DR COLIN CLARK Results
All the results of the previous section, and the estimates of the August 2019 flood, are shown in Table 10, including the estimates of bankfull discharge which have a return period in the range 1.1-1.5 years[43,25].
Table 10. Estimates of index floods and the flood of 31/7/2019 at the survey site just above Toddbrook reservoir, CA = 15.41 km2
As noted above the return period of a flood event during the summer when the return period of critical SMD is more than 1.0 means that the rarity of the summer rainfall flood will be 2 x 2.15 = 4.6 years. The return period of the 1 in 10,000 and PMF must therefore be higher than the notional return periods of 104 and 106 years by a similar factor, that is 2.15. However, there may be an early part of the design storm which would reduce that factor but determining its size is beyond the scope of the present work. Suffice to say that a critical test of these results is that they must form a Log linear relationship with the modified reduced variate. Figure 8 shows this to be the case.
Also shown in Figure 8 are the results using the ReFH and the PMF, as reported by
Hughes[28], which was calculated using FEH methods[32], including the FSR[38] PMP. These alternative results have several problems: firstly, the 1.1 year flood of 7.4 cumecs is far too high, as is confirmed by the field survey. Secondly, as a result, the growth curve of floods is much lower, all else being equal. Thirdly, the rate of increase of peak discharge with rarity does not fulfil the criteria for flood data, namely that the less rare floods must align themselves in a linear way on the modified Log Gumbel scale. Fourthly, the July 2019 flood has a rarity in excess of 100 years, but the storm has a rarity of less than 20 years. This implies a rarity of 1 in 5 years for the critical SMD, which as we have seen is far too high. Fifthly, the run-off rate for the PMF is midway between the normal maximum flood and the extreme catastrophic flood of Allard et al[1]. Thus, historic floods have already exceeded the PMF at Toddbrook. Since the catchment is high, steep and in an area of high rainfall the lower estimate of PMF
38 DAM ENGINEERING Vol XXXII Issue 1
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