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data collected during the study. Frequency of flooding was therefore expressed as a proportion of the total number of storm events observed. This unfortunately cannot be extrapolated to a conventional 'return period'. |
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A second problem arises in defining independent flood events. Rainfall and water level patterns are not discrete, and distinguishing one flood event from another is sometimes difficult. Suppose, for example, that a high-intensity storm event leads to flooding. After the rainfall intensity recedes, the water level begins a gradual drop. If a second rainfall event of much lower intensity now occurs, a flood may result only because the channel is still nearly full from the antecedent event. For these situations, it was decided that the hydraulic response of the antecedent event was 'over' when the water level had dropped by 90 per cent from its maximum. In two cases, this criterion distinguished two distinct events occurring close together; on the other hand, four storms that lasted 10 hours or more contained multiple, but hydraulically linked, peaks. Because the hydraulic response to the latter peaks could not be distinguished from that of the antecedent rainfall, these longer storms were treated as single events. |
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Figure 14.6 shows how the frequency of flooding (as a fraction of recorded storms) and the number of flooded sample nodes increases with increasing solids levels. The graph naturally shows that some nodes are more prone to flooding than others. For any given number of flooded nodes, an 80 per cent blockage of the channel increases the frequency of flooding by a factor between 2 and 3, while a 50 per cent blockage generally accounts for an approximate 50 per cent increase in flooding frequency. Note that this figure does not show anything about the severity (depth and duration) of flooding at any of the nodes, but only that the water rose above ground level. |
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14.3.3
Fraction of Runoff Lost from the Catchment |
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Figure 14.7 shows the percentage of runoff lost from the catchment through overland flow as a function of solids blockage |
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