The Importance of Selecting the Correctly Sized Drainage System

By Todd Amerson, National Trench Drain Product Manager

Drainage professionals are witnessing a new and concerning trend in the market place. Many projects are being specified with trench drains that are oversized for their typical drainage needs. This is particularly prevalent in areas that have suffered severe storms, such as Houston in the post-hurricane Harvey era. It is understandable why people react this way; they believe that by installing a larger drain, in preparation for the next “400 year storm event,” they will be able to manage more flow and prevent damage and flooding.

Drainage channels are calculated and designed for anticipated volume to generate sufficient water height, which, in turn enables proper velocity of flow to carry the stormwater and wastewater through the channel. This flow activates the “self-scouring” properties of the channel, which assists in the maintenance in that debris such as silt and soil are carried out with the water. Without sufficient water height, the captured water run-off fails to flow with enough velocity, resulting in a buildup of debris.  With an oversized channel, this buildup of debris prevents the system from functioning as required to drain the area effectively.

The other problem with oversizing the channels is cost. Larger channels require more material, more freight, and more labor. And rather than improving the performance, they do nothing but increase the scope of work cost, negatively impact performance, and require the owner to conduct maintenance more often than what would be needed with a properly sized trench drain.

To ensure the selection of an appropriate drainage system, a comprehensive hydraulic analysis by a competent drainage expert is essential. This will ensure a sound drainage plan, which requires a correctly sized system with the required capacity, options for layout, strategic grading, and optimal outlet locations.

To reach a SIGMA drainage expert to request a hydraulic analysis or a drainage plan, please contact