Design Exfiltration Trench

The Trench command lets you proportion an exfiltration trench to provide underground storage for flow peak attenuation and also to promote return of runoff to the groundwater. 

The trench usually consists of a trench of roughly trapezoidal cross-section filled with clear stone with a voids ratio of around 40% and with one or more perforated pipes to distribute the inflow along the length of the trench.

The exfiltration trench splits the inflow hydrograph into two components. One of these is the flow which infiltrates into the ground water; the balance of the inflow is transmitted as an outflow hydrograph. Obviously an exfiltration trench requires reasonable porosity of the soil and a water table below the trench invert.

The design involves several steps including definition of the trench and soil characteristics, definition of the number, size and type of pipes in the trench and description of the outflow control device comprising orifice and weir controls as used in the Pond command.

The outflow control devices are similar to those used in the detention Pond command. Water from the inflow hydrograph enters the stone fill through one or more perforated pipes running the length of the trench. The trench may also have a conventional, un-perforated storm sewer between the manholes to convey the Outflow. The positioning of the various pipes in the trench can be defined graphically using the Trench pipes window. The diameter and type (perforated or non-perforated) can be specified and the location set by dragging the pipe to the desired position or by editing the numerical data in a grid. During the drag and drop procedure the current pipe cover is shown to assist in ensuring adequate clearance.

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