Natural Hazards: Causes and Effects - Disaster Management Center ...

Seasonality —Inundation of land during a growing season can have a completely destructive
effect on agricultural production, as severe in fact as a prolonged drought. If flood waters occur
during cold weather and if they derive predominantly from snow-melt with possible ice flows,
general discomfort and subsistency levels of affected communities are also considerably
influenced. Seasonality in large floods is therefore an important influence on severity of flood
hazard. 7
Forecasting, Warning, and Monitoring Systems
Forecasting
The ability to forecast flooding is limited to the time during which changes in the hydrological
conditions necessary for flooding to occur have begun to develop. The formulation of a forecast
for flood conditions requires information on current hydrological conditions such as precipitation,
river stage, water equivalent of snowpack, temperature, soil conditions over the entire drainage
basin, as well as weather reports and forecasts.
In small headwater regions a forecast of crest height and time of occurrence is all the
information required to initiate effective adjustments; the relatively rapid rate of rise and fall
makes the period of time above flood stage relatively short. In lower reaches of large river
systems where rates of rise and fall are slower, it is important to forecast the time when various
critical stages of flow will be reached over the rise and fall. Reliability of forecasts for large
downstream river systems is generally higher than for headwater systems.
Warning
Warning time for peak or overbank conditions can range from a few minutes in cloudburst
conditions to a few hours in small headwater drainages to several days in the lower reaches of
large river systems. As with forecasting, the time and reliability of the warning increase with
distance downstream where adequate knowledge of upstream conditions exists. (Additional
discussion on warning is found under the section, Disaster Mitigation Strategies.)
Clearly the amount of information required, the data collection network necessary for collecting
the information, the technical expertise required for interpretation, and the communication
system needed to present timely information to potential victims are services that many poor
and developing nations find difficult to provide. The World Meteorological Organization of the
United Nations, through its World Weather Watch and Global Data Processing System, hopes
to coordinate efforts to improve forecasting. 8 This is especially important (and difficult) when
conditions creating floods lie outside of the national boundaries of the downstream region.
Remote Sensing Monitoring
One of the most effective methods of monitoring floodplains is through remote sensing. Of the
various techniques available Landsat is perhaps the most versatile. Landsat satellites (formally
called Earth Resources Technology Satellites or ERTS) are used to map, inventory, and monitor
earth features.
The major sensing instrument in Landsat is a multispectral scanner that produces images of the
earth in green, red, and two wavelengths of infrared radiation. The products are black-andwhite
images, and images in digital form that can be processed by computers.