FLOOD FREQUENCY ESTIMATION OF TODDBROOK FOLLOWING THE WHALEY BRIDGE INCIDENT
over the auxillary spillway equates with a head of 0.282m. Photographs taken of the spillway during the flood showed that the head was 0.3m ± 0.02m. Bearing in mind that the photograph did not show the water level behind the crest of the spillway, and also the slight drawdown effect as shown at Bourton Mill[19], a head of about 0.3m is realistic.
Discussion
The flood of 31st July has been assessed as having a rarity of about 1 in 30 years. Had the 1 in 100 year flood taken place then Toddbrook Dam would almost certainly have been breached, leading to loss of life. The safety check flood of 1 in 10,000 – with an estimated peak discharge in excess of 100 cumecs – would have caused considerable flooding even in the absence of dam-break. The previously reported 1 in 10,000 flood of 61 cumecs (Bennett, Pers Com.) is clearly too low, as is the PMF of 164 cumecs[28] which is over 100 cumecs below the value reported here. This finding chimes in with the flood study of the upper Brue[8], and supported by Black & Veatch[4], which found that the original design inflow PMF had a run-off rate half way between the normal maximum flood (NMF) and the extreme catastrophic flood (ECF) of Allard et al[1].
Since there have been at least seven well-documented floods which have run-off rates close to the ECF, and given the nature of the Toddbrook catchment area, the lower estimates can only be given little if any credibility. The main reasons for the higher estimates using the non-linear flow model[16] are, firstly, a much higher estimate of PMP[10,23]; secondly, higher percentage run-off rates than the FEH, which were obtained by statistical analysis rather than direct field measurements. Thirdly, a time to peak of the UH which is related to the value of Ksat of catchment soils since those soils which have a low permeability will produce a rapid and high percentage run-off. The greatest differences in estimates of PMF will be for catchments 15km2 and below since the area reduction factors of the FEH do not change with rarity, whereas the New Guide ARF’s are lowest for the PMP, increasing with decreasing rarity, but are highest for small areas where the difference between 1 hour PMP (FEH) and Clark[15] is greatest. Those who doubt the higher estimates of floods at Toddbrook should remember that, pro rata, the 1768 flood at Bruton[9] has already exceeded the original design run-off rate for the PMF at Toddbrook. This lower rate of run-off when applied to Toddbrook at Whaley Bridge has been accepted by the Canal & River Trust, undertakers for the dam at Whaley Bridge. Careful re-analysis of the existing data, and a proper recognition of historic flood data gathered elsewhere, should now be undertaken. Finally, greater openness regarding dam safety assessments should be made with greater public involvement. This would help to improve trust between the different stakeholders. Hiding the identity of Whaley Bridge, as in the paper by Brown et al[6] does not give the public a chance to change what they may do in the future. If Toddbrook Dam failed completely the legacy of secrecy written by Dam Inspectors would have been made apparent, so that the near miss incident of 2019 should cause those involved both now and in the future to consider who they are serving when carrying out their duties.
Vol XXXII Issue 1 DAM ENGINEERING 41
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