Projects | estimation at Todd Brook
Colin Clark provides details on a flood frequency estimation of Todd Brook UK following the Whaley Bridge incident in July 2019
Author details Dr Colin Clark, CHRS Bruton UK BA10 0BJ. Email:
colin4chrs@hotmail.com
FOLLOWING INTENSE RAINFALL CAUSED by a deep area of low pressure centred over southern England, nearly 100mm of rainfall were recorded over the catchment of Todd Brook 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 runoff from an area of 17.04km2 18- 20m3
. The auxiliary spillway discharged about
/sec which under good maintenance would have not 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; 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 and Reservoirs Ltd have not been willing to discuss the present estimate of the PMF, even though it is more than 100m3
than the figure being used in the construction of the new spillway. Newly revised estimates of probable maximum
precipitation (PMP) for the British Isles (Clark & Dent, 2021) show that the standard values as given in the Flood Estimation Handbook are far too low and cannot reproduce the discharges of many historic floods. Since the original PMP estimates (Clark, 2002) 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
Figure 1: The Toddbrook catchment area
12 | August 2022 |
www.waterpowermagazine.com
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) and which came into effect in 1986, (ICE, 1996). The biggest risks to dam safety in the UK are extreme floods and recognised by the 4th edition of Floods and Reservoir Safety (ICE 2015). The Environment Agency (2013) guide to risk assessment and the Health and Safety Executive (2001) 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 (IOH, 1999). 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
Flood frequency
/sec higher
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37
