All the above comments are correct - your problem is caused by charge injection because you used a power FET with a huge gate capacitance. This is a tough problem to solve well with a pcb design because very few manufacturers of analog switches focus on gate charge - what sells today is low Rds...
CVG: Thanks for the tip. The four foot number was close to my guess of 3 feet. If I go another foot down, I will spend about $300 more in concrete, but that is cheap insurance. BTW, I am getting permits, but the only ones needed for this project are a county grading permit, and a hydraulic...
I am placing a 9 ft wide, 22.5 ft long bottomless arch box culvert in a creek. There is no firm data for 100 year peak volume. The dimensions are set by fish passage requirements, rather than hydraulic. The capacity is more than 3X compared to upstream culverts on the same creek.
The design...
