For gravity flow distribution pipes, how should they be installed in absorption trenches?

• A. At a constant level with the trench bottom
• B. At an upward slope of 50 mm every 10 m
• C. At a downward slope of 50 mm every 10 m
• D. Downward slope of 5 mm per 10 m

Answer: (C)

Gravity-driven flow needs a gentle downward grade along the distribution pipe so the effluent moves by gravity from the inlet toward the end of the trench and can infiltrate evenly into the soil. A downward drop of 50 mm over 10 m provides a small but reliable gradient (about 0.5%) that keeps the liquid moving without causing rapid, erosive flow. This promotes uniform distribution through the perforated pipe and throughout the trench.

Installing at a constant level would create stagnation with no drive for flow, leading to pooling. An upward slope would push liquid uphill, which isn’t possible by gravity. A very small slope, like 5 mm per 10 m, may be too shallow to overcome friction and infiltration losses, risking poor distribution and standing water near the inlet.