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If you did not make any changes in your land to alter the natural flow of water and there is a wall in place and he was the one who excavated, then his argument would be very weak in court. If there were no previous issues and you did not change your land to alter the flow of water, then the liability for what is happening to his land is his liability as the evidence would show his actions caused it.
You need to consider either negotiating with him to share the cost of the wall perhaps (only because that would be less than legal costs for the other recourse). If you cannot or do not want to negotiate this, then I am afraid you need a real estate
litigation attorney to go to court on this for his altering the natural flow of water on his land and get the court to order him to pay for the wall and you would have to get a surveyor to come in as an expert to testify as to the causes of his actions resulting in the damage to his land.
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