The public are being warned to stay away from the coastline with high tides and strong winds posing a danger to life.
As of 7.30am the Environment Agency (EA) had issued 19 flood warnings and 90 flood alerts in the South East.
Shoreham Harbour is expected to be at particular risk with high tide due at 12.15pm.
An EA alert warned of tidal waters reaching the nearby Sussex Yacht Club, Riverside Business Centre, A259 Brighton Road, Ferry Road and land at Shoreham Airport.
The coastal areas around Littlehampton and Sidlesham are also expected to see severe flooding.
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An agency spokesman said the combination of high tides, strong winds and heavy rain would combine to produce a dangerous cocktail.
He added “Coastal paths and promenades could be highly dangerous as there is an increased risk of being swept out to sea. People are warned to stay away from the shoreline.
“We are monitoring the situation closely, working alongside partners including the Met Office and local authorities. We also have teams out on the ground making sure that flood defences are in good working order, monitoring sea levels and preparing to issue flood alerts and warnings.”
A number of areas are already underwater this morning. Heavy rain caused extensive flooding near Alfriston while fields were submerged outside Offham near Lewes.
In Brighton high winds closed St Luke’s Swimming Pool in St Luke’s Terrace.
A spokeswoman for Brighton and Hove City Council added that due to recent storms the beach had changed shape leaving some areas extremely steep and dangerous to walk close to the water's edge.
She urged people to stay well away and to be extra vigilant of young children, holding their hands, and also keeping dogs on leads.
The Big Beach Cafe at Hove Lagoon also said they had been told to close for the day.
Elsewhere the A29 has been closed this morning at Pulborough and Bognor due to flooding.
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