Monday, 10 February 2014

Dredging the River Parrett

 4. February 2014

The recent flooding of the Somerset levels has led to many calls for renewed dredging of the River Parrett and for reversion of wildlife conservation measures within the Somerset Levels. Calls to dredge are borne out of historic practices, but it is perhaps worth reflecting that the current problems may be far more complicated than can be resolved simply by dredging and draining.

Dredging within tidal rivers can have significant unforeseen consequences. One factor that needs to be carefully considered in this instance is the way in which deepening and widening may lead to elevated high tide levels. This is a feature of several of Europe's largest rivers that have been deepened to improve navigability. Some quite extreme examples can be seen in the Elbe, the Ems and Seine. But, there have been impacts closer to home: deepening of the Thames also led to increases in tide heights in London. In the case of the Thames, this was a long while ago and effectively beyond living memory. Modern flood-risk management counters that effect, but it is worth bearing in mind that, without deepening, the tidal flood risks to London would be lower. Quite how much lower the risks might have been are probably un-quantified, but it would be an interesting exercise to translate this into the additional cost of flood defences!

Moving on to the Parrett, it is worth reflecting that cessation of dredging will have led to siltation and narrowing and shallowing of the tidal channel. This in turn would have led to a reduction in tidal propagation and thus there would have been flood risk management benefits where tidal flooding was concerned. Why might this be?

The speed at which the incoming tide enters a river is governed by the dimensions of the river in cross-section. Conversely, the dimensions of the cross-section at any point are a function of the volumes of water passing on flood and ebb tides, and the speeds at which they travel. The cross-section reaches a dynamic equilibrium provided sufficient sediment is available to infill any void where flow rates fall below the threshold at which deposited sediment is re-mobilised. Dredging creates an un-natural void and deeper water. There are three important consequences of such a morphological change:

  • Incoming flood tides will travel upstream at a faster rate than they did previously, shortening the duration of the flood tide and reducing the scouring effect of the ebb tide which will recede more slowly.
  • More sediment will be drawn into the river and will be deposited onto the green foreshore and in the dredged channel. Consequently, dredging is not a 'one-off' and will have to be repeated regularly.
  • Depending upon the gains in tidal elevation, it may be necessary to raise the height of the flood banks to provide the same level of protection under the un-dredged regime.
In general, the carrying capacity of the Parrett has been adequate to meet needs in 'normal' conditions of rainfall (not perfect but at levels commensurate with cost-benefit analyses). Rainfall in January 2014 has reportedly been around twice normal levels for this time of year and appear to be higher than previous records. This is therefore an extreme event on a scale that could not have been predicted. It falls outside the cost-benefit calculations that would have supported the maintenance of the sorts of measures that are being called for.

Thus, it seems to me that it would be wise to reflect and evaluate the relative merits of increasing the risk of elevated tidal propagation against the likely costs and risks of a future extreme event. One possible way of reducing the flood risk from tidal propagation is to realign the banks and create much larger tidal washlands. Such measures may appear to be 'environmental' and seemingly lack any obvious human benefit but in fact they have the potential to be a highly cost-effective way of countering exceptional tidal events, as well as providing some of the very necessary accommodation space for excess fluvial discharges that might otherwise threaten Bridgwater.

I would therefore argue that current measures to manage fluvial and marine flooding events should not simply rely on a knee-jerk return to dredge and drain on the levels. There is a real need to determine precisely where, when and why the rivers over-topped their defences and to consider whether there might not be a place for win-win for people and for wildlife. My instincts say that the measures that had been in practice sought such a solution but were overwhelmed by an extreme event that would probably have caused similar devastation even if the Parrett had been dredged. After all, increased tidal propagation and an elevated high tide could coincide with a similar rainfall event and there would be the same result! Once the land is flooded the key issue is the capacity of the pumps and not the capacity of the Parrett on an ebb tide.

One possible solution is to consider a more pragmatic approach to former wetlands. Perhaps it is time to return to the use of summer pastures along similar lines to those one sees on many northern European rivers. In these cases, a small outer flood defence prevents normal tidal flooding but is over-topped by extreme events. Secondary defence lines some distance inland then provide a barrier between people and the river and accommodation space to counter major tidal or fluvial events.

This is arguably a solution that will not please the purists who seek more inter-tidal wildlife habitat but, equally, it will have detractors amongst the agricultural community! The sad reality is that we must expect more of these events if model predictions for our changing climate are to be believed. Importantly, this debate must consider predicted changes to weather patterns. We must learn to adapt in the face of rising sea levels, increased storminess and the potential for more frequent extreme events regardless of arguing about the causes which have probably got a long while to run and will not be brought under control in the foreseeable future.

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