After a violent storm in late December undermined four beachfront homes and imperiled several others on the Newbury portion of Plum Island, residents and officials are determining how to more permanently protect the barrier island's real estate. One short-term measure has been the installation of giant sandbags in front of Annapolis Way, where sand was washed away from beneath four homes — a costly endeavor that will be paid for by the owners, with support from fund-raising efforts. Previously, in October, some residents of Annapolis Way were allowed to perform beach scraping through a long-sought permit from the Army Corps of Engineers. Estimated to cost about $9,000, and paid for by homeowners, the process involved mechanically shoring up sand in front of the houses, according to Bob Connors, a resident advocate. During a recent meeting of the Merrimack River Beach Alliance, residents and local, state, and federal officials and representatives of the corps discussed the option of installing more permanent structures such as large rock piles or walls. The corps also recently began to fix the south jetty at the mouth of the Merrimack River. Residents and officials have long complained that the lack of repair and maintenance of the jetties has disrupted the natural flow of sand on the barrier island.