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The Jubilee River story - Oct 2003 - The River Thames Society


Flood Committee Report

The River Thames Society, following the January 2003 floods, decided at a meeting of the Council on 24 February to set up a Flood Committee to look into the background to the flooding, the causes and its effects particularly on residents, businesses and members of the Society.

The Committee was set up as follows:

RTS Chairman:

Sir Anthony Durant (Chairman

RTS Director

Geoff K Payne (Secretary)

RTS Vice-Chairman, Non-Tidal

Leslie Jones

RTS Vice-Chairman, Tidal

Brian Hardwick

Middle Thames Branch – Planning

Val Mason

Upper Thames/Thameshead, Chairman

Colin Reynolds


A number of meetings were held and this report draws on papers from all members of the Committee. The RTS Council endorsed the report in October 2003 for national release.                                                

This report, and the background papers, are available on the society’s website,

CONTENTS                                                                                                         PAGE





January 2003 Floods and the Jubilee River



Flooding and climate change



Planning policy and climate change



Planning policy in Windsor and Maidenhead



Insurance and compensation



Operation of weirs and the Thames Barrier



Dredging of the Thames



Tributaries of the Thames



Flood warning, emergency response and sewerage



Flood issues and questions






Association of British Insurers


cubic metres per second


Department for Environment, Food and Rural Affairs


Environment Agency


European Union

FD closure

Flood Defence closure (of the Thames Barrier)


Flood Risk Action Group


Indicative Floodplain Maps


Local Authority


Local Planning Authority


Lower Thames Flood Study


Member of Parliament


Maidenhead, Windsor and Eton Flood Alleviation Scheme


National Rivers Authority


Port of London Authority


(Government) Planning Policy Guidance


Royal Borough of Windsor and Maidenhead


River Thames Society


Supplementary Planning Guidance


UK Climate Impacts Programme


1.      The January 2003 floods and the Jubilee River:

The committee felt that the Jubilee River has succeeded in improving the flooding situation in Maidenhead and Windsor. Work needs to be done to make the river function more successfully by bank improvements and improving weir efficiency.  We welcome the fact that the independently chaired Flood Risk Action Groups (FRAGs) are reviewing the cause of the floods and the operation of the Jubilee River, particularly its effect on downstream areas.

We recommend that the FRAGs are given adequate resources to complete this review and for their extensive action plan for minimising the impact of future flooding.

2.      Flooding and climate change:

Global warming is yet to be fully proved, however there is no doubt that the weather patterns are changing.  Weather experts are predicting wetter winters and drier summers.  The River Thames Society feels that the Environment Agency and local councils should start to prepare for wetter winters with possible subsequent flooding.  They should build this projection into their project design, planning policies, maintenance work, etc. We believe the newly- formed Thames Alliance should consider addressing this problem.

3.      Floodplains and planning policies:

The constant building on the floodplains against all historic advice needs to be examined.  PPG25 is a very worthwhile document but should be implemented by the EA, councils and developers.  DEFRA should also re-examine these guidance notes and tighten up the planning procedures.  We feel strongly in the Society that floodplains are there primarily for alleviating the pressure on the river and to take surplus water.

4.      Insurance and compensation:

The insurance industry is less inclined than in the past to offer insurance against flood.  Flooding is a national disaster, and therefore the government should be the fall back for Insurers, when people are left distressed and their properties become valueless.

Since the Thames Valley floods in 2003, many householders have experienced difficulties obtaining, and renewing, flood insurance cover. While some insurance companies continue to consider flood risks for each individual property, others are unreasonably only considering flood risks for whole postcodes.

We recommend that insurance companies assess flood risks on each individual property, not simply on a postcode basis.

5.      Operation of weirs:

All weirs but one (non-critical) were eventually fully open during the flooding: the timing of their opening may need examination for the future.

The operation of the EA’s telemetry systems for measuring flood heights and flows was lacking in some stretches of the river, particularly the systems which monitor flow downstream of the Jubilee River. It is not yet clear whether this reduced the effectiveness of the EA’s flow control actions during the January 2003 floods.

6.   Operation of the Thames Barrier:

The Thames Barrier was raised a number of times during the flooding.  The committee was satisfied that it was raised principally to avoid a tidal surge coming up the river against the flood water coming down through London which might have caused further flooding and put London in danger from floods. The future adequacy of the Thames Barrier should be reviewed.

7.   Dredging of the Thames:

The question of the value of dredging was fully considered by the committee.  There is a need for a dredging plan for improved navigation and flood control.  We understand that limited dredging has been taking place in 2003, removing mud banks in the river and deepening some navigation channels.

8.   Tributaries

There are large numbers of tributaries feeding into the Thames.  Many of these are overgrown and associated ditches are often blocked with rubbish.  We recommend that those responsible for these feeder tributaries and associated ditches clear them as soon as possible to encourage a smooth flow of water.

9.   Flood warning, emergency response, and sewerage flooding

A major criticism of the 2003 floods, as far as the Environment Agency was concerned, was the lack of adequate information and warning.  An urgent review is required, and better co-ordination with other authorities, such as councils and the police, should be planned.

Thames Water should examine the sewers in the river valley to see what can be done to prevent raw sewage escaping from drains and flooding properties.

A.         JANUARY 2003 FLOODS and the JUBILEE RIVER

The Environment Agency (EA) report that the Thames Valley floods in January 2003 were the result of heavy rain in autumn followed by several days of significant rainfall at the end of December 2002 and early in January 2003. There was serious flooding along much of the non-tidal Thames. It was particularly serious, however, downstream of the new Jubilee River,

(the Maidenhead Windsor and Eton Flood Alleviation Scheme - MWEFAS). This was the first major flood since the Jubilee River was completed by the EA and deemed operational, in October 2001.

While the Jubilee River alleviated the flooding in Maidenhead and Windsor, the alleviation was not as great as predicted. This was because major design problems with the new river soon became apparent during the floods. Erosion of the banks occurred in several places, exacerbated by the lack of tree roots to strengthen the banks in these early years after the new planting. Severe erosion of the bank threatened to flood Eton Wick. The flow down the Jubilee River had to be reduced to allow emergency repairs to this bank. Full repairs could not be made until flows were lower later in the year. If the floods had lasted longer or the peak had been larger, there is some doubt whether the flood channel would have coped, and Maidenhead homes could have been flooded after all.

Flood victims downstream of the Jubilee River, particularly in Wraysbury, Staines and Chertsey, became convinced that the Jubilee River was to blame for the extent of flooding they had experienced. The government were persuaded that the new River’s effect should be investigated urgently. However, a public inquiry was ruled out, in favour of a speedier but less-resourced system of review. Flood Risk Action Groups (FRAGs), covering the Thames (from Hurley to Teddington) and the Chertsey Bourne, were set up in July 2003. With an independent chair, they have members from the EA, local authorities, Thames Water, The River Thames Society and two flood action groups which represent residents upstream and downstream of the Jubilee River. The EA and FRAGs are now looking at the effect and operation of the Jubilee River.

The downstream effects of the Jubilee River are yet to be established. Clearly flood water arrived faster and to a greater extent than if it had been allowed into the Maidenhead floodplain area. However, the EA are saying that their original modelling predictions from the early 1990s were proved correct. They say that these downstream effects of the Jubilee River were only slight, and that the main problem was caused by the exceptional rainfall pattern.

Near the Jubilee River in Datchet, land and buildings, which were supposed to be protected as a result of constructing the flood channel, actually flooded in January 2003. Some Datchet residents are also concerned about whether the channel’s raised banks near their homes might be breached in more extreme floods.

In addition, an upstream design problem, which exacerbated the flooding in Cookham, has been discovered. The flood defence bank, or ‘bund,’ across the north of Maidenhead was holding back more water than it should. Local residents are also concerned that new building and bridges may have held flow back in this upstream area.

This RTS report describes firstly  the generally-accepted causes of river flooding, and secondly the  future effects of climate change, before looking at policies and actions specific to the Thames.


B.1.     Causes of flooding and the impact of climate change


Extracts from Government Planning Policy Guidance, PPG25, Appendix A, July 2001, edited to refer only to river flooding, so exclude coastal flooding.


“A1.   Flooding occurs when the amount of water arriving on land (from rainfall, snow- melt, surface flow, flow in watercourses or inundation by the sea) exceeds the capacity of the land to discharge that water (by infiltration, surface flow, piped drainage or surface watercourses)….


A2.    The principal cause of river flooding is excessive rainfall or snow-melt within a limited period, which overwhelms the drainage capacity of land, particularly when the ground is already saturated or when channels become blocked….

(In addition, locally, (non-fluvial) flooding may occur due to groundwater overflowing, overland sheet flow or run-off exceeding the capacity of piped drainage during periods of heavy or prolonged rainfall. Such local flooding can only be addressed on a site-specific basis.)



The impacts of river flooding can be aggravated by:

·         growth of built development in catchments and other changes in land use, which increase the rate and volume of run-off;

·         sediment movement that has changed river cross-sections and affected flood levels;

·         lack of maintenance of flood defence systems, watercourses, culverts (including the flood relief areas around them) and road gullies, particularly where this leads to channel blockage;

·         canalisation, modification and diversion of rivers and watercourses, which increase the rate of flow and decrease the time taken for water to travel within a catchment; and

·         building of structures (e.g. embankments), which restrict flows over historical flood plains and thereby create additional flood risks both upstream and downstream.


A3.    Flooding is, therefore, a combination of human activity and natural physical conditions… In determining the risk posed by flooding, account needs to be taken of the likely depth, speed and extent of inundation and the potential for anticipatory action to be taken as a result of flood warnings. The consequences also vary with land use. For example, overtopping and possible failure of a flood defence defending a densely populated urban area is an extreme risk; the same event affecting agricultural land is unlikely to involve a serious threat to human life.


Impact of climate change

A4.    There is mounting evidence that the global climate is changing as a result of human activity….


A7.    The latest climate change scenarios suggest that annual rainfall is expected to increase by 0-10% by the 2050s with the largest increases in the north-west. A shift in the seasonal pattern of rainfall is also expected, with winters and autumn becoming wetter over the whole of the UK, by as much as 20% under some scenarios. It is also suggested that the number of rain-days and the average intensity of rainfall are expected to increase slightly and that average seasonal wind speeds could increase over most of the country. There is much less certainty regarding this potential increased storminess and the consequences for extreme wave activity on the coasts or for river run-off.


A8.    Initial research has suggested that, for the Thames and Severn catchments, increases in peak flow of up to 20% for a given return period could be experienced within 50 years. These are preliminary findings and further work is required but they give added incentive to maintain current defences, where they are justified, and to adopt robust and sustainable solutions where defences are replaced. Such considerations also add importance to the need to evaluate the potential impact of extreme events even where it may not be economic to contemplate high levels of protection.


A9.    Recent work has shown that there is some evidence of an apparent increase in more intense winter rainfall events in the UK since the 1960s. These are likely to contribute to Increased flooding. This is in line with a general expectation of a more extreme hydrological cycle with climate change. Climate models also predict more extreme precipitation events in the UK.


A10. The Government has established the UK Climate Impacts Programme to help organisations Including local authorities and planning bodies, to assess their vulnerability to climate change and plan appropriate adaptation strategies.  In 1998, the programme issued climate change scenarios for the UK. These are being revised to provide more information on extreme-weather events and greater detail at a regional scale. Publication is expected in early 2002. ”    (Ref: PPG25, July,2001)


B.2.     The latest data from the UK Climate Impacts Programme

            This update of climate change data, referred to in PPG25, was published by the UK Climate Impacts Programme in April 2002.[1]


It is based on an improved Hadley Centre model, which takes into account all the known factors affecting climate change. The authors looked at a range of periods and of scenarios, assuming low, medium and high emissions of greenhouse gases and other pollutants.


Concentrating here on winter rainfall projections, which are the most relevant to Thames Valley flooding, their results include:

·         The 2002 data confirm the 1998 data for winter rainfall projections.

·         The authors have ‘high confidence’ that ‘winter precipitation will increase’, and that the ‘intensity of winter precipitation will increase.’

·         For all periods and all scenarios winter precipitation will increase, eg by 2080, between 10% and 35% for different scenarios.’

·         Published results from ten other models were compared, and all ten agreed that, ‘over the UK, winters will become wetter.’ (The Hadley Centre model predicted change in the middle of the range of other models.)

·         Heavy winter precipitation will become more frequent. When physical structures are designed to withstand, say, a ‘once in 100-year event’, it is currently assumed that the magnitude of such an event is constant. For many aspects of climate this is unlikely to be true in the future. For example, in winter, the intensity of the daily precipitation event that has a 50% chance of occurring in a given year may increase over all the UK, apart from perhaps the NW Scotland. In some areas by the 2080s – SE England and SE Scotland – the increase in intensity may be more than 20% for the Medium-High Emissions and the High Emission scenarios.’





PPG25 stipulated that new housing should be protected against the ‘1 in 100 year’ flood allowing for the effects of climate change. For the Thames Catchment, 20% is to be added to the flood flows to account for climate change. This is no small amount. It is, for example, equivalent to half the capacity of the Jubilee River flood channel. PPG25 also indicates that the 20% climate change increase in flows should apply to the much wider ‘1 in 1000 year’ floodplain.


While some planning consultants are advising developers to follow this climate change guidance, many Thames Valley councils seem unaware of it. They have no planning policies or supplementary planning guidance on the matter. They clearly have responsibilities under PPG25, which they are not yet meeting. They tend to rely on the EA for guidance, but this is very slow in coming, and it is up to the councils to develop interim policies. It took nearly a year after the publication of PPG25 for the EA at Frimley to agree to commission consultants to model this 20% increase in peak flow, as part of their Lower Thames Flood Study  (LTFS), Hurley to Teddington, and the results are still awaited.


New properties built in the floodplain increase the risks for existing properties. Also if they are built without reference to the guidance on climate change, their owners may have difficulty obtaining insurance. Since January 2003, the Association of British Insurers (ABI) state that they cannot guarantee flood insurance for properties not defended to the government’s minimum standard, ie against a ‘1 in 75 year’ flood. Insurance for other properties will depend on flood history and risk. Premiums will take climate change estimates into account. (See also Section E.)[2]


The EA now aim to develop policies to take account of climate change within the ‘1 in 100 year’ floodplain, and to map a Climate Change Zone, as a buffer zone along the edge of the ‘1 in 100 year’ floodplain, within which flood protection would also be required.


Although not suggested in PPG25, it would seem logical to incorporate the Climate Change Zone into the ‘1 in 100 year’ floodplain, since the increased flows effectively shorten the return periods of floods. For example, what was a ‘1 in 100 year’ flood, will become more likely and may well become only the ‘1 in 75’ or ‘1 in 80 year’ flood. So, what is the ‘1 in 100 +20%’ should become the updated ‘1 in 100 year’ floodplain. Councils should plan to apply their full flood policies to this zone, to restrict new building, not just raise floor levels. This will require a major shift in policy, away from development, and towards flood protection for existing residents and property owners.


The results of the EA’s modelling for the ‘1 in 100 + 20%’ flood were due in July 2002, but are now not expected until 2004. So, in the absence of the full data, the EA has now decided to use preliminary data as it becomes available. In April 2003, the EA advised the RTS that, since March, the floor levels of redeveloped buildings in the floodplain are now to be raised to protect against the ‘1 in 100 +20% year’ to take into account the 20% increase of flow for climate change. The appropriate heights for floors will be estimated from the Lower Thames Flood Study as the results become available. However, if these height estimates are not available, floor levels are to be raised by a standard 600mm (approx. 2 feet) above the ‘1 in 100 year’ flood level.


However, no formal announcement of this new policy has been made and the modelled flood levels are held only by the EA, so are only referred to in cases where the EA is consulted.


Furthermore, quite illogically and against PPG25 guidance, estimates for climate change are not yet applied consistently to areas outside the current ‘1 in 100 year’  floodplain.  In the case of Maidenhead, they are not applied consistently to the floodplain protected by the MWEFAS. Thus while, within the ‘1 in 100 year’ mapped area, floor levels of new properties may be raised higher than in the past to account for climate change, they are not yet generally to be raised in neighbouring new properties just outside the mapped floodplain. We all know that flood water does not flow in steps and does not stop at a line on a map. There is no glass wall holding the flood water back.

All these matters are being discussed by a Development in the Floodplain Sub-Group of the Flood Risk Action Groups (FRAGs). The RTS is represented on this group, with an aim of reviewing and improving floodplain planning policies throughout the lower Thames area.






The EA and the Royal Borough of Windsor and Maidenhead (RBWM) have been discussing changes to flood policies during the last two years.  Progress is painfully slow.  There has been little commitment from either of the authorities or the councillors to produce updated planning guidance to take into account the government’s PPG25. They argue that parts of the Maidenhead area are now greatly alleviated from flooding by the MWEFAS.


The EA no longer wish to be consulted about planning applications in the floodplain protected by the Scheme (except for some larger developments with reference only to run-off and drainage). They stress that the Borough’s policy is for the Borough, not the EA, to determine.


The RBWM regards the Protected Area as out of the floodplain, and has no planning guidance covering it. They do not inform owners and developers that sites are in the floodplain protected by the Scheme, and do not refer them to PPG25 guidance.


PPG25 obliges planners and developers to take a precautionary approach to flood risk. New development must not be built in high risk zones if there are options available to lower risk areas. This guidance has not been followed.


As a result, new development is being permitted in the floodplain without any reference to flood risk, or to climate change effects.[3] This development is irresponsible, just as was the new building permitted after the 1947 floods, and after the construction of the Maidenhead Flood Relief Ditch in the 1960’s.


The RTS has argued that interim policies are needed urgently to increase flood protection to take account of two factors:


a)      The effects of climate change.

b)       The doubts about the effectiveness of the MWEFAS.


Some of the information required by the Borough, if it is to adhere to PPG25, is not available. For example, the EA has not yet produced the flood maps for the 1:1000 year floodplain, nor the mapping of their proposed Climate Change Zone. However, this lack of information can be overcome and is no reason for the Borough not to require precautions against possible floods. Owners and developers should be required to follow the principles of PPG25, rather be left ignorant of them, or be allowed to ignore them.





In October 2002, the Association of British Insurers (ABI) announced  that, from January 2003, they could no longer guarantee flood insurance for properties not defended to the government’s minimum standard, ie against a ‘1 in 75 year’ flood. Insurance for other properties will depend on flood history and risk. Premiums will take climate change estimates into account. change.[4]


The ABI stated, for example,

‘In areas where no improvements in defences are planned, (before 2007 to the Government’s minimum standards of 1 in 75 years or better) insurers cannot guarantee to provide cover in all cases. However, where there is a history of flooding they will use best efforts to work with policyholders to establish on a case-by-case basis, what action they, the EA and the LA can take to enable cover to be continued.’


‘Premiums and policy terms must reflect the level of risk – the annual probability of flooding - and the likely damage levels. Experience over the last 40 years is not necessarily a good indicator of future flood risk since the climate was relatively dry from 1960 – 1990. In fact many high-risk areas were flooded more regularly in the 1940’s and 1950’s and we are experiencing a return to similar conditions now. And, of course, there has been a lot of additional development in floodplains since that time. Climate change is likely to increase the risk of flooding further.’



Since the Thames Valley floods in 2003, many householders have experienced difficulties obtaining, and renewing, flood insurance cover. While some insurance companies continue to consider flood risks for each individual property, others are unreasonably only considering flood risks for whole postcodes.


If the insurance industry is less inclined than in the past to offer insurance against flood, the RTS suggests that the government has a role.  Flooding is a national disaster, and therefore the government should be the fall back for Insurers, when people are left distressed and their properties become valueless.





F.1.      Operation of weirs

All weirs but one (non-critical) were eventually fully open during the flooding: the timing of their opening may need examination for the future.


The operation of the EA’s telemetry systems for measuring flood heights and flows was lacking in some stretches of the river, particularly the systems which monitor flow downstream of the Jubilee River. It is not yet clear whether this reduced the effectiveness of the EA’s flow control actions during the January 2003 floods.


F.2.      Thames Barrier Operations January 2003

The Thames Barrier was raised a number of times during the flooding.  The committee was satisfied that it was raised principally to avoid a tidal surge coming up the river against the flood water coming down through London which might have caused further flooding and putting London in danger from floods. 


Thames Barrier closures can have two effects on water movement. As well as blocking a (surge) tide ingress in the tidal zone, they increase the upstream capacity to receive water from the non-tidal reaches, accelerating the onward discharge of upstream flood waters, providing the total flow rate is in the range 7000 to 9000 mgd (million gallons per day). The data available show that, for the flood period from 1st Jan - 8th Jan 2003, the flow rate described a hump between 8946 mgd and 7270 mgd, i.e. just within this range, so these operations managed to shift the non-tidal floodwaters downstream unhindered.


An ‘FD closure’ of the Barrier is a flood defence closure. (Other, non-FD closures of the barrier are also regularly made for other reasons.) The records show that FD Closure 69 took place on 01/01/2003. (FD closure 68 had been sometime in November 2002, i.e. no FD closures in December.) FD Closure 70 then took place on 02/01/2003, followed by two FD closures each day from 3rd January to 8th January inclusive, i.e. for both tides each day. This made a total of 14 FD closures (Nos. 69 to 82 inclusive) in just 8 days.


F.3.      Development of flood  modelling for the Thames Barrier

Originally, the decision to close the Barrier (from a tidal point of view) depended simply on the East Coast Storm Tide Warning System based at Bracknell. In 1984, it was considered that ominous conditions could be forecast at least 12 hours ahead. Action at the Barrier would be taken about one hour after low tide, fully four hours before an incoming surge tide could reach this point. The gates operate on average in 90 minutes.


The EA continues to develop models to assist the prediction of water/flood levels in response to tide and weather conditions. An “Event Plot” for the period 30/12/2002 - 09/01/2003 projected water height fluctuations and also indicated the smoothing effect expected from operation of the barrier. Measured data indicated some deviations from the projected values, and will lead to further refinement, but both plot and data displayed the effective influence of Barrier operation under the flow conditions for the period in question.


F.4.      Questions for the future of the Thames Barrier

The design life of the Thames Barrier was 120 years, starting 1984. The eastern seaboard is still tilting slowly downwards, and climate change will increase autumn/winter rainfall by about 20%, which would take the potential non-tidal discharge above the 9000 mgd cited above. So:

·         Who is planning what will supersede the Thames Barrier, and when?

·         What are the implications for the Teddington - Molesey reach for an earlier fluvial discharge rate above 9000 mgd?


G         DREDGING


G.1      General

The authorities who managed the Thames prior to the EA, (ie. NRA, Thames Water and Thames Conservancy) all carried out a structured and well-planned programme of dredging the River Thames.  This dredging programme was pursued to facilitate multi-function improvement of the River, including flood defence and navigation.  It was obviously carried out in a way that was as compatible as possible with maintaining the improving local environments.


Subsequently the general dredging programme has ceased, as it is not considered cost-effective.  Dredging is now carried out on an ad hoc basis merely to remove individual obstacles to navigation, The River Thames Society firmly believes that this development has added to a marked deterioration of the efficiency of the River Thames as  water carrier.   The EA will deny this and use hydrological statistics to support their case, but our contention is that the lack of dredging has had a gradual but significant effect on flooding, particularly from a local point of view.


In this context we note that the number of employees maintaining the River Thames used to be some 650 and is now a fraction of this with employees only held for emergency purposes.  This has also generally contributed to the reduced carrying capacity and efficiency of the River.


G2.      Dredging of the Non-tidal Thames


G2.1    Responsibility for dredging

Responsibility for dredging for the purposes of navigation and of flood control lies with the EA.


For navigation purposes, the dredging resources of the EA are focused only on the main channel (roughly the central third of its prevailing width) and on access to locks and associated moorings.

Riparian interests are expected to pay for any bankside or backwater dredging they deem necessary, though their ability to dredge may be curtailed by environmental policies of the EA, the influence of English Nature or constraints applied by local authorities.


G.2.2   Current EA policies on dredging

In response to questions raised at a “Public Surgery” concerning the January 2003 Flood Event, a written response on 7th March 2003 by Trevor Goodhew [Flood Defence Manager, South East Area, Thames Region, Environment Agency, {Frimley}] comprised the following three paragraphs:


“Lowering the river level to create “spare capacity” prior to a major flood event would have only a minor effect which would last only for a short time - very likely a matter of hours. Given the volumes of water we recently had to deal with in the Thames and its tributaries, this is not considered to be a practicable approach to flood management.


Dredging for the River Thames is carried out for two main reasons: Navigation and Flood Defence. For the former this is to maintain navigational depths in the main channels, on the lock cuts, and in areas close to moorings. The vast majority of this work consists of removal of shoals, based upon regular monitoring and survey work.


During the 1980’s and 1990’s as surveying and modeling techniques improved we continued to investigate further large scale dredging proposals. To carry out any such work we must demonstrate that our proposals are technically sound, environmentally acceptable and economically justified. A recently completed comprehensive dredging study on the River Thames was undertaken by consultants. This concluded that large scale dredging of the Thames is not justified on cost grounds. In addition, major technical difficulties and environmental damage could occur by undertaking such an exercise. Our efforts are focused on the removal of fallen or damaged trees that pose a risk to blocking of weirs. However we will be reviewing the option of dredging the Thames further when considering a flood alleviation scheme.”.


In 2003 the EA set up the Lower Thames Flood Study [covering the lowest of three non-tidal zones]. The responsibilities of its Project Manager, based at Frimley, include a re-evaluation of dredging policy. A report on this is due to be made available to the FRAGs. [Flood Risk Action Groups, collectively considering the flood risks between Hurley and Teddington].


The RTS is concerned that, in casual discussions with EA staff, it has been said that the EA have become reluctant to carry out any dredging within 30 yards of the river’s edge, because in some circumstances this can induce or enhance scouring or undermining of the bank and hence, these days, a risk of litigation by riparian owners.


G.2.3   The history of dredging on the Thames

[The following notes are based on a brief discussion with Colin Candish plus notes subsequently provided by Bill Ellison, [An EA Flood Defence Officer, also based at Frimley].]

The channel has been surveyed every 10 years since 1948. The surveys have shown that where the channel has been dredged it has not changed shape. There are isolated areas where large shoals form and these are removed as required. There is no evidence even with our improved surveying techniques that the Thames is becoming shallower. Contrary to expectations, the flow is having a scouring effect on the main channel. The general pattern indicates that in some reaches a small amount of erosion takes place, in others small amounts of deposition. However, overall, the channel seems to be in a stable condition (apart from the shoals described above).


From a Flood Defence perspective, the most important recent dredging initiative followed the Thames floods of 1947. In 1948 a dredging programme was adopted to lower the bed level of the Thames by 1 foot (300 mm) from Teddington to Reading. This may not sound much, but it was a major task that took fifty years to achieve. It meant shifting out a lot of material (up to 100 000 tons per year), mostly into old gravel basins or other landfill sites, which are now mostly full.


Whilst this was a major scheme in terms of the vast volume of material involved (and the consequent cost of dredging and disposal), this work was not intended to prevent flooding of the magnitude of the 1947 event, but to make a contribution to increasing flows and therefore reducing flood levels for all events.


G.2.4   Future prospects for dredging.

Compared with the old Thames Conservancy, the EA has much less public funding available. EA procedures require justification of projects in terms of economic benefits, acceptability in terms of environmental impact and avoidance of damage.


Economic benefits are related to the effective lifetime of the scheme being implemented, and required to be greater than the costs. Dredging is perceived as having a relatively short assured lifetime, compared with “hard works” constructions. Dredging costs are high, and disposal of excavated materials is becoming ever more difficult and costly. Most of the suitable sites were filled by the 20th century exercise, and environmental constraints are higher. With its focus everywhere on just “the main channel” (central third), the prospects of dredging by the EA are reduced accordingly.


G.2.5   Public perceptions regarding dredging.

Many of the public have strongly criticized the contraction of dredging activity on the non-tidal Thames from 60 dredgers under the Thames Conservancy to just 2 deployed by the EA, and their confinement to simply shoals in the central third of the main channel. They affirmed silting up of the border zones of the river, eg in the vicinity of Halliford Bend, which they perceived as an undeniable reduction in the capacity of the reach to either contain or transfer water under flood conditions.


For users, silting up of the river borders and backwaters, compared with conditions under the Thames Conservancy, creates a constraint on local boat movements (cruising or sailing) and property access. River use is not just about the through passages of the trip boats or the touring holidaymaker along the main channel between each pair of locks. For sailing clubs, in particular, the areas of relevant depths of water when nearing the banks extend well beyond their specific riparian locations.

 (Accounts of the EA refusing landowners (farmers) permission to protect or reinstate banks that have been eroded by flood scouring, or to dredge bankside, e.g. as requested by the RTS for a fraction of the waterfront at Bondig Bank, reflect some instances of considerable ill feeling.).


G.2.6   Questions and suggestions re dredging - Non-tidal Thames.

a) The public perceives dredging as a means to increase the water capacity of every reach to thereby enhance the containment and/ or rate of transfer of flood waters.

It also perceives dredging of the river border zones as an aid to local use, as distinct from use of the main channel by through traffic.


The sight of active EA dredgers would be a positive public image, to counterbalance the negative aspect of their officious roles and attitudes re licensing of boats, landing stages and the nature of bank maintenance.

·         Does/ could the EA value analysis for dredging operations cover this ‘public image’ effect?

·         Where did the old dredgers go?


b) The public does not appreciate the increased difficulty of movement and disposal of dredged material.

Maybe more imaginative solutions are required. Constructions by canal and railways companies at least could project longitudinal “cut and fill” policies to help maintain their levels.

·         Should management of rivers prone to flooding think laterally, shifting excavated material sideways to create supplementary dykes (though not necessarily in the immediate vicinity)?

·         When/ how could the constraints and options be presented more widely, in the pursuit of greater understanding and fresh suggestions for solutions?


G.3.     Dredging of the Tidal Thames


G.3.1   Responsibility

For the purposes of navigation and of flood control, much responsibility lies with the Port of London Authority [PLA]. There are other zones of influence, such as the “London Gateway” Scheme.


Dredging in the tidal zone of the Thames is primarily carried out as necessary to ensure safe navigation: 85 % of this island’s imports still arrive by ship.


G.3.2   Constraints

Prior to dredging, consultations take place with the EA and with English Nature concerning potential disturbance and whenever possible alternative re-use of excavated material is pre-planned. If territorial relocation of material is problematic, a written justification and a licence is required before dumping at sea is permitted.

Despite such precautions, a fresh potential burden/ constraint is a European  Union (EU) Directive aimed at protecting molluscs from physical disturbance and/or exposure to suspended material, written without regard to the requirements of Port operation and maintenance in a long-established estuary and river geography and settlement location.


Question re dredging - Tidal Thames.

·         Who is tackling any problems posed by the EU “Molluscs” Directive?




H.1      Maintenance of Tributaries

The reduction in the Environment Agency’s capacity to maintain the River Thames is also manifest in the current standard of maintenance of the major tributaries to the River.  The standard of maintenance by local authorities of the minor tributaries for which they were responsible was also very patchy. The problems observed include excessive vegetation, obstruction by rubbish or compromise by local changes of ground use.  The RTS firmly believes that the efficiency of the tributaries has been undermined due to a general lack of maintenance and this has again had a significant marginal effect on the carrying capacity of the River Thames and its tributaries.  We also believe that the cumulative effect of marginal factors affecting water capacity and transfer throughout the Thames catchment area is, in part, responsible for the poor flood defence performance in recent years.”


Modern telemetry, telephony and information systems are now so flexible and robust that we believe the Agency should be able to provide specific warnings for each tributary system, tailor-made to local circumstances.  The Agency must reassure us that this is their objective and provide us with an ambitious time scale for its achievement in the Thames catchment.


H.2      Tributary types and numbers

The society’s full list of the tributaries is available on It covers the Thames from the source to Warden’s Point in the estuary (i.e. just including the Medway)


Table 1                                                                                                           Source: RTS 






























In each category, some have a significant catchment and others are comparatively short and insignificant. This does not include the far greater number of local agricultural, highway or land drainage ditches, drains, dykes, gullies and associated cuts which lead their waters eventually to some such tributary.


H.3      Responsibilities for flood defence

Overall responsibility for flood defence rests with the Department for Environment, Food and Rural Affairs [DEFRA] as the relevant Government Department.


DEFRA has assigned responsibility for flood defence for the Thames and its major tributaries to the Environment Agency [EA]. Responsibility for minor waterways is mostly assigned to local authorities. Riparian owners have some responsibilities, e.g. not to interfere with or obstruct the course of the waterways.


There are some exceptions, e.g. Thames Water is responsible for the New River, which is really a flow bypass in East London. In some locations, highway authorities are responsible for at least part of a watercourse, e.g. where it has been diverted or culverted as a result of motorway construction. Canals vary, usually a specific trust.


H.4      Perceptions of condition

The list of tributaries summarised in Table 1 was compiled by an RTS member, for a different purpose, before the flood events of January 2003. Many of the tributaries had been visited by the author for a superficial acquaintance, not necessarily at more than one place, during the period October 2002 to March 2003. The pattern of waterways in a given locality was however often explored by cycling along minor roads, byways and bridleways. The condition of roadside and land drainage channels thereby observed incidentally varied considerably from clear to overgrown or choked: since January 2003 many appear to have been “recently cleared”.

Comments made at the meeting of Spelthorne residents called by their MP on 12 June declared that, for example, Sweeps Ditch and several other drainage channels had not been properly cleared for up to five years.


H.5      Future management of tributaries re flood defence

According to a Daily Telegraph report on 6th June, Lord Haskins, as the relevant Government minister, is proposing that the EA should be primarily a regulator re European directives on water, soil and waste, and that hence its flood defence role should be devolved to regional authorities.

Curiously, neither the MP, David Wilshire, nor Clive Onions nor any of the EA staff to whose attention this was drawn at the Spelthorne meeting on 12th June were aware of Lord Haskins statement made nearly a week earlier. At the very least, this must be expected to undermine EA staff morale and enthusiasm re flood work whilst offering nothing in their place for the next winter or two


It is not clear why the new pseudo-European regional authorities should be any better operationally than the old counties or local authorities re flood control over a total river catchment area, when once again the river is likely to be a peripheral or segmental element for two or more regional authorities having boundaries drawn up for other (political population management) reasons.


Flooding is a geographic event with environmental origins and consequences, and a two-way interaction with the much smaller network of navigation systems. It is practical to make local authorities responsible for the detailed maintenance of drains and gullies, but overall flood management should be based on catchment patterns.






I.1.       Flood warning

The water rose in the 2003 floods faster than in previous years’ floods, which gave little time for residents and businesses to prepare. A major criticism of the Environment Agency together with Thames Water was that there was a tremendous lack of information to the riparian owners about what was happening.  There was little or no warning to householders and businesses up and down the river that flood water was on its way down the river. 


Flood warning is an extremely important aspect of Flood Management.  It is absolutely vital that, where homes are in danger of flooding, residents should be given plenty of time to take appropriate mitigating action.


The performance of the Environment Agency’s website during the January 2003 floods was poor.  Although the Agency did well with national statements and warnings, it was much more patchy on a local basis.


I.2        Emergency response

The public was confused about which authorities were responsible for different actions and services during the floods. Better communication between the authorities and a co-ordinated response to flood emergencies are required.


I.3        Sewerage flooding

The drains, particularly sewerage drains, overflowed causing great distress to householders.

We appreciate in certain areas the foul water sewers are very old and are overloaded, this situation must be examined by Thames Water and remedied urgently before future floods.












During the compilation and research of this RTS report, essential questions were put to the EA and other agencies. Some are listed on these pages. These were copied to the Chairman of the Flood Risk Action Group (FRAG) in August 2003.


A small number of these questions have been answered satisfactorily, but the majority remain unanswered despite reminders.



A          Flood measures – rainfall, river flow, flood height and return periods

1.         We should be grateful for clarification on a number of technical terms and issues.


2.         Do Thames flood heights correspond exactly to flood flows?

There is confusion because flood events are described sometimes in terms of flood heights (eg. 24.2 metres) and sometimes in terms of flood flows (eg 404 cumecs). The two measures seem to be used interchangeably, eg in the EA’s report on the January 2003 floods, and in defining the return periods of floods. We understand that flood flows have only been accurately measured since gauges were introduced (eg at Old Windsor), but have been estimated for previous flood events, and presumably for return periods into the future. The more accurate historical data is therefore the heights of floods at particular places.


3.         Do we now start with the flood flows, and estimate the heights, which may vary over time reflecting new development in the floodplain and new flood defences? Hence the need to constantly update floodplain maps, to show updated heights.



4.         Does the flood flow during a particular flood event vary at different points of the Thames, depending on the pattern of rainfall along the length of the Thames and from the many tributaries, and on the extraction of water by Thames Water? If so, why do we refer to just one flow volume for each flood event, eg 404 cumecs for 2003?


5.         Does the flow associated with the ‘1 in 100 year’ flood vary at different points along the Thames, or is it constant along the whole river?


6.         If we use only the increased flow at Old Windsor lock to determine the opening of the Jubilee River sluices, are we, by default, increasing the height of the floods at Old Windsor and downstream?


7.         As opening the sluices on the Jubilee River will increase the speed of flow, and volume of flow, arriving at Old Windsor, did this lead to greater/faster height of flooding?


8.         How are return periods calculated? There seems to be uncertainty about the return period of particular floods, eg the 1990, 2000 and 2003 floods. Is it possible to give firm return periods, (disregarding for this purpose the need to take climate change into account in the future)?




B.        Floodplain modelling and mapping


1.         Can the EA provide a detailed proposed timetable for production of all currently planned modelling and mapping:

eg - to take account of the flood patterns in January 2003

-  updated post-MWEFAS Residual floodplain

-  1 in 100 + 20% flow for climate change for IFM and post-MWEFAS Residual     floodplain

-  1 in 1000 year

-  1 in 75 year, for insurance industry (ABI),

-  and others ?


2.         What are the margins of error allowed on floodplain maps? Are they cautious estimates, or should planning policies take more account of flood risks just outside the mapped line of the floodplain? How are lines on the floodplain maps drawn between two locks (or points where flood heights are known) – for,  example, is the line more accurate (or more cautious) at the top of a reach than at the lower end of the reach?


3.         What are the arguments for and against estimating the floodplain if the bund across the north of Maidenhead were breached. (NB. The author of PPG 25 recommended that this be done for sequential flood-risk testing.)




C         Effectiveness of the MWEFAS (inc Jubilee River)


1.         According to the EA Maidenhead Project Office staff, the MWEFAS should ‘protect’ against a 515 cumec flow, currently estimated as a ‘1 in 65 year’ flood, with:

295 cumecs    in main Thames,

+ 5 cumecs   in Maidenhead Flood Ditch, and

215 cumecs   in Jubilee River.


2.         What does ‘protect’ mean here? Does this mean that this flow should stay within banks, (as stated by the EA Maidenhead Project Office staff), or that the river banks, gardens, some roads, buildings with low floor levels (eg Maidenhead Rowing Club offices) will be flooded, but no residential property?


3.         Is the 295 cumec figure for the capacity of the main river being revised since the January 2003 floods? - With a flow of only 260 cumecs in the main river in the January 2003 floods, why did it flood some river banks, gardens, roads, buildings with low floor levels (eg Maidenhead Rowing Club offices) in Maidenhead and Windsor?


4.         If this flooding was not expected:

            -  does the EA have records of it in January 2003?

            -  What are the possible reasons for it?

Eg. Has new development in the floodplain increased the flood heights since the MWEFAS design was finalised?

Has the flood storage capacity of the watercourse reduced because of lack of dredging and blocked tributaries?


5.         Does the Navigation Act require certain minimum depths in the main river channel?




6.         If more dredging were carried out, could the level of the river be reduced at the beginning of the winter, in order to provide extra capacity during any flood?

7.         We would like to have comparable trend data for flow and height at Boulters Lock, not to switch to measures at the point of flow into the Jubilee River. What are the pre-MWEFAS and post-MWEFAS flood flows and heights at Boulters Lock for the ‘1 in 100 year’ flood?  So, for example is the MWEFAS expected to reduce the ‘1 in 100 year’ height at Boulters Lock by 1.0m, or more than that?


8.         What was the flow, height and return period of the 1990 flood? What aspects of the 1990 flood were used to plot the Post-MWEFAS floodplain map?


9.         How was the Interim Post-MWEFAS floodplain map drawn up? – is it based on topographically mapped heights, as well as on EA staff observation during the 1990 floods?


10.       Similar to the possible breaching of the channel’s banks near Eton Wick in 2003, might the channel’s raised banks near Datchet be breached in more extreme flood events?


11.       What are the operating instructions for the Jubilee River for more extreme flood events? So, for example, in an extreme event, would/could more than 215 cumecs be diverted down the channel, in order to reduce flooding in Maidenhead and Windsor, at the risk of flooding Eton Wick and the Dorney floodplain?


12.       The Maidenhead Flood Ditch

            The Maidenhead Flood Ditch was constructed in the 1960s, and has played a vital role in reducing flooding in Maidenhead. Is its capacity and role as a flood defence being reduced now?


             If there is doubt about the capacity of the main river, the future Flood Ditch capacity might usefully be reconsidered. The Ditch is only expected to take 5 (-7?) cumecs. Were there not plans to increase this?


            This coming winter, the Ditch seems unlikely to take that amount, because lack of maintenance means that most of the channel is blocked? Can this be remedied?


            The primary purpose of the Ditch is as a flood defence, but this is at risk being further compromised by giving greater emphasis to wildlife considerations, and to the appearance of the York Stream in Maidenhead  town centre. Proposals are now being developed to improve the all-year flow to the York Stream, and flood issues may not be given priority. How important is it to preserve or enhance the flood capacity of the Ditch?



D         Floodplain planning policies and the MWEFAS (inc Jubilee River)


1.         There is widespread ignorance, among property owners/developers, councillors and residents groups, of the floodplain maps currently in force in the different LPAs.


2.         Should an LPA implement Supplementary Planning Guidance (SPG) which has been issued for consultation, but never discussed and agreed by councillors (as has the RBWM over the last year)?




3.         Why does the EA currently advise that the MWEFAS-Protected area is in Zone 2 (not in the 1% floodplain), when according to PPG25, and its author, it is in Zone 3 (ie. in the 1% floodplain but protected by flood defences)? This error means that the EA draft policies for the Protected Area are insufficient, the  Flood Risk Assessments, required by PPG25, are not requested, developers are not made aware of the need for precautionary flood protection, and climate change effects are not taken into account.


4.         Please, can the EA and LPAs provide readily accessible large-scale copies of the current Interim post-MWEFAS floodplain map, showing sufficient detail to permit identification of development sites and street names in the floodplain? (Currently this is only available at the offices of the EA and (some) LPAs.) Can the web version of the map be made available to the public? Can FRAG members have a paper copy if requested?


5.         Can LPAs advise whether they have adopted the current Interim post-MWEFAS floodplain map, either formally or by default, and since when?


6.         Can LPAs provide copies of their current floodplain policies, any floodplain Supplementary Planning Guidance (SPG), and details of any current draft updates?


7.         How do we strike the right balance between following PPG3, which promotes high-density housing, and PPG25, which promotes a precautionary approach to flood risk?




E.         Floodplain planning policies and climate change

1.         The RTS argues that the EA and LPA’s are not yet complying with PPG25 guidance on climate change, and that more could and should be done urgently on this. (Please see ‘Thames Valley flooding and the impact of climate change’ Background Paper (RTS 1) for FRAGs, by Val Mason, River Thames Society, 9.6.03)


2.         Can the EA provide a detailed proposed timetable for production of data/mapping, and a draft policy, on climate change?


3.         What is the EA’s current advice for flood protection to take account of climate change, for development in the Thames Catchment generally, and specifically in the MWEFAS-alleviated area – in the Residual floodplain and the Protected area?


4.         Can the EA provide copies of its current interim draft climate change guidance, for consultation and agreement?


5.         Are we agreed that the Climate Change Zone should logically become part of the ‘1 in 100 year’ floodplain, (as climate change shortens the expected return periods of floods)? If yes, what action is needed to ensure this? If no, what policies are sufficiently cautious for this zone?


6.         What are the EA’s draft policies on ensuring ‘dry escape’ during flooding, and do they also take account of climate change effects? If not, should they?






7.         Do we need to clarify the legal liabilities for the various aspects of flood precaution/protection of the various parties described in PPG25, ie EA, LPAs, owners and developers?



F          The need for Interim Planning Policies


1.       The RTS argues for Interim Floodplain Planning policies to ensure compliance with PPG25 and to halt unsustainable development in the floodplain and in the EA’s proposed Climate Change Zone. These interim policies are needed until updated floodplain and climate change mapping is available, and the doubts about the effectiveness of the MWEFAS are resolved.

(Please see ‘Floodplain Policy – the need for Supplementary Planning Guidance (SPG)’, Background Paper (RTS 2) for FRAGs, by Val Mason, River Thames Society, 9.6.03)


2.       Are FRAG parties in agreement that Interim Floodplain policies of this type should be drafted for proposed adoption by  LPA’s, and the EA?


3.       If yes, can we get a working group together to draft SPG for consultation and agreement this summer/autumn?


What development has been approved by LPAs in the post-MWEFAS Protected area? How many properties ( residential and non-residential)/sq metres/dwellings/households? Can these developments be serial numbered for quick reference and then marked on detailed street maps?



G.        Groundwater flooding


1.        PPG25 gave the EA some responsibility for non-fluvial flooding, in addition to its role regarding fluvial flooding. As a result, EA (Frimley) have undertaken, since May 2002, to advise on non-fluvial flooding cases referred to them. However, not all cases are referred to them. There are several examples of new development proceeding, without any reference to the groundwater flooding which is known by local residents to occur on the sites. Furthermore, local residents’ knowledge of groundwater flooding is often not taken into account, even when LPAs are informed.


2.        For example, the Local Plan policy of the RBWM does cover groundwater flooding, but because the EA did not previously have responsibility for this flooding, no EA professional advice on it was available to the LPA.


3.        What can be done to increase understanding of groundwater, non-fluvial flooding, and to identify areas at risk, for planning purposes?


4.        Was groundwater flooding responsible for the higher than expected flooding within the Cookham defences in 2003?


5.        What can be done to monitor groundwater levels over the year, to help predict possible flooding? 


6.        Can more be done by Thames Water?


7.        Can Thames Water reduce river heights and reservoir storage at the beginning of winter, to provide extra flood capacity?


8.        How can we limit run-off and drainage problems?


9.        How can we reduce sewerage overflow?


10.      Groundwater and the MWEFAS

In addition to the erosion problems along the Jubilee River, the contribution of groundwater changes during floods, following the introduction of the channel across floodplain, do not appear to be understood.


11.      Is it not possible that, because the water level in the channel rises during floods within the raised banks, the water will leach into the surrounding floodplain, just as it does along the stretches of the Thames where the banks are higher than the surrounding land? Was this not the reason for the sewerage works near Manor Farm Weir being flooded?





[1] (Ref: ‘Climate Change Scenarios for the UK – The UKCIP02 Briefing Report’, April 2002.   See


[2] Among other things, the ABI also require the Government to supply a full report on the level of compliance with PPG25 by local authorities. Councils will presumably need to prove full compliance to any property owners experiencing difficulty obtaining flood insurance.


[3] Except in a few cases referred directly by local residents to the EA, who then required flood protection measures or full Flood Risk Assessments in accordance with PPG25..

[4] Ref. document supplied by EA: ‘ABI Statement of Principles on the Provision of Flood Insurance, includes questions and answers.. Taken from a briefing paper by ABI, October 2002’