If ending dredging and maintenance has reduced tidal channel capacities at the design water level, what is the solution?
Your interview with Jean Venables shows that there are engineers with the perception, background knowledge and understanding to make meaningful comments about the floods in the Somerset Levels (NCE 6 March).
It appears that many other commentators base their views and opinions on generalised perceptions, unburdened by the benefit of indepth knowledge or understanding of the unique and complex man made drainage systems that operate over the area. The topic of dredging on the Rivers Parrett and Tone is one which has suffered from this uninformed comment.
It is generally accepted that in a natural fluvial system, ill-considered new capital dredging can make things worse downstream.
The drainage in the Somerset moors has developed over at least the last 1,000 years and is now a completely man made system. All major rivers in Somerset, except the Parrett and Tone system, have tidal exclusion sluices at the coast that stop the heavily silt-laden water from the Severn estuary travelling and being deposited upstream.
All that the affected property owners in Somerset are requesting is that the badly silted tidal river channels are restored to the capacity they had before essential maintenance was summarily stopped around 1990.
Cessation of maintenance has significantly reduced the tidal channel capacities by at least 50% at the design water level. This means the Parrett and Tone flood more frequently, spillways operate sooner, run for longer, fill the moors deeper, and take longer to pump and drain out when river levels eventually fall.
The dredging proposed to reinstate the design capacity will not adversely affect Bridgwater well downstream, and certainly not Taunton well upstream. It will have a positive effect in reducing the extent, duration and depth of the flooding to properties, infrastructure and farmland situated in and on the extreme edges of these flood storage areas, while also reducing the significant costs of any emergency pumping that might be required in the future.
It has never been suggested that restoration of this 20-year delayed maintenance dredging would stop the flooding, only that it would help to stop the situation continuing to get significantly worse than it need be.
- David Ayers(M), firstname.lastname@example.org
Your recent articles have highlighted various views on solutions to resolve the recent flooding problems in Somerset. Indeed, the BBC also had a piece calling for the construction of a Bridgwater barrage. But this only tells half the story.
From late 2012 and through all last year I, with my team supported by four of the world’s leading engineering companies (and ably encouraged by Cardiff University professor of water management Roger Falconer), vigorously promoted our proposal for a Severn barrage with an associated Bridgwater bund.
This proposal offered all the anti-flooding benefits outlined in your articles, not just for the Somerset Levels, but for the whole area bordering the Severn upstream of Cardiff and all the Severn’s tributaries.
Despite providing a large body of evidence to the energy select committee and innumerable briefings within government, and to politicians of all stripes, we were unable to persuade the select committee, Department of Energy & Climate Change, the Environment Agency or non-governmental organisations of the need for action.
They seemed not to grasp that doing nothing would result in more coastal erosion and major loss of both valuable wetlands and breeding grounds for wildlife, as well as foregoing the benefit of generating 5% to 7% of the UK’s national electricity requirement on a green basis that is extremely cheap over the lifetime of the barrage.
Who would not want this protection and the related electricity, especially when such a structure would not cost the taxpayer a penny?
- Tony Pryor (F), Ex-CEO, Hafren Power, email@example.com
It surprises me that, in all the programmes on the flooding in Somerset, nobody has mentioned the main problem concerning dredging the rivers Parrett and Tone.
The Parrett is tidal for about 30km inland from where it discharges into the Bristol Channel near Burnham-on-sea, and the Tone, which is a tributary of the Parrett, is tidal over its downstream 5km.
The Bristol Channel carries large loads of suspended silt. The tidal curve in the Parrett is not sinusoidal and therein lies the problem.
The rising limb of the tide is short and sharp, particularly on spring tides, while the ebbing tide is slow and over a long period. The fast incoming tide can carry large quantities of silt. At dead water it deposits much of this silt load. The ebbing tide is much slower and is unable to carry the same quantity of solids in suspension.
Therefore, on each tide and particularly very high tides, a further layer of silt is deposited. It can be seen, therefore, that only continuous dredging would keep the rivers at an increased cross-section. A one-off dredge will succeed only for a very limited period of time.
A study of the siltation process on the Parrett was carried out by the Hydraulics Research Station at Wallingford in the late 1970s, and one hopes that those investigating the floods will avail themselves of this study because one finding was that a barrage downstream of Bridgwater would create heavy siltation downstream of the barrage if all tides were excluded from travelling upstream.
As others have intimated, the solution to excessive flooding in this catchment is a combination of works: a relief channel from the Parrett (already partly in existence); ringbanks round villages; raised roads for access; more pumping; but a barrage only if the correct operating procedures to prevent downstream siltation are put in place. Annual flooding to the Levels will still take place, but of a more acceptable duration and extent.
- Basil Tinkler (M), Ex-flood defences manager (Somerset), Wessex Water Authority and National Rivers Authority, firstname.lastname@example.org
Rail: Closing the gap on the Cowley flooding issue
The flooding situation at Cowley Bridge Junction is more complex than your cover headline suggested (NCE 6 March).
The Environment Agency and Network Rail’s plans are neither in conflict nor incompatible. We are, in fact, working together to address different aspects of the problem that, for around 25km north of Exeter, the railway was constructed in the floodplain.
Network Rail’s predicament is that, just north of Exeter itself, the railway embankment crosses diagonally from one side of the River Exe valley to the other, effectively creating a 2.5km long dam across the floodplain, with Cowley at its downstream end. When flood flows exceed the capacity of the limited number of railway bridge openings and culverts through the embankment, they back up and eventually overtop the railway at Cowley Junction.
Network Rail is currently investigating how it could reduce the frequency with which this occurs, and improve the resilience of the railway line to flooding when it does.
The separate issue for the Environment Agency, and our partners Exeter City Council and Devon County Council, is that the railway unavoidably crosses the line of the Exeter Flood Defence Scheme.
This only becomes an issue during more extreme flood events when the railway line creates a hole in the defences. Our basic proposal is therefore to install a flood gate or temporary barrier across the railway, which Network Rail could deploy to ‘close the gap’. This is not in any way at the expense of the railway, and because we are addressing a different level of flood risk, this barrier would only need to be operated once the railway line was already closed due to widespread flooding further upstream.
We are currently awaiting the conclusions of Network Rail’s flood risk study before progressing our barrier design any further, and we have been clear that we are very open to alternative solutions that might arise from it.
- George Arnison (M), Environment Agency - Exeter Flood Defence Scheme, email@example.com
Rail: It is imperative to reinforce the line at Dawlish
It goes without saying that something must be done to ensure that the rail link to Devon and Cornwall is not at the mercy of winter storms at Dawlish.
While the various options - a diversionary route or new alignments - are being evaluated, it is essential to reinforce and/or improve, as far as reasonably possible, the resilience of the existing coastal stretch of line.
Even if the railway along the coast were to be abandoned eventually, the structure would need to be properly maintained in order to protect the properties behind it.
If in due course a more direct line were to be built between Exeter and say Newton Abbot, the coastal route could usefully be retained for local services.
And at the very least, it must be possible to somehow improve the sea defences, while a diversionary route is being planned, designed and implemented.
By the way, may I remind Kate Purver (Letters 6 March) that High Speed 2 (HS2) is not just about saving 20 minutes to Birmingham, but saving an hour to Manchester, Leeds and most major towns beyond.
And anyway, the solution to the Dawlish problem would cost a tiny fraction of that estimated for HS2 - it must be considered as a separate issue.
- Roger Hand, Stoke St Gregory, Taunton TA3 6EW
Rail: Nothing to lose but everything to gain
Like Kate Purver (NCE 6 March) I rely on an efficient rail link from the South West. However, her dismissal as unpopular of reopening the inland route via Okehampton needs to be challenged. There should be no question of permanently closing Brunel’s route via Dawlish, even if an alternative route is opened. Having an alternative, even one that adds half an hour to the journey, is far better than the major disruption caused by bus substitution.
When evaluating alternatives, as Network Rail is doing, it is vital to consider the wider economic benefits of the options. Purver is wrong to deny those benefits. A park-and-ride station, where the inland route crosses the A30 west of Okehampton, would provide an enormous boost to vast areas of west Devon and north Cornwall.
Perhaps I can allay her concerns, which appear to be that a degraded normal service from towns such as Newton Abbot and Totnes would result were the Okehampton route to open. I am sure she agrees that when Dawlish is closed due to storm damage, a slightly degraded service is acceptable.
There are currently around 30 trains each way per day between Exeter and Plymouth, roughly half of which continue to Penzance. In addition, Newton Abbot is served by trains between Exeter and Paignton. Now suppose the Okehampton route were open.
All the Exeter to Penzance trains would run as existing via Dawlish, avoiding a reverse at Plymouth. The Exeter to Paignton trains would be unaffected, as would most of the Exeter to Plymouth terminating services, but some would run via Okehampton. Hopefully there would be additional services; a neat, historically-correct option would be for South West Trains to extend some of its Waterloo to Exeter services to Plymouth via Okehampton.
So Purver has nothing to lose from the north of Dartmoor alternative; those of us north of the moor have everything to gain.
- David Hill-Smith (F), firstname.lastname@example.org
Rail: A round of applause for a job well done
I was so surprised to hear that the railway line at Dawlish was going to open at the beginning of April (NCE 6 March). It really is an extraordinary achievement and all concerned have to be congratulated.
When I saw the extent of destruction shown on the television news, I just assumed that it would be many months before it would reopen. You could envisage that there would be no end of discussions and meetings between Network Rail, politicians, government departments, the Environment Agency and Health & Safety Executive - the list could have been endless. And where would they find a contractor and machinery to carry out the job?
Not the case - somehow they just got on with the job. The spokesman from Network Rail who was thrust onto television, with waves breaking over the destroyed railway, said it would take a few weeks - and he was spot on.
I just hope that NCE devotes a special edition to finding out how it was done and find out who were the characters that drove it forward so successfully. Brunel could not have done better.
- Malcolm Dawes (M retd), email@example.com
- Editor’s note: A fine suggestion and something we are looking into.
Energy: Nuclear versus fossil fuel risks
I felt compelled to respond to Jim McCluskey’s letter (Letters 13 March) regarding the risks of nuclear power, as I find it odd that these risks often appear to be viewed in isolation, and not weighed against the risks of continuing to burn fossil fuels.
Of course, we need to keep pushing for the development and uptake of renewable energy sources, but realistically, that is going to take time. Until then, we still need to meet society’s energy needs.
McCluskey highlights a suggestion that it is impossible to make nuclear power stations acceptably safe. However, if we continue down the path of burning fossil fuels, we will be exceeding what many consider the safe limit for carbon released into the atmosphere in the near future (as Neil Kermode pointed out in his letter in the same issue). Surely the risks involved in this cannot be considered acceptably safe?
I see it as strange that some people have such a strong objection to burying nuclear waste in a known location deep underground, when right now the realistic alternative is to continue to increase atmospheric pollution.
A case in point, Germany has recently begun closing nuclear power plants after the Fukushima disaster and is bridging the energy gap by burning more coal.
- Graham Clarke, firstname.lastname@example.org
Education: An unorthodox career route can pay off
I left school with no GCEs, let alone A-levels, to do an apprenticeship at a Nottingham steel fabricator offering day-release college - a general building course, followed
by an ONC.
At 16 I designed my first roof truss, and no, it didn’t fall down.
Whereas an HNC should have been the next step, good ONC results opened up an entirely new scenario and I decided (with advice from seniors) on a civils degree at Trent Polytechnic. Talk about ambitious, this seemed complete madness at the time, especially for someone who had left school with only teen spirit to show.
Nevertheless, I buckled down and got on with it, struggling with the pure maths and science parts but excelling in those with a practical bias - drawing on that huge benefit of the apprenticeship/day-release years, and a familiarity with everyday construction terms and methods.
Graduating in 1982 with an honours degree set me free to work up and down the country and around the world, earning chartered status en-route. I still have trouble believing it today.
An unorthodox approach by current standards, certainly, but it seemed to work back then - and it could again. Perhaps I’m living proof and I’m delighted it should now return for debate.
- Ben Zabulis (M), 135 Victoria Road, Kirkby-in-Ashfield, Nottingham NG17 8AX
Professional: Tools are there for those who seek them
A striking editorial by Mark Hansford (Comment last week). I wonder whether he’s heard of HEAT? It’s a free tool to monetise health impacts in the built environment. As usual, the tools are there.
Engineers just have to start using them, and sensibly so too. As well as explaining them to their client money-keepers.
It also requires CPD to be taken seriously by our engineering profession to stay abreast in our fast-changing world.
- Katja Leyendecker (M), email@example.com