Thoughts on OFAS

Why do we need a flood scheme at all?

We have had five major floods since 2000 and climate change will make things worse (indeed there is evidence from Oxford University that it already has). Oxford residents, and the economic well-being of the City, will suffer badly if something is not done. While the scheme will cause disruption during its construction it is imperative that Oxford is protected.

But what about downstream, won’t they be worse off?

All the detailed computer modelling for the Scheme (and we have recently heard on the grapevine that a totally independent consultancy has confirmed this ) says that flooding will not be made worse downstream. We know that comparisons are made with the Jubilee River – this scheme is nothing like that. Indeed OFAS is actually increasing the capacity of the floodplain, which together with bunding and rerouting of flow will reduce flood risk to many hundreds of properties.

Is it a concrete channel?

No, it isn’t. What it is is a much more naturalistic 2-stage channel, used around the world for flood relief.

How will the environment be affected?

While there will, regrettably, be some environmental losses, we are pressing hard – collaborating with the Environment Agency, and with support from others, particularly the Freshwater Habitats Trust – for environmental enhancements as part of the scheme. While one cannot compare one environmental loss directly with another environmental gain we believe the positives will be considerable.

Maintenance.

It’s so important that there is a plan now for the very long-term maintenance of the Scheme. In our experience over the last 10 years  “if maintenance can be neglected it probably will be”. There are some honourable exceptions and we certainly have we have no criticism whatever of the local EA maintenance team, who achieve a huge amount with very limited resources. Others do nothing or very little unless goaded and embarrassed into action. This Scheme, being “natural” will deteriorate quickly if not proactively maintained. The initial intent was to plan maintenance for 10 years: that is simply not good enough for such an expensive and important project. We have proposed that maintenance be planned for in perpetuity by setting up a responsible, funded, local body, maybe as a charitable trust (or similar).

A Green group

thinks that Oxford could and should be protected by very different means – while they are short on specifics, their main idea seems to be that planting enough trees upstream in the Cotswolds would solve Oxford’s problems. It wouldn’t. Expert opinion at our 2015 Symposium of Natural Flood Management (NFM)  made that very clear. Even afforesting the whole of the Cotswolds (not that that would ever happen) would not do the job. Oxford is simply too far downstream for that. That’s not to say that NFM can’t work in smaller catchments, nor that it might not make a contribution.

Flooding land upstream?

The Environment Agency’s Oxford Flood Risk Management Strategy (OFRMS) suggests this may be needed one day if climate change makes things sufficiently bad. Involving as it would temporary flooding large areas of farmland and other land it is never likely to be easy to implement.

 

 

Advertisements

OFA symposium on natural flood management techniques : Summary Report

OFA symposium on natural flood management techniques : Summary Report
26 March 2015, Oxford, in collaboration with the Environmental Change Institute, University of Oxford

Could techniques such as planting trees and creating more wetlands in the upper Thames help to reduce flood risk in Oxford? This was the question we asked a number of experts to address at a symposium attended by around 60 people – a mix of academics, local residents and officials from local councils and the Environment Agency – on 26 March 2015.

Professor Mike Acreman, a rivers and wetlands expert from the Centre for Ecology and Hydrology at Wallingford, gave an overview of what science today can tell us. Prof. Acreman started by pointing out that fluvial flooding is natural, with benefits to the environment. We cannot stop it happening. He said that in theory tree cover should help to reduce runoff, through ‘canopy intervention’ and’ improved soil infiltration’ but the scientific data on this are weak. A study at Pontbren, in the Brecon Beacons, showed that soil infiltration was increased by hedges and trees. Restoration of mires on Exmoor has also shown some benefit in improved water retention. But the volumes involved are small compared to the total volume of water involved in a flood event. Foresting small areas of a catchment might bring localised benefits, but would have no measurable effect at the level of the wider catchment. To impact the whole catchment change to the landscape would be required on a massive scale, and even then this would have very limited impact in a major flood: soil can only retain so much water. During a period of prolonged rainfall the ground becomes saturated, as happened in late 2013 in the Thames catchment. Any additional rainfall simply drains into the river system. He also explained that interventions which have local benefits may have unintended consequences downstream. Flooding has temporal as well as spatial scale effects, and delaying the peak from a tributary can result in it being superimposed on the peak in the main river, rather than arriving before it, so increasing the overall peak level. Prof. Acreman argued that we need a mix of conventional engineering approaches, and enhancement of natural processes, to effectively manage risks.

Lydia Burgess-Gamble, a research scientist with the Environment Agency, talked about the work the agency is doing on natural flood-management (NFM) techniques. She explained that since the Pitt Report on the 2007 floods the EA has been engaged in more work of this kind and a small number of schemes have been established, with positive early results. These are small, localised interventions such as the 30 attenuation ponds (10,000 m3 of storage) built on 10 km2 of farmland around the village of Belford in Northumberland. In Pickering in North Yorkshire conventional flood defences have been supplemented by woody dams and tree planting by the Forestry Commission. Here the aim is to reduce flood risk from 25% a year to 4% by arresting flow in a ‘flashy’ beck which runs through the town. In the Thames valley there has been work including leaky dams slowing runoff into the Evenlode at Honeydale Farm. Flooding in Stockton-on-Tees is being tackled through building 40 storage ponds in locations identified through modelling. This scheme is at the initial stages of implementation. The EA plans more of these projects, creating ‘opportunity maps’ and working with natural processes where there is potential to achieve ‘healthy catchments’. ‘Catchment laboratories’ (including data collation and workshops) are proposed, building on ‘green engineering’, but there remain many knowledge gaps and the full impact of these types of interventions will not be understood for some time. Lydia said NFM was one tool only.

Derek Holliday of the CLA, which represents farmers and other landowners who together manage 70% of the land in England, described the major shift which has occurred in the last 10–15 years away from subsidies for crop production towards payment for ‘ecosystem services’. He argued that even bigger shifts need to happen but this requires a clear policy framework – farmers need to make spot decisions between land management options, and his members say they tend not to have the necessary information to favour flood-friendly options. They also worry about the `reversibility’ of these options, policies being changed. Without a clear framework, landowners will not make the investments which change would require.

With the fourth speaker, Nathalie Schaller from the Environmental Change Institute at the University of Oxford, we changed tack slightly and looked at climate change and its likely implications. After each extreme weather event the public wants to know if ‘climate change’ was a contributing factor – but limited research has so far been done. Nathalie presented the results of a study of the degree to which human-generated greenhouse gases increased the likelihood of the Jan/Feb 2014 floods in the UK: this type of approach is known as ‘event attribution’. A large number of computer simulations were run for different scenarios to try to separate out anthropogenic carbon from underlying climate variation. The study found that human activity had on average increased the risk of the 2014 floods happening by 40%. The implication of this for the future is that extreme weather events are likely to be more common.

Following the presentations there was discussion, with the presenters responding to points from the audience. The conversation ranged across many issues and it is not possible to capture all the points made. The overall conclusion was that natural flood-management techniques can bring benefits in smaller catchments, especially in lower-order flooding events. Isolated projects in the upper Thames would, however, have no measurable impact on Oxford with its large catchment (about 2,500 km2 upstream). A complete re-landscaping of the Thames catchment might, if it could be achieved, reduce flood peaks by 10–15%. Achieving the transformations in land-use necessary to deliver this benefit would require radical new legislation and/or a new culture, taking ‘around 40 years’ to implement. As a means of addressing flood risk in the city ‘natural’ methods would not replace the proposed flood relief  scheme for Oxford and Abingdon, but might help prolong its life expectancy.

An interesting point of detail concerned the role of insurance companies which in theory should have an interest in funding flood-risk reduction measures for land, but which are not incentivised to make such investments.

There was also some discussion of groundwater, which is a major contributor to flooding in Oxford, over and above fluvial flooding. There appeared to be little which could be done to reduce groundwater at a macro level directly. Lowering surface water levels tends to lead in due course to lower groundwater levels.

There is potential scope for natural flood management techniques to benefit small communities, depending on the local catchment characteristics. There is still much to understand about what does and doesn’t work, and in what context, but the Environment Agency and others are establishing more schemes and are keen to implement projects in the Thames catchment if suitable sites can be identified.

These links download pdfs of this Summary Report, and of three of the speakers’ presentations:

OFA Symposium 2015, NFM and Oxford, Summary Report + links

Presentation by Mike Acreman

Presentation by Lydia Burgess-Gamble

Presentation by Nathalie Schaller.

Natural flood management, climate change and Oxford: a symposium

Oxford Flood Alliance are hosting their Second Flood Symposium this week, in collaboration with the Environmental Change Institute, University of Oxford. The meeting is fully subscribed.

The topic is Natural flood management, climate change and Oxford.
We will publish a summary of the proceedings on this website.