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.