The Environment Agency yesterday published its first detailed investigation into events that led to last autumn's devastating floods. Chris Baker reports on its findings.

Most of Britain was lashed by gales and rain as a series of low pressure areas swept in from the Atlantic in the days leading up to October 12.

As the fronts piled up, they formed a single massive area of intense low pressure, sucking moisture from the Bay of Biscay which was driven upwards as it crossed the Sussex coast to clash with extremely cold Arctic air.

By the time it tracked across the catchment of the river Ouse on the night of October 11/12 it was the perfect storm.

A series of severe thunderstorms dropped 88mm (3.46ins) of rain on the Ouse and its tributaries that night, 95 per cent of what would normally be expected to fall during October.

Localised rainfall of 134mm (5.27ins) was recorded in parts of the river's catchment area, mostly in the first eight hours of October 12.

Not only was the night's rainfall unprecedented but the ground was already completely saturated. Rainwater simply had nowhere to go but into streams and rivers, causing water levels to rise rapidly.

Towns, villages and outlying buildings in the Ouse valley are particularly vulnerable to flooding in the conditions that existed that morning, according to consultants who produced yesterday's Environment Agency report.

It said: "We believe that the 12th October 2000 flood in the river Ouse catchment was the result of an extreme natural event, which was neither caused by, nor could have been prevented by any individual, organisation or action."

The conditions on October 12 were much the same in 1960, the only other floods of similar severity to hit Lewes and Uckfield in living memory.

Last autumn, however, peak water levels in both Lewes and Uckfield were about one metre higher than 40 years ago, making the floods probably the most severe since 1801.

There is a long history of flooding in the Ouse catchment, dating back to at least 1671. More than 70 major floods have been recorded, mostly in the last 150 years; 31 of them serious or extreme floods causing damage to property.

Flooding of some sort occurs on average once every other year. Rarely though do they approach anything like the severity of last autumn.

The first property to be flooded in Uckfield was Bridge Cottage, at 3am. In the next two hours water levels rose dramatically, inundating the Somerfield car park and Mill Lane areas causing the river Uck, bypass channel and floodplain to merge into one.

By 6am the council had abandoned its efforts to protect shops with sandbags and as levels rose the floodwater began to flow through High Street shops with increasing force.

Cars were swept 50 metres and large quanitities of stock were washed downstream, some of it reported as far away as Isfield and Barcombe.

The flooding peaked between 9am and 10am, when water levels reached nearly two metres in places. At 1pm water levels began falling quickly as if "a plug had been pulled" and by 5pm the town was largely clear of water.

Around the same time the plug was pulled in Uckfield people in Lewes reported seeing a wave of water coming down the river Ouse.

The wave prompted bitter criticism of the Environment Agency, which was accused by some of opening flood gates and sluices upstream, causing the flood which devastated the historic town centre.

The report said there was no evidence to support the claims and the phenomenon was due to a combination of intense rain, rapid run-off and the natural characteristics of the Ouse catchment.

Nor did the midday high tide in Lewes have a major part to play in the catastrophe. Infact, the volume of water flowing downstream completely drowned four full tidal cycles.

Between the midday high tide and 1.30pm flood defences in central Lewes were breached in a number of places in quick succession.

Thirty minutes later floodwater spilled out of Malling playing fields flowing rapidly down Spences Lane and into the Malling Brooks area. At about the same time river levels overtopped flood defence walls throughout the centre of Lewes.

As flows continued to increase flood water weired over river walls, filling the urban flood plain to a depth of one metre in about half-an-hour. While water levels of a metre were common, the low-lying Malling area experienced depths of up to 3.6 metres.

By early evening Lewes was effectively cut in two. The flooding did not peak until about 9.30pm and river levels did not start falling noticably until the early hours of the following morning.

Most of the flood water remained trapped by the flood walls, however, so water levels remained high. It was two days before the bulk of the flood water was removed.

In all 2,000 hectares and 1,033 properties were flooded and 682 vehicles damaged or written off across the catchment, the report estimates.

In Lewes, 27 per cent of properties damaged were businesses or public buildings, the rest were homes. Two-hundred-and-thirty were listed or traditional buildings, needing specialist repair.

Homes were less badly hit in Uckfield, where 86 per cent of flooding was to businesses or public buildings.

The report said the flooding caused no significant environmental or public health impacts and generated no significant related crime.

Successive developments on the floodplain come in for particular criticism in the report.

Widespread building on the floodplain did not begin until the 19th century, when the Cliffe, Malling and Phoenix areas of Lewes were developed to service the booming port trade.

It was notable that it was not until this time there started to be regular report of flooding in the Ouse valley, the report said.

The next significant stage of development was in the late Fifties and early Sixties when commercial and residential development went ahead in both towns.

The number of properties in Lewes prone to flooding is 858, compared to 600 in 1960. In Uckfield the respective figures are 133 today compared to about 40 in 1960.

Building meant the floods were worse than they might otherwise have been, increasing the rate of surface run-off, reducing the area available to hold excess water and contributing to the speed in which the two towns were inundated.

The report said "Floodplain developments over the last 40 years have had a particularly significant effect on the impact of those extreme flood events which do exceed the engineered capacity of the river channel."