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findings on this aspect of performance are also difficult to interpret, and are most likely to vary from catchment to catchment depending upon topography and the layout of the drainage network. The sensitivity of frequency of minor flooding to solids level, however, is quite clear; this makes intuitive sense given the large number of small storms that can be managed by clean drainage networks of conventional design. |
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Limitations of the Analysis |
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There are of course questions about both the validity of the findings presented above and their applicability in other contexts. For example, the data are collected from only part of one monsoon, which was generally viewed locally as a 'good' or wet one. How representative is this of Indore, or of other parts of the world? The model also has a limited capacity to simulate the complexity of drainage in Pardeshipura, especially considering the high levels of solids in the drain, and the crude simplicity of the flooding model. Nevertheless, the above findings about performance make sense on an intuitive and physical level, and several had been postulated at the outset of the study (Kolsky, 1992). The overall conclusion is that solids levels significantly affect most measures of performance, and do not affect depth only because other flow routes are available. A question thus arises about how to respond to the almost inevitable capacity reduction of conventional slum drainage networks with deposition of solids. |
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14.4.2
Inadequacy of Current Practice |
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Engineers design drains as if they were free of solids. It is sometimes argued that poor design and construction practice are responsible for the solids deposition evident in so many drains in developing countries; if conventional practice was properly followed, it is argued, such drains would be self-cleansing. We suspect that the solids in drains in developing countries are, like those we measured in Pardeshipura, larger than those in the industrialized world. Construction debris, sediment from unpaved areas, temporary road surfacing materials and solid waste are |
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