SCIENTIFIC CHALLENGES: FLOODS

Many factors contribute to how floods develop: rainfall intensity and duration, soil type and saturation, vegetation, and topography. The antecedent rainfall, soil cohesion, soil saturation levels, recent fire histories, water routes through the landscape, and hillslope angles determine how flood waves will travel through a catchment and how much sediment will be eroded, transported, and deposited during a storm.[1]

Remotely sensed data are of extreme importance in flood prediction because of the ability to monitor changes to land properties over time. In addition, remotely sensed data can determine properties of both the land surface and the atmosphere in real time. Because disastrous floods are often triggered by intense, short-lived storm cells, there is a need for remotely sensed data with higher spatial and temporal resolutions.

Most current detailed flood maps produced under FEMA's National Flood Insurance Program were prepared in the late 1970s and early 1980s. Many of them were based on "flood probability estimates that, in many cases, are now-out-of-date."[2] Presently even "well monitored" river catchments only have a few gauges measuring precipitation and discharge. It is estimated that only 42% of USGS-specified area are adequately gaged. This 1996 map provided by the USGS indicates the areas not adequately gaged in magenta.[3]

Furthermore, comprehensive data about soil moisture, thickness, and strength, or on vegetation cover, fire history is scarce and the most practical solution is the implementation of a broad-based remote sensing program. During storms height and width of rivers, as well as rainfall intensity and amounts, need to be measured hourly and while vegetation can be measured seasonally. Soil thickness would require less frequent measurement- once ever 5 to 10 years.[4]

NASA's recent SRTM (Shuttle Radar Topography Mission) mission provides detailed global topography data. Flood maps require data with 1 foot accuracy or better. Future generations of SRTM missions with greater accuracy will be especially useful because NASA's datasets are global and more consistent in resolution than field surveys and traditional mapping. There will be a great need for data processing and integration into USGS flood models as well as repetition of the mapping missions as landscapes change.[5] To develop a process-based understanding of flood hazards, one needs to study previous and on-going events and the factors that contribute to them.

References:

[1] SESWG Report 2002 http://solidearth.jpl.nasa.gov

[2] "Updating Flood Inundation Maps Efficiently:Building on Existing Hydraulic Information and Modern Elevation Data with a GIS," Joseph L. Jones, Tana L. Haluska, Alex K. Williamson, and Martha L. Erwin. 1998 U.S. Geological Survey Open-File Report 98-200 http://wa.water.usgs.gov/reports/floodgis/

[3] "A New Evaluation of the USGS Streamgaging Network--A Report to Congress," USGS http://water.usgs.gov/streamgaging/network.html#HDRflooding

[4] SESWG Report, see reference #1

[5] SRTM program website http://www.jpl.nasa.gov/srtm/