Shared property line within storm drain

My neighber and I share a property line down the center of a storm drain he recently opened a home care facility and wanted to use the storm drain as an easment for entry to the side area of his property for an employee parking lot. I told him no because the area floods and was not designed for a large number of vehicles to use daily and I did not want my side to become a public easment. So he bacame angry and has told me he is going to install a fence down the center of the wash. What are my rights I feel this fence will interfere with the flow of water and besides I have been using this wash for convenient access to the rear of my property for care and maintenance for years.

If you both need and rely on the common storm drain, a court may very well give an order preventing him from changing it unless he provides adequate methods to address the drainage etc. He would very likely lose in court.

Thank you for the reply could you help me and explain how I would go about getting a "court order" should I wait until he starts building this fence?