The ABCs of Storm Surge for Asset Managers

• Storm surge is caused by wind pushing water toward the coast and low atmospheric pressure in the storm's center, leading to significant coastal flooding.

• Rapid water rise and hydrodynamic pressure from storm surges can damage properties by breaking doors, windows, and walls.

• Coastal cities like New York, Miami, London, and Singapore are highly susceptible to storm surge damage despite engineering defenses. Historical data on coastal flooding, combined with climate change, indicate increasing risks to assets, necessitating better planning and adaptation strategies.

As a storm moves toward land, the wind blowing across the ocean pushes the water in front of it.

The greater the distance that the wind blows unobstructedly over the water, which is called fetch, the more water that gets pushed toward the coast. The water piles up along the shoreline, making the tide seem to be much higher - and so sometimes called a ‘storm tide’.

In the meantime, especially for tropical cyclones and other rotating storms, the lower atmospheric pressure in the centre sucks water up, raising the ocean’s level by around one centimetre for each millibar that atmospheric pressure drops. This can easily add 60-70 centimetres to the sea’s level in the storm’s centre.

These two factors, fetch and atmospheric pressure, are called storm surge, leading to coastal flooding.

If the surge level matches the tide’s level and it strikes at low tide, then it might look as if the tide has not gone out - but a storm surge’s height can easily exceed the entire tidal range.

If the surge hits at hide tide, then storm surge can inundate up to around ten metres above the usual high tide mark, with tide and waves adding several metres more of height.

It can overtop sea walls, flooding properties that owners thought were protected. Then, waves on top of the storm surge can lead to even more damage. Storm surges can travel upstream along rivers and estuaries, impeding the drainage of rainfall (which often accompanies a storm).

In addition to coastal flooding - exacerbated by subsidence from extracting groundwater and fossil fuels, as well as the weight of buildings sometimes - surface flooding and river flooding damage assets.

The forces and energies associated with storm surge can bring highly damaging physical risks.

• When the water rises rapidly outside of a property, perhaps due to a sea wall breaching, then the pressure outside is much greater than the pressure inside, possibly blowing open doors, smashing windows, and breaking walls.
• If the water rises slowly enough outside that it infiltrates into a property at a similar rate, then the pressure difference is limited, but everything inside including electrics, plumbing, and furniture could be ruined by the salty, dirty water. Above the final flood depth, walls can be become damp and damaged from humidity and capillary rise which is when water creeps up through materials.
• As storm surge moves in, including from a sea wall breaking, the water imparts a pressure proportional to the square of the water’s speed on any property that it encounters. This hydrodynamic pressure can break doors, windows, and walls while ripping off exterior features such as cladding and drains.
• Once a property is surrounded by water and inundated, a buoyancy force tries to push the structure upward. If the building is not fastened securely to the foundation, then it can float. Mobile homes and caravans are a common example. These floating properties can collide with others, causing further damage and even pushing other properties off their own foundations.
• Fast-moving or turbulent storm surge water and related flooding can scour around properties, eroding foundations or the earth around the foundations. Properties might be undermined, collapsing into the water or floating away.
• Other debris is picked up by the floods, including trees, vehicles, and propane or diesel tanks. This debris crashes into properties, again possibly smashing through doors and windows or cracking walls. Large enough debris can knock small enough properties off their foundations.
• Waves, especially when they break, bring immense forces on properties, while also scouring and undermining under the surface.
• Chemical contaminants might be oil, petrol, diesel, household cleaners, and industrial solvents, all damaging assets and increasing the cost of clean-up.
• Many places need to contend with floods flushing out fire ants, snakes, spiders, scorpions, and other critters, all of which could enter properties. Mammals might be flushed out of their habitats by an incoming surge, escaping through urban areas. When a flood retreats rapidly, it can leave behind sea creatures from fish to seaweed, all requiring clean-up to make a property useable again.

These factors add up to significant damage.

Along the US east coast, storm surge can account for up to half of a hurricane’s losses. Along England’s east coast, some places are so low-lying that storm surge would result in the bulk of the damage.

Ultimately, any location prone to sea storms near a coast without steep topography could be flooded by a storm surge.

Even with steep topography, assets atop a cliff can disappear into the sea if a storm surge or waves take out the cliff. Eastern England, such as Scarborough and Dunwich, experience these physical risks.

For the US coastline, major cities from New York down to Miami and then along the Gulf Coast to New Orleans and Houston display large physical risks to storm surges.

The most lethal US landfall tropical cyclone (hurricane) so far ripped through Galveston, Texas - just southeast of Houston - in 1900, leaving large debris piles where thousands of people had lived.

Other major cities of the world that could easily experience a damaging storm surge include Hamburg, London, Rotterdam, St. Petersburg, and Singapore.

All of them have built a large barrage or barrier, along with associated walls, which aim to keep out most storm surges from their centres.

Extreme coastal flooding could surpass the design specifications along with the ever-present threat of sabotage or mechanical failure.

Shanghai, Manila, and Ho Chi Minh City are among others with long histories of storm surges and plenty of assets exposed to the physical risks or coastal flooding.

Most major cities susceptible to storm surge offer a long data set of previous coastal flooding, the physical risks, and the consequences.

However, this past data does not always suffice to indicate the current or future physical risks from storm surge.

The primary change is from human engineering. Miami built barrier islands, laden with chic properties. The world financial centre of Canary Wharf in London developed after the storm surge barrier across the Thames started operating. The city of Singapore has engineered and built alongside most of its rivers and bays.

As humanity changes the climate, generally intensifying storm surge and coastal flooding, the biggest change to the physical risks is from building assets in places that could get wet.

For some cities, detailed data describes asset materials, sizes, shapes, and values.

At Climate X, we modelled and calculated storm surge risks for our users, especially under different tropical cyclone, coastal engineering, and climate change scenarios. And more important, its financial impact on existing and future assets.

These analyses represent an important step forward in planning for and adaption to the future.

Asset managers who know what storm surge is, how it is changing, and why and where it could be a concern are opening doors for assessing, managing, and adapting to the physical risks to their assets.