The Upazilas are Sandwip Upazila, Banshkhali Upazila, Anwara Upazila and Sitakunda.
Local people and officials said several unions of the four Upazilas had been flooded.
Officials said inhabitants of the most of the coastal area had been evacuated.
A trader in Sandwip, Sajal Saha, told bdnews24.com, “Sawdargor Haat area has been flooded after an embankment was broken. Several houses were also damaged. All shops and other business are shut.”
Sandwip Upazila Executive Officer (UNO) Saiful Islam said the administration was mending the embankments with sand sacks.
“Inhabitants of all coastal areas have been shifted to cyclone shelters,” he said.
Roanu was crossing the Chittagong coast with a wind speed of 62 km per hour, according to the Met office.