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Letters: Dredging does improve flows

To dredge or not to dredge: that is the question.

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Clearing silt from the rivers on the Somerset Levels has helped combat flooding

Professor Falconer states that dredging would have little or no effect in the article “Tidal power barrage could protect Somerset Levels” (NCE 13 March). As any civil engineering student will know, the conveyance of an open channel is a function of hydraulic gradient, bed friction, channel shape and cross sectional area. If the first three remain the same then an increase in flow area by dredging will increase conveyance by a similar proportion.

As project manager of the £5.5M Lower Tone flood defence scheme in 2001/02, I am very familiar with the hydraulics of the Lower Tone and Parrett. During the recent floods, that scheme protected over 120 properties in Stan Moor and helped to keep the main Taunton to Castle Cary to London rail line open for all but a few days at the height of the flood.

The capacity of the Curry Moor and Hay Moor flood storage area is around 12M.m3. Due to the steep nature of the upstream catchments, it would take a very large number of storage areas and dams to make a significant difference to the overall catchment storage. These, or land management changes, might increase the time of concentration of a flood by hours or days, but the flood event that causes problems on the Somerset Levels is measured in weeks and months.

As with all flood storage schemes, the problem arises when the storage is full and has to spill in a controlled manner into other areas. In the case of the Curry/Hay Moor storage, the spill is mainly via Athelney Spillway onto Salt Moor and North Moor. If flows in the Lower Tone could be increased by dredging downstream, it would reduce flows into the storage area and would enable pumping out to start sooner.

The river system is manmade, and most of the sediment that it deposits is brought in daily on the tide from the Severn estuary. Unlike natural rivers, human intervention is required to maintain conveyance. Without it, these moors would revert back to the inland tidal marshes that were present before the Abbots of Athelney diverted the Lower Tone onto its current alignment in the 14th century. Dredging would make a difference. The question is how much dredging can be justified under current Treasury rules?

  • Murray Palmer (M), pittscornerprjcts@btinternet.com

 

The national media all seems to have difficulty in understanding the complexities of the dredging issues on the Somerset Levels, and occasionally misunderstandings creep into NCE too. May I clarify two things.

Firstly, the proposed dredging is to remove all accumulated silt to take the river channel back to its 1960s’ profile. It is not to deepen the river bed, which would be pointless. When this silt is removed and the channel widened, its capacity will increase pro rata.

Second, as a resident of the Levels, I can assure professor Falconer that no one in their right mind around here thinks that dredging alone will solve the serious flooding problems.

But they do believe, and in my view rightly so, that increasing the capacity of the rivers Tone and Parrett would have probably have avoided the sudden and catastrophic “last straw” flooding of the village of Moorland. It would have also reduced the length of time that the village of Muchelney was only accessible by boat and various roads have been closed. The main Taunton to Bristol railway (still not fully open after six weeks) might not have had to be closed at all.

The effectiveness of the dredging is to be reviewed when it is to be completed, and there will be further questions to be answered. I suspect that the Environment Agency may be reluctant to continue with regular dredging for ecological reasons. Compromises may be necessary, but we must have the complete picture so that a proper cost benefit analysis of dredging can be made.

  • Roger Hand, Woodhill, Stoke St Gregory, Taunton TA3 6EW

 

Before the dredging debate dries up I felt compelled to comment, based on over 40 years of involvement in the management of rivers and drainage systems.

Let me firstly register that I am in total agreement with the comments made by ICE past president Jean Venables in which she robustly supports the need to undertake more channel dredging, and that neglect to do so has at best exacerbated flooding and at worse failed to prevent it (NCE 6 March). There is no discussion among those wearing Wellington boots and operationally involved as to whether dredging is beneficial; and in all drainage systems and the majority of lowland rivers, including some tidal, it is essential.

This view is supported by the hard on-site evidence, of which there is plenty available, but which is not always reflected in NCE’s letters pages, as the experience/knowledge is generally vested in those who are not members of the ICE. In my opinion those who dismiss dredging as irrelevant are, in the main, from the academic wing of the Institution, with little or no hands-on experience, driving a theoretical rather than pragmatic approach.

The management of rivers and drainage systems is not rocket science; it’s not even a science: it is the understanding of channel behaviour, recognising the detrimental impacts to conveyance and channel stability, and dealing with them in an effective and timely way.

  • David Noble (F), david.noble123@btinternet.com

 

Health & safety: Call to action over mobile phones

Lafarge Tarmac makes reference to its new mobile phone policy whereby from June nobody in the firm will be making calls - hands free or otherwise - on their mobile phones while driving (NCE 13 March).

This initiative is the correct one to reduce the occurrence of dangerous driving and the potential to kill.

In Balfour Beatty this has been in place globally for everyone since 1 January this year, as part of our global safety principles strategy.

I applaud such direction and therefore all attempts by other companies in our industry must be fully supported.

Let the industry collectively take a lead on this matter; not just individual companies.

  • Steve Crossland (M), General manager engineering process, Balfour Beatty, Park Square, Newton Chambers Road, Thorncliffe Park, Sheffield S35 2PH

 

Profession: Engineering is a suitable professsion

I refer to the ongoing debate about the image of civil engineers in modern society. My son’s primary school ran a “come dressed as a scientist/engineer” day last week and it was gratifying to see him win the best dressed prize in his class - dressed as me! Sadly though, he was the only child dressed as a modern civil engineer that we could see. Which is ironic, given that we live in Telford.

  • Matthew Wyld (M), matwyld@hotmail.co.uk

 

Profession: Why we are always on tap but not on top

Recent debates in these pages surely show how far we engineers are removed from our roots. The first professional engineers sprang from the ranks of those able to apply emerging sciences to produce better widgets and infrastructure.

The early Telford/Stephenson/Brunel years of the Industrial Revolution were brilliant, but by the close of the 19th century professional engineers had become essentially middlemen between the aristo/landowner/financier class and labour. Despite this, the professions continued to attract candidates for whom achieving a sophisticated solution in spite of technical difficulties was satisfaction enough.

My involvement with the ICE began over 60 years ago. In that time I have seen governments lobbied; reports on national policies published, schools visited, and disasters responded to. I was active in much of this.

Throughout, politicians and civil servants were entirely consistent - despite all efforts and persuasions, maintaining that engineers should always be on tap but never on top. Education has remained under the control of absolutely anybody provided they are ignorant of the needs of technology.

I see nothing to suggest that this will change. We are a small minority within that sort of nation. If you really wish to influence policy making, the profession is not the place. Politics is.

  • Don Bennett (F), donovan.bennett@btinternet.com

 

Transport: African lessons still need to be learned

I read the article a “Cure for Africa’s Rustic Roads” with an ever increasing sense of frustration (NCE 13 February). I first became responsible for both black top and gravel roads as a consultant working in Zambia in 1971 when we were using Road Note 31 and the local government’s design codes. I retired from active participation in 1999, having been responsible by then for much road construction of all types in both tropical high rainfall and sub-tropical semi-arid climates over the ensuing period.

Why do we keep re-inventing the wheel? A trawl through the literature, and followed up by meetings at the main British, French, American, Australian and South African road research bodies, would have given solid evidence of the best ways of dealing with low cost, low traffic volume road construction problems throughout Africa going back over the last 50 or more years. Department for International Development must have bulging files on the matter, having funded innumerable projects over the years.

Such construction is all about finding and, if necessary, modifying suitable naturally occurring local materials, making sure drainage is robust, the pavement design is adequate for the anticipated traffic, and that poor sub-grade materials are dealt with to provide a durable foundation. A minimum usage of expensive manufactured material such as cement, road lime, and bitumen should be the aim.

The article mentions concrete strips in wheel paths: similar strips were developed in north and southern Rhodesia [now Zimbabwe] before the Second World War and were an unmitigated disaster as traffic volumes rose through passing traffic making large ruts either side of the strips, which led to accidents, ruined tyres and drainage problems in the wet season. Construction across black cotton soil is best dealt with by similar methods to those famously used by Stephenson across Chat Moss, and concrete pavements are an expensive luxury.

However, no methodology is any good without well executed long-term maintenance.

  • Michael JA Dunn (F), michael.dunn709@o2.co.uk

Water: Pay farmers for their valuable water service

Flooding

Flooding: Should we pay farmers to store floodwater?

Referring to your article on droughts and flooding (NCE 13 March), we need to be mindful that farmers are the main custodians of our land and water resources, and we must not take their efforts in bearing the brunt of floods and droughts on our behalf for granted.

Holding floodwater back on fields is an obvious example; it is a direct service to downstream communities.

So too is maintaining on-farm drainage systems to control water releases, and changing agronomic practices to improve infiltration and soil water holding capacity.

Drought services are not so obvious, but farmers again have much to offer. Some build reservoirs and store winter water for use in the drier summers.

But these costly and risky investments do not consistently attract government support, and the decision to build is essentially a commercial one. Yet farmers who decide to store winter water and give up direct summer abstraction rights leave more summer river flows for others to use. This service does not attract income from those who benefit, and nor does the possibility of augmenting summer river flows from farm storage.

Farmers can and do provide valuable water management services to communities and to the environment, but these do not come free of charge. So rather than talking about paying compensation or providing grants, which all sounds like more government hand-outs to farmers, should we not be changing our language and talking about paying them for services that we value.

By doing this we may well be making positive steps towards getting the level of water and food security that we all desire.

  • Melvyn Kay (M), UK Irrigation ­Association, c/o Moorland House, Hayway, Rushden, Northants NN10 6AG

 

The article ‘Diversion Tactics’ refers to the Leigh Barrier protecting Tonbridge.

I have an interest in the Leigh scheme because I prepared the original report recommending the construction of the Leigh Barrier - at that time named “The Straight Mile Reservoir” - in about 1969. The Leigh Barrier was built some 12 years later.

David Smith’s letter ‘Act now to cut the impact of future floods’ (NCE, 13 March) seems relevant. If it takes 12 years to prepare, fund and build a pretty small scheme like Leigh, how long will it take to really enable flood alleviation measures to have a significant effect on flooding in UK?

  • Barry Downs (F), jbandsa@talktalk.net
  • NCE welcomes letters from readers. We attempt to print as many as possible, which means letters longer than 200 words are likely to be condensed. Send your views and opinions to: The Editor, NCE, Telephone House, 69-77 Paul Street, London, EC2A 4NQ; email: nceedit@emap.com

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