On grade conditions occur when runoff is diverted away from the drain inlet.

On grade conditions occur when runoff is diverted away from the drain inlet.

• A. Runoff is directed toward the drain inlet
• B. Runoff evaporates before reaching the inlet
• C. Runoff is diverted away from the drain inlet
• D. Runoff leaks into the ground nearby

Answer: (C)

On-grade conditions describe water that moves along the ground surface rather than entering a drainage structure. When runoff is diverted away from the drain inlet, it cannot enter the inlet and instead continues to flow across the surface of the site. This overland flow staying on the ground is exactly what defines on-grade conditions. If water were directed toward the inlet, it would be carried into the drainage system rather than staying on the surface; evaporation before reaching the inlet or infiltration into the soil would remove water from surface flow altogether, which also does not describe surface overland flow across the site.

On-grade conditions describe water that moves along the ground surface rather than entering a drainage structure. When runoff is diverted away from the drain inlet, it cannot enter the inlet and instead continues to flow across the surface of the site. This overland flow staying on the ground is exactly what defines on-grade conditions. If water were directed toward the inlet, it would be carried into the drainage system rather than staying on the surface; evaporation before reaching the inlet or infiltration into the soil would remove water from surface flow altogether, which also does not describe surface overland flow across the site.