|by John Weier - July 2, 1999
|Millions of people in the United States and
around the world have no
idea that they live on a floodplain. Worse yet, they probably will not
find out until its too late. Their ignorance does not stem from crooked
developers or shady land deals, but from a lack of cartography. Very few
extreme, one-hundred-year floods are mapped out as they occur. Since
these natural events are so rare and cover such a large area, people
lose track of which lands were flooded. Without the benefit of
historical records, residents build on ground that once was and may
again be underwater.
Bob Brakenridge and a team of scientists at Dartmouth
College are working to correct this situation. For the past four years
they have been searching the world with orbiting imaging satellites,
trying to spot extreme floods. When the members of the team identify a
fifty-year flood or a hundred-year flood, they obtain an image of the
inundated area with the satellites sensors. From these images the
scientists create floodplain maps, which they then catalog on a database
for everyone to use. The teams efforts have already helped hydrologists
and floodplain managers to better understand floods. In the future, the
techniques Brakenridge and his colleagues develop could allow people to
see extreme floods as they progress in real time.
A worker from the Federal Emergency Management Agency (FEMA) inspects flood damage in Clear Lake, California. (Photograph by Andrea Booher and Greg Mathieson, FEMA Emergency Information and Media Affairs)
|Advanced Very High Resolution Radiometer images before (left) and during (right) a flood in Northern Russian, near the Barents Sea. Ice flows formed during the unusually cold winter of 1997-98 and blocked the mouth of the Pechora River, flooding several towns. Scientists used the data to make a map of the flood. (Images by E. Anderson and R. Brakenridge, Dartmouth Flood Observatory)|
|The Plain Truth About Floodplains|
|"Some hundreds of hundred-year floods occur around the world each year," said
Brakenridge. He explained the severity of a flood is measured by the number of years that pass before
a flood of equal magnitude (having the same amount of discharge) occurs along the same river. Though
hundred-year floods are obviously uncommon for any given river, many of them take place around the
globe each year. They vary greatly in size and can encompass an area as small as a subdivision or as large as
the State of Iowa.
|One of the goals of the Dartmouth team is to map and catalog as many large, hundred-year
floodplains throughout the world as technology permits. Brakenridge said the most precise way to
determine the shape and size of a floodplain is to outline the extent of the water during a flood. By
taking images of floods all around the globe with NASA and National Oceanic and Atmospheric
Administration (NOAA) satellites, the team is able to map more than a hundred floodplains each year.
"Ultimately documenting what happens now gives us the ability to predict which lands will be
underwater during future floods," he said.
There are many instances where people could have benefited from an accurate record of local floodplains. Last June Brakenridge mapped a flood in west India that wiped out an entire village. A cyclone offshore in the Port of kandla caused the flood. Since cyclones usually come ashore in eastern India, the government and the people living near the port did not know what to expect, he said. When the storm surge hit land, a flash flood engulfed the salt flats just north of the port, killing more than 1,100 people. Brakenridge feels that people in the area would benefit from accurate and available records of this event (Associated Press, June 16, 1998, by Ramola).
"Mapping out floods as they occur worldwide may seem like a simple thing to do, but no one else is doing it," said Brakenridge. Many countries do not map floodplains at all. In the United States, maps of hundred-year floodplains do exist, but they are largely inaccurate. The maps are usually made from topographic data and a patchwork of observations and predictions. Often the data are old or the maps are not updated regularly, said Brakenridge. Over a period of decades the elevation of the land around a river changes due to levies, erosion, and construction. The next time that river floods it becomes apparent that the area affected by flooding has changed. "We always superimpose our satellite images of floods on top of these old floodplain maps, and there are some big differences," said Brakenridge.
Satellite imaging technology gave Brakenridge's team the ability to monitor any number of floods at once. With support from NASA's Earth Observing System, Brakenridge's EOS Interdisciplinary Science team at Dartmouth constructed more than 200 floodplain maps over 1997 and 1998. To make the maps accessible to the public, they posted a listing of them on their Web site for anyone to order. The most interesting maps can be accessed directly on the Web site along with descriptions of the devastation the flooding caused.
Some of the more dramatic flood images from 1998 are those of the Pechora River in Northern Russia. An unusually cold winter caused large amounts of ice to form along the river, which drains into the Barents Sea. When the river began to thaw, huge chunks of ice floated downstream, gathered at the mouth of the river, and formed a dam. The waters backed up for hundreds of kilometers and threatened to pour in on the 35,000 residents in the city of Naryen-Mar. In a last-minute attempt to save the town, the Russian air force brought in four SU-24 assault aircraft and dropped ninety-six 250-kg bombs on the ice, with little effect. Eventually the town had to be evacuated (Associated Press, June 4, 1998).
Red shaded areas on this map (large version) represent areas flooded by a tropical storm that struck land near Kandla, western India. Thousands of people along the coast were killed or driven from their homes by the storm surge, and the salt flats to the north of the city remained inundated with water more than a week after the storm had passed. (Map by E. Anderson and R. Brakenridge, Dartmouth Flood Observatory)
|How to Document a Deluge|
|Nearly all the maps on the Dartmouth site are constructed using data from remote sensors like the
Advanced Very High Resolution Radiometer (AVHRR) on the NOAA operational satellites, and the Thematic
Mapper (TM) on NASAs Landsat 5. Both satellites move in nearly circular
obits approximately from pole to pole around the Earth and measure the infrared and solar radiation
from the surface of our revolving planet. These data are beamed to the surface where scientists like
Brakenridge convert them into meaningful images.
While locating floods from these images is easier than mapping them from the ground, the process involves more work than drawing a line around what looks like a flood. The water in the satellite images is frequently sitting under a forest canopy or is mixed in with the soil. "Often times you cannot see the flood [in an image] until you compare it to an image of the land before the flood occurred," said Brakenridge.
As anyone who has been caught in a rainstorm knows, dirt, pavement, trees, and clothes all change color slightly when wet. Brakenridges team uses this principle to discern flooded land from the dry land. They look for subtle changes in the "before" and "after" pictures of small sections of land known to be flooded and try to find the differences in the colors of light reflected off the land. Once these differences are found, the team highlights areas of the "after" picture that display the change. The researchers then plot the outline of flooded areas in the image onto a map. "The maps are very accurate. We try to provide full resolution image quality," said Brakenridge.
The precision of the maps encourages other scientists to incorporate them into their research. Last
October for instance, Robb Jacobson, a research hydrologist at the U. S. Geological Survey (USGS),
used Brakenridges 1998 maps of the Missouri River to study flood waters on publicly-owned
conservation lands where levee breaks had occurred. Jacobson said he and other USGS scientists are
trying to better understand how to restore the environment of large rivers and their floodplains.
This pair of Landsat images and animation (combined with elevation data) of Rocheport, Missouri were taken in September 1992 (top) and September 1993 (bottom). Note the brown and pink regionsagricultural fieldsalong the floodplain that can be seen in 1992, but are submerged in 1993. Floodplains are often fertile because of the sediment deposited there by previous floods. (Images and animation by Jesse Allen, GSFC Visualization Analysis Lab)
Many types of fish spawn in slow-moving, shallow water habitats. In general, these habitats are created when a river overflows its banks during the spring and summer months. However, over the past century levees and embankments have been built all along many of Americas larger rivers, cordoning off many valuable habitats. By observing flooded areas of the Missouri River that no longer have levees, Jacobson and his team are able to measure when, where, and how this habitat is created on natural floodplains. "Dr. Brakenridges maps of the levee breaks provide an opportunity to quantify areas inundated by water discharges over broad reaches of the Missouri River," said Jacobson. The information could give the USGS insights on ways to rehabilitate areas in and along these large rivers.
Jacobson explained that he is taking advantage of his own research to see how levee breaks affect flooding further down the river. "These [publicly-owned conservation] lands upstream have the potential to store floodwaters, thereby decreasing flood peaks downstream," said Jacobson. By either not repairing or destroying levees on conservation land, state and local governments may someday help prevent floods from hitting populated areas such as St. Louis County and Jefferson City, Missouri.
Brakenridge indicated that researchers will find more uses for the maps as knowledge of their existence spreads throughout the scientific community. The maps will probably be particularly helpful in drawing correlations between rainfall and flooding in an area, he said. Developers could also use them to survey an area before buying land.
Brakenridge explained that over the next few years he hopes to use the Moderate resolution Imaging Spectroradiometer (MODIS) instrument aboard the Terra satellite to map very large floods at even closer intervals in time. MODIS will return more detailed images than AVHRR, and it will scan the surface of the Earth every two days. The Dartmouth team should be able to turn these data into a global flood map every week. With the way satellite-imaging technology is advancing, satellites could soon monitor any large flood on the globe in real time. "If turnaround time becomes quick enough, we may be able to get images of floods soon enough to alert people," said Brakenridge.
Flooded houses in Missouri, 1993. Once floodwaters broach or overflow flood control levees, land can stay submerged under standing water for months. (Photograph courtesy Andrea Booher, Federal Emergency Management Agency (FEMA))
|This map shows the flooding (red lines) in Kandla,
India, and the salt flats to the north and east. The red dot on the map at right shows
the location of the flooding, adjacent to the Gulf of Kulch.
(Map by E. Anderson and R. Brakenridge, Dartmouth Flood Observatory)
|This map shows the extent of flooding (red shading)
on the Pechora River, Russia, on June 14, 1998. The red dot on the map at right shows
the location of the flooding.
(Map by E. Anderson and R. Brakenridge, Dartmouth Flood Observatory)