Depends what you mean by last resort. Willow is used down on the Somerset Levels to help hold the ‘drains’ together, which down there are actually a properly engineered network of ditches and mini-canals. More local to me they are used on floodplains. For example, one local school is built on a floodplain and the architects and landscapers have used Salix to keep the playground from turning into a bog.