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
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
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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