The ABCs of Coastal Erosion for Asset Managers

Coastlines are inherently dynamic, they always change. That is nature. 

Main mechanisms are waves, currents, and sea-level rise. Waves pound the shore and currents sweep in and out, carving through rocks and cliffs. They deposit or sweep away sediment.

Waves and currents are not just in-and-out. They can move parallel or near-parallel to a coastline, shifting sediment in a process called longshore transport, sometimes still referred to as longshore drift.

These processes happen day-to-day, layering on the tidal cycle, with the water moving in and out in its daily, monthly, and annual rhythms. Sediment is always being dropped by this water-in-motion as it also undercuts and pulls away soft material.

Storms, even far away, and tsunamis add immense power to waves and currents.They can create step changes in a coast by ripping inland or dumping huge amounts of material to extend the land into the sea.

Another step change in coastlines occurs during earthquakes.

Land can drop or rise several metres in under a minute, with subsidence submerging nearshore assets or suddenly offering much more land on which to build. Some types of soil liquefy during earthquakes, permitting them to be swept away.

Earthquake-induced landslides can add or remove material from coastlines. Aside from tsunamis and landslides, earthquakes can produce seiching in bays, harbours, lakes, and reservoirs when the water sloshes back and forth. These waves cut into coasts or deposit sediment to extend them.

Other processes change the height of the ocean relative to the land.

Relative sea-level rise or relative sea-level fall is when land rises or falls while the ocean remains steady. Absolute sea-level rise or absolute sea-level fall is when the ocean itself changes its height.

During the last Ice Age, so much water was frozen in the huge glaciers that sea level was over 100 metres lower than today. More recently around the Pacific, the fourteenth century witnessed a major fall in sea levels stranding many communities inland which were abandoned.

Today, the Ice Age continues to show its consequences.

The mass of the ice sheets was so great that it pushed down country-sized chunks of land. With the ice sheets gone, this land rebounds, adding metres of new land per century in places such as Norway and Scotland. Boathouses built in Norway before its full independence in 1905 now sit by the forest, too far to be useful as a launching pad.

This phenomenon is called isostatic uplift with Scotland’s gain being England’s loss, because the rebound in the northwest tilts Britain so that it subsides in the southeast. Assets in uplifting parts of Scotland are much less affected by sea flooding than those in subsiding parts of England.

Other locations are being flooded by relative sea-level rise, because groundwater or fossil fuels are being extracted from underneath the land.

Where wetlands are degraded, perhaps due to pollution or cutting channels, the land can subside, permitting the sea to encroach onto assets. The Isle de Jean Charles, Louisiana experiences all these, so the people living there are deciding how and where to move.

Meanwhile, absolute sea-level rise is a major concern now and will accelerate in coming years.

As the world’s air temperature rises due to climate change, snow, ice, and permafrost melt, adding volume to the ocean. The ocean also absorbs heat from the atmosphere, reducing its density and causing it to expand in volume. Both these phenomena lead to rising sea levels globally, impacting coastal erosion and sedimentation.

If the huge ice sheets over Kalaallit Nunaat (Greenland) and Antarctica melt substantially, then shorelines around the world will be inundated and eroded over coming centuries, including for cities such as London and New York.

Never-ending coastal changes create difficulties for asset managers and real estate valuation.

• If a property sits at a coast which is eroding, then it will be damaged by waves, tides, and storms–or simply fall into the water when the shoreline changes.
• If a property sits at a coast which is gaining land, then the value of overlooking the ocean or being by a beach could be lost.

To avoid too much loss of asset value, many adaptation measures try to keep coastlines static or try to avoid coastal erosion hitting properties.

Continual replenishing of sand keeps beaches suitable for tourists from Hilton Head, South Carolina to Dubai. Dubai has also constructed a pumping and drainage system to reduce beach erosion rates.

Data and modelling monitor how these actions are and will impact the coastlines, suggesting where it would be safer and less safe to invest in coastal and near-shore assets.

Similar analyses indicate the effectiveness of engineered works trapping sediment and preventing its removal. Groynes built out into the ocean break up currents and reduce wave power, keeping sand where it is and avoiding too much longshore transport.

Coastal engineering applies a range of approaches to maintain coastal stability. Examples are sloped walls, rocks, and tetrapod barriers, dissipating wave energy at the shore.

Honolulu built vertical walls in the sea several metres from the coast, reducing the ocean’s power at the beach.

Areas around Happisburgh in eastern England instead chose rock breakwaters – also called barriers or reefs – spending millions of pounds for varying success in erosion prevention. In some instances, erosion was displaced to other coastlines.

The Netherlands and Belgium use dunes extensively to keep beaches from disappearing. They are often combined with other techniques and the focus is typically on the assets behind the dunes.

The goal is not necessarily to stop the shifting sands, but rather to ensure that the sea never reaches beyond the dunes. When infrastructure is built on the beach, notably holiday units, they are not necessarily expected to be permanent structures with large values. Conversely, places in the eastern US and eastern England created and maintain wetlands and tidal zones as a buffer between the ocean and assets.

All these techniques, and their various combinations, trade off drawbacks.

Cost is the big one, since huge expenditures can be needed for fairly short stretches of coastline. Then, as with Happisburgh, action in one location can undermine another place. Significant ecosystem changes are forced, possibly running into problems with protected areas, heritage sites, and preferences for nature.

Where ecosystems are created, they might not be pleasant or relaxing. Marshes can bring mosquitos and few people enjoy traipsing across a swamp, even if on a boardwalk, in order to reach a beach. Combining local observations with more wide-scale climate data indicates to asset managers where they need to invest and divest.

Yet, no matter what we do, in the end the sea must always win.

Whether it advances or retreats, even as we tackle human-caused climate change, we will never be able to stop the global natural changes over centuries which filter down to local coastline evolution.

Any asset manager must use data and models to determine the timeframe over which they will continue to invest in a specific coastal location.