Determining flood extent and damages by digital image processing and GIS techniques: A case study of the Brazos River flooding of December, 1991

To mitigate the hazards of flooding, practical and scientific developments have been attempted throughout the history of human civilization. These measures range from simple building of levees to more complex prediction of the arrival of flood events. Installing and operating most of these warning systems are expensive and not economically viable in less developed countries where flood protection is severely needed. None of these complex conventional systems are complete and all can be improved to provide better protection. Post flood damage assessments are an integral part of these processes to alleviate the suffering of the affected population. Digital processing of remotely sensed images of flood events has become a significantly better alternative to the conventional methods of field investigation to determine flood extent and damages. Using the Brazos River Flooding of December 1991, an effort has been made to use remote sensing and GIS methodologies in an improved way to provide a more comprehensive solution. Landsat TM image and aerial photography have been successfully used to determine the land use-land cover, flood extent and damages. Based on USGS gaging stations discharge and velocity data, Digital Elevation Models were used to calculate volume and determine flood extent and depth within six sections of the river reach. The values obtained for depth and extent matched well with the field values near the cross section sites. The calculated depths varied more as the distance increased from them. These results were used to build a permanent GIS database for the study area which can be updated and maintained at low cost. The reasonably acceptable match between the experimental and real values makes this method useful for predicting future flood extent and depth immediately after the upstream discharge and velocity measurements become available. This information can be used to better warn the people in danger which can save lives and reduce damages. Above all this method is relatively inexpensive and can be used in other regions of the world to provide better protection from flooding.