flow from high pressure to low pressure. So if the change in pressure is negative (in the -direction) then the flow will be positive (in the -direction). The above equation works well for a horizontal tube, but if the tube was inclined so that point b was a different elevation than point a, the equation would not work. The effect of elevation is accounted for by replacing the pore pressure by excess pore pressure, defined as:

where is the depth measured from an arbitrary elevation reference (datum).