Calculate flow rate of wetland for gray water treatment
What you're really looking at is a fairly basic groundwater flow problem. What you need are hydraulic conductivity values for the "gravel" sediment in question. Given the fixed 1% grade, what you want to find is the "tank" length that maximizes pollutant residence time, and is relatively easy to calculate. 

I believe that this is the basic design around which your illustration comes from:

Horizontal Subsurface Flow CW

I don't know how you're going to solve the problem of finding and keeping enough water in your system during the dry season. You're system depends on growing plants!