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.