Climate change mitigation and adaptation – implications for inland waterways in England and Wales, April 2009
6.1
Climate change context
The predicted increase in winter precipitation discussed in Section 2, combined with any increase in extreme weather events (including summer storms), will have a number of implications not only for flooding but also for navigation. During flood conditions (i.e. when a river is out of bank), navigation may have to be restricted or suspended in the interests of safety because of increased flow velocities or currents or because the line of the channel is not visible. The same impacts on navigation may occur even if there is no flooding as such (i.e. if the river remains within its banks): closures or restrictions may become necessary because even relatively moderate increases in flow rates (e.g. as little as 1m/s on some waterways) can result in ‘Strong Stream’ conditions. There has historically been little attempt on most rivers to collate and correlate data on navigation restrictions with information on occurrence of high water levels or Strong Stream events. Further, whilst the various climate models appear to agree on the likelihood of increased winter precipitation in England and Wales, recent work by the Environment Agency suggests that the relationship between increased precipitation and Strong Stream events is not necessarily straightforward. The results of the Environment Agency’s 2007 modelling of flow rates for the Thames, Nene and Great Ouse were surprising in that they appear to suggest that the frequency of Strong Stream events will decrease under the UKCIP02 medium-low and medium-high scenarios for a time horizon up to 2080. One theory put forward for this conclusion is that greater evaporation will have a smoothing effect on soil moisture deficits. However it is also important to be aware that the research did not assess the full range of climate model and emission scenarios, nor did it take into account changes in seasonality or storminess.
More research is probably therefore needed before definitive conclusions can be drawn. In the meantime, however, this scoping report explores the range of options which might need to be considered in the event that increased winter precipitation does cause problems for the inland navigation sector. In addition to possible changes in the frequency of Strong Stream events, reduced channel freeboard and hence operating clearance (i.e. headroom) under bridges, in tunnels, etc. is another important consideration for the safety of navigation. High water levels and flood flows can also affect the effectiveness of navigational infrastructure. A lack of freeboard combined with potential for increased bank erosion, scour, seepage or overtopping can threaten the stability or integrity of embankments, supports and other structures, potentially increasing the risk of failure. The capacity of culverts, weirs and sluices might similarly be compromised by high flow levels. Flood waters typically carry more debris which then requires clearance from screens, behind structures, and there will be extra wear and tear on locks and sluices as a result of the need to use such structures to facilitate flood conveyance. Indeed, the use of navigation infrastructure to assist in flood risk management raises issues of its own.
Embankment failures on canals present risks to life and property and repairs are often costly, as seen on the Monmouthshire and Brecon Canal in 2008
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