A rising line on a monitor meant one thing: The deluge had begun

Shortly before 7am on 9 October, Steve Taylor sat glued to a computer screen, watching with growing concern as a line began to climb vertically on his monitor. Up it rose until, near the top of the screen, it triggered an alarm: soft at first, then louder until it sounded like a foghorn.

Shortly before 7am on 9 October, Steve Taylor sat glued to a computer screen, watching with growing concern as a line began to climb vertically on his monitor. Up it rose until, near the top of the screen, it triggered an alarm: soft at first, then louder until it sounded like a foghorn.

It was the moment when the Great Floods of 2000, as they will surely come to be known, began. Within hours the river Uck had spilt into Uckfield in East Sussex, while Lewes had been inundated by the Ouse.

For Mr Taylor, a 33-year-old hydrologist, and his colleagues on the Sussex flood defence team, this was the start of the longest "flood event" in at least a century. The team, responsible for issuing flood warnings that have doubtless saved lives, has been working round the clock - monitoring all the region's waterways, assessing weather patterns and alerting the emergency services to problems before they happen.

Since 7 October they have put out 25 severe flood warnings where lives and property were seen to be at risk. Normally, they issue about six warnings a year.

Living on takeaway junk food and taking cat naps when they can, members of the 19-strong team have been operating from an incident room at the Environment Agency offices in Worthing, complete with maps, charts and a bank of computers.

"It's been crazy here," said David Bonner, 33, the leader of a four-man flood-warning unit within the team. "On the worst day, people were working up to 23 hours, but we've been averaging 12 to 15 a day."

The unit's most vital work is done by the four-man warning team - Mr Taylor, Mr Bonner, Graham McLelland and Jonathan Hunter - whose task is to monitor water levels constantly and predict where and when floods will occur. The aim is to give at least two hours' notice to the population and the emergency services so that lives and property might be saved. During the latest crisis, they have more than achieved that target. "Two hours can make all the difference," said Mr Bonner. "It gives the emergency services time to get to the sick and elderly, and it lets the able-bodied move their possessions upstairs, their cars to higher ground and do whatever they can to protect their property."

At their disposal is a system introduced three years ago when the Environment Agency inherited responsibility for flood prevention from the National Rivers Authority. Known as a "regional telemetry system", it uses dozens of sensors at various points on the area's rivers, tributaries and streams, which send back data on water height and rate of flow.

This is collated by computer and presented in graph form; it shows the state of a stretch of waterway at any given moment alongside a recent history that shows whether it is on the way up or down. Each stretch has its own designated danger levels. Once these are reached, an alarm sounds - as Mr Taylor experienced on 9 October. "We hadn't experienced anything like that before - where the line just started to go up vertically," he said.

Surrounding the team are charts and boards with up-to-the-minute information on any stretch of waterway. Each area has a code number, and its status is constantly updated. F4A5 (Barcombe to Lewes on the Ouse) is marked with a red flood warning, as is F3C1 (Billings-hurst to Pulborough on the Arun). Despite the technology, it is these hand-written boards that give the incident room an atmosphere more akin to the Battle of Britain than to Star Wars. Once the alarm sounds, the team can alert the emergency services, media and general population by a simple but ingenious system known as AVM: automatic voice messaging.

After deciding on the level of warning - either a simple warning if property is at risk, or a more severe alert if lives are at stake - faxes or telephone messages are sent instantly to anyone registered on the team's database.

So far, 12,000 of the region's 50,000 households in flood-risk areas have signed up. Within minutes of a flood alarm, they are automatically called and given a recorded message with information and advice. The agency sends leaflets to homes each year and hopes that eventually everyone will register.

"We have to be very careful about assessing information and issuing warnings," said Mr Bonner. "If we don't issue them in time, people will accuse us of not doing our job. But if we jump the gun and nothing happens, we will be accused of crying wolf and people will ignore us."

In the meantime, while preparing for predicted flooding in Chichester tonight, the exhausted team keeps up its spirits with gallows humour and e-mails featuring sharks in deluged streets.