Studying storm surge and coastal hazards

Dr Rob Bell, a NIWA scientist, studies storm surges and their impact on New Zealand coasts. A storm surge is a very long wave formed by low-pressure storms over the ocean. Because they are occasional occurrences, storm surges have rarely affected the New Zealand coast in the past. However, Rob says that, as the sea level rises in association with climate change, storm surges (especially when combined with high spring tides) will cause considerably more damage to low-lying coastal areas of New Zealand.

It’s important to be able to predict storm surges and their associated risks so that action can be taken to keep people and property safe. New Zealand’s regional, city and district councils need information on storm surge risk so they can build this information into future coastal development plans and building design.

Behaviour of storm-surge waves

The danger of storm-surge waves is in their wavelength. Storm-surge waves can be 100 km long and contain a large volume of water. These waves carry a lot of potential energy because of their height (up to 1 m in New Zealand and up to several metres high in Bangladesh and during hurricanes in the Caribbean). Unlike storm-surge waves, ordinary swells breaking as surf don’t go very far up the beach. They have small wavelengths of only 30–100 m with a small volume of water per wave.

Once the long storm-surge wave reaches the coast, the water level rises above the predicted tide level and can cause a lot of inundation (flooding), especially at high tide. Surf waves, produced by the storm’s strong winds, ride on the back of the longer storm-surge wave. They can also cause direct damage. Ways to combat damage from storm surges are through coastal dune care (such as planting and maintaining natural dunes) and making sure houses are built out of reach of future storm surges.

Storm surges in New Zealand

In New Zealand, storm-surge waves are seldom more than 0.7m above the predicted tide, which doesn’t cause a lot of flooding. However, when storm surges combine with a high spring tide, the resulting waves can overtop the coastline. This causes flooding by seawater. Flooding occurs because of constructive interference, which can result in much higher water levels. Says Rob, “A lot of our research is just looking at the probability of a large spring tide occurring at the same time as a storm surge.”

The topography of coastal land determines how far inland coastal flooding can occur. For example, past storm-surge events in the 1930s greatly affected large areas of farmland on the low-lying Hauraki Plains. In response, a stopbank was built along the coast. Swell waves combined with storm-surge waves (even small ones) also affect the low-lying gravel coasts south of Napier.

Storm-surge research methods

Rob explains that storm-surge research, just like tsunami research, is done using three key techniques: numerical modelling, data analysis and historical information. Rob calls these the “three legs of a stool”.

• Numerical modelling of storm surges involves using computers to solve a set of mathematical equations. These describe the physics of storm-surge waves, the winds that affect the surface of the water, friction on the bottom of the seabed and other factors. The area of ocean being studied is divided into cells or chunks. The size of the chunks depends on the area studied and the resolution or detail required. Computer modelling calculates the average information for each chunk and builds an overall picture of the situation at each point in time.
• Data analysis involves collecting long-term data from instruments such as sea-level gauges (bubbler gauges) and satellites and using that information to tune computer models so they are as close to reality as possible. Rob says, “We’ve got a number of sea-level gauges all around New Zealand, and so I analyse them to bits basically.”
• Historical information on storm surges is consulted by researchers so we can learn from the past.

Together, the predictions from computer modelling, sea-level measurements and the historical analysis have led to the development of a simple red-alert tide warning system. NIWA also runs complex models on the supercomputer Fitzroy to produce storm-surge forecasts up to 2 days ahead.