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1.1.1 Stormwater characteristics

Peak flows of urban stormwater runoff may increase by a factor of 3 to 8 as urbanization progresses depending largely on the proportion of the urban watershed covered by impervious surfaces. Cases of poor land-use planning and failure to maintain drainage and other urban facilities in good conditions abound in Africa (Oyebande, 1990). These conditions often aggravate flooding and pollution hazards as reported in Ibadan, Nigeria (1984 floods), and Dar es Salaam, Tanzania (1987). The case of Gadarif in Sudan, is a real catastrophe. The urban centre lies within the Khor Abu Farga watershed. There was a population explosion and the resultant expansion of urban land-use was not properly and often informally undertaken, with many houses built in the flood plains. On September 9, 1973, a storm rainfall of 90mm occurred. The 5-year storm resulted in a 25-year flood which caused the worst damage ever reported in the Sudan to that date.

In most African urban areas the high rainfall intensities are prevalent and poor land-use planning is predominant. For instance Table 1.7 shows the relatively much higher intensities for tropical Africa than for the temperate region. Given such circumstances, the lag time in an urban area with a storm water drainage system is much shorter, being only between 12 and 20% of that of a comparable natural system. And for an increase in imperviousness of from 1% to 30% (developed urban areas), the flow rate itself increased by a factor of 9 for a 2-year rainfall event and by a factor of 5 for a 50-year storm. The corresponding total runoff volume can be double that of a natural area, hence the amplification of Gadarif flood level and disaster.

Urban stormwater designers and planners are having to deal with the increases in runoff volumes and peaks due to urbanization by using more sophisticated approaches for design and planning of stormwater drainage systems.

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