[fluent_newbie]

I am trying to represent a laboratory flume in FLUENT with a velocity inlet on one end, pressure outlet with gauge pressure of zero on the other end, and a pressure outlet with gauge pressure of zero leaving the bottom of the domain (near one of the vertical walls, halfway along the length of the domain in the direction of fluid flow, and only covering about half of the bottom width). The top of the domain is a pressure inlet with zero total pressure. So all of the fluid flow (water) in the horizontal direction should be driven by the velocity inlet. Gravity is turned on.

The goal is to measure how much water gets captured by a storm sewer grate (the outlet in the bottom of the domain) and how much bypasses the grate and leaves through the other outlet.

I am having problems with getting water to leave the domain through the bottom outlet. In simple terms, the pressure gradient is overpowering what should be gravity pulling the water down through the grate and out of the bottom of the domain. What am I specifying wrong with the bottom pressure outlet?