August 1, 2019

expert reaction to news that the Whaley Bridge dam has been damaged

The Whaley Bridge dam in Derbyshire has suffered damage during a period of heavy rainfall, subsequently there have been concerns raised, and the town has been evacuated due to fears of flooding.

Comments Issued on Tuesday 6th August

Jo Parker, Chartered Civil Engineer and has previously been a Supervising Engineer for over 30 years and responsible for large reservoirs in Thames Water, Wessex Water and Affinity Water, said:

“I was the project manager for a project run by UK Water Industry Research on ‘long life low probability of failure assets in the water industry’ completed in 2012. This recognises that some structures are built to last far longer than 100 years and developed good practice guidelines for monitoring these structures and developing models to predict deterioration and ensure timely maintenance.

“The designers and builders of these dams in the past designed them to last. The fact that they are more than 100 years old is not necessarily an issue – there are water retaining structures which were built in medieval times which still perform well!

“The vegetation shown on the dam in certain photos being circulated, looks as though it is purely from one season. It does not appear deep rooted which means that the Canals and Rivers Trust have been ensuring that such vegetation is kept under control and removed – if it was not done so regularly you would see e.g substantial saplings rather than the shallow rooted weeds that are actually there. An inspecting engineer would certainly have commented on any excessive vegetation as this is something specifically required in their reports and in the annual report from the Supervising Engineer.

“A much more likely cause of the damage would be high flows over the spillway combined with a rapid change in water level . This could increase the internal pressure in the dam without allowing it to dissipate and this pressure could push the blocks off the dam. This came from the unprecedented amount of water – a month’s rainfall in 2 days was reported. To my knowledge it is still standing which to me shows that our dam management systems are working. Emergency measures were put in place. The requirements for the sizing of spillways and the management of flood flows has been reviewed and no doubt will continue to be reviewed regularly. The Environment Agency ensures that every incident is reviewed in detail and learning is captured. As an example, an incident occurred at Ulley reservoir near Rotherham several years ago where some cracks were found in routine inspections following high rainfall. Remedial action was put in pace and the design of side spillways was reviewed as a result. The problem was caused by high flows of water scouring the walls of the spillway. This is an example of the 1975 reservoir act in action. Other reservoirs have been strengthened as a result and once again major issues avoided in spite of more intensive rainfall events.”

Dr Helen Gavin, Knowledge Exchange Researcher and Programme Manager, Oxford Martin Programme on Integrating Renewable, University of Oxford, said:

Are we able to speculate on the maintenance quality of the dam from photos alone?

“Speculation is all that can be done: a full understanding of the structural integrity of the dam would need measurements and information derived by other means (e.g. ultrasound) and from below the below the water line.

Does vegetation on a slipway cause issues for the functioning of a dam? If so, in what way?

“If the slipway is paved then the roots of woody vegetation can penetrate the concrete / surface and cause internal erosion, and open up pathways for water to flow under the concrete structure potentially causing instability.

Just because a dam is 100+ years old, does that mean they have exceeded their design lifespan?

“No.

What’s been done to upgrade/ improve older dams? Are they fit for purpose or does more need to be done?

“The Reservoir Act places a duty on owners to ensure it meets the required standards.  Different sized dams and reservoirs have different requirements.   Inspection need to be made regular by a panel engineer (i.e. an approved engineer) and any recommended works completed.  See: https://www.gov.uk/guidance/reservoirs-owner-and-operator-requirements

With climate change, are we prepared for more extreme weather events? Are those involved in dam safety and regulation already looking into this and making these changes? How do you make these structures ready for these harsher weather events? Do they need renovation or complete rebuilding?

“National Policy Statements are in place which require climate change projections to be taken into account when developing new major infrastructure projects: for critical safety aspects they recommend a high emissions scenario (which includes within it a 4°C rise in global temperature above pre-industrial levels by 2100; and associated changes in rainfall quantity, frequency, duration, and timing) be used for engineering calculations. 

“Currently water companies have been using datasets such as the UK Climate Projections (UKCP09), Future Flows and others to forecast the additional rainfall and flow volumes etc when sizing new infrastructure, and are looking to use other datasets such as UKCP19 where possible.

“However, in terms of existing infrastructures, I believe that it’s important to properly recognise that they may need to be rebuilt or renovated as extreme weather events become more common. As far as I am aware, there is no comprehensive policy or initiative that cover such aspects, such as a national standard for resilience. There is a large number of different groups responsible for different aspects, on different governance levels: much is devolved, and there is no overall organisation that has responsibility. The effects of climate change in changing temperature and rainfall patterns, coupled with land-use change, notably an increase in impermeable surfaces and increased housing density, has meant that the drainage system in many areas can no longer cope because the quantity / quality / intensity of rainwater is much greater than when the asset was originally designed. An example is the increase in combined sewer overflows (CSOs) as conditions have moved beyond what these combined drainage systems were designed for, resulting in raw or partially untreated sewage discharging into rivers and the sea.”

Prof Roger Falconer FREng, Emeritus Professor of Water Engineering, Hydro-environmental Research Centre, Cardiff University, said:

“The role of fluid mechanics should not be forgotten in the situation we are observing at Whaley Bridge dam. In response to the unprecedented amount of rainfall in the area, there would have been a much greater amount of water, flowing at a higher velocity than normal over the spillway.

“In much the same way you get lift with air flow over the wings of an aeroplane, with the high velocity water going over the face of the spillway this creates low pressure underneath, which could be enough to generate a sufficient lift force by which the concrete slabs may be lifted or material broken away from the surface, especially at any vulnerable points.”

Prof Peter White, Emeritus Professor of Thermofluid Dynamics, Coventry University, said:

“There are several aspects of the dam, all of which could have contributed to failure, and should therefore feature in the repair and subsequent maintenance. When the spillway is subject to high flows as it was prior to failure, any obstruction to the flow which results in separation from the face will result in low pressure regions and/or cavitation, both of which could result in the movement and breaking of the slabs and water ingress between the slab joints. Another problem is the side wall which cuts across the spillway face and will cause vortex roll up at the intersection of the wall and spillway face, which will result in another area of low pressure and possible failure.

“With regards to the presence of vegetation on the spillway this is a major problem if the roots are in the joints between the slabs, or in cracks within the slabs as the result will be weaknesses for water to ingress under the slabs.

“In the repair or rebuilding of the spillway face at Whaley Bridge, it should be ensured that there are no areas for possible water ingress, for example a reinforced concrete structure, and that all possible causes of flow separation from the spillway face should be removed to ensure it can withstand high water flows in future in addition to regular maintenance with the removal of all vegetation from between the slabs and any resultant repairs to the joints.”

Comments Issued on Thursday 1st and Friday 2nd August

Jeremy Benn, Executive Chairman JBA Consulting Engineers & Scientists and a member of the UK Reservoir Supervising Engineer panel who undertake safety inspections of reservoirs, said:

“Toddbrook reservoir is one of 2,055 ‘large raised reservoirs’ in England.  These are defined as reservoirs that can store more than 25,000 cubic metres of water.  It has a capacity of 300 million gallons (1.36 million cubic metres) and dates from 1838.  It was built (and is still used today) to supply water to the Peak Forest Canal, and is owned and maintained by the Canal and River Trust (formerly British Waterways).

“Although there have been no recent reservoir failures that have resulted in loss of life, it was the death of 21 people after the failure of two dams in 1925 that led to the passing of the Reservoir (Safety Provisions) Act 1930. This Act was updated and superseded by the Reservoirs Act 1975, which in turn was amended by the Flood and Water Management Act 2010.  This legislation places a strict safety and maintenance regime on the owners of all reservoirs that are defined as ‘large raised reservoirs’. This includes registration of the reservoir, compilation and maintenance of a ‘log book’ called the Prescribed Form of Record, and a regular regime of inspections.  The latter are undertaken by specialist ‘Panel Engineers’ who must be qualified and competent and approved by the UK Government.  Regular (at least annually) inspection is undertaken by a Supervising Engineer and who is available at all times to advise the owner on safety.  At least every 10 years a Section 10 Inspection is undertaken which is a thorough examination of the safety of the dam.  Following the inspection a Safety Certificate is issued which confirms if the reservoir is safe to fill and to what level water can be impounded.  If the Section 10 inspection finds any matter that could impact on safety then a notice is issued that sets out the remedial work required.  This work has by law to be carried out.  In England, The Environment Agency is the enforcement authority and will and does take action if owners do not implement the safety measures. There is no record of any failure to comply with safety measures at this reservoir by the Canal & River Trust.

“In addition to the above, for the reservoirs with the highest consequence should there be a breach, risk maps have been produced by computer modelling.  These show the areas that could be flooded and to what depth. These maps are shared with the Local Resilience Forums and the emergency bodies and will have been available to all those needing access to them. Most owners also produce an emergency plan which sets out what they would do if there is a risk of the reservoir dam breaching or overtopping.  This includes a plan of how the authorities will be alerted and how the reservoir water level will be drawn down.

“In the emergency we are currently seeing at Toddbrook the problem seems to be damage to the auxiliary spillway.  From its form of construction this seems to be a latter addition to the dam and results from past safety inspections which have required additional capacity to cope with the very large floods.  The original spillway on the north side of the dam would have on its own been unable to cope with the current rainfall.  As this is a canal supply reservoir, it would have been as full as possible at this time of year to provide water for the summer boating season.  As soon as the damage was spotted, then measures have been put in place to reduce reservoir water levels – both to stop water flowing over the damaged spillway and to reduce the volume of water being stored.  This is being done by opening the ‘scour’ or low level outlet (like a plug in a bath) – but the water can only be released in a controlled manner in order to prevent flooding downstream and also damage to the dam (an earth embankment dam like this one could suffer landslips if the reservoir level was dropped very rapidly).  It may be that pumps will or are being brought in to help lower the water level quicker.

“While this event is obviously very concerning, it is in some ways illustrating how well the legislation and safety inspection regime works in the UK.  For one, instances like this are very rare, and also there are plans in place of what to do.  There will of course be lessons to learn but they can wait until after the dam and reservoir are made safe.”

Professor Tim Broyd, Professor of Built Environment Foresight, University College London (UCL), and Past President of the Institution of Civil Engineers, said:

“Reservoirs are built to impound water, and are a critical part of England’s water supply infrastructure.  In simple terms, a reservoir is generally a flooded river valley, with the valley ‘cut off’ by a dam, which prevents water escaping and is normally designed to allow water to be released in necessary and manageable quantities throughout the year.  There are various types of dam, but the type typically used in England is an embankment dam, in which the force of water against a dam’s inner surface is balanced by the gravitational weight of the dam itself.  If the river that feeds water into a reservoir exceeds the flowrate for which the dam was designed to operate, there needs to be a means of relieving the extra pressure that the dam must then resist.  Engineered structures are used for this task, known as spillways, and it is important that a spillway is designed to meet a realistically ‘highest’ spillway flowrate.  If a spillway flowrate is excessively over that for which it was designed, then problems can potentially occur.

“Toddbrook Reservoir sits just above the village of Whaley Bridge, and was built in 1831.  From photographs on the internet, it appears to have an embankment dam with a central spillway, part of which collapsed on the morning of 1 August 2019.  The cause of damage is currently uncertain, but it might be that water overtopping the dam onto its spillway exceeded its design limits, and that the spillway surface succumbed to scour, which is a process whereby fast moving water creates pockets of high negative pressure that ‘suck’ the surface off whatever the water is flowing over.  Whilst the dam appears to be safe at the moment, there would be risks that further heavy rain might lead to further scour, or that the force of water behind the dam will become higher than the gravitational resistance force of the weakened structure, and that further parts of the dam might collapse leading to a dam breach.  If that happened, then the water impounded by the dam could very quickly flush out, with a very high flowrate and speed.

“Dams are highly regulated structures, which includes regular structural inspections by highly qualified engineers.  It is unlikely therefore that the dam was in a previously unsafe condition.  What may have been the cause, however, is that the flowrate into the reservoir was exceptionally high, as a result of extreme local rainflows.  As we have seen on numerous occasions over the last few years, rain and wind storm incidents are becoming much more extreme than observed in recorded history, potentially as a result of climate change.  It is becoming necessary to maintain a much closer eye on our engineered infrastructure as a result.”

Professor Roderick Smith, Imperial College London and Formerly Chief Scientific Advisor Department for Transport and Past President of the Institution of Mechanical Engineers, said:

“Toddbrook (the reservoir behind the Wharley Dam) is an old canal feeder reservoir built c. 1840 for the Peak Forest Canal.

“It is made watertight by a clay core supported by an earth embankment. The wall collapse appears on the pictures I have seen to be in the spillway. As long as the core is not damaged then the wall itself should be ok. Within the last few years new valves have been placed in the dam to expedite rapid drawdown in emergencies: presumably, this is happening now.

“In terms of volume, this is a rather small dam with a surface area of 0.15 km2 originally containing about 300 million gallons: a figure probably much lower now because of silting. Ladybower, for example, contains about 20 times this volume of water.

“Reservoir safety is maintained by inspections under an act of 1930 strengthened in 1975. Extreme weather events mean that there is increasing unease about the safety of older dams: particularly the need to release excess water safely and easily.

“Toddbrook has in the past had some issues with an inadequate valve system, but these are now replaced. Some have, in the past, expressed some concerns about old coal mines in the area with worries that should they collapse it has the potential to undermine some stability of the dam wall, although what we’re seeing today in unlikely to be related to this.”

Chris Binnie, Visiting Professor, University of Exeter, Water Engineer specialising in dams and water resources development and Fellow of the Academy of Engineering, said:

“The only direct knowledge I have is seeing the clips of yesterdays flow over the dam and todays images of the current situation. However, I was a panel engineer under the Reservoirs Act for some 30 years and designed three dams over 50m high so I have some general knowledge. I have not previously been involved in this dam.

“This dam is an old earth dam built about 1830.  What would appear to have happened is that the dam was originally built with a small spillway on the hillside beside the dam. The design standards for spillways then increased, i.e. it became a requirement for the dam to be able to pass a larger flood flow. To provide that, a large central spillway was constructed on the dam surface using slabs laid on the downstream face of the earth embankment. This has the risk that, if the slabs crack, then high velocity water can get underneath the slabs and erode the underlying earthfill and the slabs then settle, and failure of the slabs takes place. This is what appears to have happened here.

“The original side spillway at the end of the dam is being used to lower the water level in the reservoir. This is the correct response. This has lowered the water level in the reservoir such that there is no longer any water going over the central spillway.

“Provided that the side spillway can cope with the amount of water entering the reservoir, the situation should not get worse. Should the rainfall be such that the current side spillway cannot cope, then  there is a risk that the reservoir water level might rise to the extent that water could then go  over the central spillway again and it would then be possible for the unprotected section of the dam to erode further and fail but, on the limited evidence available to me, that seems unlikely.”

Dr Mohammed Heidarzadeh, Assistant Professor, Head of Coastal Engineering and Resilience LAB (CERLAB), Department of Civil & Environmental Engineering, Brunel University London, said:

“Whaley Bridge dam is an embankment dam which is equipped with a concrete spillway. In embankment dams, spillways are extremely important because they prevent embankment dams from overtopping. Note that overtopping of embankment dams is very dangerous because they can cause the dam to be washed away in a few hours and consequently a big flood may occur. Note that embankment dams are made from soil and so can be washed away rapidly. Due to heavy rainfall in Whaley Bridge area, the spillway is now broken and a big chunk of its concrete structure is damaged. There is a possibility that the spillway could then become fully broken in a few hours. If the spillway is fully gone, the embankment dam will be washed away very rapidly which could cause a massive flood.”

Declared interests

None received.