Ten years from the date of its acquisition, property acquired by a port district pursuant to this chapter shall revert to the county to be used the same as property acquired by tax foreclosure, and upon demand by the county commissioners the port commission shall convey the property to the county, unless before the expiration of the ten year period, the port district has adopted a comprehensive plan of harbor improvement which provides for the improvement of an industrial development district which includes such lands or the district has freed the land from the trust imposed upon it as provided in this chapter.