John,

The transition zone for the slot at the bottom is parabolic and not trapezoidal but that area has mimimal impact if you are talking about a 600 foot tall slot. I estimate an increase of 28.29% in volume if you use a slot width of 0.005 times the pipe width of 13.5 feet. If you lower the slot width to 0.0001 times the pipe width then the increase in volume is only 0.57%. Of course you will have to use a much small time step to make your model stable.

A ten foot tall slot only increases the volume by 0.47% with a slot width of 0.005 times the pipe width and only 0.01% with a slot width of 0.0001 times the pipe width. I do not know if the slot ( a mathematical construct to connect the top of the pipe to the ground surface so that the St. Venant equation can be used for all flow situations) was intended to be so much taller than the pipe diameter.

Regards,
Bob Dickinson
DickinsonRE@CDM.COM