How to Install a Weir
Weirs are sensitive to installation conditions. Poor installation is many times not correctable, so it is important that the following be used when installing a weir plate.
Installing the Weir
- The crest (flow edge) of the weir must be set 2-3 times Hmax above the floor of the channel.
- The crest of the weir must be set at least 2-inches [5.08 cm] above the maximum downstream water level.
- The widest portion of the weir crest at Hmax should be at least 2 times Hmax from the sidewall of the channel (on each side).
- The weir must be centered in the channel.
- The weir must be level from front-to-back and from side-to-side.
- The crest must be horizontally level.
- If present, the bevel of the weir crest must be set downstream.
In earthen channel applications, the channel downstream of the weir should be reinforced / armored so that scour does not occur. Scouring can lead to the weir plate shifting out of position or flow bypassing the weir plate.
Water Measurement with Flumes and Weirs (ISBN 90-70754-55-X) provides details of channel reinforcement, however as a rule of thumb protection should extend at least 4 Hmax downstream (and not less than 5-feet [1.5 m].