water problem

I own 20 acres and an office park was recently built next to my property. The builders raised their property about 4 feet, they built the required 8' fence at the original grade level. The buildings have a sloped roof ( high in front low to the back, toward my property the buildings have no gutter system) all the water came under the fence and flooded my property. I have live here for 11 years and have never had this type of water problem. The builders were told to put up a berm to retain and redirect water to the back of the property. This did not help, so I had a 12 in concrete retaining wall (curb) poured on my side with a channel between their fence and my small curb to direct the water to the back of the propery. It was dug out 6 inches before it was poured. I brought in sand and clay to build up my side of the area, this is a riding arena for my horses--never flooded before the offices were built. I did this at my own expense after the city told me that the builders had no further legal obligation since they had built the berm on their side. It stopped the water for a short time, but now I have an area where the water is coming underneath my wall in a torrent, flooding thru the property and running down into my pond. I have a stable in the area which has running water along the outside. The ground is soggy for days- I live in TX so there are few days of rain, but when it does there is a lot of runoff. I have lost most of the fill that I put into the arena. I have asked the builders to check their side, I feel that there is a low spot where the water is pooling and then getting under the walls. I was told by the firm putting in their sprinklers that there was a low spot. They have refused to help and they have told me that the water is my problem and that I am harrassing them. I don't know what to do next. This last rain left a 6 inch deep foot wide rut thru about 300 feet of my property, flooded my horse pens and washed sand into the pond. This happens to some degree everytime it rains, the degree depending on the amount of rain. This last rain was 2". Any advice from anyone? Thank you. Sorry to be so wordy!