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.