The UK experienced widespread flooding during October and November 2000. Despite negative reports and misunderstandings in the local media, the Ashford flood alleviation scheme in Kent was able to prove its worth. Suzanne Pritchard reports
On 6 November 2000, overtopping of Aldington flood storage area in Kent prompted nationwide media coverage in the UK. Much to the dismay of those closely involved with the operation of the flood alleviation scheme, a documentary television programme claimed that the embankment had failed and had flooded land and properties in the area surrounding the town of Ashford.
Ashford is situated at the confluence of the Great Stour and East Stour rivers which are joined by a number of minor tributaries. During the 1970s the town’s urban and rural areas experienced extensive flooding and by 1980 plans to develop a flood defence system were under way. Constructed between 1989-91, the Ashford flood alleviation scheme aims to reduce the risk of widespread flooding in and around Ashford. The town has developed significantly in recent years and, most importantly, is home to the international railway station which is linked via the Channel Tunnel to France.
The flood alleviation scheme has two onstream storage areas (Aldington and Hothfield) which each comprise an earthfill water retaining embankment with a clay core and vortex flow control device. The design of the main embankments had to allow for a PMF discharge over the embankments, while retaining floods of up to a 100-year return period with a controlled discharge. The spillway section of the embankment includes a concrete kerb sill. The grassed crest and downstream slope are both reinforced with a geotextile fabric.
Working under pressure
Heavy rainfall from 9 October to 15 November 2000 prompted widespread flooding across the UK. Although Hothfield storage area impounded to its highest level, it did not overtop the spillway, as happened at Aldington.
‘We are very pleased with how Aldington performed,’ says Geoffrey M Gibbs, regional flood defence regulation engineer for the Environment Agency which operates the scheme. ‘And we are disturbed by how this was portrayed in the media. Aldington performed as we had expected it to. It worked as designed. For the first time in its ten-year history Aldington was fully impounded and overtopped the spillway.’
Gibbs explained that overtopping had caused limited flooding in the East Stour valley. Farmland was mostly affected but a small number of properties were also flooded, as had been highlighted by the television programme. ‘But,’ Gibbs added, ‘historically these are areas that flooded annually.’
Without the scheme more properties and farmland would have been inundated in the East Stour valley, and it is likely that widespread flooding would have occurred in Ashford town centre. ‘Without such schemes there would have been utter devastation,’ said Gibbs. ‘This is a successful project, like the Leigh flood barrier which protected Tonbridge town centre in Kent. And these are successes that the industry should be celebrating.’
Unfortunately, a lack of understanding on how the facility operates caused widespread media concern. Reports suggested that the facility had failed or collapsed in the heavy rain. As Ken Allison, regional flood defence manager for the southern region of the Environment Agency explains, because the scheme did not actually prevent flooding it attracted a lot of media attention. ‘The problem was,’ he said, ‘this was the first time the facility did its job in anger.’
The Environment Agency cannot fault how the Ashford flood alleviation scheme performed and Ted Craker, liaison officer for Ashford Borough Council, agrees. ‘Without Aldington and Hothfield,’ he says, ‘Ashford would have suffered enormously. Older parts of the town, which are close to the international railway station, would have flooded and about 100 houses would be under water. As it was, only two to three houses in Mersham, immediately downstream of Aldington, flooded. Aldington and Hothfield were a great success.’
Such events at Aldington may become more frequent, Gibbs warns. Focusing on climate change, he said that although annual rainfall may decrease in the UK, there may be more intense storms at certain times of the year. The year 2000 proved to be an exceptional period for rainfall. There were three extremely wet months up to the end of October. Land was totally saturated and rain ran straight off agricultural land. From 27 October to 6 November there were three periods of heavy rainfall which led to an increase in water levels at Aldington flood storage area.
‘I believe that climate change is happening all the time,’ Gibbs said, ‘and it has been happening since the beginning of time. But until we fully understand it and its consequences, we may find that such flood alleviation schemes will continue to impound more frequently.’
|WHAT HAPPENED AT ALDINGTON…
| Even though the Environment Agency prefers to call Aldington and Hothfield ‘flood storage areas’, as they are not intended to contain water on a permanent basis, they are classified as reservoirs under the Reservoirs Act of 1975.
Aldington has a maximum storage capacity of 1.3M m3. And although normally empty, the 6m high embankment does impound from time to time. During this recent flooding event on:
• 27 October 2000 – the flood storage area started to fill.
• 6 November 2000 – the spillway started to overtop and lasted for a period of 28 hours.
• 27 December 2000 – the storage area was drained completely.
| A Vortex flow control device was used at Aldington as it allows substantially greater flows at low heads and restricts flows at higher heads more effectively than orifice plates. As Geoffrey Gibbs from the Environment Agency explained, Vortex controls permit a constant discharge which restricts the flow downstream to a level that the channel can cope with.
Initial proposals at the Ashford flood alleviation scheme suggested the use of orifice plates for the main outlet controls. But these can over-restrict the flow at an early stage so that the storage areas will begin to fill earlier, and take longer to drain down after the event.
This is an important consideration.
‘Upstream of the flood storage areas we had to purchase the right to flood the land,’ says Gibbs, ‘but this land is still farmed to a certain extent. So we need to decrease the flooding time and endeavour to drain it as quickly as possible.’
Adoption of the vortex control device has various benefits:
‘The impressive thing about Aldington and Hothfield flood alleviation schemes are that they are designed to make the optimum use of downstream river courses,’ said Gibbs. During the flooding in October/November 2000, rivers downstream of Aldington remained