Sounds a little like a 'better mouse trap' idear.
It really depends on the type of motion you're trying to detect, as well as what kind of area you need to cover, and then what you intend to do with said input.
A couple of thin metal plates with a shim between one end will work if something steps on it, but it's not going to do anything for water flowing over it.
An IR Emitter and Receiver 'side by side' isn't going to work that great, as it implies they're both facing the same direction. If you're talking about the Emitter on one side or edge of the pipe and the Receiver on the other, then it would work fine.
You may need too debounce or delay that setup also, since it will be fast enough to pick up more than a few pulses in a very short time-frame, so you could end up with far, far more readings than what actually happened, again depending on what motion you're trying to detect. For example, take your hand and pass it between the Emitter and Receiver breaking the beam really fast and you'll get 1 pulse, but if you had you fingers fanned out and passed them thru it, it would count 4 pulses as it goes from being blocked by a finger to open, then blocked again by the next finger then open again and so on. While you only passed you 'hand' thru there once, it's registered 4 pulses, but again, it all depends on what you're trying to detect there.
