DEQ Stormwater Management Inspector Practice Test 2026 – All-in-One Guide to Master Your Certification!

The key idea is to drain water out of the porous pavement effectively. In level one porous concrete, the water that infiltrates the pavement needs a path to escape without blocking the pore space. Placing the underdrain beneath the surface of the concrete does exactly that: it sits below the porous layer where it can collect the infiltrated water as it drains downward and carry it away through the drainage system.

Putting an underdrain on top would clog or block the pores and prevent infiltration. Placing it within the growing media would mix drainage paths with soil and reduce efficiency. An underdrain next to the foundation doesn’t capture the water that comes through the pavement. So, installing the underdrain under the surface of the concrete provides the proper drainage path while preserving the porosity and performance of the pavement.