Link directly to the PDF. I recommend taking a look at the Typical Cross-Section. This is one of the more interesting drainage details I"ve come across. I couldn’t figure out exactly what was going on until I read the article. But in case you don’t want to take the time, the larger drain is actually to handle a shallow groundwater table which, depending on the season, can apparently rise to within a few feet of the ground surface. There is a geomembrane or GCL separating that drainage layer with the one that handles the chemical laden runoff and is therefore collected and transported to various tanks for disposal and/or recycling. Pretty slick.
