other map shows corresponding areas for 100 year flood cycles, A flood event which may be equalled or exceeded only in 100 years (i.e. a flood with a probability of occurrence of 1 per cent), would inundate large areas of the flood plains, whereas a 10 year flood (i.e. a flood with a probability of occurrence of 10 per cent) would cover a much smaller area, mainly in the neigh- bourhood of the river banks. The methods employed by hydrologists can readily be applied for the preparation of maps for return periods other than those mentioned. Such a requirement should be decided in accordance with local data and experience. In hydrology, the usual practice is to characterize a flood by its peak discharge, or peak stage. In principle, it is necessary to estimate the peak stage at every point along the river chan- nel. The raw material for such an investigation consists mainly of the available rainfall and streamflow data. These data are analysed and used with catchment, and other hydrological models, to estimate flood frequencies and extent of inundation. 11 5.4 Storm surges r Storm surge is caused by strong winds and low barometric pressures (usually generated by a cyclone) blowing over a large sea surface, Water is thereby lifted and driven towards the coast. Where the depth is shallow, the return flow is retarded by friction at the sea bed, and the excess water piles up on the shore line until it eventually invades the hinterland. The originating phenom- enon will probably be accompanied by heavy rains. Thus, the sum total of destruction may prove to be exceptionally high because of the contribution of three major factors - storm surge, heavy rainfall and increased discharge, if not actual flooding, from rivers. Although the worst storm 
**Analysis**
The text appears to be discussing natural hazards and their impact on the environment and human settlements. It focuses on flood cycles, storm surges, and their effects on floodplains and coastlines. The language used is technical and scientific, suggesting a focus on hydrological and environmental aspects.

**Category**
Natural resources and environment, Science and technology