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Hello, I'm Glenn Paterson. Several lectures in this course emphasize that water is a scarce
and often precious resource that needs to be carefully stored, allocated and conserved.
But as we know, water is unevenly distributed. Too much of a good thing can also be a problem.
We're looking at the mouth of the Big Thompson Canyon a few miles west of Loveland, Colorado.
The Big Thompson River flows from its headwaters in Rocky Mountain National Park through the
town of Estes Park and down through this canyon on its way to the Great Plains and its confluence
with the South Platte River. US36 follows the river through the canyon passing numerous
cabins and resorts along the way. On July 31, 1976 a huge thunderstorm stalled
over the mountains just east of Estes Park and dumped a foot of rain in just four hours.
That night, the water level in the Big Thompson River abruptly rose 20 feet creating a raging
flood that demolished roads, campgrounds, cabins, and resorts. It was Colorado's deadliest
flood. Heroic actions saved many of the 4,000 people who were in the canyon, but 143 people
lost their lives in the churning water. Many observers were sure they had witnessed a once
in a lifetime experience. The people who rebuilt the highway felt that it would have a good
chance of withstanding future floods. But, as we film this video, construction crews
are hard at work again, 37 years later, rebuilding the same highway after it was once again destroyed
by a flood on the Big Thompson River. During September 8-15, 2013, a heavy flow of warm
moist air from the south collided with a stationary cold front dropping as much as 17 inches of
rain on the same mountains. This time the rainfall was more widespread and 8 South Platte
tributaries rose to flood levels. Fortunately this time there were fewer tourists in the
canyon, there was a little more time for warning, and we had learned some lessons from 1976
about flood safety and flood plain management. These factors keep the loss of life to 9 this
time in the whole flooded region. But again, Highway 36 in this canyon and other roads
in similar canyons were washed out, structures were damaged and destroyed, and people were
stranded for days. These and many more similar occurrences remind us that while water is
a precious resource, its uneven distribution in space and time can also make it a powerful
agent of destruction. The topic of this lecture, not surprisingly,
is water disasters. We'll talk about what types of disasters are related to water, what
type of damages they cause, how they occur, how people respond to them, and what can be
done to reduce disaster losses. These images from the 1976 Big Thompson Flood
remind us of the destructive power of a flooding river.
The storm was typical of a flash flood, starting with a huge thunderstorm that remained parked
over the mountains near Estes Park for 24 hours while it dropped a foot of rain. The
thin soils and steep slopes did not allow much water to infiltrate into the soil. Nearly
all the water ran off quickly into streams. The steep narrow canyon quickly funneled the
flood into the confined channel where the water level and velocity both rose abruptly
and dramatically. The pictures from 2013 from the same place
carry the same message of destruction. This time, the atmosphere over the southwestern
states was like an airborne river of moist air flowing in from the subtropical Pacific
Ocean, converging from the southeast on the Front Range of the Rockies.
The Big Thompson floods are not unique. Floods are one of the most frequent and destructive
of natural disasters. Colorado, a fairly dry state most of the time, has had its share
of destructive floods. Floods can be even larger and more destructive
in other areas around the world. Floods on steep mountain canyons tend to be flash floods,
as we saw before. On the other hand, widespread rain and snowmelt over large flatter river
basins cause big rivers to rise more slowly, but remain over flood stage for longer periods
of weeks or months. Flooding is a natural part of the variability of the hydrologic
regime of the river, but when floodwaters encounter people and their structures and
farms, significant damage occurs. Large natural floods occur less frequently
than smaller ones and have a lower probability of occurring in any given year. This flood
frequency or probability relationship for a given point on a river can be depicted in
a graph such as this. Floods near the top of the graph are larger in terms of water
flow than floods lower down on the graph. The average frequency of occurrence for a
flood on a given magnitude is shown on the bottom axis in terms of recurrence interval,
the average number of years between floods of this magnitude. The probability of occurrence
during a given year is shown on the top axis. In the example shown, a flood of 1800 cubic
meters per second has an average recurrence interval of 10 years and a probability of
occurrence of 10% in any given year. This could be known as a 10-year flood. To construct
a curve like this, you need a fairly long series, at least 30 years or so of annual
peak flows. Of course, the 1% or 100 year flood may occur in back-to-back years, but
statistically speaking, the average recurrence interval over a long period, would be 100
years. Most floods are caused by natural events such
as precipitation. But some are caused or exacerbated by failure of artificial structures such as
dams. The most destructive dam break flood in the United States was the 1889 failure
of the South Fork Dam on the Conemaugh near Johnstown, Pennsylvania which killed 2,209
people, the largest loss of civilian lives on a single day in the United States until
September 11, 2001. Other notable dam break floods in the US included
failures of the St. Francis Dam near Santa Clarita, California in the LA area in 1923
which killed about 600 people and the Teton Dam failure near Rexburg, Idaho in 1976 which
killed 11. Dam break floods can also occur during natural events that create temporary
dams, such as ice jams, glaciers, earthquakes, and landslides.
In the United States alone, annual flood damage to property in dollars averages over 8 billion
dollars a year. If crop losses are included, the annual average tops 20 billion dollars
a year. You can see that there's an increase in trend with time over the last 80 years
or so. Despite increasing expenditures for flood hazard mitigation, the trend in flood
damage is still increasing. This is due in part to the increasing value of real estate,
in part to population growth in general, and perhaps in part to changing climate that puts
more heat energy into the atmosphere and may be contributing to more frequent and larger
storms. But the primary reason for the increasing trend in flood damages is our propensity to
keep putting people and expensive structures in flood prone areas. Why do we do this? Because
most of the time flood prone areas are attractive sites near aesthetically pleasing rivers and
streams. After nearly every destructive, local communities and the federal government wrestle
with the question of how to balance economic development and the freedom to put things
where you like with the need to keep people and structures out of risk-prone areas such
as floodplains. The Netherlands is a country that has hundreds
of years of experience dealing with this question and has spent billions of dollars on flood
protection projects such as dikes, dams, pumps, and storm surge barriers. In recent years,
partly in response to growing awareness that a changing climate and rising sea level are
likely to exacerbate future flooding, this country has decided to move people and structures
farther away from the rivers. The concept, called Room for the River incorporates undeveloped
floodways into rural and urban planning. Not all flooding is caused by rivers. Along
the coast, gradual flooding can be caused by rising sea level or sinking land or both.
Catastrophic coastal flooding, one of the most destructive types, can be caused by hurricanes
and typhoons. Examples shown here include the Galveston Hurricane of 1900 which killed
6-8,000 people over several days and the typhoon that struck the Philippines in 2013 which
is estimated to have killed about 10,000 people. Another type of coastal flooding disaster
can be triggered by earthquakes near or under the sea. Tsunamis, sometimes known as tidal
waves, are a result of such an event. A tsunami is triggered by a sudden displacement of water
by earth movement, such as an earthquake. The energy from the displacement can travel
rapidly through the ocean, long distances fairly quickly, to adjacent land masses. The
arrival of the wave at land first causes water at the shore to drop as though the water were
being drained away. The wave itself then arrives in force as a wall of water moving rapidly
inland. Other types of storms besides rainstorms and
hurricanes can cause water related disasters. Blizzards have their own set of problems related
to water in frozen form. Hailstorms each year cause millions of dollars
of damage to houses, cars, and crops, especially in parts of the country subject to heavy thunderstorms.
Not all water related disasters involve too much water. On the opposite end of the spectrum,
drought is a frequent type of disaster as well. Drought develops slowly and may persist
for months or years. Sometimes it's hard to tell when a drought begins. A long drought
may even include periods of brief flooding. One of the primary means of mitigating droughts
is through water storage projects, but a multiyear drought can exhaust the storage supply in
even large reservoirs. Droughts result in large costs for lost crops nearly every year
in the US and in other countries, too. One of the best ways to tell with droughts
is to obtain and disseminate accurate information about them. In the US, this function is provided
by the US Drought Monitor. In addition to water quantity, water quality
can also be involved in water related disasters. Unfortunately, spills of hazardous materials
into rivers, lakes, and aquifers, is a common occurrence. One notable example is the 1986
fire and subsequent release of toxic chemicals from a factory near Basel, Switzerland into
the Rhine River. The spill turned the river red for many miles, killed thousands of fish,
and interrupted the supply of drinking water in several countries.
Farther east in Europe, a spill of toxic sludge into the Marcal River in Hungary, a tributary
to the Danube also resulted in severe damage to the aquatic ecosystem and drinking water
supplies. In conclusion, floods are part of the natural
regime of a river, but when they encounter people and structures they can be very destructive.
We're gradually learning to avoid putting people and structures at risk in flood-prone
areas, which is the best way in the long-term to mitigate flood losses. Floods can also
be caused by failure of artificial and natural dams, and by hurricanes and tsunamis. Other
serious water-related disasters include blizzards, hailstorms, drought, and chemical spills.
Thank you.