In the third article in the series “A New Approach to Storm-Water Management and Flood Reduction“, Ed Hill looks at the Sustainable Drainage Systems (SuDS) project called “Slowing the Flow” in Pickering, North Yorkshire, England.
Pickering is a market town in North Yorkshire described as a ‘notorious flood spot’ owing to its location “at the bottom of a steep gorge draining much of the North Yorks Moors”. Pickering rose to prominence recently as a result of two factors: cost saving and flood resilience.
The town has been flooded by the Pickering Beck (River) on four occasions between 1999 and 2007, with an estimated £7m in damages caused by the 2007 floods. The cost of a building concrete flood walls to keep water in the river was estimated in 2007 to be in the order of £20m, an amount which the authorities said could not be justified by the size of population to be protected from flooding. As a result, DEFRA called for proposals for alternative means of addressing the flood risk in Pickering.
The result was a partnership between local residents, academics from Oxford, Durham and Newcastle Universities, the Environment Agency, and the Forestry Commission, amongst others, to investigate the restoration of water flow conditions in the river catchment area in order to reduce runoff into the river itself during high rainfall events. The concept involved low-tech, low impact interventions such as ‘leaky dams’ (which allow normal river flow, but restrict higher flow volumes), heather re-seeding, tree planting and small bunds or water retention areas.
According to the Forestry Research final report, “the overall aim of the project was to demonstrate how the integrated application of a range of land management interventions can help reduce flood risk at the catchment scale, as well as provide wider multiple benefits for local communities.”
The project was repeatedly modelled in order to earmark optimum locations for implementation, and the overall cost was found to be between £2m and £4m, or 10% to 20% of the cost of a conventional solution.
The ‘Slowing the Flow at Pickering’ project involved building a 120 cubic metre clay-wall bund, installing over 150 woody debris ‘leaky’ dams in water courses at strategic points in the Pickering Beck and neighbouring River Seven catchments, and a similar number of bales of cut heather in smaller watercourses and gullies, as well as planting up to 80ha of trees. To reduce erosion, ‘no-burn’ buffers were to be established along water courses, and heather re-seeding was to take place.
The partnership commenced in 2009 and implementation of the second phase of the ongoing project was completed by September 2015.
One of the challenges to implementation was resistance from some landowners to afforestation of grazing land, and only 44ha of trees were eventually planted – mostly on public land. The number of woody debris dams installed totalled 187, and two wooden bunds were installed in the beck as a pilot project.
Modelling based on the December 2009 near-flooding event indicated that an 85,000 cubic metre capacity bund with a culvert allowing a flow of up to 15 cubic metres per second, would alone protect the 50 most flood-prone properties in Pickering from smaller flood events such as the 1999, 2001 and 2002 floods, but would not fully protect them against a 2007-scale event.
In order to locate and design this facility to minimize negative impacts, various proposals were explored, with final agreement reached on a scheme designed to reconnect the main river to a 30 hectare storage area of usable flood plain. The flood storage area, which was completed in September 2015, used a low recycled-clay wall bund to temporarily store up to 120,000 cubic metres of flood water forced out of channel by a culvert-style control structure. The pasture land involved can be used for grazing when the bund is not in use.
Evaluation of the Project Implementation
Following ‘near flood’ conditions in December 2009, the Environment Agency (and its consultants Arup) were able to refine their modelling of the critical flow thresholds for the onset of flooding.
With this data, the construction of the 120,000 cubic metre bund was modelled to reduce the forecast flooding likelihood for Pickering from 25% per annum to 4% per annum, as per the project target. The impact of the additional measures could not be included in the modelling, but could be expected to reduce and retard water flow and thus reduce the ‘hydrographic peak’ of any flood events.
During the November 2012 ‘one in 8 year’ event, only one of 120 constructed woody debris dams failed, and its logs were found lodged close downstream where they remained until removed two years later. Unfortunately, the effects of the project implementation could not be accurately confirmed from modelling of the event, owing to its multiple-peak nature.
The Pickering project hit headlines in January 2016, following the Christmas 2015 flooding of much of northern England, with an article by Geoffrey Lean in the Independent, attributing the lack of flooding in Pickering to the success of its ‘Slowing the Flow’ approach. This was soon followed by a riposte in the Guardian by Jeremy Biggs, director of the Freshwater Habitats Trust, stating that the lack of flooding in Pickering was actually the result of a lack of rainfall in the catchment, rather than the success of the STFAP project. Lean’s response in his personal blog defends his standpoint, indicating that the rainfall figures used by Biggs were for a different catchment.
In April 2016, it was reported that the Environment Agency had released figures indicating that the flood reduction measures at Pickering had “reduced river peak flow by around 15-20 per cent during the December floods … when a total of 50mm of rainfall fell over a 36-hour period over Christmas 2015.”
It is estimated by the Environment Agency that approximately half of the reduction was due to the bunded flood storage area and the remainder to upstream land management measures. Analysis by project hydrologists indicate that peak flood flow was reduced by up to 20%, as expected, but Jenny Walker, the ‘Slowing the Flow at Pickering’ chairperson cautioned that “the measures installed have their limits and would not be enough to prevent flooding in the event of rainfall on the scale experienced for example in 2007.”
Nevertheless, Elizabeth Truss, DEFRA Secretary of State, endorsed the scheme as an example of best practice at its official opening in November 2015.