Dudley,

The physics sounds logical to me but maybe I am missing something ? I have spent many years on Oil Tankers dealing with fluid dynamics.

The downriser will be coupled to the existing horizontal culvert and be about 8' higher than what I have now to handle the water.

The overflowing water will be dropping 8' and charging the horizontal tube for the water velocity to be greater than the static head velocity it now handles.

I felt that a 90 degree elbow at the interface between the downriser and bottom horizontal discharge tube would add to a turbulent restriction where the flow changes direction so I am going to use a double mitered elbow made out of two 45s instead. This should help the flow to be smoother where it goes around a bend.

Granted the upwelling water coming up the 3' diameter upriser and feeding the 2' diameter downriser wont have the same velocity as the falling water. The 3' upriser outlet will have an open top about 12 inches higher than the downriser inlet so any excess water not upwelling fast enough can crest over the top of the upriser and access the downriser.

Worst case scenarion is that overlow water will be able to fully charge the downriser.. I hope.

As for Joe G's wager, if he goes on the internet he can access the 'Save the Children - Tsunami' web site. I forgot to add the cost of the 3' upriser so that means his check should now be written for 10 times the amount of a 20' section of 3' diameter double wall HDPE tube along with a 20' section of 2' double wall HDPE tube. Ha Ha Ha. Joe's profile says he is an Engineer so maybe he is up on his physics more than I am.