My belief is there are NO specific requirements. You are going to have to work out the RA from first principles. Having been Mr safety on a flammables site with a river running through the middle, fire exits were top priority and prevention form flooding was a priority. So you have to balance them and be workable.
I also see many flood defences from my narrow boat on canals and rivers.
In non flood times the barriers (usually gates or slide in boards) must be removed out of the way and allow free egress.
When there is a flood risk then revised fire escape routes fully briefed to all staff must be in place, along with the barriers.
Here I must stop as your circumstances will dictate the result. are you on a hill - one side dry other side flooded, can you be surrounded like a moat. Can you step over the barriers or are they 4ft high? At what stage do you actually close the site/premises and send everone home?
regards Peter