Helen Marcus in H&H

There is worrying disparity between the factual evidence and the claims made by the City’s engineering advisors about proposals to build 8ft dams round some of the Hampstead Heath ponds. Of course the City must keep the dams in good repair and pay proper regard to the safety of the public.  But the exaggerations, contradictions,  and inconsistencies,  are deeply concerning when set against the damage these proposals will do and the money that will be wasted doing it. 

In all their 300 year history, the  dams on the Heath ponds have never collapsed and  no-one has ever been killed by water overflowing from them.  The flooding in the streets of north London in various torrential rain storms was entirely due to the failure of the sewers to cope with the volume of rainwater.

Camden and GLC reports of these events over many years never mention the ponds or the dams as being in any way involved in the flooding of surrounding streets.  In Gospel Oak in 1975, for instance, the Camden investigations found that flooding was “concentrated along the lines of the GLC trunk sewers”.

Nor do the reports ever record any death, which they would have been legally obliged to do.  The “urban myth” of the 1975 “fatality” has been touted by the City and its engineers in support of their doom-laden warnings without bothering to verify it.

As we all know, what you get out of a computer model is what is put into it ; any model is as good as the assumptions it relies on; change the assumptions and you get a different answer.  The City’s proposals regrettably are based not on the facts as they are recorded, but solely on computer models invariably using only the worst case assumptions of total dam collapse: that every dam in every pond would breach instantaneously and simultaneously, and great waves of water would flood from the ponds drowning 1,400 people.

It bears no relationship to real figures for storms on the Heath or, indeed, to fatalities in recent terrible storms across the UK,  which, where they have sadly occurred,  have been in the tens, not hundreds or thousands, and not due to dam collapse.  The Environment Agency states categorically that:

“… since 1925, there has been no loss of life due to dam disasters in the UK.  …….. the likelihood of a complete dam failure is considered to be very low. The likelihood of flooding from a reservoir is far lower than from other forms of flooding.”  Surely the engineers must know that; it contrasts strangely with the City’s disaster scenarios.

Surely they must also be aware of Modelling Uncertainty connected with floods in urban areas.  The UNESCO  website advises  that “modelling of their dynamics and impacts should not be considered as a precise activity”.  One of the Stakeholder groups has also suggested that the scale of proposed construction actually introduces an engineering risk.

Another point of concern is that the bases for the City calculations are so variable  as to be of the  “think of a number and double it”  variety.  Depending on which report you read the risk factors of the Probable Maximum Flood (PMF) vary from a 1:400,000 years storm probability down to a 1:23 or even a 1 in 5.  Government design standard reservoir requirements vary from 1:100 years to  1:10,000 ( which, incidentally, would include Noah’s flood!).  Take your pick – they all appear to be arbitrary.

Attempts to query them are stonewalled with “the figures have come from the computer models” and “industry guidance”, implying that we must therefore simply  trust them. And anyone who tries to query the “fatality” figures is made to feel that they must be callous monsters who do not care about people’s lives. Stakeholder groups were told that local rainfall data from the Hampstead Scientific Society  were discounted as they didn’t fit the model that had been designed.  The Camden & GLC factual reports were brought to the attention of the City’s engineers, but dismissed as irrelevant.

This is really about democracy and common sense. The government’s appointed Panel of Dam Engineers is a narrowly specialised group of people who have been given enormous power by legislation that they have lobbied for, to do as they please. They have not hesitated to make clear that if their proposals are rejected,  they have powers under the law to force them through anyway. No doubt they can claim to be following the legal guidelines and standards, “in the interests of public safety”,  but who has set them?

Furthermore the City reports make clear that, even if the dams are built, “storms will still cause floods in the area downstream after the work is complete”  and  “…..will not prohibit associated flooding from occurring”.

If this was  really about public safety why is the City not working with Thames Water and Camden Council to address the deficiencies of the area’s sewage and water systems, and develop robust early warning plans?  This much more likely danger is ignored: the City reports make clear that if a thousand people were to be drowned by failure of the sewers, they do not regard it as their problem. But the Heath will have been permanently and irrevocably disfigured for no good reason.


Helen Marcus
(published in “Ham & High” newspaper)