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

Size and Type of Outlet

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 catch basin - usually same width as trench, but deeper. Allows use of trash bucket to collect debris. Offers superior outlet capacity as larger pipes can be connected and increased depth gives significant head of water pressure.

Catch basin - large basin wider and deeper than trench. Allows use of trash bucket to collect debris. Offers best outlet capacity as largest pipes can be used and increased depth gives significant head of water pressure.

Position and size of outlet pipe

A trench drain is ultimately connected to the underground pipe system. If the trench drain is designed with an end outlet, the water builds up along the trench and may flood before reaching the outlet. Alternatively a larger and more costly trench drain and/or more outlets are required to prevent flooding. If there is zero ground slope and underground pipe work position is not a factor, the trench drain can be designed with a central outlet. Run lengths to the outlet are shorter and less likely to exceed capacity and flood. This allows a smaller, more economic trench drain and/or fewer outlets to drain the same volume.