Complex sentence: It was not settled until 1970, until then only seaonally.Because of riverbank erosion in nearby areas, especially Hatiya, Shahbajpur and Ramgati people migrated to the island as new settlers.
Split into simpler sentences: It was not settled until 1970, until then only seaonally. Then because of riverbank erosion in nearby areas, especially Hatiya, Shahbajpur and Ramgati people migrated to the island as new settlers.