As the canoe became the preferred mode of transport in large parts of Yorkshire during last Christmas’s floods, two areas of the North York Moors with a history of inundations were being congratulated in the media for remaining distinctly dry.
One of them, the town of Pickering, is thought to have survived as a result of a flood defence scheme which involved a catchment management approach incorporating a combination of soft and hard engineering measures. The other, Sinnington village, avoided floods as a result of an exclusively soft engineering approach to flood prevention (see box).
While the precise amount of rainfall the river catchments were subjected to was a matter of some conjecture in the media, engineers will look to evidence of the long term performance of this approach to draw conclusions about the success of the £2M project.
Environment Agency flood and coastal risk management senior advisor Dean Hamblin was the Agency’s project manager for the Pickering flood defence scheme and even he admits that it has yet to be comprehensively proven.
Pickering flood defence scheme
“The evidence is there, certainly anecdotally, that these measures are having a significant impact but I think the science has yet to be conclusive and we need that longer record of rainfall to prove their effectiveness,” he says.
Hamblin is speaking at the Forestry Commission’s Pickering headquarters, having invited New Civil Engineer to see the project and judge the theory for itself. He is joined by the Forestry Commission project manager for the scheme, Philip Roe, and Arup global flood resilience leader David Wilkes whose company designed some of the more traditional water storage measures in the project. The fact that they are all together, points to the unusually collaborative nature of the scheme.
“The Slowing the Flow at Pickering project is one of three pilot projects funded by the Department for Environment Food and Rural Affairs (Defra) in response to the Pitt Review [of the 2007 floods],” explains Roe. “The review called for Defra, the Environment Agency and Natural England to work with partners to deliver flood risk management involving greater work with natural processes.”
The Forestry Commission was one such partner in the project. It controls a third of the land covered by the scheme and, as such, was able to implement some of the more natural river management measures he is talking about.
Roe describes the 66km2 area of the project as being split into two butterfly wing shapes. It includes the neighbouring catchment of the River Seven, to the west of the site, and the catchment for the Pickering Beck to the east.
Pickering flood defence scheme
Previously the steep sides of the valley, the narrowness of the catchment and elements of historical straightening had led to repeated flash floods in the towns of Sinnington, downstream of the River Seven, and Pickering, which is downstream of Pickering Beck.
“The issue in Pickering itself is [that there are] constrictions on the river as it flows through the town, primarily due to bridges and road crossings,” says Hamblin.
The principal pinch point was Ropery Bridge which runs through the centre of the town. This was shown to have a safe conveyance level of 14.5m3/s.
“We decided that what was needed was a comprehensive scheme all the way through the catchment,” says Hamblin. “We needed to attenuate and drop peak flow upstream so that when it comes into Pickering it’s about 14.5m3/s.”
Wilkes describes the final engineering solution for Pickering as involving “two strands”. The first, led by the Forestry Commission, involved natural catchment management measures while the second, led by the Environment Agency, with design input from Arup, involved more traditional flood plain storage.
Using a specific catchment scale overflow model, which combined hydrology and hydraulic modelling developed by Durham University, the Forestry Commission identified 17 areas in the Pickering catchment where it could plant trees and construct large wood debris dams to slow the flow of the river. This was later extended to include the River Seven catchment. The delicate nature of the project was evident in the fact that of out of 96 sites surveyed, only a third were identified as having positive impacts.
Pickering flood defence scheme
In total, 129 large timber debris dams were constructed in the upper catchment of the Pickering Beck in an attempt to reconnect the water course with its flood plain and slow the flow.
“We were looking at the tributaries as inputs into the wider hydrograph and trying to disaggregate each of them rather than amalgamate them,” says Hamblin. “If you spread them out and slow some of the small ones down you can stretch that hydrograph and reduce the peak flow.”
A survey of 100 of the dams revealed that the volume of water that could be temporarily stored on their banks in times of peak rainfall ranged from 0.1m3 up to 110m3 depending on the size of the dam. Together they combine to provide 1,300m3 of storage for Pickering Beck alone.
“The [dams] are leaky structures so there is no impedance to fish migration,” says Hamblin. “So at low flow the channel flow is unimpeded, maybe 300mm below the bottom log.”
Roe says the low cost of constructing the dams has helped to keep the overall project price down. “This kind of structure would be a three-man day build in one day, so you’re looking at £550 to £600 to build them,” he says.
There was a realisation from start that what was going to be done wouldn’t solve all flood risk, and so there was engagement with the community to understand what their needs were.
Dean Hamblin, Environment Agency
In addition to these measures, the Forestry Commission planted 19ha of woodland along the banks of the water courses, 15ha of farm woodland, carried out 3.2ha of heather reseeding and installed a total of 187 heather bale check dams to stop immediate run off into the moorland drainage system.
The model calculated that all of these measures together would contribute to reducing the height of a 1 in 25 year flood peak by 4% and that of a 1 in 100 year event by 8%. This represented a significant contribution to efforts to reduce flood flows in the river but would still only succeed in cutting a peak flow of 18.9m3/s, recorded during the 2000 flood in Pickering, to 17.5m3/s. This was still short of the 14.5m3/s safe conveyance figure for the Ropery Bridge in Pickering town centre.
It was at this point that more traditional measures came into play. The Environment Agency took the lead with design input from Arup on a project to build a bund or “large raised reservoir” as Wilkes puts it, 2km upstream from the village. This consists of roughly 1km of embankment split into two main parts: a spillway some 2m to 2.5m above natural ground level and a lateral embankment that runs alongside the North York Moors railway to protect the railway should the reservoir overflow. (To watch a video of Hamblin on site at the reservoir, click here).
Pickering flood defence scheme
In the centre of the reservoir, a concrete control structure was constructed and completed in September 2015 that throttles the flow of water heading towards Pickering, restricting it to the target flow rate of 14.5m3/s. In total the reservoir is designed to hold 120,000m3.
Hamblin says the local community was consulted and involved in all of the decisions about where best to target flood protection measures and, because of the limited funds available, some tough decisions had to be made.
“There was a realisation from start that what was going to be done wouldn’t solve all flood risk, and so there was engagement with the community to understand what their needs were,” he says.
Pickering flood defence scheme
“There’s a low lying area in the village called Beck Isle which consists of a few residential properties and a museum. To protect that area through these means would have reduced the effectiveness of the volume of the storage that was available, so actually, through working with the community, we worked on providing those properties with property-level protection.”
Put together, Hamblin likes to think of the measures as a ”patchwork” of soft and hard measures that dovetail to provide protection against a 1 in 25 year event. He adds that the project is a model of stakeholder engagement.
“The catchment sensitive aspects are led through Natural England, the North York Moors National Park has done work on top of the moors, blocking moorland drains and addressing other areas of increased run off such as footpaths as well as establishing no-burn buffer zones of heather adjacent to watercourses,” he says.
“The Forest Estate has installed wooded debris dams, carried out planting and drainage management and on top of that that there’s the Environment Agency-led flood storage reservoir.”
In the area of Pickering known as Beck Isle, the local community has been working on property protection and the Pickering Town Council is contributing £5,000 a year towards the operating and maintenance costs of the flood storage reservoir.
The fact that there were only three landowners to deal with – the Duchy of Lancaster, the Forestry Commission and the National Park also made life easier as far as Hamblin is concerned.
“Buy-in from landowners is a key success criterion to flood risk management, especially when you’re taking it out to the catchment scale and the land management scale,” he says.
Two flood peaks
Since the project was completed in September 2015, Roe says there have been two flood peaks and the town remained dry. Initial estimates suggest that the 9.9m3 peak flow experienced was up to 2m3 less than would have been expected. This is thought to be around 50% down to the reservoir and 50% down to the other measures. What the peak flows have also done is show the predictive model to be fairly accurate.
“No two flood events are the same. There are differences in site wetness, levels of water flow, the nature of the storm event, its duration and the intensity of rain,” he says. “To determine the effectiveness of the measures, we’re plotting what has happened against what we theoretically thought might have happened and they’re pretty close, so we think we’ve got it fairly right.”
Hamblin argues that the project offers the optimum amount of protection for the money spent but will not protect against the most extreme weather.
“They’ve got 1 in 25 year protection that will combat the middle order and regular flood events but it’s not a panacea,” he says. “After that, it’s about being prepared and making sure the community is prepared for flood risk and that it signs up to flood warnings.”
Given that the town flooded four times between 1999 and 2007, with the last of those disasters causing £7M of damage, this would so far appear to be £2M well spent.
The River Seven catchment
While Pickering is protected by a blend of traditional and natural flood defence measures, Sinnington that sits to its west below the River Seven is protected exclusively by natural measures.
Once again the Forestry Commission has sought to increase “roughness” in the river catchment by planting 10ha of woodland along its banks and constricting the river at 38 separate points with large wooded debris dams. It has also constructed two timber mini-bunds. These are more substantial structures that Hamblin describes as a natural evolution of the wooded debris dams.
“We thought, why don’t we build a big one?” he says. “Part of the experimental part was seeing what the measures do, how much they attenuate and how much they kick out into the flood plain.”
The furthest mini bund upstream is designed to store 3,620m3 of water. Further downstream the other bund can store 1,260m3. These figures are added to the 300m3 of storage provided by the wooded debris dams and the 2,500m3 of water held as a result of the planting of trees to give roughly 7,000m3 to 8,000m3 of storage to attenuate flow through the town of Sinnington.
To watch a video of Philip Roe at the site of one of the timber mini-bunds, click here.
The River Belford flood defence scheme
The £2M Pickering flood defence scheme is not the first to employ natural measures high in the catchment to slow the flow of a river in a flooding blackspot. In 2008, the Environment Agency, working with Newcastle University, proposed a series of small-scale, interventions to slow the rush of water downstream to the town of Belford in Northumberland.
The town flooded seven times between 1997 and 2007 but the £3M price tag for a traditional flood defence scheme was too great for the 35 properties at risk. Without the evidence to back up the new solution, Grant-in-Aid funding for flood defence schemes could not be made available so a request was made to the Northumbria Regional Flood & Coastal Committee. Funding was granted and in 2008 the project started.
Online Stream Structure diverting flow into a field
Like Pickering, the flood defence scheme includes large wooded debris dams to increase hydraulic roughness and to create storage in the channel and across the flood plain.
The project also includes overland flow interceptors. These are bunds, quite often in locations away from the watercourses, which intercept the overland flows that occur in storms. Intercepting these flow pathways, with leaky soil, wood or stone barriers, slows rainwater’s progression to the watercourses. These features also have a significant additional benefit of trapping huge amounts of rich sediment that is washed from fields when this overland flow occurs.
Finally the scheme includes offline storage areas. These are small bunded structures located on the floodplain where water is diverted during high flows. The diversion can take the form of a restriction within the channel or a lowering of the riverbank. The earth bunds are drained with outflow structures while timber dams can be designed to leak.
Research by Newcastle University has quantified the impacts of the individual features in the Belford catchment. By analysing historical events in this relatively small catchment (less than 10km2), the modelling has determined that a network of attenuating features has the potential to reduce peak flow by up to 30% in the type of storms that caused the more frequent flooding. It is clear that the approach is transferable to other catchments but clearly for larger catchments the approach must be scaled up.
During the recent storms in Northern England the Belford scheme operated and protected property from flooding, with only one case of minor inundation. It is clear that the risk has not gone away as flooding has been close on a couple of other occasions. The research has shown that in Belford the natural flood management techniques have reduced the frequency of flooding and is very effective in small to medium-sized storms.