High Tides a Predictor for Storm Surge Risk

On February 21, 2015, locations along the Bristol Channel experienced their highest tides of the first quarter of the 21st century, which were predicted to reach as high as 14.6 m in Avonmouth. When high tides are coupled with stormy weather, the risk of devastating storm surge is at its peak.

Storm surge is an abnormal rise of water above the predicted astronomical tide generated by a storm, and the U.K. is subject to some of the largest tides in the world, which makes its coastlines very prone to storm surge.

The sensitivity of storm surge to extreme tides is an important consideration for managing coastal flood risk. While it’s not possible to reliably predict the occurrence or track of windstorms—even a few days before they strike land—it is at least possible to predict years with a higher probability of storm surge well in advance—which can help in risk mitigation operation planning, insurance risk management, and pricing.

Perfect timing is the key to a devastating storm surge. The point at which a storm strikes a coast relative to the time and magnitude of the highest tide will dictate the size of the surge. A strong storm on a neap tide can produce a very large storm surge without producing dangerously high water levels. Conversely, a medium storm on a spring tide may produce a smaller storm surge, but the highest water level can lead to extensive flooding. The configuration of the coastal geometry, topography, bathymetry, and sea defenses can all have a significant impact on the damage caused and the extent of any coastal flooding.

This weekend’s high tides in the U.K. remind us of the prevailing conditions of the catastrophic 1607 Flood, which also occurred in winter. The tides reached an estimated 14.3 m in Avonmouth which, combined with stormy conditions at the time, produced a storm surge that caused the largest loss of life in the U.K. from a sudden onset natural catastrophe. Records estimate between 500 and 2,000 people drowned in villages and isolated farms on low-lying coastlines around the Bristol Channel and Severn Estuary. The return period of such an event is probably over 500 years and potentially longer.

The catastrophic 1953 Flood is another example of a U.K. storm surge event. These floods caused unprecedented property damage along the North Sea coast in the U.K. and claimed more than 2,000 lives along northern European coastlines. This flood occurred close to a Spring tide, but not on an exceptional tide. Water level return periods along the east coast are varied, peaking at just over 200 years in Essex and just less than 100 years in the Thames. So, while the 1953 event is rightfully a benchmark event for the insurance industry, it was not as “extreme” as the 1607 Flood, which coincided with an exceptionally high astronomical tide.

Thankfully, there were no strong storms that struck the west coast of the U.K. this weekend. So, while the high tides may have caused some coastal flooding, they were not catastrophic.