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


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