Damage of The Surge

A Storm Surge measuring just under 1 metre hit the East Anglian coast on the evening of 14^th/15^th January 2024 with the surge persisting for around 24 hours.

The Surge, which coincided with Spring Tides resulted in extremely High-Water levels causing flooding in places, with the Thames Barrier closing for the 211^th time, since its inception. High Waves also caused severe damage at Hemsby and other locations. Therefore, this discussion will continue the 2^nd part of a two-part look at whether Surges have a particular strength to cause serious damage. In this context, Damage is defined both as physical destruction as well as overtopping of structures and/or flooding.

The 1^st part of this discussion looked at the damage caused by two surges in November and December 2023. The two surges had slightly different characteristics, the first on Friday 24^th November was the lower of the two surges with high wind speeds and wave heights. It seems that this surge was also slow-moving with a longer duration and fetch, which is the surface area over which the wind blows over the ocean. The second Surge on 21^st & 22^nd of December was produced from much steeper pressure gradients with a deeper low-pressure system, but wave and wind heights were lower with a much shorter spike in the level and persistence of the surge.

It is generally accepted that determinants as to the magnitude of a surge include wind strength, direction, and distance over which the wind is blowing, the “fetch”. In addition to how these forces combine with atmospheric pressure, the track of a low-pressure system and how these influence the elevation and the velocity of the sea. But the level of a storm surge does not necessarily give an indication of the power of a surge. Surges are rarely similar, as they are produced from differing weather systems, and display different features. But is it the case that whatever their traits, Surges possess a particular strength to cause severe damage to coastal landscapes.

It is said the North Sea is particularly vulnerable to Storm Surges. One reason for this is the effect of Coriolis forces, which describe the way the earth rotates, which influence the acceleration of ocean currents. In the Northern Hemisphere this means movements of sea water are accelerated to the right. If a storm moves down the North Sea, elevated sea levels will be piled up against the coastline. This point is also relevant because the North Sea is rather shallow.

Therefore, it is thought that as two flowing elements connect with each other a transaction occurs between the quicker moving element and the slower element. If this process is translated to wind blowing over the ocean, a wind speed of 23 metres per second blowing over 200m of water, with a depth of 30 km, could cause a rise in sea level of 0.85m. If the gale force increases to 23 metres per second, sea level could increase to 1.60 m.

The effects of Wind tension on the surface of the sea, produces differences in the speed of the current in relation to the depth of the water. It is thought that water is propelled at around 3% of the wind speed applied to the ocean’s surface. As an ocean current runs down a coastline, it rises progressively in a manner that operates inversely with the depth of water. Movement of the current and alterations in ocean levels can be magnified if the tension of the wind on the surface moves water in the direction the wind is blowing.

But do these surge characteristics provide evidence regarding the particular destructive power of surges. Especially as Wind Wave storms with Significant Wave Height persistently above Storm Alert Thresholds also occur in the North Sea. Such storms can also cause severe erosion features and harm to coastlines.

Additionally, it is said, sea water elevated by storms can simply mean tidal water is kept longer on beaches between tides. Specific forceful sea water episodes produced by storms that coincide with a tide, could simply be the product of a collection of storms, A series of storms can generate wave power equal to a single extreme weather event. Is it possible within these tempestuous sea states, to pick out a surge, as the sole culprit, of a culmination of damaging coastal events.

It is valid to query the usefulness of isolating one type of agitated, sea state that predominantly presents itself simply as the high wave heights or water levels that cause the damage. It could be more useful to coastal communities and urban centres affected to warn of higher than usual tide levels, with faster wind speeds and waves than normal. However, it isn’t the case that such a sea state will remain as a volatile phenomenon to be viewed from a distance.

With the surges in late 2023 and in the last few days, it is the large scale, almost clean sweep, for want of another description, of the destruction, that marks these surges as significant. At the time of the largest surge on 14^th/15^th January 2024, wind speeds at Happisburgh were under 30 metres a second. At Felixstowe, measures were just above 20 metres per second around about the time of the highest surge on Monday 15^th January. Yet a Flood Warning was in place for Sunday 14^th /Monday 15^th January for flooding at Bawsdsey Quay and Felixstowe Ferry. With sea levels at mAODN (height above average sea level) of 2.92, which is 0.88m above tides tables.

Therefore, to conclude this debate, even though every storm surge can be different, generated, as they are, by numerous systems acting on the ocean’s surface. Each surge is a demonstration of the transfer of momentum between atmospheric systems and the sea. Alterations in pressure systems alter energies down though the surface of the sea which are felt at all levels. With wind tension applied to both the physical surface and as well as equidistant to it.

The deepening of low-pressure, the strength of the wind and size of the area over which it blows, and the duration can initiate systems outside of normal tidal processes. These mechanisms can elevate and give Surges a particular force. When this is ranged against coastlines and tidal estuaries it can generate abnormally high-water levels. It is the intensity of these energy processes that can cause severe disruption to coastal landscapes and infrastructure.