Earth Processes and Natural Hazards

Transcription

1 PART TWO Earth Processes and Natural Hazards Our focus in Part 2 turns to the major natural hazards: an introduction to hazardous processes (Chapter 5), earthquakes, including tsunamis (Chapter 6), volcanic activity (Chapter 7), flooding (Chapter 8), landslides (Chapter 9), coastal processes, including hurricanes (Chapter 10 ), and impacts of asteroids or comets (Chapter 11). The purpose is not to provide extensive amounts of detailed information concerning these natural processes that we define as hazards but to focus on the basics involved and the environmental concerns resulting from interactions between people and natural processes and hazards. The major principles presented are: 1) Earth is a dynamic environment, and change resulting from natural processes is the norm rather than an exception; 2) we must strive to learn all we can about natural processes and hazards so that effects on human society may be minimized; and 3) human population increase and changing land use are greatly increasing the threat of loss of life and property to natural hazards. Of particular importance is the recognition of locations where hazardous processes are going to occur and their natural or human-induced return period (time between events). We will learn that the environmentally preferred adjustment to natural hazards is environmental planning to avoid those locations where the hazards are most likely to occur and to zone the land appropriately. Environmental planning involves detailed study of natural processes and mapping of those processes to produce environmental maps useful in the planning process. 131

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3 Introduction to Natural Hazards Learning Objectives Natural hazards are naturally occurring processes that may be dangerous to human life and structures. Volcanic eruptions, earthquakes, floods, and hurricanes are all examples of natural hazards. Human population continues to increase, and there is a need to develop environmentally sound strategies to minimize the loss of life and property damage from hazards, especially in urban areas. The study of hazardous processes, therefore, constitutes one of the main activities of environmental geology. The learning objectives for this chapter are Understand why increasing population and changing land use increase the threat of loss of life and property from a natural disaster to the level of a catastrophe Know the conditions that make some natural Earth processes hazardous to people Understand how a natural process that gives rise to disasters may also be beneficial to people Understand the various natural processes that constitute hazards to people and property Know why history, linkages between processes, prediction, and risk assessment are important in determining the threat from natural hazards Know how people perceive and adjust to potential natural hazards Know the stages of recovery following natural disasters and catastrophes Hurricane Katrina, a giant storm Hurricane Katrina approaching New Orleans region aerial image. (NOAA) 133

4 134 Chapter 5 Introduction to Natural Hazards CASE HISTORY Hurricane Katrina, Most Serious Natural Catastrophe in U.S. History Hurricane Katrina (see opening photograph) made landfall in the early evening of August 29, 2005, about 45 km (30 miles) to the east of New Orleans. Katrina was a huge storm that caused serious damage up to 160 km (100 mi) from its center. The storm produced a storm surge (a mound of water pushed onshore by the storm) of 3 to 6 meters (9 to 20 feet). Much of the coastline of Louisiana and Mississippi was devastated as coastal barrier islands and beaches were eroded and homes destroyed. Property damage from Hurricane Katrina and costs to rehabilitate or rebuild the area may exceed $100 billion, making it the most costly hurricane in the history of the United States. The number of human deaths will never be known for sure, as many bodies may have been washed out to sea or buried too deep to be found. The official number of deaths is 1,836. The hurricane and subsequent flooding set into motion a series of events that caused significant environmental consequences. Initial loss of life and property from wind damage and storm surge was immense. Entire coastal communities disappeared with their fishing industry. At first it was thought the city of New Orleans had been spared, as the hurricane did not make a direct hit. The situation turned into a catastrophe when water from Lake Pontchartrain, north of the city and connected to the Gulf, flooded the city. Levees capped with walls, constructed to keep the water in the lake and protect low-lying parts of the city, collapsed in two locations and water poured in. Another levee failed on the Gulf side of the city and contributed to the flooding. Approximately 80 percent of New Orleans was under water from knee deep to rooftop or greater depths (Figure 5.1). People who could have evacuated but didn t, and those who couldn t because they lacked transportation, took the brunt of the storm. A part of the city that wasn t flooded was in the French Quarter (Old Town), which is the area of New Orleans famous for music and Mardi Gras. The people who built New Orleans over two hundred years ago realized that much of the area was low in elevation and built on the natural levees of the Mississippi River. The natural levees formed by periodic overbank deposition of sediment from the river over thousands of years. They are parallel to the river channel and are higher than the adjacent land, providing natural flood protection. As marshes and swamps were drained in lower areas, the city expanded into low areas with a much greater flood hazard. Much of the city is in a natural bowl and parts are a meter or so (3 to 9 feet) below sea level (Figure 5.2). It has been known for a long time that if a large hurricane were to make a direct or near-direct hit on the city, extensive flooding and losses would result. The warnings were not completely ignored, but sufficient funds were not forthcoming to maintain the levee and system of floodwalls to protect low-lying areas of the city from a large hurricane. The fact that the region is subsiding at highly variable rates from 1 to 4 m (3 to 12 feet) per 100 years contributed to the flood hazard. Over short periods, 50 percent to 75 percent of the subsidence in some areas is natural, resulting from geologic processes (movement along faults) that formed the Gulf of Mexico and the Mississippi River delta. 1 As much as several meters (more than 10 feet) of subsidence has occurred in the last 100 years, and during that period sea level has risen about 20 cm (8 in.). The rise is due in part to global warming. As the Gulf water and ocean water warm, they expand, raising sea level. The subsidence results in part from a number of human processes including extraction of groundwater, oil, and gas, as well as loss of freshwater wetlands that compact and sink when denied sediment from the Mississippi River. Because the Mississippi River has artificial levees (embankments constructed by humans), it no longer delivers sediment to the wetlands. Therefore wetlands have stopped building up from sediment accumulation. Before the levees were constructed in the Mississippi River delta, floodwater with its sediment spread across the delta helping maintain wetland soils and plants. The freshwater wetlands near New Orleans were largely removed during past decades. As wetlands are removed, they are replaced by saltwater ecosystems as sea level rises and the land continues to subside. The freshwater wetlands are a better buffer to winds and storm waves than are saltwater wetlands. Tall trees such as cypress and other plants of freshwater wetlands (Figure 5.3) provide roughness that slows down water from high waves or storm surge moving inland. It s well known that one of the natural service functions of both saltwater and freshwater coastal wetlands is to provide protection to inland areas from storms. Figure 5.1 Katrina floods New Orleans New Orleans was flooded when flood defenses failed during Hurricane Katrina. (N. Smiley/Pool/ Dallas Morning News/Corbis) New Orleans Will Be Rebuilt. The people of New Orleans are resilient and will move back into this famous historic city. We have learned from this event and better measures are being taken to ensure that this kind of catastrophe is less likely to occur in the future. For example, in some areas where flood waters, even with future levee failure, will be less than one story high, reconstruction includes flood-proof buildings by constructing living areas on the second floor. The U.S. Army Corps of Engineers, which is largely responsible for the flood protection of New Orleans, produced a draft report in June 2006 concerning the hurricane and flood protection system. They acknowledged that the system had evolved piecemeal over a number of decades and

5 Introduction to Natural Hazards 135 N km Mi Lake Pontchartrain University Of New Orleans New Orleans Lakefront Airport 10 Lake Willow City Park Golf Course A B Earhart Expy 61 Pontchartrain Expy 90 Alton Ochsner Medical Foundation Notre Dame Seminary New Orleans Tulane University Of Louisiana Chalmette National Historical Park Audubon Golf Club Mississippi River Gretna 90 Our Lady Of Holy Cross College Canal St. Flood A Univ. New Orleans Protection Downtown B ~ 7m Old Town New Orleans Flood Latin Quarter Protection OX Level ~ 6m Sea Level Mississippi River Natural Levee Drained Marsh & Swamp Levee Lake Pontchartrain Natural bowl (subsiding) Figure 5.2 Map and cross section of New Orleans Much of the city is below sea level between the Mississippi River and Lake Pontchartrain. (Edward A. Keller) that it was a system in name only. Nevertheless, the flood protection structures were constructed to protect the city and their failure was responsible for a majority of the flooding that resulted from Hurricane Katrina in Some conclusions of the report 2 are There were no fallback redundancies (second-tier protection) to the flood protection if the primary flood control structures failed. Pumping stations designed to remove floodwaters were the only example of a redundant system and these were not designed to function in a major hurricane with extensive flooding. Hurricane Katrina exceed the design criteria of the flood protection structures. Some levee and floodwall failures resulted from being overtopped by floodwaters. Others

6 136 Chapter 5 Introduction to Natural Hazards failed, without being overtopped, by erosion from their front side. Regional subsidence has been faster than was appreciated. The heights of the flood protection structures were not adjusted for subsidence, and some of the floodwalls and levees were as much as 1m (3 ft) below the elevation they were designed at. Although scientific knowledge about hurricanes and storm surges has increased, this did not lead to updating the flood control plan. What adjustment did occur was fragmented rather than consistent and uniform. Consequences of flooding were also concentrated. More than 75 percent of the people who died were over 60 years old and were located in areas with greatest depth of flooding. The larger number of deaths of the elderly occurred because poor, elderly people and disabled people were the least able to evacuate without assistance. Parts of the flood and hurricane protection system have been repaired at a cost of about $800 million since Katrina, and these are the strongest parts of the flood protection. At present, the protection level remains the same as before Katrina. In fairness to the Corps, flood control structures are often underfunded, and as with New Orleans, construction is spread over many years. Hopefully, we have learned from Katrina and will build a stronger, more effective hurricane protection system for the city of New Orleans, as well as other U.S. cities where hurricanes are likely to occur. An important question is, can flooding occur again even if higher, stronger flood defenses are constructed? Of course it can when a bigger storm strikes, future damage is inevitable. If freshwater marshes are restored and the river waters of the Mississippi allowed to flow through them again, they will help provide a buffer from winds and waves. As previously mentioned, freshwater marshes have trees that provide a roughness to the land that slows wind and retards the advance of waves from the gulf. Every 1.5 km (1 mile) of marsh land can reduce waves by about 25 cm (1 ft). Tremendous amounts of money will need to be spent to make New Orleans more resistant to future storms. In light of the many billions of dollars in damages from catastrophes, it seems prudent to spend the money in a proactive way to protect important resources, particularly in our major cities. In the remainder of this chapter, we will discuss some of the principles of natural processes we know as hazards and how they produce disasters and catastrophes. You will learn how poor land uses and changing land uses, coupled with population increase, greatly increase the risk of some hazards. Figure 5.3 Cypress tree freshwater wetlands along the coast of Louisiana. (Tim Fitzharris/Minden Pictures) 5.1 Hazards, Disasters, and Natural Processes Natural Disasters: Loss of Life and Property Damages Natural disasters, which are events that cause great loss of life or property damage, or both, such as earthquakes, floods, cyclones (hurricanes), have in the past few decades killed several million people, with an average worldwide annual loss of life of about 150,000 people. The financial losses resulting from natural disasters now exceed $50 billion per year and do not include social impacts such as loss of employment, mental anguish, and reduced productivity. Three individual disasters, a cyclone accompanied by flooding in Bangladesh in 1970, an earthquake in China in 1976, and a tsunami in the Indian Ocean in 2004, each claimed over 250,000

7 Hazards, Disasters, and Natural Processes 137 lives. These terrible disasters (catastrophes) were caused by natural hazards that have always existed atmospheric disturbance and tectonic movement but their extent was affected by human population density and land-use patterns. Why Natural Processes Are Sometimes Hazards Natural hazards are basically natural processes. These processes become hazardous when people live or work in areas where they occur. Natural processes can also become hazards when land-use changes, such as urbanization or deforestation, affect natural processes, causing flooding or landsliding. It is the environmental geologist s role to identify potentially hazardous processes and make this information available to planners and decision makers so that they can formulate various alternatives to avoid or minimize the threat to human life or property. However, the naturalness of hazards is a philosophical barrier that we encounter whenever we try to minimize their adverse effects. For example, we try to educate people that a river and floodplains, the flat land adjacent to the river, are part of the same natural system and that we should expect floods on floodplains as the name suggests. Minimizing the flood hazard may be as simple as not building on floodplains! However, this seemingly logical solution is difficult to get across to people who see floodplains as flat land on which to build houses. Magnitude and Frequency The impact of a disastrous event is in part a function of its magnitude, or amount of energy released, and frequency, or recurrence interval; however, it is influenced by many other factors, including climate, geology, vegetation, population, and land use. In general, the frequency of such an event is inversely related to the magnitude. Small earthquakes, for example, occur more often than do large ones (see A Closer Look: The Magnitude-Frequency Concept). Benefits of Natural Hazards It is ironic that the same natural events that take human life and destroy property also provide us with important benefits or natural service functions. For example, periodic flooding of the Mississippi River supplies nutrients to the floodplain, on which form the fertile soils used for farming. Flooding, which causes erosion on mountain slopes, also delivers river sediment to beaches (Figure 5.4) and flushes (a) (b) Figure 5.4 Dams and beaches (a) Sediment from the Ventura River in southern California is delivering some sand to beaches in the region; however, an old upstream dam (b) is storing sand that might otherwise nourish the beach. The dam is scheduled to be removed. ([a] Pacific Western; [b] Edward A. Keller)

8 138 Chapter 5 Introduction to Natural Hazards A CLOSER LOOK The Magnitude-Frequency Concept The magnitude-frequency concept states that there is generally an inverse relationship between the magnitude of an event and its frequency. For example, the larger the flood, the less frequently such a flood occurs. The concept also includes the idea that much of the work of forming Earth s surface occurs through events of moderate magnitude and frequency rather than by common natural processes of low magnitude and high frequency or by extreme events of high magnitude and low frequency. As an analogy to the magnitude-frequency concept, consider the work of reducing the extent of a forest by resident termites, human loggers, and elephants (Figure 5.A). The termites are numerous and work quite steadily, but they are so small that they can never do enough work to destroy all the trees. The people are fewer and work less often, but, being stronger than termites, they can accomplish more work in a given time. Unlike the termites, the people can eventually fell most of the trees. The elephants are stronger still and can knock down many trees in a short time, but there are only a few of them and they rarely visit the forest. In the long run the elephants do less work than the people and bring about less change. In our analogy it is humans who, with a moderate expenditure of energy and time, do the most work and change the forest most drastically. Similarly, natural events with moderate energy expenditure and moderate frequency are often the most important shapers of the landscape. For example, Figure 5.A Human scale of change Human beings with our high technology are able to down even the largest trees in our old-growth forests. The lumberjack shown here is working in a national forest in the Pacific Northwest. (William Campbell/ Sygma Photo News) pollutants from estuaries in the coastal environment. Landslides may bring benefits to people when landslide debris forms dams, creating lakes in mountainous areas (Figure 5.5). Although some landslide-created dams will collapse and cause hazardous downstream flooding, dams that remain stable can provide valuable water storage and are an important aesthetic resource. Volcanic eruptions have the potential to produce catastrophes; however, they also provide us with numerous benefits. They often create new land, as in the case of the Hawaiian Islands, which are completely volcanic in origin (Figure 5.6). Nutrient-rich volcanic ash may settle on existing soils and quickly become incorporated, creating soil suitable for wild plants and crops. Earthquakes can also provide us with valuable services. When rocks are pulverized during an earthquake, they may form an impervious clay zone known as a fault gouge along the fault. In many places, fault gouge has formed groundwater barriers upslope from a fault, producing natural subsurface dams and water resources. Along some parts of the San Andreas fault in the arid Coachella Valley near Indio, California, this process has produced oases, in which pools of water are surrounded by native palm trees in an otherwise desert environment (Figure 5.7). In addition, earthquakes are also important in mountain building and thus are directly responsible for many of the scenic resources of the western United States.

9 Hazards, Disasters, and Natural Processes 139 most of the sediment carried by rivers in regions within a subhumid climate (most of the eastern United States) is transported by flows of moderate magnitude and frequency. However, there are many exceptions. In arid regions, for example, much of the sediment in normally dry channels may be transported by rare high-magnitude flows produced by intense but infrequent rainstorms. Along the barrier-island coasts of the eastern United States, high-magnitude storms often cut inlets that cause major changes in the pattern and flow of sediment (Figure 5.B). Salt marsh Frontal sand dune line Beach Wavefront Ocean (a) Before hurricane Barrier island, sand 0 1 Km Salt marsh New sand spit New inlet (tidal flow) Wavefront Ocean (b) After hurricane New inlet and sand spit Dune vegetation denuded Frontal dunes eroded Salt marsh vegetation eroded Salt marsh vegetation Small trees on back dune areas Movement of sediment and water Figure 5.B Hurricanes change coasts Idealized diagram showing the formation of an inlet through a barrier island resulting from erosion during a hurricane. (a) Before and (b) after hurricane. Lake Dam Figure 5.5 Landslide dam Landslide dam forming a lake in Utah. (Michael Collier)

10 140 Chapter 5 Introduction to Natural Hazards (a) (b) Figure 5.6 New land from volcanic eruption New land being added to the island of Hawaii. (a) The plume of smoke in the central part of the photograph is where hot lava is entering the sea. (b) Closeup of an advancing lava front near the smoke plume. (Edward A. Keller) Death and Damage Caused by Natural Hazards When we compare the effects of various natural hazards, we find that those that cause the greatest loss of human life are not necessarily the same as those that cause the most extensive property damage. Table 5.1 summarizes selected information about the effects of natural hazards in the United States. The largest number of deaths each year is associated with tornadoes (Figure 5.8) and windstorms, although lightning (Figure 5.9), floods, and hurricanes also take a heavy toll. Loss of life due to earthquakes can vary considerably from one year to the next, as a single great quake can cause tremendous human loss. It is estimated that a large, damaging earthquake in a densely populated part of California could inflict $100 billion in damages while killing several thousand people. 3 The 1994 Northridge earthquake in the Los Angeles area killed approximately 60 people and caused more than $30 billion in property damage. In fact, property damage from individual hazards is considerable. Floods, landslides, frost, and expansive soils each cause mean annual damages in the United States in excess of $1.5 billion. Surprisingly, expansive soils, clay-rich soils that expand and contract with wetting and drying, are one of the most costly hazards, causing over $3 billion in damages (a) (b) Figure 5.7 Oases and faults (a) Native palm trees along the San Andreas fault, Coachella Valley, California. The fault dams groundwater that the trees use. (b) In some cases the water forms surface pools and an oasis. (Edward A. Keller)

11 Hazards, Disasters, and Natural Processes 141 TABLE 5.1 Effects of Selected Hazards in the United States No. of Occurrence Deaths Influenced by Catastrophe Hazard per Year Human Use Potential 2 Flood 86 Yes H Earthquake 1 50? Yes H Landslide 25 Yes M Volcano 1 <1 No H Coastal erosion 0 Yes L Expansive soils 0 No L Hurricane 55 Perhaps H Tornado and windstorm 218 Perhaps H Lightning 120 Perhaps L Drought 0 Perhaps M Frost and freeze 0 Yes L 1 Estimate based on recent or predicted loss over 150-year period. Actual loss of life and/or property could be much greater. 2 Catastrophe potential: high (H), medium (M), low (L). Source: Modified after White, G. F., and Haas, J. E Assessment of research on natural hazards. Cambridge, MA: MIT Press. annually to building foundations, sidewalks (Figure 5.10), driveways, and swimming pools. An important aspect of all natural hazards is their potential to produce a catastrophe. A catastrophe is any situation in which the damages to people, property, or society in general are sufficient such that recovery or rehabilitation is a (a) (b) Figure 5.8 Tornado hazard (a) Tornado in Tampa Bay, Florida, on July 12, (Brian Baer/ St. Petersburg Times/AP/Wide World Photos) (b) Mobile homes destroyed by tornado that struck Benton, Louisiana, on April 4, (Eric Gay/AP/Wide World Photos)

12 142 Chapter 5 Introduction to Natural Hazards Figure 5.9 Lightning strike Lightning is responsible for more than 100 deaths each year in the United States. Shown here are lightning strikes near Walton, Nebraska. (Joel Sartore/NGS Image Collection) long, involved process. 4 Table 5.1 shows the catastrophe potential for the hazards considered. The events most likely to produce a catastrophe are floods, hurricanes, tornadoes, earthquakes, volcanic eruptions, and large wildfires (not included in Table 5.1). Landslides, which generally cover a smaller area, have only a moderate catastrophe potential. The catastrophe potential of drought is also moderate: though a drought may cover a wide area with high financial losses, there is usually plenty of warning time before its worst effects are experienced. Hazards with a low catastrophe potential include coastal erosion, frost, lightning, and expansive soils. The effects of natural hazards change with time. Changes in land-use patterns that influence people to develop on marginal lands, urbanization that changes the physical properties of Earth materials, and increasing population all alter the effects of natural hazards. Although damage from most hazards in the United States is increasing, the number of deaths from many hazards is decreasing because of better hazard forecasting and warning to the public. Figure 5.10 Soil hazard Organic-rich expansive soils are cracking the walls of this building in Spain. (Edward A. Keller)

13 Evaluating Hazards: History, Linkages, Disaster Prediction, and Risk Assessment Evaluating Hazards: History, Linkages, Disaster Prediction, and Risk Assessment Fundamental Principles Concerning Natural Hazards The understanding of natural hazards and how we might minimize their impact on people and the environment is facilitated through the recognition of five principles: 1. Hazards are known from scientific evaluation. Natural hazards, such as earthquakes, volcanic eruptions, landslides, and floods, are natural processes that can be identified and studied using the scientific method. Most hazardous events and processes can be monitored and mapped, and their future activity can be evaluated based on the frequency of past events, patterns, and types of precursor events. 2. Risk analysis is an important component in understanding impacts resulting from hazardous processes. Hazardous processes are amenable to risk analysis based on the probability of an event occurring and the consequences resulting from that event; for example, if we were to estimate that in any given year in Los Angeles or Seattle there is a 5 percent chance of a moderate earthquake occurring. If we know the consequence of that earthquake in terms of loss of life and damage, then we can calculate the risk to society of that earthquake actually happening. 3. Hazards are linked. Hazardous processes are linked in many ways, from simple to complex. For example, earthquakes can produce landslides and giant sea waves called tsunamis, and hurricanes often cause flooding and erosion. 4. Hazardous events that previously produced disasters are often now producing catastrophes. The size of the natural hazardous event as well as its frequency is influenced by human activity. As a result of increasing human population and poor land-use practices, what used to be disasters are often now catastrophes. 5. Consequences of hazards can be minimized. Minimizing the potential adverse consequences and effects of natural hazards requires an integrated approach that includes: scientific understanding; land-use planning (regulation and engineering); and proactive disaster preparedness. Role of History in Understanding Hazards A fundamental principle of understanding natural hazards is that they are repetitive events, and therefore studying their history provides much needed information in any hazard reduction plan. Whether we are studying flood events, landslides, volcanic eruptions, or earthquakes, the historical and recent geologic history of an area is a primary data set. For example, if we wish to evaluate the flooding history of a particular river, one of the first tasks is to study the previous floods of that river system. This study should include detailed evaluation of aerial photographs and maps reaching as far back as the record allows. For prehistoric events, we can study the geologic environment for evidence of past floods, such as the sequence of flood deposits on a floodplain. Often, these contain organic material that may be dated to provide a history of prehistoric flood events. This history is then linked with the documented historical record of high flows, providing a perspective on flooding of the river system being evaluated. Similarly, if we are investigating landslides in a particular river valley, studying the documented historical occurrence of these events and linking that information to prehistoric landslides will provide basic data necessary to better predict landslides. The hydrologists role in flood analysis is to evaluate stream flow records taken from sites, known as gauging stations, where stream flow recorders have been established (Figure 5.11). Unfortunately, except for larger rivers, the records are usually relatively short, covering only a few years. Most small streams have no

14 144 Chapter 5 Introduction to Natural Hazards Figure 5.11 Monitoring stream flow Stream gauging station on the Merced River in Yosemite National Park continuously monitors the flow of water in the river. Solar cells provide power. This is not a run of the mill station. It is designed to educate park visitors on how stream flow is recorded. (Edward A. Keller) gauging station at all. Geologists have the observation skills, tools, and training to read the landscape. They can evaluate prehistoric evidence for natural hazards and link this information with the modern record to provide the perspective of time on a particular process. Environmental geologists also have the ability to recognize landforms associated with hazardous processes. In addition, they recognize that the nature and extent of the hazard varies as the assemblage of landforms varies. For example, flooding that occurs in a river valley with a flat adjacent floodplain is very different from flooding on a delta. The river and floodplain constitute a relatively simple system consisting of a single channel bordered by the floodplain (Figure 5.12). Deltas, however, are more complex landforms, produced when a river enters a lake or an ocean (Figure 5.13). Deltas often have multiple channels that receive floodwaters at various times and places, varying the position Figure 5.12 Floodplain Mission Creek, California (left), and floodplain (right) with an urban park on it. The location of the park is an example of good use of a floodplain. (Edward A. Keller)

15 Evaluating Hazards: History, Linkages, Disaster Prediction, and Risk Assessment 145 Mediterranean Sea Nile Delta Figure 5.13 Delta Infrared image of the Nile Delta (upper left) and surrounding region. Healthy vegetation is red. The white strips at the delta edge are sandy islands that have a serious erosion problem since the construction of the Aswan Dam (not shown) in (Earth Satellite Corporation/SPL/Photo Researchers, Inc.) Egypt Nile River of the channel and thus the energy of a flood. Processes on deltas are discussed in detail in Chapter 8, but the general principle of instability of channels associated with different types of landforms is the idea we wish to emphasize here. In summary, before we can truly understand the nature and extent of a natural hazard, for example, flooding at a particular site, we must study in detail the history of the site, especially the occurrence, location, and effects of past floods. Understanding this history provides a perspective on the hazard that allows for the big picture to be better understood and appreciated. Integrating historical information with both present conditions and land-use change of the recent past, such as deforestation and urbanization, allows for better understanding of the hazard. This results because land-use changes can increase the impact of hazards such as landslides and floods. Studying the record also enables more reliable prediction of future events. Linkages between Hazardous Events Linkages between natural processes that are hazardous to people generally fall into two categories. First, many of the hazards themselves are linked. For example, hurricanes are often associated with flooding, and intense precipitation associated with hurricanes causes coastal erosion and landslides on inland slopes. Natural hazards and the characteristics of Earth materials provide a second type of linkage. For example, the sedimentary rock known as shale is composed of loosely cemented or compacted tiny sediments that are prone to landslides. Granite provides another example of the linkage between natural hazards and Earth material characteristics. Although generally strong and durable, granite is prone to sliding along fractures within the rock. Disaster Forecast, Prediction, and Warning A prediction of a hazardous event such as an earthquake involves specifying the date, time, and size of the event. This is different from predicting where or how often a particular event such as a flood will occur. A forecast, on the other hand, has ranges of certainty. The weather forecast for tomorrow may state there is a 40 percent chance of showers. Learning how to predict or forecast disasters in order to minimize loss of life and property damage is an important endeavor. For each particular hazard, we have a certain amount of information; in some cases, this information allows us to predict or forecast events accurately. When insufficient information is available, the best we can do is P R to E L locate I M I N A R areas Y P R O O where F S disastrous

16 146 Chapter 5 Introduction to Natural Hazards events have occurred and infer where and when similar future events might take place. If we know both the probability and the possible consequences of an event s occurring at a particular location, we can assess the risk the event poses to people and property, even if we cannot accurately predict when it will occur. The effects of a specific disaster can be reduced if we can forecast or predict the event and issue a warning. In a given situation, most or all of the following elements are involved: Identifying the location where a hazardous event will likely occur Determining the probability that an event of a given magnitude will occur Observing precursor events Forecasting or predicting the event Warning the public Location. For the most part, we know where a particular kind of event is likely to occur. On a global scale, the major zones for earthquakes and volcanic eruptions have been delineated by mapping earthquake foci and the locations of recent volcanic rocks and volcanoes. On a regional scale, we can predict from past eruptions which areas in the vicinity of certain volcanoes are most likely to be threatened by large mudflows or ash in the event of future eruptions. This risk has been delineated for most large volcanoes, including the Pacific Northwest s Cascade Range and volcanoes in Alaska, Japan, Italy, Mexico, Central and South America, Hawaii, and numerous other volcanic islands in the oceans of the world. On a local scale, detailed work with soils, rocks, and hydrology may identify slopes that are likely to fail and cause a landslide or where expansive soils exist. Certainly we can predict where flooding is likely to occur from the location of the floodplain and evidence from recent floods such as the location of flood debris and high-water line. Probability of Occurrence. Determining the probability that a particular event will occur in a particular location within a particular time span is an essential goal of hazard evaluation. For many large rivers we have sufficient records of flow to develop probability models that can reasonably predict the average number of floods of a given magnitude that will occur in a given time period. Likewise, droughts may be assigned a probability on the basis of past occurrence of rainfall in the region. However, these probabilities are similar to the chances of throwing a particular number on a die or drawing an inside straight in poker; the element of chance is always present. For example, the 10 year flood may occur on the average of every 10 years, but it is possible for several floods of this magnitude to occur in any one year, just as it is possible to throw two straight sixes with a die. Precursor Events. Many hazardous events are preceded by precursor events. For example, the surface of the ground may creep, or move slowly down a slope, for a period of time, days to months, before a landslide. Often the rate of creep increases up to when the landslide occurs. Volcanoes sometimes swell or bulge before an eruption, and often emissions of volcanic gases accompanied by seismic activity significantly increase in local areas surrounding the volcano. Foreshocks and anomalous, or unusual, uplift may precede earthquakes. Precursor events help predict when and where an event is likely to happen. For example, landslide creep or swelling of a volcano may result in the issuance of a warning, allowing people to evacuate a hazardous area. Forecast. When a forecast of an event is issued, the certainty of the event is given, usually as the percent chance of something happening. When we hear a forecast of a hazardous event, it means we should be prepared for the event. Prediction. It is sometimes possible to accurately predict when certain natural events will occur. Flooding of the Mississippi River, which occurs in the spring in

17 Evaluating Hazards: History, Linkages, Disaster Prediction, and Risk Assessment 147 Scientists Prediction review group Figure 5.14 Hazard prediction or warning Possible flow path for issuance of a natural disaster prediction or warning. D A T A W A R N I N G Local officials Regional officials P R E D I C T I O N PUBLIC response to snowmelt or very large regional storm systems, is fairly common, and we can often predict when the river will reach a particular flood stage, or water level. When hurricanes are spotted far out to sea and tracked toward the shore, we can predict when and where they will likely strike land. Tsunamis, or seismic sea waves, generated by disturbance of ocean waters by earthquakes or submarine volcanoes, may also be predicted. The tsunami warning system has been fairly successful in the Pacific Basin and can predict the arrival of the waves. A short time prediction of a hazardous event such as a hurricane motivates us to act now to reduce potential consequences before the event happens. Warning. After a hazardous event has been predicted or a forecast has been made, the public must be warned. Information leading to the warning of a possible disaster such as a large earthquake or flood should move along a path similar to that shown in Figure The public does not always welcome such warnings, however, especially when the predicted event does not come to pass. In 1982, when geologists advised that a volcanic eruption near Mammoth Lakes, California, was quite likely, the advisory caused a loss of tourist business and apprehension on the part of the residents. The eruption did not occur, and the advisory was eventually lifted. In July 1986, a series of earthquakes occurred over a 4 day period in the vicinity of Bishop, California, in the eastern Sierra Nevada. The initial earthquake was relatively small and was felt only locally; but a later, larger earthquake causing some damage also occurred. Investigators concluded there was a high probability an even larger quake would occur in the same area in the near future and issued a warning. Local business owners, who feared the loss of summer tourism, felt that the warning was irresponsible; in fact, the predicted quake never materialized. Incidents of this kind have led some people to conclude that scientific predictions are worthless and that advisory warnings should not be issued. Part of the problem is poor communication between the investigating scientists and reporters for the media (see A Closer Look: Scientists, Hazards, and the Media). Newspaper, television, and radio reports may fail to explain the evidence or the probabilistic nature of disaster prediction. This failure leads the public to expect completely accurate statements as to what will happen. Although scientific predictions of volcanic eruptions and earthquakes are not always accurate, scientists have a responsibility to publicize their informed judgments. An informed public is better able to act responsibly than an uninformed public, even if the subject makes people uncomfortable. Ship captains, who depend on weather advisories and warnings of changing conditions, do not suggest that they would be better off not knowing about an impending storm, even though the storm might veer and miss the ship.

18 148 Chapter 5 Introduction to Natural Hazards A CLOSER LOOK Scientists, Hazards, and the Media People today learn what is happening in the world by watching television, listening to the radio, surfing the Internet, or reading newspapers and magazines. Reporters for the media are generally more interested in the impact of a particular event on people than in its scientific aspects. Even major volcanic eruptions or earthquakes in unpopulated areas may receive little media attention, whereas moderate or even small events in populated areas are reported in great detail. The news media want to sell stories, and spectacular events that affect people and property sell. 5 Establishing good relations between scientists and the news media is a goal that may be difficult to always achieve. In general, scientists tend to be conservative, critical people who are afraid of being misquoted. They may perceive reporters as pushy and aggressive or as willing to present half-truths while emphasizing differences in scientific opinion to embellish a story. Reporters, on the other hand, may perceive scientists as an uncooperative and aloof group who speak in an impenetrable jargon and are unappreciative of the deadlines that reporters face. 5 These statements about scientists and media reporters are obviously stereotypic. In fact, both groups have high ethical and professional standards; nevertheless, communication problems and conflicts of interest often occur, affecting the objectivity of both groups. Because scientists have an obligation to provide the public with information about natural hazards, it is good policy for a research team to pick one spokesperson to interact with the media to ensure that information is presented as consistently as possible. Suppose, for example, that scientists are studying a swarm of earthquakes near Los Angeles and speculation exists among them regarding the significance of the swarm. Standard operating procedure for Earth scientists working on a problem is to develop several working hypotheses and future scenarios. However, when scientists are working with the news media on a topic that concerns people s lives and property, their reports should be conservative evaluations of the evidence at hand, presented with as little jargon as possible. Reporters, for their part, should strive to provide their readers, viewers, or listeners with accurate information that the scientists have verified. Embarrassing scientists by misquoting them will only lead to mistrust and poor communication between scientists and journalists. Just as weather warnings have proved very useful for planning ships routes, official warnings of hazards such as earthquakes, landslides, and floods are also useful to people making decisions about where they live, work, and travel. Consider once more the prediction of a volcanic eruption in the Mammoth Lakes area of California. The seismic data suggested to scientists that molten rock was moving toward the surface. In view of the high probability that the volcano would erupt and the possible loss of life if it did, it would have been irresponsible for scientists not to issue an advisory. Although the eruption did not occur, the warning led to the development of evacuation routes and consideration of disaster preparedness. This planning may prove useful in the future; it is likely that a volcanic eruption will occur in the Mammoth Lakes area in the future. The most recent event occurred only 600 years ago! In the end, the result of the prediction is a better informed community that is better able to deal with an eruption when it does occur. Risk Assessment Before discussing and considering adjustments to hazards, people must have a good idea of the risk that they face under various scenarios. Risk assessment is a rapidly growing field in the analysis of hazards, and its use should probably be expanded. Risk Determination. The risk of a particular event is defined as the product of the probability of that event s occurring multiplied by the consequences should it actually occur. 6 Consequences, such as damages to people, property, economic activity, and public service, may be expressed in a variety of scales. If, for example, we are considering the risk from earthquake damage to a nuclear reactor, we may evaluate the consequences in terms of radiation released, which can further be translated into damage to people and other living things. In any such assessment, it is important to calculate the risks of various possible events in this example, earthquakes of various magnitudes. A large earthquake has a lower probability of occurring than does a small one, but its consequences are likely to be greater. Acceptable Risk. Determining acceptable risk is more complicated. The risk that an individual is willing to endure is dependent upon the situation. Driving an automobile is fairly P risky, R E L I M I N but A R Y most P R O O F of S us accept that risk as part of living in a

19 The Human Response to Hazards 149 modern world. However, acceptable risk from a nuclear power plant is very low because we consider almost any risk of radiation poisoning unacceptable. Nuclear power plants are controversial because many people perceive them as high-risk facilities. Even though the probability of a nuclear accident due to a geologic hazard such as an earthquake may be quite low, the associated consequences could be high, resulting in a relatively high risk. Institutions, such as the government and banks, approach the topic of acceptable risk from an economic point of view rather than a personal perception of the risk. For example, a bank will consider how much risk they can tolerate with respect to flooding. The federal government may require that any property that receives a loan from them not have a flood hazard that exceeds 1 percent per year, that is, protection up to and including the 100 year flood. Problems and Opportunities for Risk Assessment. A frequent problem of risk analysis, with the exception of flooding on a river with a long record of past floods, is lack of reliable data available for analyzing the probability of an event. It can be difficult to assign probabilities to geologic events such as earthquakes and volcanic eruptions, because the known chronology of past events is often inadequate. 6 Similarly, it may be very difficult to determine the consequences of an event or series of events. For example, if we are concerned about the consequences of releasing radiation into the environment, local biological, geologic, hydrologic, and meteorological information must be gathered to evaluate the radiation s effects. This information may be complex and difficult to analyze. Despite these limitations, methods of determining the probability of earthquakes and volcanic eruptions are improving, as is our ability to estimate consequences of hazardous events. Certainly, risk assessment is a step in the right direction and should be expanded. As more is learned about determining the probability and consequences of a hazardous event, we will be able to provide more reliable forecasts and predictions necessary for decision making, such as when to issue a warning or evacuate people from harm s way. 5.3 The Human Response to Hazards Often, the manner in which we deal with hazards is primarily reactive. After a disaster we engage in searching for and rescuing survivors, firefighting, and providing emergency food, water, and shelter. There is no denying that these activities reduce loss of life and property and need to be continued. However, the move to a higher level of hazard reduction will require increased efforts to anticipate disasters and their impact. Land-use planning to avoid hazardous locations, hazard-resistant construction, and hazard modification or control such as flood control channels are some of the adjustments that anticipate future disastrous events and may reduce our vulnerability to them. 4 Reactive Response: Impact of and Recovery from Disasters The impact of a disaster upon a population may be either direct or indirect. Direct effects include people killed, injured, displaced, or otherwise damaged by a particular event. Indirect effects generally include responses to the disaster such as emotional distress, donation of money or goods, and paying taxes to finance the recovery. Direct effects are felt only by those individuals immediately affected by the disaster, whereas indirect effects are felt by the populace in general. 7,8 The stages of recovery following a disaster are emergency work, restoration of services and communication lines, and reconstruction. Figure 5.15 shows an idealized model of recovery. This model can be applied to actual recovery activities following events such as the 1994 Northridge earthquake in the Los Angeles area. Restoration began almost immediately after the earthquake. For example, in the first few weeks and months after the earthquake, roads were repaired and utilities were restored with the help of an influx of dollars from federal programs, insurance companies, and other sources. The damaged areas P R E in L I M Northridge I N A R Y P R O O F Smoved quickly