The City of London is embarking on a multi-million pound scheme in an attempt to stave off the threat of flooding on Hampstead Heath, according to plans revealed to NCE.
“1,500 at risk”
Engineers were called in to tackle the Heath’s system of ponds, or reservoirs, which desperately need upgrading.
If they were breached, the City of London has calculated that the lives of up to 1,500 people would be at risk.
Severe renovations are required to make the Heath’s much-loved ponds safe in light of the findings of a hydrological study and new guidelines (see box). Given the new information, work will need to be carried out within the next two to three years at a cost to the City of London of between £10M and £15M.
The East Heath has two long established pond chains that originally formed from tributaries of the River Fleet. They were dammed over 300 years ago and now form the well-used ponds.
As the Heath’s steward, the City of London must now install spillways on at least three, but potentially all, of these 13 ponds (see map), to enable them to withstand extreme flooding conditions.
This will involve drawing down the ponds, then installing a reinforced earth structure to bolster the embankment (see picture).Environmental consultant Haycock Associates has been carrying out hydrological studies for the City of London, and the results have revealed that without the work the pond dams are unable to cope with the 1 in 10,000 year flood standard set by government guidelines, City of London assistant director for engineering Paul Monaghan told NCE.
Dense pedestrian traffic on the Heath has compacted the grass and made it act like concrete, which has resulted in a much greater run-off than is normally associated with grassed areas. Monaghan added that the Heath’s ponds are “historic earth filled dams” that do not have engineered spillways and can therefore be overtopped.”
“The results allowed us to fully understand how the system would work in a severe flood”
At the moment, three of the ponds are covered by the Reservoirs Act 1975 – the Highgate Men’s Bathing and Model Boating ponds on the east chain and Hampstead No 1 on the west – which regulates reservoirs over 25,000m³ in volume.
However, the new Flood and Water Management Act 2010 regulations, which will come into effect in 2012, now means that all 13 ponds will be covered. In addition to providing a full understanding of the risks, the hydrological study also helped to create a better mitigation plan.
“The [study] gave us some interesting results,” said Monaghan. “The results allowed us to fully understand how the system would work in a severe flood.”
That coupled with the new Act means that the combined effect of one dam failure spilling flood water into another reservoir can now be fully addressed.
“The new Act allows us to consider the ponds as a chain and analyses the cascade effect of dams failing and doesn’t treat each one in isolation,” dam expert and ICE panel engineer responsible for the scheme Andy Hughes said.
This gives more flexibility in how the team deals with the problem, added Hughes, because it allows them to build capacity upstream to help reservoirs further downstream. The key principle behind the work will be to ensure that the dams cannot be overtopped. “If the water goes over then there is a possibility that the dam could be breached,” said Hughes.
“The engineering solution is relatively simple, the clever bit is talking to people and finding the right solution”
If this happened, the results could be catastrophic for housing in the flood zone, which covers areas to the south and east of Hampstead Heath. This part of the borough of Camden includes some of the most prosperous residences in the country.
Monaghan hopes to raise the embankment between the Bird Sanctuary Pond and Model Boating Pond by 2.5m to enhance water storage and attempt to reduce the size of the spillways in ponds downstream (see image).
Trees are adding to the difficulties – particularly regarding the spillway planned for Hampstead No 2 Pond.
“There are 20 oak trees in that area, we will need to remove some of them, but there will be no net loss of trees on the Heath,” said a City of London spokesman, adding that whatever is lost will be replanted elsewhere.
Gaining permission “challenging”
While the planned improvements will reduce the risk of potential loss of life to just double figures, gaining permission to carry out the work is expected to be challenging.
The Hampstead Heath Act of 1871 decrees that the area must be preserved in its current state and local residents are vehement in their defiance against changes to the Heath and the surrounding area.
However, the Reservoirs Act overrules the 1871 Act and Hughes insisted that there was a “need to engineer a solution that will be sympathetic to the surroundings”.
In addition to the local residents, there are a large number of stakeholders, including over 7M visitors each year, who will want to have a say.
The City of London together with Hughes and the design and construction teams, yet to be appointed, will need to carefully balance the needs and wants of the local population with ensuring the Heath meets enhanced safety standards.
“The engineering solution is relatively simple, the clever bit is talking to people and finding the right solution,” added Hughes.
The City of London is aware of the need to engage the local population in the process because there will be a lot of disruption when the improvement works get underway.
It will begin formal talks with user groups and stakeholders in spring 2011. Following their conclusion, it will apply for planning permission to Camden Council and then set out options for work on the Heath.
It is hoped work will begin in 2012 and could last for either one or two summers depending on the outcome of the talks, detailed design and contractor considerations.
History of the Heath
Hampstead Heath is an ancient London park of around 320ha. It sits within the London Borough of Camden.
Ownership was transferred to the City of London in 1989 from Camden with a £20M dowry. That has long since run out and the City of London spends £6M per year of the Heath’s upkeep.
All reservoirs under the Reservoirs Act 1975 must be inspected and supervised by a panel engineer.
Panel engineers are a group of specialist civil engineers appointed by the Secretary of State. All reservoirs operating under the Reservoirs Act 1975 must be inspected and supervised by a panel engineer.
The independent panel engineer will decide whether a reservoir should be assessed to a 1 in 10,000 year worst-case flooding scenario using a risk-based analysis. The 1 in 10,000 year flood scenario was first introduced in 1996 guidance Floods and Reservoir Safety 3rd edition.