Early warning systems for the detection and response to severe floods

William J. R. Alexander
Department of Civil Engineering, University of Pretoria, Pretoria, South Africa.
Tel.: +27-12-348 1795, Fax: +27-12-348 8980,
E-mail: alexwjr@iafrica.com

The efficiency of early warning systems in saving lives during severe floods was well illustrated in the floods on the Rhine River in Europe and the Mississippi River in the USA in recent years. These are large rivers in developed countries and accurate warnings to downstream residents could be issued days in advance and evacuation procedures could be set in motion.

However, there are many large rivers in developing countries, and small rivers in many other countries where no such warnings can be issued in time to prevent a tragedy occurring. In South Africa for example, the largest loss of life due to floods is in communities located on the banks of a small rivers that rise within a matter of hours after a severe storm.

Sophisticated flood routing procedures are of no use in these situations. The paper describes a simple but effective method based on antecedent precipitation indices. Most floods in South Africa are caused by large area, long duration rainfall of the order of more than 500 mm in four days, culminating in a severe rainfall event. By observing the extent and severity of rainfall, communities at risk can develop their own flood watch systems based on the build up of rainfall as an indicator of the likelihood of floods developing.