Flood Update — River Thames Scheme interim answers

Following questions from PPDRA about the predicted effects of the proposed River Thames flood diversion channels — designed to discharge an additional 150 cubic metres of water per second back into the river at Weybridge — the Environment Agency River Thames Scheme team has helpfully provided some interim answers.  See below.

These are reassuring in some respects, but show that the updated flow modelling, drawing on recent new evidence, has some way to go yet.  We await further news.

From: River Thames Scheme
Sent: 20 January 2016 17:08
To: ‘Miles Macleod’
Subject: Query regards modelling of tidal interface and thames barrier

Dear Mr Macleod

I apologise for my delayed reply.

Please see below our table of responses to your recent questions regarding modelling  and tidal interface and Thames Barrier from our specialist teams.

Please be aware we are still awaiting the river modelling which we anticipate will be available in summer 2016.

Please do get in touch with us again if you have any further questions.

Yours sincerely

Felicitas Wappler
River Thames Scheme
Environment Agency Kings Meadow House, Kings Meadow Road, Reading, Berkshire RG1 8DQ

No. Question Answer
1 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) The hydraulic model that will be used to assess the impact of the RTS (including downstream of Teddington) is currently being finalised.  The potential impact of the RTS will be tested with and without the Thames Barrier closed.
2 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? The hydraulic model of the lower Thames to test the RTS design is currently being finalised.  The model extends from Hurley to Southend.Design flood conditions are represented in the model as inflow hydrographs for the Thames and its main tributaries, and a tidal water level boundary at Southend.  The magnitudes of flows tested in the model are based on analysis of long-term flow records at gauging stations such as Windsor, Staines and Kingston.  Therefore, the inflows to the model represent the effect of extreme rainfall and groundwater conditions, rather than modelling rainfall or groundwater directly.  The model has been tested for a range of design flood magnitudes covering the following chances of occurring in any given year: 1 in 2 (50%); 1 in 5 (20%); 1 in 10 (10%); 1 in 20 (5%); 1 in 50 (2%); 1 in 75 (1.3%); 1 in 100 (1%); 1 in 200 (0.5%); and 1 in 1000 (0.1%).  The downstream impact of the RTS is being assessed for each of these design events.The modelling work is being independently reviewed by CH2M.  Their review covers the appropriateness of the model boundary conditions.
3 What is the current calculated effect of the RTS on levels in the Thames between Shepperton and Teddington, in worst case scenarios? The hydraulic model that will be used to assess the impact of the RTS (including between Shepperton and Teddington) is currently being finalised.The objective of the downstream compensation measures (at the Desborough Cut and Sunbury, Molesey and Teddington weirs) is to reduce flood risk at all locations in this reach of the river with the RTS in place.
4 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? No, there is not a conceivable scenario where the lower Thames weir complexes would be operated to deliberately limit flows passing downstream to prevent flooding downstream of Teddington.  With the RTS in place, the gates at Sunbury, Molesey and Teddington will continue to be operated in the same manner as they are currently.  The gates at the weir complexes are progressively opened as river flows increase, so that in major floods all gates would be fully open.
5 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? The magnitude of the design floods used in the modelling work have been reassessed as part of the update to the hydraulic model.  This update makes use of the additional recorded flow data available in the last decade.  This has resulted in changes to the peak flows to be used in the model, although these changes are relatively small.


FURTHER UPDATE  10 February 2016

From: River Thames Scheme [mailto:rts@environment-agency.gov.uk]
Sent: 10 February 2016 09:16
To: ‘Miles Macleod’
Subject: FW: Query regards modelling of tidal interface and thames barrier

Dear Mr Macleod

Please find below the response from our specialist addressing your questions about peak flows and appropriateness of the model boundary conditions.

The peaks flows used in the modelling work are based on a statistical analysis of long-term records at flow gauging stations.  The key station for the Lower Thames is Kingston, for which records are available since 1883 and is the longest continuous flow record available in the UK.  The record has been extended up to the present day and this showed that including the Jan/Feb 2014 flood in the statistical analysis does not have a large impact on the peak flows used in the model compared to the estimates used previously in the Lower Thames Strategy.  Partly this is because such a long flow record was already available.  The second important factor is that the peak flow observed in February 2014 (500m3/s), whilst significant, is smaller than several other previous floods that will have been considered when deriving the Strategy peak flows.  These floods include November 1894 (800m3/s), March 1947 (700m3/s), January 1915 (600m3/s), September 1968 (600m3/s), December 1929 (550m3/s) and November 1974 (550m3/s).

The 2014 flood was more exceptional in terms of its overall flood volume than its peak flow.  That is, the flows remained very high for a longer duration than previous floods.  The 2014 flood volume has been accounted for in the modelling work in the same way as peak flow, utilising statistical analyses of the long term flow records over a range of durations.

The scope of the CH2M review of the modelling work does cover the appropriateness of the flow peaks and flood volumes, and their assigned probabilities.

Please do contact us again if you require any further information.

Many thanks.

Kind regards

Felicitas Wappler
River Thames Scheme
Environment Agency
Kings Meadow House, Kings Meadow Road, Reading, Berkshire RG1 8DQ


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