Bathymetry and Shoreline Topography’s Affect on Tsunami Propagation
There are many things that affect the propagation of a tsunami. One major impact is the bathymetry of the seafloor in between the origin of the tsunami and the shoreline which it impacts. The factors of bathymetry can be separated into two major types; those that make the wave have a larger impact, and those that lessen the impact.
» Natural Barriers that Disrupt Tsunami Waves
The general detriments to the propagation of a tsunami wave have characteristics of drastically interrupting the sea floor, and therefore taking a portion of the wave’s energy away from it. They essentially cause the wave to dissipate some of its energy before it reaches the shoreline, therefore having a lesser impact on the area. An example of a detriment of a wave is a coral reef. When a tsunami impacts with a coral reef it can cause the tsunami to have a premature break, thereby loosing a portion of its energy. Although this greatly damages the coral reef and the wave does regain some speed after it passes over the reef, it can lower the level of damage to the shoreline. Berms also help to remove some energy from waves. Berms are embankments, and have the general same effect on a tsunami wave as a coral reef. Berms, however, do not get destroyed as easily as coral reefs because they do not have the extensive ecosystem associated with them like coral reefs do. A final example of a detriment to a tsunami wave is a deep trench parallel to the shoreline. This hurts the tsunami wave because when there are drastic changes in the seafloor it removes energy from the wave when the wave interacts with it.
» Natural Barriers that Enhance Tsunami Waves
There are also many enhancements to tsunami waves due to bathymetry. One aspect is trenches that run perpendicular to a shoreline. As opposed to the trenches that run parallel with shore and act as a buffer, the trenches that run perpendicular essentially funnel the wave in towards the shore. It focuses the strength of the tsunami to the portion of the shore that is exactly opposite the opening of the trench. In addition if the seafloor is smooth and gradually slopes up to shallow water the wave will have a larger impact on the shore simply because there is nothing for the wave to interfere with, and limited places for the energy to be dissipated.
» When a Wave Hits the Shore
Once the wave reaches shore there is also a series of interactions to be considered when thinking about the potential damage of an area. The coastal topography can either protect and area from most major damage or ensure the destruction of the infrastructure, land, and lives in the area. There are some important aspects of costal topography to consider when looking specifically at Peru and Micronesia. There are again two categories in which topography features can be divided into. Within the first category of protecting the shoreline the topic of bluffs or seaside cliffs that could potentially be present in an area is found. This feature offers significant protection to an area, but is unfortunately hard to replicate except perhaps by a seawall. Luckily Lima, the capitol of Peru, has significant cliffs protecting most of it from a tsunami. Another feature that can protect the shore is mangroves or other densely packed vegetation on the shore. Although this is not as sturdy as cliffs, a large portion of the tsunami’s energy will be dissipated on the vegetation which will significantly help to lower the damage. Finally, levees provide temporary protection for an area as well. They can hold back flood water and provide a barrier that can shield an area. Levees however are not exceptionally sturdy as was seen in New Orleans, and if they are heavily relied on the consequences can be horrific.
The affect that bathymetry and coastal topography has on the tsunami waves allows for important information to be predicted about future waves. Algorithms can be created using data sets of bathymetry and topography to determine the risk of an area. Other important pieces of information that should be included within the algorithm would be data like past tsunamis, past earthquakes, the magnitude and damage occurred due to them. This coupled with the bathymetry and topography data can be used to determine what areas are at the highest risk so they can be the most prepared and all areas can plan accordingly.
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Lida, K, and T Iwasaki. Tsunamis: Their Science and Engineering. Boston: D. Reidel, 1983.
Pennisi, Elizabeth. "Powerful Tsunami's Impact on Coral Reefs Was Hit and Miss." Science 4 Feb. 2005: 657. MIT. 20 Sept. 2005.
Yalciner, Ahmet C., Efim N. Pelinovsky, Emile Okal, and Costas E. Synolakis. Submarine Landslides and Tsunamis. Boston: Kluwer Academic, 2003.
Methods of Discussing Risk Assessment
A group of scientists decided in 2000 that they wanted to determine an effective and simple way of assessing the risk for Japan’s coastline to the impact of a tsunami. This, they believed was very important due to the infrastructure along Japan’s coastline and its location of being exposed directly to the entire Pacific Ocean which is very seismically active. In order to come up with a quantitative risk assessment they designed a series of algorithms which could be solved in order to determine a numeric value of risk for a given area. They mainly wanted to determine the way that was most effective evacuate people so the citizens would have the least amount of risk of death or injury as possible. The main basis for their assessment of risk was based on the fault lines of the surrounding area.
These are the three general equations that the scientists came up with. The second and third terms in equations one and two do not apply in depths over 50 m.1
Equation 6 is based on the linear propagation of waves.1
The equations are evaluated under certain parameters (P) and a range is determined in which certain areas are understood to have the wave hit sooner and with more power based on the specific coastline in the area. The areas with the highest risk will have the larges run up and the soonest impact time. The numbers obtained through these calculations were used to inform the public within the studied areas in Japan the situation in which they were living, and how they should act in case of a tsunami.
1. Sato, Hiroaki, Hitoshi Murakami, Yasunori Kozuki, and Naoaki Yamamoto. "Study on a Simplified Method of Tsunami Risk Assessment." Natural Hazards (2003): 325-340.
» International Cooperation
A tsunami is by nature a global natural disaster. Propagation of tsunami waves most often occurs in open ocean, where the waves can spread to many countries. This was seen in the December 2004 tsunami that spread from Indonesia to parts of Asia and Africa and eventually the Americas. An earthquake that occurs underwater near Japan could create a tsunami that hits not only Japan but Micronesia as well. The ideal warning system must allow for the possibility that a tsunami may originate at a close or far location from the area being studied. At present, the international community is in the process of implementing such a network around the world as priority and funds allow. As a Mission group, we worked to determine the best sensors and sensor placement for the areas which we are studying in reference to the plans already in the works, with the hope that the planned sensors will be put in place soon.
» Tsunami Formation and Propagation
Tsunamis are caused in three ways. The most common, and usually most devastating, are earthquakes. Underwater landslides can also cause tsunami waves, as they abruptly distort the sea floor morphology. Finally, tsunamis can be caused by meteor impacts over open water; however, these are infrequent enough that our team did not specifically plan this cause.
Once a tsunami or landslide occurs, the strength of the wave once it reaches land is determined by several factors. The sea floor morphology will either channel or dissipate the wave’s energy. The more mountainous features underwater, the more the wave will be disrupted and the tsunami wave will be smaller. Coral reefs work to dissipate wave energy in this way. At high tides, a tsunami will be much larger than at a low tide. Areas with higher sedimentation rates have more rock shifting during the earthquake or tsunami, resulting in larger waves.
» Sensor Design - Satellites
Our team looked at several methods of tsunami detection. The two main proposed methods were remote satellite sensing and buoys utilizing a bottom-pressure recorder. Satellites can “see” the tsunami waves as they propagate across open water. A satellite happened to be over the Indian Ocean at the time of the Sumatran tsunami and pictures taken from the satellite showed how the tsunami wave moved around the world. While this data was very useful in reconstructing the event after it occurred, it was not part of the warning system. The particular satellites that have the instruments that can detect tsunamis are not in geosynchronous orbit and there are not enough of them to ensure that a satellite would be within range at the point in time when a tsunami would occur. The data gathered from the satellite in the Sumatran tsunami was not accessed until several hours after the earthquake hit, much too long for it to be useful during an evacuation. Currently, the Global Earth Observation System of Systems (GEOSS) project is planning for such a sensing network of satellites that measure many different things that would be put in place over the next ten years. However, our team has determined that a more immediate, accessible method of detection is needed.
» Sensor Design - Buoys
At present, the most effective warning system utilizes DART II (Deep-ocean Assessment and Reporting of Tsunamis) buoys designed and monitored by the National Oceanic and Atmospheric Administration (NOAA). These “buoys” consist of two main parts – the bottom package and the buoy. The bottom package consists of a Bottom Pressure Recorder (BPR) which uses a quartz-crystal resonator that can detect changes in bottom pressure and water height within millimeters. There is also a thermometer to counteract the influence of temperature on the pressure readings. There is a tilt sensor, a battery and a transmitter that connects the bottom package to a surface buoy (Meinig, 2005). We propose to add to the DART II buoy a seismometer to the bottom package to gather additional earthquake data.
The surface buoy is a 2.5 meter diameter fiberglass foam disk buoy that displaces 4000 kg of water (National Data Buoy Center). It has two electronic systems in case one system malfunctions or fails. It also includes a receiver to receive transmissions from the bottom package and a transmitter to send the data to GOES GPS satellites, from where it is sent to warning centers. The buoy is capable of two-way communication. We propose that a short-wave radio transmitter also be added to the DART II buoy that could send the data to a site on land close to the buoy. That way, the time delay that results from sending the information to the international warning center is eliminated and the area about to be hit will have a few extra minutes to begin preparing.
Generally, the BPR will continuously monitor the pressure and record fifteen second averages, which are sent together in a single transmission every six hours. This is known as “standard mode.” When a tsunami occurs, the buoy detects high pressures and enters “event mode.” The above-average pressure rating for the fifteen second interval is sent immediately as message #0 via satellite, and then pressure readings are continually sent detailing the 15 second intervals every few minutes until the algorithms within the buoy determine that the tsunami parameters identifying a tsunami are no longer met. It takes from three to five minutes from when the first abnormal pressure reading is made to when the satellite transmission reaches a warning center (Meinig, 2005).
The DART design has been tested rigorously and currently has a 96% success rate at data return since 1998 (National Data Buoy Center). It can detect differences of less than a millimeter in water column height, and has a reporting delay of three minutes (Meining, 2005).
DART I system diagram, excluding our suggested improvements of a short-wave radio transmitter and a seismometer. Diagram from http://www.ndbc.noaa.gov/Dart/dart.shtml
» Financial Information
There are currently 11 DART buoys along the Alaskan, US Pacific, and parts of the South American coast line, as well as Hawaii. Each buoy costs approximately $250,000 as an initial cost, and requires an additional $125,000 per year to operate and maintain. To maintain the current system and expand it further, NOAA has estimated that it would require $1,200,000 a year. The four buoys around Micronesia would cost $1,500,000 initially to buy and maintain for one year; the one buoy in Peru would cost $375,000 to buy and maintain for one year (National Buoy Data Center).
» Sensor Deployment
Because DART II-style buoys are large, special boats are required to deploy them. BPRs have a standard life of two years and the surface buoys must be replaced yearly. The buoys are deployed in water about 6000 feet deep, beyond the continental shelf. The BPRs can get the best pressure readings at this depth, where surface currents do not cause disruption. This distance can be one hundred miles or more from the coast. The boat must be large and must be equipped with a crane, large winches, and an A-frame that extends over the back of the boat. For buoy deployment, the boat will use a buoy-first, anchor-last deployment technique in which the buoy will be lowered into the water and then, at a short distance away, the attached anchor mooring is dropped (Gates, 1998). The anchor is allowed to drop to the ocean bottom and the buoy will remain on top of it. The bottom package is designed to be lowered to the water’s surface and then allowed to free-fall through the water, with its own anchor already attached. The buoy and BPR transmitters should be check to ensure they are working before the boat returns to shore (Dever 2001).
» Sensor Placement - Peru
The continental shelf around Peru can change rapidly, which makes it more difficult to place a sensor there. A buoy should be placed every 1000 miles of shoreline. Chile has a buoy on its southern western coast, and is planning on placing one soon near the border of Peru on its northern shore. In light of this, we recommend that a buoy be placed at 8°25’ S, 85°43’W. This is a prime spot because it overlooks the rest of the country. It is at an area where it gives enough time for warning from a potential tsunami.
Diagram prepared using Google Earth
» Sensor Placement - Micronesia
In the area of Micronesia we are proposing four buoys to encompass the islands. The islands have two main trenches around them, and are also open to tsunami threats from a distance. The locations were decided upon by studying detailed geographic maps showing plate movements and seismic activity. By placing four buoys around the islands, we hope to maximize the detection capabilities of the buoys by preparing for a tsunami originating from a number of different directions.
The buoys are placed at:
Diagram prepared using Google Earth
» Warning Centers and Further Preparation
As a tsunami is by nature a global issue, it is essential to use global resources to coordinate efforts. At present, several international warning centers exist. Two of these are a center in Ewa Beach, Oahu, Hawaii, and a center in Palmer, Alaska in conjunction with the National Weather Service. Both of these sites are run by the US National Oceanic and Atmospheric Association. Due to its central location in the Pacific Ocean, we have designated Hawaii as the primary tsunami response center for the Pacific, where international warnings will be issued and analysis will take place. In the case that the Hawaiian center is unable to function in this capacity, the Alaskan center will assist in these tasks.
We also recommend that detailed analysis of the bathymetry and sea floor morphology of the areas around Micronesia and Peru take place. In a similar manner to a Japanese study (see Risk Assessment), we recommend that the algorithms taking varying hypothetical values for time, gravity accelerator, water level lift from still water level, water depth, friction coefficient of the ocean bottom, flux in the x and y direction, and the vertical amount of displacement (Sato, 2003). Running computer simulations for each area could create a database of that projects an estimated time of arrival and wave height from the data gathered by a buoy and data gathered about the earthquake or landslide. This is an important recommendation for other coastal countries as well.
Bernard, E.N. (2005, May). The U.S. National Tsunami Hazard Mitigation Program: A Successful State–Federal Partnership. Natural Hazards, 35(1), 5 – 24. Springer Science and Business Media B.V., Formerly Kluwer Academic Publishers B.V. Retrieved September 20, 2005 from SpringerLink database.
"Deep Ocean Assessment and Reporting of Tsunamis." National Data Buoy Center. Retrieved 10/11/2005 from http://www.ndbc.noaa.gov/Dart/dart.shtml.
Dever, E. and Harvey, P. (2001). "CoOP/WEST Buoy Deployment and Recovery Procedures." Retrieved 10/11/2005 from http://shipsked.ucsd.edu/schedules/2003/nh_2003/dever/coopdep.pdf.
Gates, P. D., Preston, G. L., Chapman, L. B. Preston. (1998). Roberts, M. (Ed.) (1999). "Fish Aggregating Device (FAD) Manual: Volume III: Deploying and Maintaining FAD Systems." Noumea, New Caledonia: Government of Taiwan/ROC. Retrieved 10/11/2005 from http://www.spc.int/coastfish/Fishing/FAD3_E/fad3_e.htm.
Meinig, C., Stalin, S.E., Nakamura, A.I. and Milburn, H.B. (2005), Real-Time Deep-Ocean Tsunami Measuring, Monitoring, and Reporting System: The NOAA DART II Description and Disclosure. Retrieved September 20, 2005 from http://www.pmel.noaa.gov/tsunami/Dart/Pdf/DART_II_Description_6_4_05.pdf
Sato, H., Murakami, H., Kozuki, Y., Yamamoto, N. (2003). Study on Simplified Method of Tsunami Risk Assessment. Natural Hazards, 29, 325-340.
National Data Buoy Center: Center of Excellence in Marine Technology. http://www.ndbc.noaa.gov/Dart/dart.shtml.