Modern ways to reduce flood damage

Forecasting The ability to forecast flooding is limited tocharacteristics. For example, Band 7 (long wavelength
the time during which changes in the hydrologicalinfrared) always shows water as black, even if it has
conditions necessary for flooding to occur have beguna high sediment load. This is most useful for mapping
to develop. The formulation of a forecast for floodcoastlines, lakes, rivers, and floods.
conditions requires information on current hydrologicalThe satellites pass over the same point every 16 or 18
conditions such as precipitation, river stage, waterdays, thus providing repetitive and comparable
equivalent of snowpack, temperature, soil conditionsobservations over time. This is particularly important for
over the entire drainage basin, as well as weatherdetection of changes and monitoring of rates and
reports and forecasts.extent of environmental change.
In small headwater regions a forecast of crest heightImages are available for almost all areas of the world
and time of occurrence is all the information required toexcept within nine degrees of the north and south
initiate effective adjustments; the relatively rapid rate ofpoles.
rise and fall makes the period of time above floodDepending on cloud cover, images may be available in
stage relatively short. In lower reaches of large riverrepetitive observations for all countries.
systems where rates of rise and fall are slower, it isMapping of flooded areas with Landsat images is not
important to forecast the time when various criticaldifficult. There are advantages and disadvantages to
stages of flow will be reached over the rise and fall.the methods, but within their specific characteristics
Reliability of forecasts for large downstream riverand limitations they can be highly useful.
systems is generally higher than for headwaterSeveral mapping image interpretation criteria are given
systems.below. They will be followed by a description of the
Warning Warning time for peak or overbank conditionsmost useful applications and limitations of the images.
can range from a few minutes in cloudburst conditionsInterpretation Criteria Floods are imaged best on
to a few hours in small headwater drainages toLandsat Band 7 (long wavelength infrared) because
several days in the lower reaches of large riverwater is always black in those images and is readily
systems. As with forecasting, the time and reliability ofdistinguished from lighter-toned areas.
the warning increase with distance downstreamLandsat color composite images show floods as dark
where adequate knowledge of upstream conditionsblue, light blue, or bluish white as the concentration of
exists.sediment in the water ranges respectively from very
(Additional discussion on warning is found under thelow, through medium, to high.
section, Disaster Mitigation Strategies.) Clearly theImages taken before floods can be overlaid with
amount of information required, the data collectionimages taken during floods; the resulting map of an
network necessary for collecting the information, theinundated area will show the difference between the
technical expertise required for interpretation, and theimages.
communication system needed to present timelyDigital processing of images may be necessary if the
information to potential victims are services that manysoils in and near flooded areas are very dark and
poor and developing nations find difficult to provide.difficult to distinguish from the flood water.
The World Meteorological Organization of the UnitedThis unusual situation is not often encountered.
Nations, through its World Weather Watch and GlobalLimitations of Application Since Landsat images have a
Data Processing System, hopes to coordinate effortsresolution of about 0.45 hectare (one acre), details
to improve forecasting.8 This is especially importantbelow that sizes are not readily visible unless they are
(and difficult) when conditions creating floods lie outsideof high contrast.
of the national boundaries of the downstream region.Because of the resolution, images of floods are best
Remote Sensing Monitoring One of the most effectivefor mapping at scales from 1/1,000,000 to about 1
methods of monitoring floodplains is through remote100,000 but only rarely at scales larger than 1/ 100,000.
sensing. Of the various techniques available Landsat isBecause of those limitations, the images are most
perhaps the most versatile. Landsat satellites (formallysuitable for broad area mapping at small scales and
called Earth Resources Technology Satellites orleast suitable for mapping in urban areas where
ERTS) are used to map, inventory, and monitor earthdelineation of a flood boundary is required at much
features.higher precision.
The major sensing instrument in Landsat is aSince Landsat images are taken at regular intervals
multispectral scanner that produces images of thethat cannot be changed, it is not possible to image a
earth in green, red, and two wavelengths of infraredflood at its peak, except by chance. It is, however,
radiation.possible to map areas that have been inundated by a
The products are black-and- white images, and imagesflood up to about two weeks after the flood has
in digital form that can be processed by computers.receded, because the wet ground left after flood
The following basic characteristics of the satelliterecession can be mapped.
system and its images are of importance in disasterAreas undergoing flooding may often be cloud
planning, management, and mitigation: Each imagecovered during the time of a Landsat overpass, thus
covers an area 171 kilometers (110 miles) on a side.preventing acquisition of usable images.
Each wavelength band has specific useful