Environment Agency presentation: ‘To Dredge or not to Dredge’
Here you can peruse a document which the Environment Agency has prepared for national consumption – so not specific to the Upper Calder Valley – but which sets out the general issues and offers analysis concerning the dredging of river channels and flooding mitigation –http://www.bidfordonavon-pc.gov.uk/pdfs/notices/dredgingpres.pdf
There is also a BBC news article about our river stewardship volunteer work in Mytholmroyd, which took place in December, available to view at: http://www.bbc.co.uk/news/uk-england-leeds-25404188
This is as live a question in the Upper Calder Valley as it was recently in the Somerset Levels, but now we have two expert documents to inform that discussion, which can also be taken up with Environment Agency and Calderdale Council staff (and Calder Future) at the drop-in event in Hebden Bridge Town Hall Wednesday 9th April 2014 3-7pm; everyone welcome. The first is a general report (so not specific to UCV) Floods and Dredging: a reality check produced by CIWEM, ‘the leading Chartered Professional Body covering all aspects of water and environmental activity’. Its conclusion, which you can find on page 4, is that: “dredging can play an important role in flood risk management in some cases, but is not a stand-alone solution. It should be considered in the context of a range of tools and the origins of different sources of flood water, and comes with significant risks that must be understood at a local and catchment scale.”
The second is Briefing note: Mytholmroyd channel maintenance which is produced by the Environment Agency, specific to Mytholmroyd, and presents the results of the computer modelling exercise they have carried in recent months for that stretch of the river. It contains detailed tables and pictures that contrast the effect of dredging the shoals, contrasted with those for clearing and managing vegetation. (Shoals are accumulated banks of silt). Its conclusions (page 6) are:
Dredging: ‘The locations with the largest shoal accumulations are located in the wider and larger sections of the river where it flows more slowly and deposits materials. The channel sections with shoal accumulations are larger, even with the shoals, than the smaller and narrower sections which have no accumulations. The removal of shoals from the wider sections would not therefore lead to an increase in channel capacity.’
Vegetation Management: ‘Why does removing vegetation have such a big impact? Vegetation grows along most of the channel through Mytholmroyd. This increases friction and slows down the flow, resulting in higher river levels and an increased risk of flooding. This impact is accumulative and acts over the length of the river through the town.’
Comparison between dredging and vegetation management: The impact of dredging reduces water levels by 2-5cm (1-2inch) whereas managing vegetation reduces water levels by 30-40cm (1-1.3feet). Dredging does not therefore improve the overall standard of protection against flooding. However, by not removing vegetation the standard of protection reduces from 1 in 25 yrs to 1 in 10 years for the centre of Mytholmroyd.
These results will come as a surprise to some and so the briefing is worth reading in full. And do come to Wednesday’s meeting to get more information and discuss the issue with the EA and others.
Because vegetation management is demonstrated to be effective it forms the centrepiece of the ‘river stewardship’ programme, using environmental volunteers, that Calder Future will be organising on behalf of the Environment Agency and Calderdale Council in Mytholmroyd and elsewhere over the next 6 months. To volunteer, and to do your bit reducing flood risk in Mytholmroyd and across the UCV, contact us at firstname.lastname@example.org or speak to us at the meeting.