Trench Drain Outlets
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Pipe Sizing

Correct sizing of outlet pipes is critical to effect surface water drainage. Reputable pipe companies offer services and data to allow correct sizing of pipes based on Manning's Pipe Theory. Outlet from trench drains (catchment water) must be less than the pipe capacity to avoid backing up of water and flooding.

If the grated trench drain is part of catchment system that requires the grated trench drain to hold water for a period of time - such as sustainable drainage system - this pipe can be used as a constriction.

Size and Type of Outlet

In modeling hydraulic performance of grated trench drains, the assumption often made is that the outlet is not a restricting factor. Designers should ensure the outlet or subsequent pipe work is not undersized and acts as a constriction to the grated trench drain.

End outlet - pipe connected horizontally at the end of the trench. Offers lowest outlet capacity.

Bottom outlet - pipe connected vertically out of the bottom of the trench. Offers improved outlet capacity due to gravity.

In-line pit - usually same width as trench, but deeper. Allows use of rubbish basket to collect debris. Offers superior outlet capacity as larger pipes can be connected and increased depth gives significant head of water pressure above pipe outlet

Universal Pit - large pit wider and deeper than trench. Allows use of rubbish basket to collect debris. Offers best outlet capacity as very large pipes can be used and increased depth gives significant head of water pressure above pipe outlet.

Position and size of outlet pipe

A grated trench drain is ultimately connected to the underground pipe system. If designed with an end outlet, the trench and may reach capacity sooner unless a larger and more costly grated trench drain and/or more outlets are used.
If there is zero ground slope along the run and underground pipe work position is fixed, the trench drain can be designed with a central outlet. Run lengths to the outlet are therefore shorter and less likely to exceed capacity and flood. This allows a smaller, more economic grated trench drain and/or fewer outlets to be positioned along the run to drain the same volume.

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