Coastal Flooding

By Alex Jackson

Causes of Coastal Flooding

Storm Surges

Storm surges are sudden rises in sea level caused by very strong winds, normally those found in hurricanes and cyclones. The strong winds essentially push the water on an ocean’s surface on top of more water, increasing the sea level and flooding coastlines. The conditions needed to create these strong winds are generally associated with low pressures, further increasing the sea level. The strong winds can create large and powerful waves that can overtop coastal defences so even if the rise in sea level doesn’t flood the coastline, the resulting waves likely will.

Logically, storm surges are most dangerous during high tides, since the sea level will already be elevated at this time.

Rising Sea Levels

Sea level rise is discussed in more detail here.

As sea levels rise due to climate change or isostatic rebound low lying coastal areas are permanently flooded by the sea. The likelihood and severity of storm surges also rises since weaker winds will also be able to increase the sea level enough to flood coastal areas.

Tsunamis

Tsunamis are giant waves resulting from earthquakes, volcanic eruptions, meteor impacts, any sort of major displacement of water in the ocean. Tsunamis are incredibly dangerous since they travel quickly and are difficult to detect. Out at sea, close to the source of the tsunami, the amplitude of the wave is relatively low making it difficult to detect but it travels very quickly (over 800kmh-1). As it approaches the shore the wave slows down significantly but its amplitude increases exponentially without the wave breaking. When the wave hits the coast it does so with an immense amount of energy and its amplitude continues to grow as it slows down even more. Tsunamis have so much energy that they can travel several miles inland.

Tsunamis flood vast expanses of land and cause immense amounts of damage due to the energy they impact the coast with and the fact that they collect debris as they inundate more areas.

Reclaimed Land

Many coastal settlements have developed onto what is known as “reclaimed land”. This is land that has been gained from the sea due to coastal management or, somewhat ironically, lowered sea levels. This land, while highly valuable, is low lying and flat, so a small rise in sea level from a mild storm surge is enough to flood it and cause extensive damage.

March 2011 Tōhoku Earthquake & Tsunami (Japan)

On March 11th 2011, a magnitude1 9.0 earthquake occurred with an epicentre that was around 70km from the Tōhoku region2 of Japan. The earthquake displaced water in the Pacific Ocean by nearly 6m creating an incredibly powerful tsunami that devastated the eastern coastline of Japan. The first wave of the tsunami took around an hour to reach the coastline and flooded Sendai airport. From this point, several waves impacted the coast with heights ranging from 3m to 20m in different areas around Japan. The height of the waves was predicted to be between 3m to 6m. In the city of Sendai, the tsunami flooded land 10km away from the coast.

Social Impacts

  • The tsunami killed thousands of people. The National Police Agency has stated that the death toll from the earthquake was 15,870 deaths of which 14,308 people drowned. Many more people are still missing. In the town of Minamisanriku, 10,000 people went missing.
  • The tsunami wiped entire towns off the map, destroying communities and uprooting & separating families. Out of 100,000 children who were forced out of their homes, many of them were separate from their parents.
  • Because of the large number of people killed, mass graves had to be dug for the dead as morgues and crematoriums were over capacity and damaged. With no where to preserve bodies, they had to be buried before disease started to spread.
  • Over 700 cultural relics were destroyed by the tsunami and many museums and libraries were flooded.
  • A level 73 nuclear meltdown at the Fukushima Daiichi power plant resulting from the tsunami forced 1700 people to evacuate their homes in the area surrounding the accident. They will not be able to return to their homes for a long time since they have been contaminated with radioactive materials.

Economic Impacts

  • Through a combination of the tsunami and earthquake (with the tsunami doing most of the damage) 45,000 buildings were destroyed and 145,000 were damaged. The estimated cost of the disaster is in the tens of billions of dollars.
  • Industry was heavily disrupted due to power cut outs caused by the loss of a power plant, the severing of electrical pylons and the loss of oil due to a fire at an oil refinery.
  • Some of the worst affected areas by the tsunami were ports and fishing villages. 319 ports were damaged by the tsunami and had to be closed some for up to a year. The tsunami caused some ¥1.3 trillion in damage to the fishing industry in the areas affected by the tsunami.
  • Most people who owned homes did not have earthquake or tsunami insurance and so will have to pay for repairs or reconstruction of their homes themselves or through government assistance.
  • 24,000Ha of farmland was destroyed by tsunami. The presence of salt on this farmland will make it un-farmable for many years now.

Environmental Impacts

  • The tsunami washed tonnes of debris out to sea including wooden planks, metal, cars, boats and oil. Debris has been found on coasts around the globe and pose risks to sea life and animals that live along coasts.
  • The nuclear meltdown at the Fukushima Daiichi power plant has released unknown but dangerous amounts of radioactive material into the atmosphere and water nearby the power plant. Food that was grown in Fukushima has now been banned because it was found to contain high levels of radiation.
  • The meltdown will have environmental impacts globally since the radioactive materials have entered the atmosphere.

Responses

  • In the immediate aftermath of the earthquake, a tsunami warning was broadcast that allowed people to get to higher ground. The tsunamis were taller than predicted though and many of the safe zones weren’t safe.
  • Urban search and rescue teams and the Japan Self Defence Force carried out search and rescue operations in the areas affected by the tsunami and earthquake.
  • The Japanese government requested assistance from South Korea, New Zealand, Australia, Germany, the USA and the UK and also made satellite data, showing the extensive damage caused by the tsunami, available to all relief efforts.
  • The Japanese Red Cross received $1 billion in donations and the American Red Cross received $120 million in donations towards their efforts in Japan.
  • Nuclear engineers from around the globe assisted the Japanese government in attempting to avert a nuclear meltdown at the Fukushima Daiichi plant.
  • Temporary housing was set up for those who lost their homes. By June 2011, 46,000 temporary homes had been set up and 20,000 people were staying in inns or hotels.
  • In the months following the disaster, the government set about rebuilding the many sea walls that had been destroyed by the tsunami and also increased the height of the sea walls still standing4.
  • Efforts were made to understand why the earthquake and resulting tsunami had been so powerful so that future earthquakes of a similar scale could be prepared for.

  1. That’s using the moment magnitude scale, not the richter scale.

  2. The Tōhoku region is located in the north east of Honshu, the biggest island that makes up Japan.

  3. That’s the highest level you can get. The only other accident to get this level was Chernobyl in 1986.

  4. The sea walls were designed for 10m waves and many of the waves were 10m in height however the east coast of Japan sunk by 1m during the earthquake so the sea walls were easily dwarfed by the waves.