You have a couple of issues here. First, the government has a certain amount of immunity called "sovereign immunity" for acts that the commit or omissions they make. YOu have to hire someone pretty quickly and have them send a notice letter pursuant to the statute or you lose your right to pursue them at all.
MOst people are automatically liable if they redirect the flow of water causing someone's land to flood. This is a lawsuit that would be brought under a form of trespass known as "nuisance". However, as I said, your rights to sue the city government are limited and your lawyer will first want to send the notice letter then they will want to do an evaluation to see if the city ignored federal or state standards, and there are a lot of potential rules and regulations dealing with water drainage, as well as looking to see whether they considered and allowed for the possibility of flooding. It is possible that the city just filled it in and never considered the fact that it might flood your property and didn't have any studies done. This could be a violation of your civil rights and considered an unlawful "taking" of your property.
You can find a lawyer to assist you by going to www.lawyers.com and in the section for Area of Practice enter Civil Litigation, Civil Rights Law, or Consumer Law. Any of those will have the skill set you need.
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