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Feature

Seeking Storm Surges

The Firth of Thames is the focus for a new system to warn of storm-related coastal flooding.

Adam Munro

Coastal flooding caused by surges of storm-tossed water has been a concern in the Firth of Thames area.

Over the last 120 years, the most serious coastal flooding has been experienced in low-lying areas around the Firth of Thames, with the most vulnerable settlements being Thames and Kaiaua. Coastal flooding around the Firth of Thames is also reasonably frequent, with events similar or more severe than the recent July 1995 and Cyclone Drena events probably having an annual probability of at least 3-5%. Total damage estimates for each of the last two events have been in excess of $4-5 million, including damage to settlements, agricultural land and roads.

Storm surge is defined as an abnormal, sudden rise of sea level associated with a storm event. Most commonly, storm surge elevation of sea level occurs in response to decreases in atmospheric pressure and to wind stress on the surface of the ocean (the latter effect "piling" a wedge of water against the shore). At some sites exposed to the open ocean, large-scale oceanic disturbances, such as coastal-trapped waves, can also contribute significantly to observed water level fluctuations. Storm surge is normally generally constant over a large area.

The Firth of Thames is particularly prone to storm surge activity due to:

  • its northern geographic position making it subject to cyclonic storms which bring the necessary ingredients to produce storm surge activity (such as strong onshore winds, high wave energies, and low atmospheric pressure)
  • the nature of the surrounding topography, in particular the Hunua and Coromandel Ranges (which run parallel with the Firth); these act in part as a wind funnel, thereby enhancing the effect of onshore winds and waves
  • the shape and exposure of the Firth to the open ocean allows swells to head parallel to the shoreline down the Firth; the shallow nature of the Firth also enhances wave height, particularly at the southern end.

Environment Waikato is undertaking work to improve its forecasting of storm-induced coastal flooding in the Firth of Thames, trialling a pilot system developed by the National Institute of Water and Atmospheric Research (NIWA). The model will be linked into Environment Waikato's telemetry warning system so that alarms can be generated automatically. The timing and accuracy of warnings to Civil Defence agencies -- and the public -- is critical because it leads to better decisions and responses which then lessen the threat to human safety.

Warnings of potential coastal flood events on the Firth of Thames at present originate from Environment Waikato's multi-sensored Thames Tide Gauge at Tararu. Along with tide height, monitoring includes wind speed and direction, atmospheric pressure, and wave height.

Providing accurate storm surge warnings is not an easy science owing to the relatively short tidal records in New Zealand and the complexities that drive the storm surge phenomena. In most cases, the main causal mechanism behind each storm surge event is different from the other, and this is particularly evident in past events on the Firth of Thames.