DR COLIN CLARK
Safety Executive[27] have long since laid down the approach to risk-based assessments. For dams whose breach would result in the loss of lives the “standards based” approach is applied, which means that the dam must safely pass the probable maximum flood (PMF) which for nearly all cases involves the use of the Flood Estimation Handbook[32]. However, the current guidance also includes the opportunity for the Inspecting Engineer, with justification, to depart from the detailed application of this method if there is a well-documented historic flood record, rare rainfall, a shorter time to peak, and/or a higher percentage run-off. This has already happened with the Bruton Flood Storage Reservoir where the original design flood – now specifically called the Safety Check Flood – was increased from 240 cumecs to just over 500 cumecs[8,4]. If a flood with the higher peak discharge had occurred the original 8m dam, whose spill- way was armoured with petriiflex block-work, would have been breached. The resulting flood would have been considerably higher than if the dam had not been present. Wisely, the dam has been strengthened and raised[39].
With all the legislation and documentation in place one would expect no major or near miss type event to occur, but since 2000 there have been incidents; one at Boltby Dam[14] and another at Whaley Bridge. These both point to the fact that safety standards are not always being met, and that subsequent analysis of the latter event[2] shows a marked lack of appreciation of the more likely risk of dam break floods in the future. The Whaley Bridge flood took place after nearly 70mm of rainfall was recorded in about 16.5 hours (Figure 1). A subsequent report[37] did not estimate the flood rarity but did quote a rarity of about 1 in 90 years for the 30-31 July storm with a duration of 28 hours and a depth of 103mm at the top of the catchment. This result, together with FEH13[42] rainfall, implies a 28 hour PMP of about 284mm. But this has already been exceeded by 170mm in 1955[12] so the rarity of 1 in 90 years is much too high, and will be recalculated later in this paper. At present an estimate for the 1 in 10,000 year flood was given as 60m3/sec by the Canal & River Trust who are the dam undertakers (Bennett, Pers Com.). An estimate of PMF at the dam site was given by Hughes[28] as 164m3/sec. Both of these results are contest- ed in this paper.
The Catchment Area
The Toddbrook catchment covers an area of 17.04km2 at the Toddbrook reservoir in the town of Whaley Bridge, Derbyshire, UK (Figure 1). There are no rain gauges within the catchment, but the 1981-2010 annual average rainfall at Buxton is 1329mm and at Leek is 864mm, which are located 6km and 16km respectively from the top of the catchment, and representative of the high and low ground of the area. The highest point is Shining Tor at 559m, and much of the area is above 300m with steep slopes. The geology consists mainly of millstone grit, with some coal measures. Glacial deposits are found in part of the valley. The combined effect of geology and topography produces soils that are thin Brown Earths which cover 46% and Stagnogleys and Podsols covering another 46%. The remaining 8% is
24 DAM ENGINEERING Vol XXXII Issue 1
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