Thank you for using our forum. My name is ***** ***** I hope to assist you today.
I am sorry to learn of this situation.
While modifying your property in such a way that it changes the natural flow of water onto your neighbor's property is actionable (it is treated as both a trespass and a damage to real property), the fact that this improvement to your property has been in place for 10 years creates a significant statute of limitations (and related "doctrine of latches") defense that you can use to your advantage.
The trick to making the maximum advantage of either of these is to utilize them in the opening pleadings of your court case - a local attorney can help you in deciding the proper defensive pleading (an answer, a motion to strike, or a demurrer) to properly open your defense against this complaint.
In some cases, you can prevail against the neighbor in the pleading phase and you will not have to go through the complexities of civil litigation.
You can do this yourself - but you must work very quickly to take advantage of this, and it takes a fair amount of research (I cannot tell you based on a quick "Q&A" which is going to be best to address the court filing that your neighbor has made) but your local law library will have resources, including a set of books called "Practice Guides" which can help you sort out which is most appropriate
You can find local attorneys using the State and local Bar Association directories, or private directories such as www.AVVO.com; www.FindLaw.com; or www.Martindale.com (I personally find www.AVVO.com to be the most user friendly).