Author: G. Robert Brakenridge, Dartmouth Flood Observatory, Dartmouth College, Hanover NH 03755 USA
Flood Origins
Quick look images from NASA's Tropical Rainfall Measuring Mission (TRMM) show Hurricane Floyd's landfall on the SE United States coastline September 15, 1999. Figure 1 shows these data and also two subsequent rainfall events that contributed to the river flooding: a frontal storm on September 21-22, and the passage of a tropical depression or wave northeastward from Florida on September 26-27. In particular, the last brought very heavy rains and greatly intensified the flooding already underway. The hurricane passage itself was relatively rapid (examine TRMM images for September 14-16). It thus appears that in this instance the very severe flooding along many North Carolina streams and rivers was partially due to the three-fold sequence of storm events. The region had been experiencing drought conditions prior to the flooding.
Figure 1. TMI Quick-look data from TRMM.
| 09/14/99 | 09/15/99 | 09/16/99 | 09/17/99 | 09/18/99 | 09/19/99 | 09/20/99 |
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| 09/21/99 | 09/22/99 | 09/23/99 | 09/24/99 | 09/25/99 | 09/26/99 | 09/27/99 |
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Flood Chronology and Areal Extent
The following four stream gaging station-based maps (Figure 2) provide a useful regional perspective that is also supported by direct satellite imaging of the overbank flood events. Fooding began in eastern North Carolina the day after storm landfall (Figure 2, map on left). It intensified a day later, and several illustrated gages were more than 10 ft above flood stage (black stars) from September 17 to shortly before October 1: thus, for two weeks. Even in early October, these gages remained 5-10 ft above flood stage (red dots; map on right). Note that the initial, Floyd-related, flooding affected the entire coastal region from northern South Carolina to New Jersey (examine map for September 17; note data gap in Virginia). River flooding within most of this broad region then gradually subsided, whereas those in North Carolina were again affected by very heavy rains on September 23 and remain high in October. The sustained nature of this flooding indicates exceptional total flow volumes and probable related increases in damage and geomorphological effects.
Figure 2. Moderate flooding (yellow dots), more intense flooding (red dots) and very severe flooding (black stars) being reported already by SW U.S. stream gages on, left to right: September 16, 17, 23, 27, and October 1, 1999.
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The Dartmouth Flood Observatory began processing LAC or HRPT AVHRR images obtained from NOAA's Satellite Active Archive during and following landfall of Hurricane Floyd. For change detection purposes, we also obtained images from the archive covering the region before the storm. By comparing the visible and (for night images) thermal bands for images obtained immediately after the storm to those obtained from immediately before, floodplains affected by severe flooding could be detected and approximate outlines of the affected lands drawn. These inundation limits are based on geocoded 1.1. km resolution images; they are most useful for discerning where flooding was occurring on a regional scale but not appropriate for detailed mapping at larger map scales than approximately 1:250,000. Figure 3 is a summary map showing the "rapid response" flood mapping based on AVHRR data, and Figure 4 shows a sample change detection image based on a night overpass and comparison of the thermal bands. As we be futher discussed, this sensor is a very sensitive detector of changes in surface water occurrence; the upcoming NASA-MODIS sensor should have enhanced capabilties due to its similar wide-area coverage but better spectral and spatial resolution and calibration.
Figure 3. AVHRR change detection-based mapping of flooding for this event. The images this map is based on were acquired by several NOAA satellites September 17-21. Compare with the in-situ data shown in Figure 2.
Figure 4. Sample thermal-band AVHRR change detection-based mapping of flooding for this event. The images this map is based on were acquired on .
This report is in
progress...