OPINION: Thames Flood Diversion – Time For A Rethink?

The River Thames Scheme: piecemeal planning based on unreliable evidence?

OPINION by Miles Macleod
– The following article expresses the views of the author, and does not necessarily reflect the views of PPDRA. 

Will the plans for River Thames flood diversion channels be an effective solution for flooding, or a source of bigger future problems downstream and increased flood risk to London?

The Environment Agency’s River Thames Scheme proposes three new flood relief channels alongside the Thames, intended to reduce flooding between Datchet and Chertsey. The final channel, with a capacity of 150 cubic metres per second, would be cut past Shepperton and discharge at Weybridge, opposite the Canoe Club.

‘The River Thames Scheme’ is perhaps a misleading name. Originally called The Lower Thames Flood Risk Management Strategy, it covers the stretch of non-tidal river between Datchet and Teddington. Not the whole River Thames.

Capacity increase funded, impact unknown
Downstream of the three planned relief channels, the River Thames Scheme proposes to widen the Desborough Cut at Weybridge, and to increase flow capacity at Hampton, Molesey and Teddington Locks, to carry extra water onwards towards London.

What would the effects be at Weybridge, and downstream towards London? Would it increase the flood risk?

At present, the Environment Agency can’t say. Why so? Because “the hydraulic model that will be used to assess the impact of the RTS (including downstream of Teddington) is currently being finalised“.

Yet over £300 million funding for the River Thames Scheme has been promised, on the basis of old assumptions and outdated flood risk and flow predictions.  Meanwhile – as we will see later in this article – senior figures in the Environment Agency itself are questioning existing thinking on flood protection.

Risk to London
In the 2014 floods, the existing River Thames flow capacity carried 500 cubic metres per second of water over Teddington Weir. The Thames Barrier had to be deployed repeatedly, in sync with normal tidal flows, to help carry the excessive river flow onwards, and hence prevent flooding in the tidal Thames.

The River Thames Scheme proposes to increase the potential flow arriving at Teddington Lock by another 150 cubic metres per second. Will that increase in flow capacity lead to an increase in the flood risk to London?  At present, the Environment Agency can’t say.

Some of us are deeply concerned at what seems like a succession of uncoordinated plans – different project schemes, with different political sponsors, looking at different parts of the river – moving the flood risk progressively downstream.

Previous experience
In 2002, a flood diversion channel bypassing Maidenhead, Windsor and Eton was opened. It discharges just upstream of Datchet. Called the Jubilee River, and costing £110 million, with a capacity of around 170 cubic metres per second, it has helped divert floods from Maidenhead, Windsor, Eton and Cookham.

In the years since the Jubilee River was opened, flooding has increased from Datchet downstream. That, according to the Environment Agency, is a coincidence. They say that the Jubilee River “operates so that flood levels downstream are not adversely affected”.

Local concerns
In 2009 the Environment Agency put forward the Lower Thames Flood Risk Management Strategy, setting out plans to address the flood risk downstream of the Jubilee River.  That raised hopes for some people, and worried others.

In Elmbridge, people were concerned by the potential downstream effects of a flood relief channel discharging at Weybridge, and also the proposals for cutting into a significant amenity, the popular stretch of the Thames Path and National Cycle Route which runs alongside the Desborough Cut. The Environment Agency called this an ‘access track’. They proposed to widen this side of the Desborough Cut by 3-4 metres, and move the ‘access track’ closer to the road (Walton Lane) – but not to widen the two bridges across the Cut, which are existing bottlenecks restricting flow.

Elmbridge objections
The author of this article, Miles Macleod, was at that time an Elmbridge Borough Councillor for Weybridge North. That is where the planned third flood relief channel would discharge, bringing 150 cubic metres a second of additional water flow capacity. He alerted Elmbridge Borough Council to the implications for the Borough of the plans. Others councillors agreed that this was a serious issue for the Borough.

Elmbridge Borough Council studied the plans, and was concerned. It concluded in January 2010 that it could not support the plans for widening the Desborough Cut – plans which would need the Council’s consent.

Elmbridge Borough Council also said that the projections for flood risk and flows needed credible independent external verification, and fluvial and tidal flood strategies should be coordinated.

PR campaign
The Environment Agency found that in the financial climate of the time it could not secure funding for its Lower Thames Flood Risk Management Strategy plans. So it spent five years actively promoting the plans and the projected benefits, using its old flow calculations as evidence. It renamed the strategy as The River Thames Scheme – a name suggesting something more far-reaching than the lower Thames between Datchet and Teddington – but still could not raise enough funding.

Then the February 2014 floods came upon us, with particularly severe effects on residential areas between Datchet and Chertsey (downstream of the Jubilee River). The result was political pressure to fund the proposed Lower Thames flood relief channels between Datchet and Weybridge (and the wider Desborough Cut).  At the time, a general election was not far away.

Lo, the remaining funding for the River Thames Scheme was secured. And it was secured despite new evidence about river flows and flood levels – experienced in the 2014 floods and more recently – which raised serious questions about existing flood risk and flow calculations and modelling, including those underpinning the River Thames Scheme.

Misguided approach
The relief channels should help moderate short term extreme peaks of flow, but if there are extended periods of very high flow they have the potential to increase the total volume of water flow arriving at Weybridge by some 30%.

The Environment Agency reassuringly says that the differences in projected flood risks are expected to be small in the revised calculations, drawing on recent evidence added to the accumulated evidence of the past 130 years. But the fact remains that the scheme begs questions which at present don’t have good answers.

The answers may continue to change: with a changing climate and more extreme events, we cannot rely so much on probabilities calculated from the frequency of past flood events. Flood risk models must build in higher probabilities of known maximum flows and levels being exceeded.

Complete rethink
There is a basic concern.

Flood defences need a complete rethink. Who says so? The Environment Agency.

Rarely has a truer word been spoken.

The Agency’s deputy chief executive, David Rooke, has said the UK’s climate is entering an era of unknown extremes, and that a complete rethink of flood protection and resilience across the country is needed.

The £300 million River Thames Scheme plans are based on calculations of risks and flows which recent evidence has already shown to be unreliable.  They reflect a limited view of flood protection, and potentially increase flood risk to London from an extreme event, by significantly increasing flow capacity into the tidal Thames.

New approach needed
Successive extreme floods in various parts of Britain have led many people to the conclusion that current flood strategies are flawed, too narrowly focused and trying to tackle flood problems in the wrong way.

Piecemeal plans which move flood risk downstream are unacceptable.

A new approach is essential. The Thames needs a better thought out whole catchment plan, to help reduce the size of flood peaks along the length of the river – even in extended periods of exceptional rainfall.

A whole catchment plan might start with initiatives to slow down inflows along the length of the river, rather than trying to increase capacity in first one problematic stretch of the river, then another downstream.

Yet the River Thames Scheme appears to be going forward with great determination, to deliver what history may judge to be a very expensive mistake: an outdated, narrow and ultimately damaging project.

Time for that complete rethink?  It is not too late!

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