When you hear the word "unconventional" used to describe swiftwater and flood hazards, you might think that they're rare enough not to worry about. Unfortunately, the only thing rare about these dangers is how often they're considered by emergency planners and responders, not to mention the public. The threat they represent is very real in many regions of North America.

Unconventional hazards include dam failure, hurricane-induced flooding, mud and debris flows, and even tsunamis. At first glance, these events may appear so unlikely that you may wonder whether they warrant separate discussion. But if history is taken as a whole, it's apparent that they occur with startling regularity. In fact, new evidence indicates some of these phenomena are far more likely than previously thought.

The nation's fire and rescue services are increasingly being held accountable for maintaining readiness even for unlikely emergencies, especially if they carry the potential for massive loss of life and property. If any of the hazards discussed herein are recognized as "potentials" within your agency's jurisdiction, prudence dictates active planning and development of realistic consequence management capabilities.

Problems of biblical proportions Dam failure certainly falls under the heading of unconventional hazard, even though history is filled with instances in which manmade or natural dams have failed, causing massive destruction and loss of life.

Some scholars and scientists are convinced that even the biblical Flood of Noah is a veiled reference to an actual flood that occurred centuries ago in what is now Turkey. New evidence indicates that the Mediterranean Sea had slowly risen to a level equal with the top of a natural land bridge that separated the Mediterranean from the Black Sea, near the Bosporus.

The Mediterranean overtopped the land bridge and began rushing over into the Black Sea, the surface of which was hundreds of feet lower in elevation. As the Mediterranean began draining into the former landlocked sea, the natural land bridge was rapidly eaten away. Finally, the land bridge failed, causing an epochal flood that swept away or inundated all of the area's settlements while raising the level of the Black Sea by dozens of meters.

The recent discovery of flood debris and alluvial material on the submerged floor of the Black Sea indicates it was once an ancient lakeside shoreline. This and other evidence support the theory of a true Great Flood in the Middle East, caused in part by the failure of a natural dam.

Natural dams develop under a variety of conditions, including landslides and rock slides that block rivers, volcanic eruptions, and the slow elevation of land masses due to earthquakes and other tectonic forces. They're inherently unstable, and their sudden failure can prove catastrophic.

Manmade dam mistakes Manmade dams fail from a number of causes, including faulty engineering, poor siting in relation to geologic hazards, sabotage and even earthquakes. During this century alone, thousands of people have been killed by collapsing dams in the Americas.

It's a little-known fact that the second-greatest life-loss incident in California history was the 1928 collapse of the St. Francis Dam. Engineered by the legendary William Mulholland in the deep San Francisquito canyon north of Los Angeles County's Santa Clarita Valley, the St. Francis Dam collapsed during the night, sending forth a 90-foot wall of water that killed more than 500 people. The exact toll of this flood will never be known because some victims were never found. Thirty miles away, when the flood met the Pacific Ocean, it was still 8 feet high and half a mile wide.

Since then, another large dam has been built in the vicinity of the original, and the population in the inundation zone has grown from thousands to well over 100,000 people. A similar event today could easily become the nation's worst disaster in terms of life loss.

The potential for manmade dam failure in that region is particularly chilling when one considers the potential damage from earthquakes. Scientists from the California State Division of Mines and Geology released a report predicting that up to 20,000 fatalities from earthquake-related dam failures could be expected in the greater Los Angeles area. With more than 200 dams in L.A. County alone, the potential for failure during earthquakes - not to mention other causes like engineering problems or terrorist acts - is ever-present.

With the exception of evacuation, almost nothing can be done for victims caught in the direct path of a dam failure inundation. However, many may be trapped and in need of rescue from the perimeters of such a flood. This is where water and debris can create conventional swiftwater rescue predicaments, such as people trapped in trees, or on homes, hillsides, bridges and automobiles.

Hurricane-induced flooding Test question: What's the most common cause of death resulting from hurricanes in the Americas?

a) The storm surge

b) Structural collapse

c) Flying debris

d) Sickness and disease

Surprisingly, the answer is none of the above. In hurricane after hurricane, the prevalent cause of death is drowning due to the intense deluge of rain and its attendant flash floods, dam failures, fast-rise flooding and other swiftwater rescue emergencies that accompany these storms.

Now, this isn't meant to downplay the danger posed by the storm surge or any other effects of hurricanes, but it's important for fire and rescue managers and emergency responders to understand that most people who die in these events are swept away by fast-moving water or trapped by fast-rising floods. This is especially true where mountains, canyons, foothills and other terrain features concentrate water in highly destructive flash floods and torrents.

Therefore, hurricane consequence management is largely a swiftwater rescue issue, complicated by the effects of storm surge, extremely high winds, structural collapse, fires and other typical hurricane problems. This was clearly demonstrated during the past couple of years on the East Coast and in South America, where tens of thousands perished in floods, landslides, and mud and debris flows.

Mud and debris flows Mud and debris flows are some of the deadliest natural events. They caused tens of thousands of fatalities when Hurricane Mitch slammed into South America in 1998. In the Venezuelan coastal city of La Guaira, mountains of rock, mud, trees and other water-borne debris roared through the downtown streets for days. The mud and debris flows were so immense that high-rise buildings acted much like boulders in a stream, creating eddies downstream while forcing millions of tons of debris to pile upstream.

In his book "The Control of Nature," John McPhee writes a simple but eloquent description of a 1978 mud and debris flow that buried a neighborhood in the Los Angeles suburb La Canada, which is nestled below the San Gabriel Mountains.

"It was not a landslide, not a mudslide, not a rock avalanche; nor by any means was it the front of a conventional flood.... In geology, it would be known as a debris flow. Debris flows amass in stream valleys and more or less resemble fresh concrete. They consist of water mixed with a good deal of solid material, most of which is above sand size. Some of it is Chevrolet size. Boulders bigger than cars ride long distances in debris flows. Boulders grouped like fish eggs pour downhill in debris flows.... It was not only full of boulders; it was so full of automobiles it was like bread dough mixed with raisins."

The La Canada mud and debris flow buried several homes to their roofs. One family and most of their furniture were floated to the ceiling by the invading mud. They were trapped for several hours, faces to the ceiling with precious little breathing room, until L.A. County firefighters tore through the roof of the house to rescue them.

Debris flows are a little-known phenomenon endemic to places where steep mountains and foothills rise above valleys and flood plains. They often occur where the mountains are cracked and fractured by earthquakes and tectonic forces, which makes them more vulnerable to erosion.

If the mountains are covered with highly flammable vegetation, the probability of debris flows increases exponentially, because vegetation is sometimes all that keeps boulders and soil clinging to the slopes. When fire denudes the vegetation, the rock and soil begin sliding and falling into the canyon bottoms. (See sidebar, page 50.) During intense rains, tremendous amounts of debris can be quickly turned to a slurry and mobilized into a huge flood.

Mud and debris flows in steep terrain commonly move faster than 20mph, and they have been known to travel faster than 100mph. Debris flows have moved rocks measuring more than 2,400 cubic feet. One debris flow in the Tujunga area of Los Angeles County transported a boulder weighing 15 tons into a residential area two miles from the San Gabriel Mountains. In places like the Santa Monica Mountains, which hug the Pacific Coast of Southern California, conditions are ripe for debris flows. The mountains there are fractured and unstable, and ancient landslides cause mudslides nearly every time it rains.

Tall waves, tall tales There's yet another earthquake-related hazard emerging in coastal zones and even large lakes: The possibility that a series of tsunamis could wipe out extensive areas of densely-populated land within minutes of a large quake.

Almost universally, the threat of tsunamis in the United States has been considered to emanate from Northern California, Oregon, Washington State, British Columbia and Alaska, all of which sit on the Cascadia Subduction Zone. It's the site of one of the world's largest earthquakes, which caused the coastline to drop nearly 30 feet in places and generated huge tsunamis that wreaked havoc across the entire Pacific Ocean.

However, that quake occurred long before Europeans had settled on the West Coast, and until recently the Native American account of a giant shaking followed by the sea rushing in to destroy all the villages was treated as a myth.

But recent findings are a chilling reminder that myths are often based on actual events. In the late 1980s, a U.S. Geological Survey scientist from Seattle set out looking for evidence to prove or disprove the legends. Working waist-deep in a marsh far from the coast, Brian Atwater sliced into peat with chain saws, discovering a thick layer of sand where there should only have been sedimentary deposits. Nearby were dead forests of cedar and spruce trees that he suspected of having been drowned in salt water - an ominous sign many miles from the coast.

By counting the tree rings, scientists estimated that the trees died en masse, possibly in a single event, sometime between 1680 and 1720. A search of historical records showed that a series of large tsunamis struck the coast of Japan at midnight Jan. 27, 1700. Knowing that tsunamis move through the ocean at more than 500mph, the researchers worked backward, determining that a magnitude 9+ earthquake occurred at 9 p.m. on Jan. 26, 1700.

From archeological excavations performed at coastal Indian village sites, scientists like Robert Losey of the University of Oregon are piecing together the effects of the quake and the tsunami. Some of the sites are accessible now only at low tide, indicating that the land itself dropped some 30 feet during the quake. Tsunamis washed across large swaths of land, flooding places that had previously been beyond the reach of the sea.

"There's an explicit tale of a mythological character called Earthquake traveling along the coast, sinking Prairie into Ocean," Losey recently said in an interview with the Los Angeles Times. "That's exactly what happened geologically."

Surprise near-source hazards In Southern California, emergency officials have long been assured that the threat of tsunamis rested with distant sources, leaving plenty of time for warning and evacuation. But that's changing as a result of the newly discovered threat of near-source tsunamis in Southern California and even Lake Tahoe.

To understand why near-source tsunamis are a serious concern, one need look no farther than Turkey. A rarely discussed effect of the recent earthquake there was the devastation caused by tsunamis, which practically wiped out portions of several coastal cities. Spawned by the 7.4 main shock, tsunamis struck land within minutes of the shaking, just as some people were emerging from their homes in the pre-dawn darkness to assess the damage and begin rescuing those trapped within.

Each tsunami, pushed by the mass of the unsettled Sea of Marmara, rushed inland like a flash flood in the dark. Walls of black water up to two stories high carried automobiles, boats and debris. As each wave receded, buildings, vehicles and people were washed back to the sea. It was a cruel blow, piling more misery on a population that had just been struck by one of the worst natural calamities of the century.

It also complicated search and rescue operations by denying or delaying access by firefighters and other rescuers. Many victims, some of whom might otherwise have survived until rescue teams reached them, drowned as sea water swept into quake-damaged buildings.

In Southern California, it was generally assumed that locally generated tsunamis were a non-issue, because there are no subduction zones along this stretch of coast. Until recently, reliable information about local tsunami hazards was difficult to find. The prevailing view was that most offshore faults in Southern California were of the strike/slip variety, which were considered incapable of generating large tsunamis.

However, the 1992 Mendocino Earthquake, which surprised seismologists by generating a small tsunami in Northern California, prompted a re-evaluation of near-source tsunami hazards in California. This was soon followed by the disastrous Northridge Earthquake that originated from a previously unidentified "hidden thrust fault," surprising scientists and beginning a quest to quantify the threat posed by thrust faults.

In the intervening years, a number of previously unknown thrust faults have been identified beneath the waters of the Pacific, off the shores of Los Angeles, Ventura and Santa Barbara counties. Researchers have also discovered evidence that large underwater landslides in deep offshore canyons pose a major tsunami risk, even before earthquakes are factored in.

New theories hold that populated coastal areas of Los Angeles County and other parts of Southern California are at moderate to high risk from large tsunamis generated by local earthquakes and underwater landslides. According to scientific researchers, the potential for heavy damage and loss of life from these events is significant. Perhaps most disturbing are new findings that local offshore faults are capable of generating large tsunamis that can strike the coast of Los Angeles County within as few as eight minutes, leaving little time for warnings or evacuation.

The message for fire/rescue personnel is clear: If tsunamis are preceded by earthquakes that cause fires, structural collapses, hazmat releases and injuries along the coast, emergency responders will be exposed to significant hazards that they may not anticipate.

Considering the potential effects of so-called "unconventional" swiftwater and flood rescue hazards, it's apparent that they deserve attention in places where there's an indication of true risk. Although it's true that we'd be hard-pressed to address all the rescue problems that would accompany some of these worst-case scenarios, it's also true that rational preparation begins with recognizing their potential and assessing the impact that something of this magnitude might have.

Wildfires play a pivotal role in creating conditions for mud and debris flows. In California, for example, chaparral litter piles up thick in the years between fires. As this debris decomposes, it gives up its high amounts of internal oils, in the form of waxy, long-chain hydrocarbons, to the soil.

When intense fires sweep the area, fed explosively by the oil-laden plants, the waxy compounds are vaporized and condense in a layer a few inches below the ground. During this reaction, the layer of soil just below this surface condensation essentially becomes hard and waterproof.

After the fire, dry unconsolidated particles of soil, rock, ashes and other material are left in thick layers on steep canyon walls. Without plants and roots to hold this material to the slopes, it cascades down hillsides in a steady dry stream, "pre-loading" the canyons for debris flows. Earthquakes add to the problem, causing tremendous landslides that rain down into the canyons.

Rain is the trigger that can literally move large portions of mountains. The top soil quickly becomes saturated with water, increasing the pore pressure just above the hydrophobic, or waterproof, layer. Soon, the topsoil liquefies and begins moving downhill.

Seeking the path of least resistance, as a stream would, the mud flow creates small canals called "rills" across the hydrophobic layer. The rills speed the water downslope, sometimes increasing the velocity threefold - while increasing the transport capacity a thousandfold. As rills develop across the face of the burned slopes, they connect, forming small tributaries leading to the bottom drainages. Massive loads of debris can now be mobilized by relatively small amounts of rainfall.

With the onset of intense rainfall, the trigger is set, the trap is sprung, and anyone down canyon is vulnerable to the unannounced arrival of huge walls of mud, rock, water, trees, and sometimes homes and cars.

For reasons still not entirely understood, wildfire burn areas seem to attract tremendously intense rain. Amazingly, the burned areas appear to act as separate microclimates, attracting or possibly creating storm cells that concentrate directly over the denuded terrain. This creates one type of worst-case scenario (denuded soil) overlaid by another (intense rain in steep topography).

As noted by John McPhee in "The Control of Nature," an inch of rain on a 10-by 10-mile area of mountain is about 7.2 million tons of water. Mix that amount of water with millions of tons of rock, soil and other debris, add the runoff factor caused by the hydrophobic layer, and the stage is set for disaster.

Tsunami hazards are widely misunderstood, not only by the general public, but also by many members of the emergency services. Contrary to common perceptions, seismic sea waves are not simply larger versions of what's generated by normal oceanographic and meteorological conditions. Tsunamis are very different - and far more dangerous - due to their inertia and their ability to sweep ashore for great distances.

While it's true that tsunamis may be quite tall, the true danger is related to the mass of energy that propels them through the ocean at great speeds. This "thrust" is generally caused by significant vertical movement of large blocks of the earth's crust during earthquakes, the occurrence of large underwater landslides or both.

When such a mass of waterborne energy strikes the coast, it may suddenly raise the level of the sea and drive walls of water far inland, causing a flash flood that can pick up ships and large buildings and carry them inland. This effect can be multiplied by common coastal zone topographic features, such as bays, inlets, and river mouths.

Consider the example of Los Angeles County's Marina Del Rey. Current studies by Prof. Costa Synolakis, University of Southern California School of Engineering, demonstrate that a large portion of this coastal community may be inundated by even a moderate 6-foot tsunami and sea rise, almost certainly causing a large loss of life.

Such a tsunami would carry with it boats, yachts and floating docks as it moved across the water. Upon striking the inland edge of the marina, the wave would come ashore, adding automobiles and buildings to its debris load as it moved onto land. Part of the wave would run up Ballona Creek, causing further damage in adjacent neighborhoods.

Although smaller in size and power, such an event would not be entirely unlike the tsunami that struck Papua, New Guinea, earlier this year. As in that event, Marina Del Rey could be the target of multiple waves, some larger than the first, that could endanger rescuers for 15 to 20 minutes. Aftershocks could cause repeated tsunamis for hours after the main shock. According to researchers, similar effects could occur at Malibu Creek and other local coastal sites.

As the waves subside, equally destructive events can occur when the water rushes back toward the ocean, carrying homes, cars, boats and other debris. For victims caught in the inundation zone, the overall effect of the incoming and outgoing waves is not dissimilar to that of multiple flash floods that completely - and repeatedly - reverse course.