DR COLIN CLARK


The dam was completed in 1840, and is constructed with a clay core[37]. The 24m high dam impounds an area of about 14Ha. The original spillway was 41.2m long, and was replaced in 1970 by an auxiliary spillway 76.2m long. This improvement came about because a fall of about 70mm on 12 December 1964[5] caused a 1m deep overflow down the original spillway which became badly damaged with the overflow lasting for 24 hours[28]. The new spillway took into account a higher estimate of the probable maximum flood of 164m3/sec (quoted by Hughes[28]). 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 in the crest and shute joints[37]. 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 185.8m 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.


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


The SMD was calculated on a daily basis using the regression equations relating to monthly rainfall and evaporation from a sunken open pan[17]. The results were applied on a daily basis and the equations relating to rainfall and pan evaporation[17] were updated to 2020 and given in Table 3.


26 DAM ENGINEERING


Vol XXXII Issue 1


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