| Radar Imagery Proves Valuable in Managing and Analyzing Floods
Red River flood demonstrates operational capabilities
By Kevin P. Corbley More than two years after the Red River flowed over its banks and flooded thousands of square miles of valuable agricultural land in Minnesota, North Dakota, and Manitoba, the devastating impacts of that flood are still etched in the lands and minds of the people.
Almost 103,000 residents were chased from their homes in April and May 1997, by the rising waters that swept northward from the United States into Canada. When the Red River finally began to recede, many people returned to find their houses, businesses and farms damaged beyond any hope of repair.
If anything positive can be taken from the Red River disaster, it is that a new flood monitoring technique was shown to be a viable solution for more effective emergency management during the flood and afterward in flood plain analysis. This remote sensing and GIS-based process contributed to saving lives in Manitoba, and will likely reduce destruction in future floods along the Red River and other basins.
"The 1997 Red River flood was the first operational demonstration of radar imaging technology for near real-time flood monitoring," says Terry Pultz, a research scientist at Canada Centre for Remote Sensing (CCRS).
Near real-time flood monitoring and mapping became a reality only recently with the emergence of commercially available synthetic aperture radar (SAR) imaging systems such as the Canadian RADARSAT and European ERS satellites, and several airborne devices. These types of SAR systems are ideal for mapping flood extent due to their all-weather imaging capabilities and delineation of land/water boundaries.
Leading the development of operational flood monitoring techniques has been Vantage Point International, Inc. (VPI) of Ottawa, Ontario, an engineering consulting company that has provided radar mapping services in numerous floods over the past four years. VPI played a pivotal role in the Red River flood by generating maps from RADARSAT imagery for the Canadian Space Agency (CSA), Canadian Prairie Farm Rehabilitation Administration (PFRA), and U.S. Army Corps of Engineers.
In the Red River and other floods, accurate and timely flood extent maps serve two key purposes. First, they provide emergency management personnel with the decision-making information they need to assess preliminary damage and deploy resources effectively . This enables them to plan evacuation measures, carry out rescues of trapped residents, and protect property.
Second, flood boundary maps allow hydrologists to calibrate their hydrologic and hydraulic models after the fact, so they can better predict the flow and distribution of water across the flood plain in future events. Results of these models are relied upon to establish land-use regulations, design dikes and levees, and to create storage reservoirs.
"One of the ironic aspects of managing floods is that emergency relief agencies are usually so busy dealing with the flood, they don't have the time to map it and they lose the opportunity to collect information that may mitigate future losses," says VPI President Ron Saper.
With partial funding from the RADARSAT User Development Program of CSA and technical support from CCRS, VPI is expanding FloodTrack, the commercial flood mapping service relied upon during the 1997 Red River disaster. Since that incident, an increasing number of emergency management agencies have called upon VPI to document large-area floods. SAR Makes Flood Mapping Possible
The ultimate objective of flood mapping is to delineate precisely where the land-water interface is located, preferably at multiple stages of inundation. Emergency relief agencies have long known the value of high water maps for use during and after the event, but have lacked the tools to accurately and quickly capture this information.
Optical satellites have enjoyed sporadic success at flood mapping, but too often the clouds that typically accompany a rain-induced flood obscure the area from their sensors while the event is unfolding. SAR satellites, on the other hand, peer through clouds, rain and even darkness to acquire scenes at every overpass opportunity.
SAR-equipped aircraft such as the Convair 580 operated by the Canadian government, have also proved adept at monitoring some rapidly changing flood events isolated within small drainage basins. However, the wide-area, synoptic coverage of the satellite is still preferred for operational mapping of flood-plain floods such as the Red River.
"The ability to monitor floods with radar satellites is guaranteed well into the future," says Saper. "The Canadian RADARSAT, which currently has the best flood mapping capabilities, is functioning perfectly and, within the next three years, Canada will launch RADARSAT 2, and Europe will launch ENVISAT."
RADARSAT is considered the ideal choice for mapping floods because its adjustable incidence angle allows for collection of repeat imagery as often as every 12 hours at high latitudes, and every three days at the equator. This makes it possible to document the rise and flow of water at multiple stages-critical data for validating flood plain models after the waters have subsided.
"RADARSAT images with large incidence angles are ideal for delineating the land-water boundary, but we can vary our processing scheme to accommodate other incidence angles," says Saper. "In a flood situation, we use whatever imagery can be acquired."
Although SAR has an inherent ability to differentiate water from land classes, VPI has written a series of custom algorithms in C++ to improve the accuracy of the classification and reduce processing time to only four hours. More importantly, the firm has developed a semi-automated process to identify partially flooded vegetation which may go undetected with the naked eye.
One secret to VPI's quick turnaround of FloodTrack products is that it usually avoids terrain corrections that are typically applied to SAR data to remove distortion. Such distortions normally affect only mountainous or hilly terrain, not the flat flood plains inundated by high water.
FloodTrack is a fully integrated service whereby VPI handles ordering of the appropriate SAR data, geocodes and processes it, and then generates a variety of end products. When requested, VPI also integrates the flood data with client GIS vectors and provides custom analysis and interpretation of the flood data.
The product requested most often is a flood mask, which is a geocoded, GIS-ready raster image showing land in black and white, with the submerged area shown in blue. VPI compresses the mask and transmits it as an e-mail attachment. Vector layers of roads, property lines, and dike locations can be overlaid as needed. VPI also produces image prints and web page images. Responding to Emergencies
VPI's involvement in the 1997 Red River flood increased as the waters rose. In that event, Canadian and U.S. agencies were aware that snow pack exceeded 200 percent of normal, and that regional flooding was likely. Several agencies submitted acquisition orders in advance for RADARSAT imagery over the basin. RADARSAT International of Richmond, British Columbia, filled these orders in record time.
The Prairie Farm Rehabilitation Administration (PFRA), a Canadian federal agency responsible for managing sustainable agriculture in the prairie provinces, was requested by the Royal Canadian Mounted Police and other emergency response organizations to lend assistance in planning the flood evacuation and relief efforts.
"PFRA has an extensive GIS of the prairie agricultural lands containing information on the location of all farms and towns," says Bill Harron, manager of the PFRA GIS unit in Regina, Saskatchewan. "The GIS showed where all the people and livestock were."
PFRA hired VPI to generate a series of GIS-compatible flood masks during the event. These were overlaid on the GIS to identify the hardest hit areas and dispatch rescue teams. When distress calls came in from stranded farmers, PFRA searched the GIS database by name to pinpoint the coordinates of the farm. These GPS coordinates were supplied to helicopter crews to fly in and save them.
"We did tactical mission planning with the flood mask during the Red River floods," says Lyle Boychuck, a PFRA GIS analyst. "We gave the pilots copies of the flood mask map so they could have some visual reference of where to land to pick up the flood victims."
Overlaying the flood mask on the existing GIS basemap provided an unexpected benefit to rescuers in boats. On more than one occasion, boat bottoms and motors were damaged by hitting raised railroad or road beds hidden beneath the water. The crews used the GIS maps to see where the submerged obstacles were so they could maneuver safely around them.
Ultimately, the flood masks proved more accurate than surveys made by the pilots of the rescue aircraft. Several times, a pilot reported that an area was not flooded because he could see the vegetation on the ground. In reality, water was covering all but the tops of the vegetation, making it look like dry land. The VPI masks had correctly classified these areas within the flooded zone. Fine Tuning Flood Models
In the months following the Red River flood, VPI was hired by the U.S. Army Corps of Engineers and the International Joint Commission (IJC) to process a backlog of RADARSAT imagery acquired during the flood. Their goal was to enhance the Red River hydraulic models by gaining knowledge of terrain characteristics and water movement from the images.
"Imagery is really the only way to identify relationships between land features and water," says Terry Birkenstock, GIS coordinator for the Corps' St. Paul District in Minnesota. "Our hydraulic engineers are using the [flood masks] to reconstruct what happened during the flood and determine what caused the water to go where it did."
Corps hydraulic engineers have used this information to fine- tune their Red River flood plain models to better predict water distribution in the basin. They likely will recommend changes to existing flood containment systems in North Dakota and Minnesota that were breached during the 1997 flood.
The other group performing post-flood analysis was the IJC, a joint Canadian-U.S. organization responsible for overseeing projects affecting boundary waters shared by the two countries. At the request of the two federal governments, IJC formed a task force to examine causes of the 1997 flood and recommend ways of reducing future flood damage. The task force studied a variety of digital flood maps generated by VPI.
"We were concerned with the accuracy of the digital elevation models used in hydraulic models of the Red River flood plain," says Bob Halliday, a water resources engineer on the task force. "We compared the output of the models with the RADARSAT flood masks."
Almost immediately, it was apparent that there was insufficient detail in the DEM to accurately portray the receding flood waters. Since precise elevations were available only for major roads and rail lines, the effects of minor roads-which in this area are elevated above the prairie land-were not captured in the model. This network of minor roads retained water.
"Essentially, our models predicted much quicker drainage of water from the basin," says Halliday. "The DEM is being improved, and the next step is to quantify exactly how much water is held back in these fields to improve the predictive capability of the model."
In addition to comparing results of the models with actual flood maps, IJC is integrating terrain elevation information gleaned from the RADARSAT images directly into the hydraulic models. Planning for Future Floods
Agencies involved in the 1997 Red River flood analysis agree that real-time flood mapping has proved extremely valuable both during and after the event. Not only were lives probably saved, but the building decisions made, based on the flood masks generated by VPI, could potentially save millions of dollars in property damage in the future.
"We are educating emergency management agencies to invest in flood mapping at the time of the flood, and the investment will pay for itself many times in the future," says Saper. About the Author:
Kevin Corbley is the principal in Corbley Communications. He is located in Littleton, Colo., and may be reached at 303-979-3232 or k[email protected] Back |
