I live in an Arizona flood plain area where rainwater and sometimes storm ditches overflow.
Water historicly drains through backyards to a nearby creek. My new downstream neighbor
Installed an earth levee on his side of the fence .blocking the flow and flooding my back yard.There is now several inches if water over my leach field. He won't give and I could clhanel the water to my behind neighbors yard but that would create another problem:)

call the zoning department, he can not build it without doing studies of the causes, Also if there is damges, you can sue him for the value of these damages in civil court.