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14.4
Conclusions and Implications for Drainage Practice |
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14.4.1
Principal Findings and their Limitations |
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Flood Depth and Solids Levels |
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The result that flood depth varies little with solids levels is not surprising. The onset of flooding suddenly increases the 'wetted surface area' over which flow is distributed. If one considers a small drain in a large street, the hydraulic capacity and storage of the drain clearly play relatively small roles in fixing the depth of flow, once flooding takes place. As noted in the literature of major/minor drainage systems, (Wisner and Kassem, 1982; Argue, 1986; WEF/ASCE, 1992) the function of the minor drainage system is to reduce the frequency of small floods, rather than to diminish the impact of rare floods. |
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Flood Duration and Solids Levels. |
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The sensitivity of duration to solids levels depends greatly upon the relative role of surface flow and channel flow in draining flood waters. The proportions of surface and channel flow will vary greatly between catchments, and between different sites within the same catchment. For the nodes examined in this analysis, all flood waters had to return to the channel at some point before leaving the catchment, so that channel capacity has a strong influence on the duration of flooding. At nodes served by a true major/minor system, the additional capacity of the surface drainage route could dramatically reduce the durations observed here. This is a powerful argument in favour of explicit recognition of surface flow in design and analysis; if urban areas are only to be drained by conduits, then flood durations will indeed depend upon the channel capacity and will be very sensitive to the degree of its blockage with solids. |
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Flood Extent and Frequency and Solids Levels |
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The 'extent of flooding' is difficult to model, and, in fact, difficult to measure in the field during a rapidly changing event. Our |
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