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The munificent seven

Prevention - Preventing floods in the Speyside town of Rothes calls for drastic action, discovers Andrew Mylius.

Seven householders in the small east Scottish town of Rothes are making an extraordinary sacrifice to resolve a 250 year old flooding problem. They will be packing up their belongings and clearing out to make way for local council wrecking teams, who will erase their homes from the landscape.

Following the flooding of some 155 homes in Rothes in 1997 and again in 2002, pressure on Moray Council to take action was huge, explains Steven Trewhella, director for coastal and river projects at consultant Royal Haskoning.

Under framework contracts with the council, Royal Haskoning and contractor Morrison were asked to look at the causes of flooding and propose solutions.

Trewhella maintains that the drastic course of demolishing homes forms part of the only really viable flood alleviation strategy. 'There's a consensus that this is the best way forward.

The people who will be affected recognise that they own the problem, and that they also own the means to solve it.' Rothes is perched on the western flank of the River Spey.

Five malt whisky distilleries crowd the three tributaries to the Spey that course through the town.

'These 'burns' are very steep and flashy - they respond fast to rainfall, and also have a very high sediment content, ' notes Trewhella. 'Normal flow rates in the burns are in the order of one cumec, but in spate they flow at about 10 cumecs. The flow rate isn't that great but because of the gradient - 1:10 - the speed is huge. The water's very powerful. In 2002 one of the road bridges was washed away.' The burns are hemmed in not only by the distilleries, but by housing and bridges, channelling high flows so that they quickly become angry torrents. 'From a hydraulic perspective, flows are governed entirely by the local section, ' Trewhella explains.

Logs frequently jam in narrow sections or are caught against bridges, and due to sediment deposition and erosion, river bed levels 'can move up and down a foot [300mm]' from season to season and year to year.

Royal Haskoning and Morrison had already carried out extensive research on rainfall and catchment behaviour in the wider Moray Firth area. This meant that even though there were no flow gauges on the burns it was possible to build up a good idea of how they behaved.

To reduce their 'flashy' behaviour, Trewhella's team looked at creating upstream water storage capacity, 'but there were few sites to choose from, and in any case a storage reservoir would effectively act as a silt trap. This could have led to unpredictable changes in channel section downstream'.

Tunnelling to connect the burns and lead them around the town was briefly considered, but any siltation of the culvert would have squeezed its capacity. 'If it blocks, what's your back up plan-' Raising the height of existing flood walls was ruled out simply because it would only aggravate the problem.

Last ctober rewhella recommended to the council that the best way forward would be a combination of modifying the burns' sections and instituting a new channel maintenance regime.

To make the channels 'completely flexible' Royal Haskoning advised removing seven foot and two road bridges crossing the burns and replacing them with structures providing greater clearance. Services such as pipes and cables will be rerouted. In some areas new flood walls and banks will be erected, while at pinch points existing flood walls will have to be moved back. Seven houses must be demolished.

'Removal of people's homes isn't something that you take lightly, but we sat down with the Scottish Environment Protection Agency, the Health & Safety Executive and local people, and looked at the consequences of each type of intervention. We've had to demonstrate that this is the most secure scheme for the future.' Planning consent for the £15M ($26M) scheme is being sought and construction/ demolition work is slated to start in a couple of years time.

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