Please see below Ian Thompson’s finalised notes on the above consultation meeting that he attended in Staines on 1st October.
Should you wish to discuss the content of these notes please do not hesitate to contact Ian direct.
Contact Mr Graham Piper Project Manager for the EA
The Lower Thames Study relating to Datchet, Wraysbury, Horton and Hythe End
Notes from EA Public Consultation held in Staines on 1st October 2009.
Flood alleviation will commence at Teddington if funding and approvals proceed on programme. The earliest start of the first phase will be no earlier than eight years time. The Datchet section is the last phase. This will probably take place in 25-30 years time in practical terms.
The scheme will be most likely be subject to a public enquiry regarding the costs and extent of the proposal.
The Datchet/Wraysbury section will only proceed on a cost benefit basis, ie the flow volume diverted relative to the construction cost related to the number of properties protected/flooding alleviated. Simplified to:- the quantities of water diverted (cubic metres/sec = cumex) relative to the cost of construction, which is appearing to be the current governing factor.
This section will proceed subject to Treasury approval which projects to approximately 17 years ahead of 2009.
A report containing modelled evidence has been completed for the total scheme. The draft strategy document is currently being compiled. Both reports will be forwarded for Government approval (assume DEFRA and the Treasury).
Should the scheme relating to the Datchet/Wraysbury section proceed it will provide welcome and much needed flood risk reduction and provide relief benefit to Wraysbury, Old Windsor and Ham Island, but does not appear to provide any benefit to Datchet.
In order to understand the extent of the intended Datchet diversion of the Thames River volume in cumex Mr Piper declined to provide this figure, which is the critical factor defining the extent and level of downstream protection. However, he did reveal that where groundwater containment within existing lakes containing a depth of 3.5 metres, the intention of the diversion would be to limit the rise in level within the new channel to an additional 0.5m. The width of the channel would generally be between 50-60 metres with sloping sides.
When Mr Piper was requested to provide the intended maximum water height for the channel, he again declined to provide this figure. The height of the water within a permeable channel contained at a level possibly at a higher level than the internal flood levels of houses in East Datchet, Wraysbury and Hythe End, is of concern to those residents.
There are a number of construction and engineering problems associated with the scheme:-
The channel has a water height restriction which is not present within the Jubilee River and that is controlled by the level of the railway line running between Datchet and Staines. In 2003 this was subject to being flooded by the height of water within the Thames backflowing from local watercourses. This scheme restricts water to flow only below the railway track and should water reach the track level of the railway being at a level lower than that of water in the Thames River, the railway track would act as a conduit to circumvent Datchet flood defences and free flow to Sunnymeads and Wraysbury. Therefore the control gates and their operation are critical in restricting water levels within the new channel when at times the water level in the Thames River is higher than the maximum height that can be accommodated within the new channel.
The intention is to use the Datchet Liquid Leisure/Sunnymeads Lake as part of the channel, which will invoke a purchase cost, as will other owned land that the channel passes through.
The channel will require crossing ‘the Common Brook’ at Sunnymeads. This is also a cost engineering problem, as the Common Brook backflows when the River Thames experiences modest water level flooding events. A control structure is required in this location as noted by Mr Piper.
A new bridge or conduit will be required at Welley Road.
Some gravel pits will be utilised as channels. Others that are subject to existing SSSIs will have three years of surveys with agreement and approval required from Natural England before they can be incorporated within the scheme, providing that approval is given.
The Wraysbury Drain or river will also be a cost engineering problem similar to the Common Brook, as this watercourse is similarly subject to backflowing and also crosses the channel.
Crossing the railway for a second time will possibly invoke the same cost as and engineering as the first crossing point at Datchet. However, this crossing is followed by an immediate tight turn at Tithe Farm, which does not comply with the design parameters for watercourse bends and similarly the proposed bend at Longfield Farm. Mr Piper advised that these bends may require to be contained within a concrete formed channel. These will invoke high construction cost relative to their channel lengths.
The current landfill site in Staines Road will probably require sheet steel piling throughout the area where the channel passes through and a new bridge within Staines Road.
There will be control weirs within the length of the channel together with inflow and outflow gated structures as part of the construction.
Concerns have been raised by residents throughout the scheme where use of the gravel pit lakes are used as part of the channel. One concern is introduction of run off fertilisers from the main river that can give rise to growth of algae in summer months when flow of water within the channel is suspended. Another is the increasing movement of groundwater from the channel increasing the local water table in East Datchet, Wraysbury, Horton and Hythe End, when the channel is in flood operation. Mr Piper advised that surprisingly, recent surveys have shown that water flow in and areas across Wraysbury gravel pits flow quite slowly. However, this may not be the case in Datchet, where ground water levels have varied quickly relatively to circumstances. Assurances will be sought on these issues.
Operation of the new channel will be coordinated with operation of the Jubilee River. The Lower Thames Scheme will be part of the River Thames Flood Control System eventually providing integrated flood control for the whole river.
Many people raised the question regarding dredging the river as an alternative. The answer provided was that the cost would be excessive even against the project cost over time and the EA did not have anywhere to dispose of the dredged material. Additionally a Halcrow survey over the past seven years has shown that the river to be generally self clearing over a long term cycle. Having seen the survey for our local area it is a first class piece of work and supported by ThamesAwash and Datchet Parish Council. However, there are locally reoccurring problem areas that require localised dredging for navigation purpose. When asked about non mainstream local silting Mr Piper stated they could be dredged, but again disposal is a problem. This is a problem that the EA could face when construction of the channel commences which could bear a high negative cost against the project.
Some conclusions can be drawn from the small amount of facts that are currently available:
There will not be any meaningful flood protection work intended to take place in the channel location for approximately 25 years. The whole scheme is speculative as to Treasury support either now or in the future. The balance of construction cost against the size and effectiveness of the channel to provide an assessable benefit to a specific quantity of properties is as yet unknown, especially when projected to 25 years ahead. Due to lack of data, there is little to assess any effect on Horton, the River Colne and Hythe End. The influences of conservation societies are as yet unknown.
Although ThamesAwash fully support the scheme in principle, should the modelling and draft strategy documents be released into the public arena be more informative to aid clarification on a number of points this may improve the probability level of the project. At present the writer considers that based on construction problems, ie fixed data limitations and construction costs, the present proposal in its current form will fail to meet scrutinised practical criteria and would almost certainly fail at the Public Enquiry level. Even the EA are currently not confident that this section of the project will proceed.
Meanwhile local flood prevention and protection must not be allowed to fall into a 17 year waiting vacuum for something that has a high probability of not happening. It is also highly probable that meanwhile we will experience at least two maybe three serious flood events based on the timing of past occurrences. Therefore it would be more practical and expedient to focus on local flood defence measures to provide and improve local flood protection for low cost and immediacy rather than an all eggs in one basket that might result in the basket being dropped and being left with no improvement in local flood defences extending into future decades, which would be totally unacceptable and irresponsible.
I would record my thanks to Mr Graham Piper of the Environment Agency for his patience in providing answers to a number of questions and hopefully we will be able to access the missing information in the future.
Ian Thompson - ThamesAwash
The RBWM Parishes Flood Liaison Group
Cllr Colin Rayner
Wraysbury Parish Council
Datchet Parish Council
Old Windsor Parish Council
Adam Afriyie MP
Jubilee River Website