Starting next month, ships over a certain size or weight or surpassing specific pollution levels can no longer ply the Giudecca Canal, a major sea artery in Venice, nor can they cruise the waters near St. Mark’s Square, the historic heart of the environmentally fragile city. Until a permanent docking place can be chosen and developed for the big cruise ships, the liners will be allowed, for now, to pull up in Marghera, an industrial suburb of Venice. The United Nations culture agency, which was considering declaring Venice an imperilled heritage site, on Wednesday hailed Italy’s ban on big cruise ships in the heart of the lagoon city. UNESCO’s director-general Audrey Azoulay wrote on Twitter that the Italian government’s decision a day earlier to ban access by the mammoth liners as of Aug. 1 is “very good news and an important step that significantly contributes to the safeguarding of this …