Unit 6 Tsunami Inundation

1 Introduction to
2 Tsunamis of
   the Past
3 Plate Tectonics
4 Tsunami Generation
5 Tsunami Propagation
6 Tsunami Inundation
7 Tsunami Aftermath
   and Response

6.1 What is Tsunami Inundation?

Essential Question:
Which factors affect how tsunamis impact coastlines and communities and how does this knowledge help us prepare for the next tsunami?

Enduring Understanding:
Understanding and modeling how tsunami waves have inundated, or flooded, coastal areas in the past helps experts issue warnings, forecast the extent of inundation after tsunami generation, and prepare inundation maps and evacuation routes to increase hazard preparedness at the local level.


Unit 6 explores how tsunami waves behave as they approach land and inundate coastlines and how a tsunami’s impact is measured. Bathymetry, coastal topography and elevation determine how tsunami waves will behave and interact and how far inland they will reach. Past events and data allow researchers to model tsunamis, increasing the capability of officials to warn communities in advance, after a tsunami is generated. Locally-generated tsunamis are the exception. In those cases, the only warning may be a strong earthquake.

What is Tsunami Inundation?

Tsunami inundation is the final and most destructive phase of tsunami evolution. The first two phases, generation and propagation, were explored in previous units. Tsunami inundation refers to the distance inland that a tsunami wave penetrates and varies for each different coast or harbor affected by a tsunami. Simulating this process is very difficult because of the complex variables associated with varying bathymetry, topography and elevation along coastlines. How tsunami waves affect coastlines is determined by the bathymetry of the ocean floor, the elevation and topography of the shoreline, the resonant period of a particular bay, basin, or coastal shelf and the direction, or flow, of the tsunami. Wave behavior at the shore depends on the relationship between wavelength and water depth and wavelength and height. Some of the greatest inundation and runup from tsunami waves have been produced as a result of seiche, when the period of oscillation of a coastal feature creates constructive interference with subsequent tsunami waves.

As tsunamis reach the coast, they most often look like a rapidly rising tide or a flash flood. Because large tsunamis are rare, people were generally unfamiliar with the way a tsunami looked until widespread, easily viewable footage of the 2004 Indian Ocean tsunami and the 2011 Japanese tsunami were made available. The image of the massive shore-breaking waves, reinforced by depictions of tsunamis in popular culture, has added to dangerous misconceptions and misinformation about tsunamis.

Tsunami waves are very unique and fundamentally different from wind waves or swells. Not only do tsunami waves have a much longer wavelength than wind-generated waves, but they also have a much longer period, or length of time between wave crests. Tsunamis have periods up to an hour between waves, which means it could take thirty minutes for a crest to pass. In contrast, a typical swell that would draw surfers would have a period from 17-20 seconds. Wind chop would have a much shorter period of only a few seconds. The “peeling” behavior seen in surfing waves is a result of a short period, where a crest takes a relatively short amount of time to pass. This is why some tsunamis start with a massive receding of the water level followed by a surging flood. In that case, the trough of the wave arrived first, and it can take more than thirty minutes for the water to surge and inundate the coastline. 


6.2 Mapping

Tsunami Inundation Mapping

Civil defense and government agencies in communities affected by tsunamis have prepared evacuation maps detailing evacuation routes and zones. In Hawai'i such maps are printed in the first pages of each island's telephone directory and are also available online. In some areas, evacuation routes are marked by road signs. Unfortunately, there are none on O'ahu, and signs differ between Hawai'i County and Maui and Kaua'i counties.

Evacuation maps are prepared based on studies of the inundation of previous tsunamis but are not completely "fool proof" due to the variability of tsunamis. In Hawai'i the tsunamis of 1946 and 1960 showed a marked difference in wave heights and inundation. Scientists concluded that while each tsunami is unique, the location of a tsunami's source may be one of the major factors in determining flooding patterns and potential damage. Tsunamis generated in the Aleutians produced very different inundation patterns than the 1960 Chilean tsunami.

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Tsunami inundation is usually determined by a combination of measurements and eyewitness accounts. Measurements are taken of erosion depth and tsunami sand deposit thickness and chemistry to determine the extent of inundation, or how far inland the water traveled. Sand grain size, distribution, composition and chemistry indicate a great deal about the path of a tsunami and tsunami inundation extent. Scientists also rely heavily on microfossils (foraminfers, diatoms, etc.) to differentiate tsunami deposits from hurricane deposits, for example.

Tsunami Evacuation Zone Maps for Hawai'i

In Hawai'i, tsunami evacuation zone maps are available in the front of phone books and online. The maps are based on actual data from past tsunamis plus new information learned from numerical models about the expected behavior of tsunamis from different source areas. State Civil Defense and the University of Hawai'i work together to improve the information using the latest scientific evidence. In addition to patterns observed from past tsunamis, traffic patterns and congestion, elevation, population and other factors must be considered when preparing the maps. The Pacific Tsunami Museum and other organizations have conducted workshops in tsunami hazard preparedness, including dissemination of tsunami evacuation
zone maps.

6.3 Instruments Used to Issue Official Warnings

With the aid of new technology and instruments, researchers are able to issue detailed warnings and inundation forecasts minutes after tsunami generation. One of the most important recently developed technologies for detection of tsunami generation has been the installation of Deep-ocean Assessment and Reporting of Tsunami (DART) buoys. The first six were developed and installed in 2001, and by March 2008, 39 buoys were located at sites with a history of generating destructive tsunamis. DART buoys play an essential role in early detection and real-time reporting of tsunamis.

How the Warning System Works

The Tsunami Warning System begins with earthquake detection. When earthquakes greater than 6.8 occur in the Hawaiian Islands or greater than 7.0 elsewhere in the Pacific, special alarms go off and scientists at the observatories then determine the epicenter of an earthquake. If an earthquake occurs near the coast and is strong enough to cause a tsunami, a “watch” is issued, placing officials on high alert. The use of DART-buoys and tide-gauging stations help determine whether or not a tsunami has been generated. A tsunami warning is issued if the warning center receives reports from DART buoys, tide gauges or observers indicating that a tsunami has been generated.


The Fire of the Rice Sheaves; or How Hamaguchi Gohei Saved Villagers' Lives

Whether advanced technological instruments detect a tsunami, or whether an observer notices natural signs, human response and alerting others is essential for saving lives. The Japanese story of the rice sheaves aptly illustrates this point, telling the story Gohei, a village leader, who notices signs of an impending tsunami and saves his village by setting fire to their food supply. Alarmed villagers climb the hill to the storage areas, thereby going to higher ground and saving themselves from the ensuing tsunami.

This story is still used today, orally, in booklet form, and as a puppet play, as a tsunami education tool as a way to educate people on how to recognize natural warning signs and urging them to take action. Because the story tells of an earthquake, the tsunami was most likely generated locally. Scientists and others have enjoyed trying to find physical evidence to pinpoint an accurate date for the earthquake and tsunami described in this story. Some point to the earthquake and tsunami of 1854, others to tsunamis of 1808.

6.4 Experimental Systems in Inundation Forecasting

Inundation modeling is essential for coastal communities. After a tsunami is generated,  modeling data assists in determining which, if any, locations will be affected by tsunami waves. The most sophisticated inundation forecasting system currently under development is NOAA’s Short-term Inundation Forecasting for Tsunami (SIFT) system. SIFT is what is known as a composite forecasting system: it synthesizes a number of real-time and modeled data sources to provide rapid forecasting data about potential inundation in coastal areas around the world.


MOST/DART and Near-Shore Modeling

The current Tsunami Warning System utilizes the Method of Splitting Tsunami (MOST) model, which captures real-time data from its array of Deep-ocean Assessment and Reporting of Tsunamis (DART) buoys. The system provides essential for­ecasting information to the Tsunami Warning System, as well as important generation and propagation data for research. Despite this, the system is not without its shortcomings. While the DART system is well suited for generating forecast warning data (reporting the generation as well as intensity of a tsunami event), MOST and DART data cannot effectively forecast near-shore wave behavior.

The typical use of MOST is to identify tsunamis during trans-oceanic propagation. For that purpose, the low-density array of DART buoys are ideal, allowing forecasting of model wave behavior throughout the deep ocean. However, once a wave enters shallower water, the increasing complexity of the bathymetry and the topography of the shoreline add a large number of highly variable factors to the behavior of the waves as they move toward shore. A composite modeling system would overcome this shortcoming by improving shallow-water and inundation forecasting.


A Composite Modeling System

While MOST modeling cannot accurately forecast the complexities of inundation and near-shore wave behavior, the SIFT system relies heavily on MOST data to develop its modeled inundation forecast from real data. When a tsunami is generated, the DART array gathers and transmits data to NOAA, which is then integrated into the SIFT forecast process. SIFT takes the data from the DART buoy array and compares that real-time data to a large database of pre-run tsunami propagation models. These propagation models provide tsunami travel estimates and arrival times for various coastal communities throughout the world. If the composite data from DART and SIFT identifies similarities between the real data and the models, it can identify severe risk to coastal communities. If those scenarios are identified, the system then begins modeling potential run-up scenarios.

An ideal tsunami forecast requires a balance of efficiency and detail. When potential runup scenarios are identified, rapid response is a necessity, but an inaccurate forecast can have massive repercussions. Some of the highest resolution near-shore bathymetric imagery is compiled in the Forecast Inundation Model (FIM), which currently holds high-resolution mapping for over forty coastal communities throughout the United States. The SIFT system has to quickly handle real-time data and ultimately integrate it into a numerical model. SIFT is able to use this modeled data to develop run-up forecasts by using DART data to set the boundary conditions for a given coastal area, which in turn provides variables for the Stand-by Inundation Model (SIM). These boundary conditions provided would not be sufficient to project inundation behavior on their own, because the propagation model assumes linear and constant behavior for trans-oceanic propagation, where the ocean floor’s depth exceeds the tsunami’s depth of influence (one half of the wavelength) and the constant of gravity. The exchange of this linear data to the Stand-by Inundation Model allows for that linear data to become the basis for numerous model scenarios of inundation wave behavior for rapid forecasting. Because response time is so essential in a tsunami situation, SIM runs at an optimized resolution for quick response, using a reduced-resolution model that represents an ideal compromise between efficiency and detail.

The SIFT system has been tested in a number of real tsunami events, including the 2006 Kuril Islands tsunami. To test the system, researchers compared the modeled forecast built from the SIM to tide gauges that recorded the run-up behavior.


6.5 Wave Behavior

Forces of Tsunami Inundation

Once a tsunami reaches a coastal community, it has massive potential for damage. Not all damage from a tsunami is due to the force of tsunami waves or surge striking structures on shore. Significant damage can occur in the flood conditions following the initial impact of a tsunami wave.

There are six major forces involved in damage caused by tsunami inundation: surge force, debris impact, hydrostatic force, hydrodynamic (drag) force and buoyant force. Of those forces, only two occur during initial impact, debris impact and surge forces. These may be the most destructive forces during tsunami inundation. The remaining three forces, hydrostatic, hydrodynamic and buoyant forces, occur following initial impact.


Surge Force

As advancing water in a tsunami wave approaches structures, the wave continues to surge inland and the wall of water runs into anything in its path. Surge force is most similar to what is typically imagined as the primary source of structural damage in tsunami inundation: the sheer force of a wave striking a structure. The effect of this force is levied on everything below the inundation height, but as a general rule broad walls are considered most susceptible to surges because more surface area is exposed to the wave. Particularly for smaller structures such as homes, if broad walls are facing the ocean’s shore, the structure will be more likely to suffer damage from surge force than a structure that has narrow walls facing the shore. Perhaps the most effective mitigation measure is to build structures above historic inundation heights. Oftentimes this is achieved through the use of pillars to build a structure that can stay above the inundation height.


Debris Impact Force

Although surge force is a significant risk during tsunami inundation, the magnitude of potential for damage due to debris impact increases that risk. Inundating tsunamis bring with them all objects in their path, from driftwood logs to automobiles, to entire structures. Tsunami waves can carry debris that poses a serious risk to any structures in the path of inundation. Debris impact force affects one localized area on a surface at any given moment; for example, when a four-foot long, two-foot diameter log strikes a building, it hits only a small area of the building, whereas surge forces affect all surfaces below the inundation height. When designing a structure to withstand inundation damage, individual components of a structure should be strong enough to not only sustain surge force pressures, but also potentially large debris impact forces.

Hydrodynamic or Drag Force

Hydrodynamic or drag force occurs because tsunami waves are very long, and is one reason why tsunami inundation can be so devastating. It can take longer than five minutes for the crest of a tsunami wave to pass due to the sheer length of the wave. After the initial surge hits a structure, water continues to run past the structure quite rapidly. Tsunami speeds during inundation can be significantly faster than a person can run, on average around fifty miles per hour. The inundating wave continues to pass at this rate, and while it does so there is a drag force on any surface that provides resistance to the flowing water. The force exerted on structures is dependent on the velocity of the tsunami and the tsunami bore. Possible structural solutions include building circular pillars or piles rather than rectangular ones.


Hydrostatic Force

When a coastal community is inundated by a tsunami waves, inland flooding is incredibly likely. Although tsunami waves eventually recede, they leave large amounts of water behind, particularly in low-lying areas. Hydrostatic force results from the pressure of water pushing on all sides of a structure when water is at a significant depth and relatively still.


Buoyant Force

Buoyant force results from the tendency of structures and many of their components to float. In an inundation situation, structures displace water that floods an area during the tsunami. That displacement causes a buoyant force as the water attempts to return to equilibrium. While buoyant force, in combination with other forces, can result in an entire structure being swept away, buoyant forces most often manifest themselves by influencing individual structural elements.


Tsunami Forces and Impacts


6.6 Review

Take the following practice quiz to review content covered in Unit 6.

  1. What are the three phases of a tsunami evolution?

  1. What is tsunami inundation?
    Tsunami inundation refers to how a tsunami is generated, or its place of origin near the earthquakes epicenter in the ocean where subduction occurs.

  1. Why is modeling tsunamis important?

  1. Why is simulating tsunami inundation difficult?

  1. What popular misconception has been dispelled by commonly available video footage from the 2004 Indian Ocean and 2011 Japanese tsunamis?

  1. How do researchers determine tsunami inundation extent, or how far inland the water traveled?

  1. What size earthquake puts scientists on alert to determine whether a tsunami has been generated?

  1. How did the Japanese village leader Gohei save the lives of 400 villagers in the tsunami story told to educate children and adults?

  1. What innovation does the SIFT modeling system currently under development offer that other modeling systems such as MOST cannot do well?

  1. What is debris impact force?