[fred cook]

When you reduce the pipe diameter by half, any period of high water flow will cause a hydraulic high pressure point at the reduction point. If you expect the pipe to flow full at any time, you will need to do something to keep the water from washing out the joint. You could encase the entire joint in concrete for example. Also, make certain that the adapter fits over the 24" pipe and inside the 12" pipe to help keep the water inside the pipe as it goes downhill. Water under pressure has a way of finding weak spots! Good luck!