floods

RTS Walton Lane flood meeting update

Flood prevention was the main topic of an informative meeting on 19 December in Walton Lane, Weybridge, between representatives of the Environment Agency River Thames Scheme and local residents. We learned more about the status of work to model flood flows and levels, and the latest thinking on options for carrying excess water around Desborough Island.

The current preferred option is to widen Desborough Channel by cutting back the north bank (the Desborough Island side of the channel), which would avoid displacing the nationally important amenity of the Thames Path, which runs along the south bank.

The north bank is overgrown with self-seeded trees, a number of which which are reaching the end of their natural life, and in recent years several have been blown down in storms, sometimes partly obstructing the channel. (UPDATE March 2017: The picture below shows a recently blown down poplar, and scrubby trees growing into the channel.)

Widening the north bank would enable this edge of Desborough island, which has been prone to fly tipping, to be made into an appealing riverside nature amenity. It seems there is a balance to be struck in the decision however, with a few residents of Spelthorne wanting the south bank cut back.

Some Walton Lane residents meanwhile are pressing for the excess river flows to be diverted around the Spelthorne side of Desborough Island, by relocating the direction of the proposed Flood Channel 3 discharge, and dredging around the north of Desborough Island.

The current plans are to discharge the water back into the Thames by D’Oyly Carte Island, reentering the Thames more or less at right angles to the flow of the main river. Miles Macleod asked if the effects of the momentum of this 150 cubic metres per second flow entering the main stream at right angles had been taken into account (as the momentum of this flow would logically push up the water level against the south bank just before the narrow first Desborough bridge). It seems this has not yet been looked at in the 1D modelling, but the more detailed 2D flow and level modelling will seek to incorporate it.

Headlines from the meeting:

Bottom line of the RTS is that it should cause no increase in risk anywhere
At the top of Desborough current modelling shows a 0.04 to 0.11 metre reduction in flood levels
Modelling has been calibrated using data from six real flood events
Calibration of the model includes past flows from all sources (fluvial and groundwater)
Target accuracy is +/- 0.15 metres (i.e. it might not actually reduce levels at Desborough…)
The river bed beneath the Desborough bridges will be lowered by around 2 metres to accommodate extra flow
Current preferred option for increasing flow around Desborough is to cut back the north bank of the Cut

We had a number of questions for which immediate answers were not available, and the EA team have very helpfully now supplied these. Some key points are that:

The total volume of water occupying the floodplain in a 1 in 20 year flood is equivalent to about six hours flow of the Thames
Extra total flow downstream of the RTS relief channels caused by that water draining from the floodplain is modelled at about 5-10 cubic metres per second (cumecs) in a total river flow of 500 cumecs
More water currently flows around the north of Desborough Island around the old river loop than flows down Desborough Cut; this position will be reversed by the changes proposed in the RTS
The width of the channel under the first Desborough bridge below water level is 23.2 metres (and this will not be increased); the modelling takes this into account

Once again, we are extremely grateful to the River Thames Scheme team for the time and effort they are putting into keeping our local community consulted and informed.

UPDATE: North bank of the Desborough Cut, viewed from the Thames Path on the south bank in February 2017. Note the poplar blown over in a recent storm, and various trees growing out into the channel obstructing flow.

Walton Lane Weybridge flood update meeting 1 Aug 2016

The latest River Thames Scheme (RTS) developments and local flood prevention measures – including a possible temporary local flood barrier – were top of the agenda when Walton Lane residents (plus PPDRA Chair Miles Macleod) met with representatives of the Environment Agency and the RTS modelling team on 1 August 2016.

Costs and Benefits of the RTS

We were told that the projected costs of the River Thames Scheme at current values have risen to £476 million, and of that around £250 million has been promised so far (£212m from the Government, £36m from partners). The scheme would have significant benefits in terms of flood relief for residents between Datchet and Shepperton.

The RTS aim for residents of Weybridge — where the proposed Flood Relief Channel 3 would discharge — is that the scheme must not cause any projected increase in local river levels.

In parallel, the Environment Agency (EA) is looking actively at what can be done to mitigate local flood risk in Weybridge.

RTS flows and levels – 1D modelling

While the RTS 2D model (which should give more accurate modelling) is still undergoing peer review, we saw some highly relevant outputs of the latest 1D modelling of local flows and levels in different states of flood, modelled without the proposed Flood Relief Channel 3 in place. The modelling projected local flood levels on land around Walton Lane which very largely coincided with past local experience of actual flood levels. This reflects well on the quality of the modelling work.

Once the 2D model is available, probably by the end of September, it will be run to simulate flows and levels with Flood Relief Channel 3 in place. Then we will be able to get a better indication of the anticipated local impact of that proposed new channel (with its 150 cubic metres per second capacity) discharging at Weybridge

Models of Desborough alternatives

Meanwhile, on 1 August, we were shown the latest 1D modelling of alternative possibilities for works on Desborough Cut or around Desborough Island.

Five possibilities were presented, each of which has now been 1D modelled:

Widening Desborough Cut by 3 metres on the south bank
Widening Desborough Cut by 3 metres on the north bank
Dredging Desborough Cut
Dredging downstream of Desborough Cut
Creating ‘Doug’s Channel’ across Point Meadow and dredging north of Desborough Island

All five possibilities would succeed in avoiding an increase in river levels locally at Weybridge, according to the latest 1D modelling. Choices will be made later — we are told this will be after 2D modelling and after further consultation — using a range of criteria, and not simply on up front cost.

Option 5 would cost considerably more than others, as it would involve a huge amount of excavating and dredging. The 1D modelling suggests it would require, as well as cutting a channel across Point Meadow, the dredging of 35000 cubic metres of riverbed (20m wide and 2.2km long) to prevent the upstream RTS works increasing flood risk locally. This option would have a neutral effect north and east of Desborough Island, and would bring the benefit of decreased flood risk at Weybridge.

Impact of silting up again after dredging or widening

1D modelling predicts that the dredging options (3 & 4) would require 12,000 cubic metres of dredging along 1.1km of Desborough Cut, or 10,000 cubic metres of dredging along 1.0km downstream of Desborough Cut, to have a neutral effect at Weybridge.

In discussion it was suggested to us that – while no decisions on preferred option are being made at present – dredging would have major maintenance implications, which are likely to make dredging Desborough Cut less desirable than widening.

We were told that the River Thames in normal flow ranges achieves a self-managing equilibrium, with silting and scouring occurring as flows change. Hence if dredged it would tend to silt up again (because dredging would allow normal flows to be carried at a slower water speeds) and it would tend to revert to its natural depth profile needed to carry its normal flows, unless regularly dredged.

One question which we did not think to ask at the time, was ‘would the same not be true of widening the Desborough Cut?’. Widening the Desborough Cut would allow normal flows to be carried at a slower water speeds, so would not a widened Desborough Cut similarly tend to silt up and become shallower, hence reducing the maximum flood flow capacity, unless it was regularly dredged back to its current depth? We are following up on this question.

Implications of widening Desborough Channel

In comparing options 1 & 2, widening Desborough Cut one side or the other, participants were agreed that there are more things to consider than simply cost and increased flow. A very significant factor is loss of amenity – something which led Elmbridge Borough Council to conclude in 2010 that it could not support the proposals for cutting back the south bank.

If 3 metres of the south bank were to be removed along the length of the Desborough Cut, this would have a serious impact on the amenity and appearance of this stretch of the Thames Path and National Cycle Route 4. It would mean moving the path in places closer to the traffic of Walton Lane, and would forever change a popular riverside path which is used by countless people. There would also be a potentially costly logistical question of how to manage works which would require a national path and cycle route to be closed during those works.

We learnt that the profile of the river bed of the Desborough Channel is not, as previously suggested, vertically deep on the piled south bank and shelving on the north. In fact, close to the south bank it has a shelving river bed, and the piling is simply there to hold back erosion. Hence cutting back the north bank would not necessarily be significantly more difficult or different in terms of flow impact.

Widening on the north bank would also enable the Environment Agency to deal with those elderly self-seeded trees on its riverside land along the north bank which are in an increasingly poor state, with branches breaking off and trees falling into the river with each major storm.

Bridge bottlenecks in Desborough Channel

A serious concern for Walton Lane residents is the bottleneck in river flow caused by the first bridge across the Desborough Cut. The bridge is much narrower than the channel either side, and it would be too costly to widen it if widening Desborough Cut.

At present, water backs up in times of high flow, with visibly different levels either side of the bridge – high enough on the upstream side that it flooded a neighbouring property in 2014. The river bed was scoured deeper beneath the bridge by those extreme flows of the 2014 floods.

This bridge would become an even more worrying bottleneck if Flood Relief Channel 3 is constructed, increased the potential maximum flow arriving at the bridge.

To facilitate flow under the bridge, the EA is proposing to dredge beneath the bridge, to create a permanently deeper section of river which will be easily scoured in future high flows; also to modify the profile of concrete banking upstream of the bridge.

As a short term measure, residents have funded a small earth bank to help hold back floodwater from overflowing the bank at this point, but more could be done, and more needs to be done if the RTS proceeds with the construction of Flood Relief Channel 3.

EA Proposals for Temporary Flood Barriers

The 1 August Walton Lane meeting was also told of proposals for local flood prevention measures, aimed to protect residents of Walton Lane and Dorney Grove.

These plans are still work in progress by the Environment Agency, but they including a possible temporary local flood barrier which could be erected at short notice to prevent flood water from the River Thames reaching Weybridge homes.

The temporary barrier could run from the bottom of Thames Street and along the Thames Path towards the first bridge across Desborough Channel.

The initial draft route for the temporary barrier would have blocked Walton Lane, and excluded some houses from protection, but residents at the meeting suggested extending the barrier slightly to the point where Walton Lane rises above flood level, avoiding the need to put a barrier across Walton Lane. On the other side of Walton Lane a length of barrier would help prevent water from the engine river reaching homes.

The EA has purchased 40 km nationally of this modular temporary flood barrier system, and Walton Lane / Dorney Grove is a target area for protection. The EA draft plans for Walton Lane are being amended following our meeting, to reflect the suggested rerouting. Further work will be done to look at the relative risk from river overflow and from rising groundwater (which the barrier could not prevent).

Potential for a permanent flood barrier?

Our meeting also discussed the possibility for a more permanent barrier along the river edge between Thames Street and the first Desborough Bridge, either in the form of a bank alongside the path or slightly raised path (as per the Wey Navigation).

Important factors beyond its effectiveness at holding back floods would include the aesthetics of flood protection works along this stretch of the Thames Path, impact on amenity, and the practicalities of easy river access for the WLA Rowing Club, Weybridge Sailing Club and the Canoe Club

Future River Thames Scheme consultation

RTS workshops are planned for the end of September, and Walton Lane Residents anticipate a further update meeting once the 2D modelling has been conducted of river flows and levels at different flood probabilities with Flood Relief Channel 3 in place.

Walton Lane Environment Agency flood meeting

Residents of Walton Lane, Weybridge, have gained new insights into the Thames Scheme flood plans by talking directly with people working for the Environment Agency on modelling river flows and levels.

A very informative meeting in Walton Lane on 25 April was attended by local residents, including PPDRA committee members Doug Myers and Miles Macleod. In the course of the meeting it was agreed that flows of specific alternatives to widening Desborough Cut would also be modelled.

Headline learnings from the meeting include:

The 2014 floods were a ‘1 in 15 year’ event, based on historic evidence
Flooding has been unusually light in the Thames Valley for the past 40 years, so public expectations of flood risk are low compared with historic reality (and possible extreme events from climate change add more risk)
Flood Relief Channel 3 which discharges at Weybridge would carry its full design flow of 150 cumecs (cubic metres per second) at much lower total river flow volumes than reached in 2014 (which was 500 cumecs)
The flood relief channels would INCREASE the projected risk of Weybridge flooding unless something is done to increase flow downstream of Weybridge
The criterion of acceptability is that the works must NOT increase the predicted river levels for projected flood flows at any point (so downstream works are essential)
Widening the Desborough Cut on its south bank is the cheapest option to increase flow downstream (requiring driving in new vertical sheet piling to define the new bank and excavation of the current bank)
Widening on the northern bank of the Desborough Cut would be more expensive, as it has developed into a more natural kind of shelving bank with many trees limiting flow
Cost is a critical factor
Modelling predicts that Desborough Cut does not need to be widened much along its lower half: widening the upper half and dredging at the first bridge would increase flow sufficiently
Widening the bridges would be expensive (and has not been budgeted for)
The 2014 floods scoured the river bed at the first Desborough bridge, significantly increasing the depth at that point
The Environment Agency are looking into potential alternatives to widening Desborough Channel (but the indications were not particularly positive)
Cutting ‘Doug’s Channel’ through Point Meadow (the north-west horn of Desborough Island) would also require widening or dredging of the remainder of the northern old river loop, which would be costly
The modelling team agreed to model the effect of this northern alternative approach on local levels and flows

Modelling of levels and flows is a highly complicated process, and not a precise science

1D modelling is relatively quick, but only considers the flow within the confines of the river
2D modelling includes flow in the adjacent flood plain, when levels are high, but is slow and very complex
Peer reviewing is a hands-on process, involving CH2M (formerly Halcrow) peer reviewing the JDA model and the design works, in “a process of questions and answers that get to an agreed design”
The overall margin of error in modelling appears to be higher than some of the predicted local changes in level which modelling suggests the scheme will produce, though the calculation of possible modelling error is far from straightforward
(Editor Comment: A truly robust approach would demand that the projected effects must be greater than the margin of error. However, logically the estimated margin of error of modelling can only be based on experience of disparity between modelled flows and actual measured flows.)

Future action timescale

Currently the River Thames Scheme has Treasury approval for the outline case (as previously reported)
Modelling with the latest data is in progress, 1D now, 2D in the coming months
A definitive outline design is planned by the end of 2016, following workshops
Scheme detailed design will follow that
Works timetable is for weirs work in 2018, relief channels in 2020

LATEST NEWS: UPDATE FROM ENVIRONMENT AGENCY 26 MAY 2016 SHOWS FUNDING SHORTFALL

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!

Responses are welcome. Interested in contributing an opinion piece? Find out more…

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.

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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.

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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