Landfall and Famous Tsunamis

When a tsunami reaches land, it hits shallower water. The shallow water and coastal land acts to compress the energy traveling through the water. This starts the transformation of the tsunami.

The topography of the seafloor and shape of the shore begins to affect the tsunami's appearance and behavior. Also, as the velocity of the wave diminishes, the wave height increases considerably -- the compressed energy forces the water upward. A typical tsunami approaching land will slow down to speeds around 30 miles per hour (50 kph), and the wave heights can reach up to 90 feet (30 meters) above sea level. As the wave heights increase during this process, the wave lengths shorten considerably. (Think of squeezing an accordion.)

A witness on the beach will see a noticeable rise and fall of beach water when a tsunami is imminent. Sometimes, the coastal water will disappear completely as it is drawn into the tsunami. This amazing event is followed by the actual trough of the tsunami reaching shore. Tsunamis most often arrive as a series of strong and fast floods of water, not one single, enormous wave. However, a bore, which is a large vertical wave that arrives with a churning front, may appear. Bores are often followed by rapid floods of water, which make them particularly destructive. Other waves can follow anywhere from five to 90 minutes after the initial strike -- the tsunami wave train, after traveling as a series of waves over a long distance, is releasing itself onto land.

Tsunamis typically result in staggering body counts. This is especially true when they strike without warning. Tsunamis can level development and strip away coastlines, pulling everything in their path out to sea.

Photo courtesy National Geophysical Data Center Tsunami breaking over pier in Hilo, Hawaii

Photo courtesy National Geophysical Data Center Wreckage of a political party clubhouse from a tsunami that hit the Aleutian Islands in 1946

The areas of greatest risk during a tsunami strike are within 1 mile (1.6 km) of the shoreline, due to the flooding and scattered debris, and less than 50 feet (15 m) above sea level, due to the height of the striking waves.

A tsunami is even capable of reaching sheltered areas due to varying land features and the underlying seascape. For instance, a protected bay area with a narrow inlet can give a tsunami a "funnel" to travel through, amplifying the destructive power of the waves. Also, a river channel can provide room for a tsunami bore to rush through, allowing it to flood tremendous tracts of land.

Until a tsunami strikes, it's difficult to predict how it will interact with the features of the affected land. The wrap-around effect occurs along island coastlines when multiple wave strikes hit different areas of surrounding land, resulting in different degrees of flooding. Harbor resonance is a chaotic and highly destructive tsunami side effect created when waves continuously reflect and bounce off of the edges of a harbor or bay. Harbor resonance can cause the amplification of circulating wave heights and even increase the duration of the wave activity within the area.