Gravity Drain Capacity

Gravity Drain Systems

• Subsurface drainage systems use gravity to discharge storm water runoff to an acceptable outlet. Pipes are buried underground for subsurface drainage. Gravity drains are a common feature in residential construction, often used for foundation drainage and French drains and where land is too flat or too wet. The capacity of the gravity system contains normal runoff that's conveyed through the system until it's discharged.

Gravity Drain Installation

• Underground pipes are laid in a trench, sloped slightly to direct drainage to the discharge point. The trench is usually lined with well-drained gravel and compacted. When the pipe is perforated, it's laid on geotextile cloth, or the cloth may be wrapped around the pipe to keep sediment from entering the openings and clogging the pipe. The pipe discharges to a swale or ditch that'connected to a storm sewer system or disperses over an open area.

Pipe Capacity

• Flow is affected by the smoothness of the pipe wall, while the velocity of flow in the pipe helps keep it clear of sediment and debris. Flexible corrugated pipe is difficult to clear of obstructions, and the material has more friction loss than smooth wall pipes, reducing flow and allowing sediment to accumulate. Perforated pipe lets water enter and exit, releasing the water slowly back into the ground, suitable for a site where runoff needs to be managed.

Gravity Drain Capacity

• Pipes are rated for the volume of internal flow as an N-value, which is used in the Manning formula for calculating the hydraulic capacity of drainpipes for gravity and pressure flows. The lower the N-value, the greater the volume of flow. The capacity of the pipe is only one part of the overall performance of the gravity drain system. Site design elements can improve the capacity by directing the discharge or overflow to open areas, vegetated buffers or filter strips.