Probe launched after video emerges of bus driving through 3ft of flood water in East Sussex
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It was captured driving through the water and the video was posted on YouTube on the urban rot channel.
The bus is then seen stopping at a bus stop with passengers disembarking, while water gushes out from the main doors as they open.
A spokesperson for Stagecoach said: "We are aware of this incident, and are taking it extremely seriously.
“The safety of our passengers and colleagues is our highest priority. We have very clear protocols in place regarding floodwater situations and we are actively investigating the incident at Bexhill on Saturday.”
Later several cars are seen attempting to pass through the water under the bridge, some succeeding, while others turning back.
At one point the fire service was called out to assist the occupants in a car that got stuck under the bridge.
Many places in the county saw torrential rainfall on Saturday, just days after Storm Ciarán brought power cuts and travel chaos to Sussex.
Heavy rain also caused severe flooding near Cooden Beach station, just outside Bexhill on Saturday, meaning no trains could stop there.
Network Rail said it was unsafe for trains to stop at Cooden Beach and passengers were advised to alight at Collington or Normans Bay.
The line was affected by flooding for most of the day.
East Sussex Fire and Rescue Service (ESFRS) said it was called to several incidents on Saturday where cars had become trapped in flood water.
A spokesperson said firefighters were called at 12.15pm to Richmond Avenue, Bexhill, to reports of three people in a car under a bridge. All persons were able to get out of the car safely, the spokesperson added.
Firefighters were also called to Westcourt Drive, Bexhill just after 2.55pm on Saturday to reports of two cars in water under a bridge. Crews confirmed no one was inside, the spokesperson said.