indicated the Average Annual Loss (AAL) expected from flooding is US$400 million with storm surge accounting for a further US$323 million. This flood AAL far exceeds that of other hazards, accounting for 87% of the total. Overall the information NZ FloodMap and NZ FloodScore provide is precisely the type that can help in planning and prioritizing investments and strategies for managing disaster risk.

Edgecumbe flooding

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 New Zealand environment
Climate variability affects the likelihood and location of flooding and 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 comprehensive information on the costs and impacts of flooding today and also under climate change scenarios.

NZ FloodMap provides 99.6% coverage of the country for fluvial, pluvial and tidal flood sources at unprecedented levels of precision and for a full range of return periods. 

The accuracy of NZ FloodMap is driven by two key data inputs: hydrology and topography. Hydrology is the scientific study of the movement, distribution and management of water.

FlooodMap 100yr Fluvial later

Visualisation of the NZ FloodMap 100yr Fluvial
(Riverine) layer at Paekakariki on the North island

For New Zealand Ambiental have extensively researched localised catchment characteristics and applied regional best practice techniques. Topography describes the shape and features of land surfaces and for this the most up to date datasets have been sourced, including high precision LiDAR modelled at 5m grid resolution in urban areas which covers over 50% of the population. More rural areas have been 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 4th February 2020. Due to the extensive floods a state of emergency was declared for the Southland region, including Gore. 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.

Queenstown flood 1878

Ballarat Street, Queenstown, NZ, flooded 1878, Queenstown
by William Hart, Hart, Campbell & Co. Te Papa (C.014174)

These flood products are calibrated and validated against a range of historic events. Whenever possible following new flood events Ambiental’s team of specialists check their floodmaps to understand how accurately they predicted flood extents. 

Validation studies are also shared with customers to provide situational awareness and help understand impacts and financial losses. Wherever model improvements are required the affected areas can be rapidly remodelled to ensure increased accuracy and also ensure they keep pace with the latest science and any land use changes.

Detailed modelling of risk to properties
The NZ 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.

FloodScore

NZ Fluvial FloodMap and FloodScore building risk data at the
Hutt River as it flows through Alicetown, North Island New Zealand.


Being able to plan for potential flood hazards can make all the difference - for insurance companies making underwriting decisions on properties, banks supplying property finance, telecommunications and utilities companies providing vital services to homes and offices, government agencies tasked with protecting communities and their assets, and of course for individual New Zealanders. Contact us today to discuss how NZ FloodMap and NZ FloodScore could benefit your business.