Scenario 3 - an Alternative Proposal to the Channel

 

Letter Outlining an Alternative Proposal published in Oxford Times 21st April 2022

Full Text of letter from Chris Sugden of HOEG and Brian Durham - two long time campaigners for improvements to the scheme who understand it in great detail.
They propose the enhancement of existing watercourses and bridges to save Hinksey Meadow and floodplain trees, while easing constructed dams at Kennington T-Junction and the railway, and avoiding the need for an artificial 2nd- stage channel between Botley Road and Old Abingdon Road.

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Full Text

"Dear Editor


You recently published a good news letter from Cllr Tanner about Oxford’s proposed flood scheme (Oxford Times 7 April), and here is even better news.  


Everyone knows that flooding is tricky and needs state of the art numerical modelling to predict what the water will do.  In July 2020 the Hinksey and Osney Environment Group (HOEG) met the Environment Agency, and in December that year we were offered `a separate run’ of their model. We have sent them and the planning authority an outline specification for that extra run, now shared with two hydraulic engineers to ensure that it is `watertight’.


We are calling the new run `Scenario A3’, the good news being that it upgrades the existing streams (natural 1st-stage channels) in order to retain the beautiful and precious existing flood plain (a natural 2nd-stage channel).  


Design elements include: matching flow capacities at one existing and three proposed bridges; a 50m-wide corridor free of `flotsam traps’; climate-change adaptation at the `Electric Road’ culvert; existing Railway Lakes to be linked as a 1st-stage channel; the long-awaited disabled ramp from the Devils Backbone up to the railway footbridge; beams for the proposed Old Abingdon Road bridging to be above the 100-year flood level; and best of all, helping Network Rail to comply with obligations set out in the Oxford Railway Act, which received the royal assent from Queen Victoria.


This is evolving towards a design that everyone can be pleased with. 


Hurrah for Queen Victoria! 


Chris Sugden - Hinksey and Osney Environment Group

Brian Durham

 
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Brian Durham's Analysis and Alternative Proposal

"The two alternative `scenarios’ in Appendix Q A1, with the channel removed from Hinksey Meadow and A2 with no stage 2 channel between Botley Road and Old Abingdon Walk are presented as performing worse hydraulically than the preferred scheme while losing all the same trees.  But neither is necessarily true.


The worse performance is questionable, because EA have admitted that they could have included `sumps’( a pit or drain) under their proposed bridges at Willow Walk, NH Causeway and Devils Backbone but didn’t.  They have clearly also modelled some flow enhancement downstream of Botley Bridge in their own preferred scenario but not in the alternative scenarios.  The latter alone accounts for roughly 70% of the advantage they claim over scenario A2.  

Re- trees, they say the smoothed section of flood plain is the same as the no-channel and would lose the same trees, but all they need is a 50m-wide corridor clear of conventional stock fencing so as not to trap flotsam - the mature trees could support the electric fencing.   

On a level playing field therefore it is likely that the A2 performs better than the preferred version. If therefore we factor in: a 50m corridor free of flotsam traps;  the linked Railway Lakes (crossed by the disabled ramp); bridge beams at T-junction installed above the 100-year flood level, and best of all, the new railway culvert, Scenario A3 should be a really useful flood scheme."

 

Additional Comments about A1 and A2 Modelling in Appendix Q


It's important to understand that any model will only reflect the data that is fed into it. If this changes then the readings will be different. Relying on models has its limits because of the complexities of hydrologies and the impact of small, random changes such as blockages or large ones such as development that happen in the real world cannot be factored in.

The modelling which the EA included for the first time the tall embankment and bund around Osney Mead (which limits the number of businesses at risk).  It also included an expanded bridge across Willow Walk. 

The way in which the water accumulates across the flood plain depends on where it falls in the catchment area. If the main rainfall is in the catchment of the Ray or the Cherwell, then it is unlikely to affect the Hinksey Flood Plain at all.  It could be argued that if the water drains much more rapidly than before near Redbridge (a new railway culvert is already installed, the plans to improve water flow around the ancient monument at Redbridge, water flow in the new culverts beside the railway under the A423 railway bridge will improve, and so will the flow down the new channels at Kendall's Copse below Redbridge) it will accumulate much more slowly than before around South Hinksey. Together with the lack of accumulation at Willow Walk (better bridge) that will cause water to drain from Oxford rapidly instead of accumulating north of Willow Walk and north of Redbridge during a peak flood because the floodplain is full up.

One problem with the modelling is the length of time over which a peak flow occurs and the fact that this is not factored in. If you have a peak flow of 240 cubic metres a second over a couple of days that may not matter much, but a peak flow every day for three weeks deposits far more water in the flood plain. In the models both will be recorded as a 1/100 flood event. The former will be much less serious than the latter, but the latter is far less likely.

So the bottom line is that models need to be treated with caution, particularly if the difference between alternative scenarios is a matter of centimetres as this may enough of a margin for error. 

 

How the A3 Proposal Can Work

What effect does the A3 proposal have on Seacourt park, Hinksey Meadows and Willow Walk ie stage 1? The section from Botley Bridge to the Seacourt nature park is the same as the EA scheme - it cannot be anything else if it is to get the extra water through by removing modern landfill.  Thereafter it follows Seacourt Stream round the back (west) of the pylon with zero effect on Hinksey Meadow except for doubling width of the existing Seacourt Stream. 
Do we still lose Seacourt Nature Reserve?  Some of becomes part of the widened stream. 

Is Hinksey Meadows compromised by losing MG4a grassland and having its hydrology affected?  It saves Hinksey Meadows totally if you accept the circa-doubling the width of Seacourt Stream - the closer willows may need to be pollarded and moved 10m back to the new stream bank. 

Are we still left with the ugly, outsized Willow Walk bridge and losing veteran willows? A more sensitive and site-specific design can be incorporated that doesn’t detract from the rural landscape. Like bridges at University Parks or Christchurch Meadows for example.

Will there still be a channel under Willow Walk and beyond to the Fishes? No continuous `channel’, a lowered bed under the bridge itself for what engineers call `increased flow capacity’, so a few metres either side of the bridge (the EA use the word `sump’). This could mean the bridges do not need to be exactly where EA put them, but the Willow Walk bridge should take out at least one of the existing pipe culverts.

How many trees and hedgerows will be lost? No trees lost, see pale blue tone for 50m-length of hedgerows.

Will there be much digging? Widening the existing streams, new channel from Hinksey Stream to Railway Lakes downstream of Hogacre Park (dotted line), new channel (matching the scheme) from downstream end of Railway Lakes to the T-junction. But this is much less than the proposed EA channel. 

Is that the advantage that the second stage channel is also diminished as well as the first? The 2nd-stage is bad full stop.  If we can also make the Hinksey Stream/Railway Lakes work, we can remove the 1st-stage (red) to maintain the integrity of Corpus Christi and City Council land for grazing and grass crop. 

What does the work around the railway contribute? The railway is fantastic because it has become a dam, including the bypass, Kennington residential, and silting at Mundays Bridge (railway).  The culvert at Kendall Copse resolves all that, as recognised by EA in 2015.

In conclusion does the A3 Scenario save Hinksey Meadows by scrapping the channel? It saves everything of Hinksey Meadow aside from the widening of Seacourt Stream.  If we get the existing channels widened (blue route) we avoid the red 1st-stage channels. 

 

DO WE WANT THIS?

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Oxford Needs a Better Flood Scheme

The Environment Agency has put in a planning application for an Oxford Flood Alleviation Scheme comprising flood walls, gates, bridges, culverts, and a giant 5k channel from the North of Botley Road to the South of Abingdon Road. The EA's own report shows the the channel is the most disruptive, costly and environmentally damaging part of the scheme, yet provides only marginal flood alleviation benefits compared to the other measures. 

The channel will take 3-5 years to build, cost £23 million and will cause:

  • 240 HGV movements a day on the A34, 40 mph speed limit on sections of the A34, massive queues at Hinksey Hill

  •  2,000+ trees, miles of hedgerow and much of the biodiversity net gain happens offsite

  • - 3 acres of rare MG4a grassland to be lost in Hinksey Meadow with the rest at grave risk due to changes in hydology

  •  South Hinksey village to be surrounded by work compounds, traffic and haul roads

  • Loss of recreation to most of the fields between Botley Road and Old Abingdon Road and forever in some areas. 

  • - The excavation of 455,000 cubic metres of soil and gravel and loss carbon sequestration

  • The scheme is intended to provide flood protection: the EA's own figures show the number of dwellings with a great than 1% annual risk of flooding reduces from 1126 to 180. However the same figures show that the channel only accounts for 5.7% of the reduction (54 properties), which could be protected by more environmentally friendly and effective methods. 

  • Surely it makes more sense to adopt the scheme without the channel:

  • we'd get most of the benefits with far fewer costs and minimal disruption. Meanwhile a pumped pipeline alternative with even fewer cost and impacts hasn't even been analysed by the EA, despite offering more flood protection both now and in the future. 

Please comment on the planning application by May 9th. 

 

News

OFAS Consultation Now Open

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THE OXFORD FLOOD ALLEVIATION SCHEME PLANNING APPLICATION IS OPEN FOR COMMENTS 7TH APRIL - MAY 9TH

January 1, 2025

The Environment Agency has put in the new planning application to Oxfordshire County Council. We would urge everyone to make make a comment.

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INITIAL RESPONSE FROM RIKI THERIVEL, PROFESSOR OF BUILT ENVIRONMENT OXFORD BROOKES UNIVERSITY

3rd April 2022

Dr Riki Therivel, a leading expert on the environment and sustainability and a visiting professor at Oxford Brookes University has analysed the new proposal and has expressed concerns about the impact of pollution, carbons emissions, vegetation loss and the lack of evidence that the costly channel is justified.

 She has highlighted the loss of mature trees that apparently will not be replaced, particularly around Seacourt Stream and Willow Walk .

The scheme will rely heavily on maintenance, yet it is only budgeted for at most 15 years. Also, the economic justification for the channel doesn't appear to add up. "In economic terms, not having a channel (iBCR) makes much more sense than having one, and having no channel through Hinksey Meadow makes as much sense as the proposed scheme."

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SERIOUS CONCERNS ABOUT THE CASE FOR THE CONTROVERSIAL CHANNEL

April 2022

  • An initial assessment highlights some key concerns. Many of these relate to  the creation of a new channel in two parts. This will be approximately 5km long, it will run from north of Botley Road down to south of the A423 southern by-pass where it re-joins the River Thames. Stage one of the flood channel will be "a natural river channel", which will always carry water diverted from other streams and stage two will be a sloping dry channel to take flood flow. 

  • - This requires 258,000 cubic metres of soil and gravel extraction. What effect will this have on the existing hydrology of the flood plain, particularly the rare MG4a grassland of Hinksey Meadow?

  • - There will be a massive loss of trees, independent calculations estimate 4,000 mainly mature trees, miles of hedgerows and other vegetation. What will be the impact of the loss of "Oxford's green lung" and the loss of a major wildlife corridor? 

  • Trees, particularly willows, are a major contributor to biodiversity, the root systems secure riverbanks and store water. Are the mitigation measures proposed by the Environment Agency enough? Could some of this massive loss be avoided? What about protected species such as bats, kingfisher and insects?

  • - The change of a grassland, pastural landscape of meadows and fields is to  wetland. Can this really be considered a net biodiversity gain?

  • -It is estimated there will be 111 journeys of lorries carrying debris a day at the height of the gravel and soil extraction. What effect will the removal of so much embodied carbon have on the environment? What about pollution levels from trucks? Pollution is a bigger killer than covid. 

  • - Why is the flood problem not being addressed further upstream where the floodwater originates? Is it the wrong way round to create massive upheaval in a floodplain with high levels of urbanisation around it, when the problem could be tackled further upstream? As eminent experts such as hydrological engineer, David Ramsbottom has commented regenerative farming, which has the potential to have massive impact because of the vast areas of land, needs to be part of the solution. 

  • - Even though some of the figures have been updated, the scheme will by the Environment Agency's own acknowledgment, not keep pace with climate change. The origins of this scheme pre-date our increased knowledge about unpredictable weather patterns. Should there be a re-think to make the scheme more future proof?

  • How will the complex scheme work if maintenance is only planned for the next 15 years? A lot of the responsibility will devolve to the landowners but will they have the necessary expertise and resources?

  • - The application cannot take account of any new development on or near the floodplain. Yet Oxford is poised to become "greater Oxford" due to massive expansion of the city.  Can the scheme be effective if there is more development with increased run-off, sewage and hard surfaces? Is it a sticking plaster that could become a waste of money?

  • Have your say make sure to comment on the planning application.