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documented historic flood record, rare rainfall, a shorter time to peak, and or a higher percentage runoff. 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 (Clark, 1997 Black & Veatch, 2005). If a flood with the higher peak discharge had occurred the original 8m dam whose spillway 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 (Pether, 2010). 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 (Clark, 2008) 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 (Allman et al 2020) shows a marked lack of appreciation of the more likely risk of dam break floods in the future, says the author. The Whaley Bridge flood took place after nearly 70mm of rainfall were recorded in about 16.5 hours (Figure 1). A subsequent report (Mason, 2020) 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 (Stewart et al 2013) rainfall implies a 28 hour PMP of about 284mm. But this has already been exceeded by 170mm in 1955 (Clark, 2005) 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 and River Trust who are the


dam undertakers, (Bennett, Pers Com.). An estimate of PMF at the dam site was given by Hughes (2020) as 164m3


/sec. Both of these results are contested in this paper.


The catchment area The Todd Brook catchment covers an area of 17.04km2


Table 1 Soil associations in the Toddbrook catchment Mapped Association Soil Association


541 g & h 651a


1011b 654a 721c 713g


Rivington 1 & Neath Belmont


Winter Hill Hafren


Wilcocks 1 Brickfield 3


Type


Brown earths Stagnopodzols


Oligo-fibrous Peat Ferric stagnopodzols Stagnohumic gleys Cambic stagnogleys


% 46 10 8


18 8


10


Table 2 Estimates of soil moisture deficit at two sites near Whaley Bridge. SMD values in brackets. For station locations see Figure 1


Cat and Fiddle


April May June July*


* until 30th July


Rainfall 113.2 56.0


123.8 229.4


PE day-1 1.28 (3.6) 2.30 (11.7) 2.17 (11.1) 1.62 (0)


Lamaload Res.


Rainfall 80.7 45.8 95.4


248.4


PE day-1 1.44 (4.3) 2.34 (3.1) 2.43 (18.2) 1.48 (0)


Table 3 Relationships between monthly rainfall and monthly pan evaporation, 1986-2020


April May June July


P = -0.1739 R + 57.654 P = -0.1496 R + 81.016 P = -0.2540 R + 96.274 P = -0.2206 R + 99.023


P = pan evaporation (mm) R = rainfall (mm)


r = -0.68 Sig 0.1% r = -0.46 Sig 1% r = -0.68 Sig 0.1% r = -0.57 Sig 0.1%


replaced in 1970 by an auxiliary spillway 76.2m long. This improvement came about because a fall of about 70mm on 12 December 1964 (British Rainfall, 1964) caused a 1m deep overflow down the original spillway which became badly damaged with the overflow lasting for 24 hours (Hughes, 2020). The new spillway took into account a higher estimate of the probable maximum flood of 164m3


/ at the Todd Brook reservoir in the town of


Whaley Bridge, Derbyshire UK (Figure 1). There are no raingauges within the catchment but the 1981- 2010 annual average rainfall at Buxton is 1329mm and Leek 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 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 covered with peat. Table 1 gives further details based on the 1:250,000 soil map for Midland and Western England. The dam was completed in 1840 and is constructed with a clay core (Mason, 2020). The 24m high dam impounds an area of about 14 Ha. The original spillway was 41.2m long and was


sec (quoted by Hughes, 2020). While the new spillway was protected by 150mm concrete slabs, photographs taken several years before 2019 showed extensive growth of small shrubs and even trees growing in the crest and shute joints (Mason, 2020). A bywash channel conveys water around the northern side of the reservoir and operates at lower flows prior to the water level reaching a nominal height of 185.80m OD.


Antecedent conditions leading to the flood


During the spring of 2019 rainfall in the area had been about average, but in June a series of depressions led to over 120mm being measured at the top of the catchment and nearly 80mm in the surrounding area. Rain continued in July so that by the 30th the soil moisture deficit (SMD) was nil. Table 2 gives further details. The SMD was calculated on a daily basis using


the regression equations relating monthly rainfall and evaporation from a sunken open pan f


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