A six-story , 146,000-square-foot mixed-use development proposed for 90 Washington St. is expected to be considered by the Zoning Board of Appeals this week. The development would include more than 125 apartment units and 13,000 square feet of commercial space divided into five storefronts, according to site plans. It would be located where Cobble Hill Plaza now exists. Developer Corcoran Jennison submitted the plans to the city’s Planning Department on May 14. The zoning board is to meet Wednesday at 6 p.m. at City Hall.