Flood Frequency Estimation of Toddbrook Following the Whaley Bridge Incident
Dr Colin Clark, CHRS, Bruton, UK BA10 0BJ. Email:
colin4chrs@hotmail.com
Abstract: Following intense rainfall caused by a deep area of low pressure centred over southern England, nearly 100mm of rainfall was recorded over the catchment of Toddbrook on 30-31 July 2019. The catchment is located in the Derbyshire Peak District just above the town of Whaley Bridge. A 24m dam impounds the run-off from an area of 17.04km2. The auxiliary spillway discharged about 18-20m3/sec which under good maintenance would not have been damaged. However, much needed maintenance had not been carried out with the result that the left flank of the spillway partly collapsed. A state of emergency was declared and many townsfolk were evacuated for fear of a major dam-break incident. This paper assesses the hydrological evidence gathered after the flood and gives an estimate of the magnitude and frequency of the event. Three methods of analysis have been used. The first is a field survey and 2D flow modelling; the second is a joint probability method for estimating flood frequency; and the third is the application of a non-linear flow model which uses soil hydraulic conductivity data gathered in the field. The results showed that the July 2019 flood had a rarity of about 1 in 30 years, and that if the 1 in 100-year event had taken place the dam would probably have been completely breached. Additional results gave an estimate of 290m3/sec for the probable maximum flood (PMF) at the dam site. The author believes this result alone calls for a major redesign of the spillway – now underway – but says the dam undertakers and Dams & Reservoirs Ltd have not been willing to discuss the present estimate of the PMF, even though it is more than 100m3/sec higher than the figure being used in the construction of the new spillway. Newly revised estimates of probable maximum precipitation (PMP) for the British Isles[23] show that the standard values given in the Flood Estimation Handbook are far too low and cannot reproduce the discharges of many historic floods. Since the original PMP estimates[10] for Britain are now over 20 years old, the need for national dam safety appraisal is now urgent.
The Background to Dam Safety
In the UK the legislation for the design and inspection of dams imposes strict rules on both dam owners and the Inspecting Engineers. Their role is to enforce the law regarding dams enshrined in the Reservoirs Act 1975, which came into effect in 1986[30]. The biggest risks to dam safety in the UK are extreme floods and recognised in the 4th edition of Floods and Reservoir Safety[31]. The Environment Agency’s[26] guide to risk assessment, and the Health &
Vol XXXII Issue 1
DAM ENGINEERING 23
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