What T time should be used to design the trenches for a fully raised leaching bed?

• A. The percolation time of the raised leaching bed fill
• B. The percolation time of the native soil
• C. The rainfall intensity
• D. The groundwater seepage rate

Answer: (A)

The design of trenches in a fully raised leaching bed relies on how fast effluent can move through the bed media itself. In a raised bed, the water first percolates through the raised bed fill material before reaching the underlying gravel and native soil, so the governing flow rate is the percolation time of that bed fill. Using this T time ensures the trenches are sized for the actual infiltration capacity of the engineered media, preventing groundwater from rising to the surface or the bed from becoming oversaturated.

Using the percolation time of native soil would ignore the engineered medium that dictates flow in a raised system, leading to an incorrect design. Rainfall intensity affects surface drainage and stormwater considerations, not the subsurface infiltration rate of the bed media. Groundwater seepage rate concerns leakage toward the water table, which is not what determines trench infiltration capacity in a raised bed.