Free & Porous Fluid Flow -- Measure of mass flux

Hi All,

I am attempting to model a stream bottom seepage collection device, but I getting some strange results.

I am starting out fairly simple with no lateral flow in the free fluid, and setting the BC's so that I have a vertical flow field. But my results show the velocity contours have local maxima along the stream bottom under the device. I was expecting the maxima to be near the outlet of the device as it is a considerable constriction.

Also, is there a way to measure the mass flux of fluid through a plane? I am trying to evaluate how much water is being captured by the device and how much fluid is going around.

Thanks,
John