Mayor Michael R. Bloomberg outlined a far-reaching plan on Tuesday to protect New York City from the threat of rising sea levels and powerful storm surges by building an extensive network of flood walls, levees and bulkheads along its 520 miles of coast. The mayor said the plan would initially cost about $20 billion, and eventually far more. The city would spend the money on fortifying infrastructure like the power grid, renovating buildings to withstand hurricanes and defending the shore, according to a 438-page report on the proposals. The proposals, in all, would change the look and fabric of the city, though not until well after the mayor leaves office at the end of the year. Still, he emphasized that Hurricane Sandy was such a devastating event that the city had to move immediately. “This plan is incredibly ambitious — and much of the work will extend far beyond the next 203 days — but we refused to pass the responsibility for creating a plan onto the next administration,” he said in a speech at the Brooklyn Navy Yard. “This is urgent work, and it must begin now.” The report details 250 recommendations, including the installation of flood walls and other measures to protect some of the areas that were hit worst by the hurricane in October. The plan covers so many parts of the city and proposes such an array of projects that the cost could change — and given the history of such large projects, it is likely to grow substantially.