A very informative talk on flood management in the tidal Thames explained how the Thames Barrier is used to manage flood risk from extreme river flow as well as tidal surges, but it left a key question unanswered: what effect will the extra river flow from the proposed River Thames Scheme have on flood risk to London (and the whole area downstream of Shepperton)?
Andy Batchelor, Operations Manager – Thames Tidal Defences, received a warm reception from a large audience in Shepperton on 13 November as he unfolded the story of flooding in the tidal Thames, and the actions taken to combat it. He told how in recent years the Thames Barrier has been deployed to defend London: not just by holding back exceptional incoming tides, but also by helping drain excess floodwater coming downstream by closing the barrier after low tide, effectively turning the tidal Thames into a partly empty reservoir.
He showed how dramatically the River Thames flow at Kingston varies: from 5 cubic metres per second (cumecs) at times of drought to as much as 500 cumecs when floodwaters are coming down the Lower Thames valley.
A question which Andy Batchelor could not answer (asked by PPDRA Chair Miles Macleod) was what effect the River Thames Scheme will have on the volume of those flows. Mr Batchelor explained that his colleagues responsible for the Lower Thames flood management, under the leadership of David Murphy, were still modelling the flows resulting from the River Thames Scheme. He offered to pass on any specific questions.
The potential impact of the scheme on local flooding was of particular interest to the audience in Shepperton, as the talk topic had originally been advertised as The River Thames Scheme with David Murphy as speaker.
The hope is that the proposed flood diversion channels (see flood diversion coming to Weybridge) will take excess water to areas of benign floodplain with a high capacity to hold and slow its flow, while peak river levels subside. The concern is that the diversion channels will have lower holding capacity — so that bypassing built floodplain upstream results in higher volumes of water (and higher river levels) in the Thames from the point where they discharge, in the event of prolonged high rainfall. The third and final proposed diversion channel discharges at Weybridge, and according to the RTS Policy Paper it has a calculated flow capacity of 150 cumecs.
Follow up questions
Miles Macleod wrote to Andy Batchelor after the talk to thank him and to set out some specific questions. Andy Batchelor gave a very helpful response (see email trail below).
We are hoping also for a response from David Murphy to the specific questions:
- How would the progressive rise in level in the tidal Thames (upstream of the barrier) from river flow, over a high tide cycle (with/without the barrier closed) be changed by the increased maximum projected possible flow over an increased capacity Teddington Weir, as proposed under the RTS? (i.e. by how much would RTS increase the water level in the tidal Thames throughout the tide cycle, in a worst case scenario)
- On what assumptions has that been calculated (worst case projected rainfall, worst case prior groundwater conditions, maximum flows upstream etc); how has it been modelled, and how has it been independently verified?
- What is the current calculated effect of the RTS on levels in the Thames between Shepperton and Teddington, in worst case scenarios?
- Is there a conceivable scenario in which the flows at Hampton / Molesey / Teddington weirs might be constrained to protect London from flooding, and what would the impact of that be on river levels between Shepperton and Teddington?
For residents downstream of Shepperton weir, these are significant questions.
Below is the text of the emails and the questions and replies
From: Miles Macleod
Sent: 17 November 2015 16:13
To: Batchelor, Andy
Subject: Questions on river levels and flows — following up from your talk in Shepperton
Dear Andy
Thank you again for giving such an interesting and informative talk in Shepperton last Friday. It is always a pleasure to hear someone speaking from a real depth of knowledge and experience. Your presentation answered most of my questions without my having to ask them!
Of particular interest was your slide showing graphs of actual/projected river levels in the tidal Thames in 2009, with/without the Thames Barrier being used to protect London from a major tidal surge (while there was a relatively low fluvial flow of 9.45 cumecs), also the figure of flows in the Thames during periods of drought and fluvial flood (range 4 to 500 cumecs), and the discussion of the large number of occasions of Thames Barrier closing/opening to help drain the fluvial floodwaters at the start of 2014.
I would be extremely interested to see equivalent graphs showing the actual/projected levels in the tidal Thames over a tide cycle at the peak of the 2014 floods, with/without use of the barrier to help drain fluvial water.
Would it be possible to have a copy of any such 2014 graphs, plus the 2009 graph from your presentation? That would hugely help us understand the issue more deeply locally.
You may recall that I asked a couple of questions about an aspect of particular local interest, the impact of the proposed River Thames Scheme works on flows and flood levels downstream of the relief channels, which you said David Murphy would be better equipped to answer.
Specifically:
- How would the progressive rise in level in the tidal Thames (upstream of the barrier) from river flow, over a high tide cycle (with/without the barrier closed) be changed by the increased maximum projected possible flow over an increased capacity Teddington Weir, as proposed under the RTS? (i.e. by how much would RTS increase the water level in the tidal Thames throughout the tide cycle, in a worst case scenario)
- On what assumptions has that been calculated (worst case projected rainfall, worst case prior groundwater conditions, maximum flows upstream etc); how has it been modelled, and how has it been independently verified?
- What is the current calculated effect of the RTS on levels in the Thames between Shepperton and Teddington, in worst case scenarios?
- Is there a conceivable scenario in which the flows at Hampton / Molesey / Teddington weirs might be constrained to protect London from flooding, and what would the impact of that be on river levels between Shepperton and Teddington?
For residents downstream of Shepperton weir, these are significant questions.
Would it be correct to assume that the thinking (and conceivable maximum flows) may have moved on a little since the original work in framing the Lower Thames Flood Relief Strategy?
I found the paper ‘The winter storms of 2013/2014 in the UK: hydrological responses and impacts’ (WEATHER, Volume 70, Issue 2, pp40-47, Feb 2015) very illuminating on the extent to which previous maxima were exceeded in some rivers. We were fortunate indeed that the storms had been preceded by an extended period of exceptionally low rainfall and river flows nearing period-of-year record minima. With the jetstream adopting patterns unheralded in recent history, it seems quite possible that next time we may not be so lucky.
Hence it would be reassuring to have some definitive answers to the above questions. If you or David Murphy can provide the answers, that would be hugely appreciated.
Thanks again.
Kind regards
Miles Macleod
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Miles Macleod
Chair, Portmore Park & District Residents Association
Here are the graphs showing the record flows arising from the exceptional rainfall of Jan Feb 2014 (fortunately following a period of exceptionally low rainfall and flows), as published in Weather, February 2015, Vol 70 No 2 (Copyright 2015 Weather)
What would be the effect on the flows and levels at Kingston and downstream of Teddington Lock, of an extra 150 cumecs of water flow potentially being introduced by the proposed River Thames Scheme channel discharging into the Thames at Weybridge?
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From: Batchelor, Andy
Sent: 27 November 2015 08:33
To: ‘Miles Macleod’
Subject: RE: Questions on river levels and flows — following up from your talk in Shepperton
Hi Miles
Thank you very much for the feedback and I am glad you found the presentation interesting.
Apologies for the delay in getting back to you but I have been away from the office a lot since we met.
I will today forward on your request to David Murphy and team and I will also find the slides you mention from my presentation and get them to you early next week.
Kind regards
Andy
Andy Batchelor
Operations Manager – Thames Tidal Defences
Environment Agency
Kent & South London Area
Thames Barrier, Eastmoor Street, Charlton,
London. SE7 8LX
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UPDATE DECEMBER 2015
Andy Batchelor has supplied some additional and very interesting information on past flows and levels. The two graphs below reflect level at times of
- low flow in the Thames (9 cumecs) with the barrier deployed to test its use in protecting against a tidal surge, and
- high river flow (266 cumecs) with the barrier in use to help manage the water coming down the Thames.
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From: Batchelor, Andy
Sent: 07 December 2015 12:10
To: Miles Macleod
Subject: RE: Questions on river levels and flows — following up from your talk in Shepperton
Hi Miles
Sorry again for the delay in getting back to you.
The slide below ( Oct 09 )is the one that was part of the RTS presentation you attended. It is the tide plot from our annual test closure where we close at low water and allow the tide to build up against the barrier gates , with low levels upstream this creates the maximum differential between upstream and downstream levels. We do this operation as part of our annual load tests.
This slide ( Feb 14 ) was the tide plot from an actual flood defence closure of the Barrier during the 23/14 winter closures. The timing of closure of the barrier gates in the tidal cycle varies upon the conditions at the time. In this example we have closed later in the tidal cycle and have just taken the top off the tide to reduce levels upstream.
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UPDATE 23 DECEMBER 2015
A promising response from the River Thames Scheme:
From: Wappler, Felicitas
Sent: 23 December 2015 18:16
To: ‘Miles Macleod’
Subject: RE: Questions on river levels and flows — following up from your talk in Shepperton
Dear Mr Macleod
Thank you for your helpful feedback regarding the recent Thames Barrier talk by Andy Batchelor.
To update you, the project manager is working on his response to your outstanding questions and is also liaising with other specialists who are on leave over the Christmas period to give as fully informed a response as possible.
We will be able to send out a finalised reply to you after we are all back from leave during week commencing 04 January 2016.
I hope this is in order with you. Please do feel free to get in touch if you have any additional questions.
Many thanks
Yours sincerely
River Thames Scheme
Environment Agency
Kings Meadow House, Kings Meadow Road, Reading, Berkshire RG1 8DQ
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