Can we borrow Formula One's ground effect for airplanes?
The reduced pressure on the car not only exerts a downward force on the car, but an equal upward force on the ground. Mythbusters did an episode where they drove an Indy car over a manhole to show that it does lift the manhole cover slightly.

The force on the ground is irrelevant because it doesn't move and is not attached to the car. But in your drawing the same thing is happening. The reduced pressure exerts an upward force on the bottom of the channel and an equal downward force on thing top of it. They cancel each other.