Check fluid dynamics simulation for buggy behavior [merge #225293]

On waterfalls map, created 3 deep impoundment with levees creating a 90 degree turn in the flow, and a 5 wide release structure of 3 height floodgates. Downstream about 50 blocks built another impoundment with a 5 wide 1 height floodgates. With the downstream floodgates set to a height of 0.5 complete river flow is able to exit without flooding, ie channel are area of 2.5 units^2 with max depth 0.5 handles all of flow. With the same flow, the upstream gate set at a height of 1, which gives a channel area of 10 units^2 with a max depth of 2, consistently floods. I know a 90 degree turn slows velocity and backs up water flow, but this seems excessive. I also note that setting gate heights of 0.5 to 2.5 produces little change in water flow in this case, but setting even a single gate to 0 height produces a sudden massive flood and rapid drainage of the reservoir. The difference in behavior between a gate setting of 0,5 and 0 also leads me to think this may be a bug.