Subdivision draining stormwater onto private property?

I am only licensed to practice law in the state of Ohio so my response is qualified by that limitation.  However, I have experience with a very similar circumstance in Ohio that might be helpful. You may have 3 separate causes of action regarding this matter: i) nuisance; ii) negligence; and/or iii) trespass. Nuisance requires a showing that the defendant's conduct created a non-trespassory interference with an individual plaintiff’s use or enjoyment of his property.  To prove nuisance, you will need to show that the additional volume and erosive effect of storm water created by the newly developed subdivision unreasonably interferes with the enjoyment of your property. Negligence requires a showing that: i) defendant owed a duty to plaintiff; ii) defendant breached that duty; and iii) defendant's breach caused plaintiff to incur damages.  In cases involving the destructive effects of storm water, it is important to determine whether the local government with appropriate jurisdiction has any storm water detention standards for new development.  In Ohio, the general engineering standard is that the pre-construction and post-construction storm water flow should be similar.  If the developer failed to follow the adopted storm water design requirements, that can constitute a breach of duty supporting damages. Trespass requires a showing that defendant unlawfully and intentionally entered onto another's land.  Trespass does not require the person to physically enter onto another person's land but instead simply requires that the person cause a destructive force to enter onto another's land.   In your circumstance, the increased volume and erosive effect of storm water created by the new development may be considered a trespass. The other issue you need to consider is the applicable statute of limitation.  In Ohio, the statute of limitations is 4 years for each of the above listed potential causes of action.  However, the 4 year statute of limitations may be tolled (extended") when the action causing the damage is "continuing" as opposed to "a one time event."  In your case, the 4 year statute of limitation should not be a problem, unless the entire subdivision was completed more than 4 years ago. If the City continues to threaten you with legal action based on your refusal to remove the culvert, you might consider filing an declaratory judgment action or an action seeking injunctive relief against the City. In any event, you will need to contact an attorney licensed to practice law in Wisconsin to represent you in any of those actions.  Good luck.