Forecasting coastal overtopping: What’s the worst that can happen?

Gold, I. and Connolly, S. (2018) Forecasting coastal overtopping: What’s the worst that can happen? In: Protections 2018 (3rd International Conference on Protection against Overtopping), 6-8 June 2018, Grange-over-Sands, UK.

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Abstract

Much is known about what is possible around the coast; Astronomical Tides follow fairly predictable rules, Physics limits the effects of weather on top of that. In recent years we have invested significantly in better forecasting for coastal extremes, attempting to determine whether the apparently random conjunction of tide and weather will overtop our sea walls and beaches, and if it does what the hazard or inundation extent, depth, speed and impact will be. Internationally, these factors may not be the sole drivers for flood risk management, but by recognising short and long term limits of these variables we can better describe the coastal risk and a “reasonable” worst case of possible impact. We are able to forecast coastal storm possibilities out to 5 days and as flooding approaches we are able to narrow the range of that forecast, which may or may not exclude the worst extremes. How do we use that as a heads-up to prepare for the possibilities without scaring people? By sharing extreme possibilities with responders who understand the low probability, but who may have to mobilise, we can prepare for that worst case. This process may escalate with the proximity of the event and the likely scenarios narrowing towards that expected. Conversely, as the worse events become less likely, we can scale down the response needed. The biggest variable around our coast is the tide. As it is driven by the pull of the moon and sun predominately, it can be predicted indefinitely ahead, with reasonable accuracy. Around 6 times a year, these work together to generate threatening sea levels. Second to the tide, storm surge is a major contributor, adding up to 3m onto the water level. On top of the sea level, wind driven wave action may give overtopping of defences well above that combined height. We will look at the way these factors combine and we forecast their impact. As we know from flooding history, these events where the factors combine to their worst effect are rare. The way they add together varies around our coast and more so around the world; that same history means that exposed locations will have been affected by similar events in the past and the risks should be well known. We have defended much of the coast where people and property are at risk, so we are dealing with the remaining vulnerability. We can’t build an un-surmountable wall round our coast and we wouldn’t want to, so we defend against events proportionately and prepare to respond to extreme events. Our defences are affected by wave action in a storm. Beaches are a major part of our defence as waves will break before they get close, but that wave action tends to mobilise sand or shingle and move it offshore. Hard defences may be damaged by heavily plunging waves or be undermined by overtopping waves. Either of these will affect the way the event is managed and need repairs planning if possible. These and the extreme events need a plan to deal with the failure of that defence. Major Incident Plans are maintained for events where serious or widespread overtopping or defence failure occurs. In some areas, demountable defences may be applicable, others may require evacuation. Knowing the worst that can happen is vital to planning this response. The uncertainty in our forecast at 5 days out means we shouldn’t mobilise at that point, acting according to our best-most likely- forecast, be prepared for the reasonable worst case and monitor for any unprecedented escalation to give as much warning as possible.

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