Where to Install a Weir
Proper site selection is important to the success or failure of any weir plate installation. The need for large upstream weir pool and a drop after the weir can make it challenging to find just the right site. Proper weir installation isn’t as simply as throwing a weir plate across a channel.
Upstream of the Weir (Weir Pool Sizing)
- Without flow condition, the channel upstream of the weir should be straight and of a uniform cross-section for at least 20 times the maximum anticipated head (Hmax).
For applications where an underflow baffle is placed upstream of the weir, the approach conditions can be eliminated.
Here the underflow baffle is usually place 1-2 Hmax upstream of the point of measurement.
- The flow approaching the weir should be smooth, tranquil, and well distributed across the channel.
- The upstream channel should be clear of vegetative growth.
- For earthen channel installations, the sidewalls and weir pool floor should be consolidated so that flow does not bypass the weir.
- Flow approaching the weir must be sub-critical and should not exceed 0.5 ft/s [0.15 m/s].
If the flow is super-critical approaching the flume a hydraulic jump must be formed well upstream of the flume or upstream energy absorbers and tranquilizing racks must be used).
Should a hydraulic jump need to be formed to slow the flow, it should be forced to occur at least 30 Hmax upstream of the flume.
The crest (flow edge) of the weir must be able to be set 2-3 times Hmax above the floor of the channel.
The crest of the weir must be able to 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).
This distance should extend at least 20 times the maximum anticipated head (Hmax) (per Weir Pool Sizing above).
The weir must be set level from side-to-side and from front-to-back.
Weirs must be able to be set horizontally / vertically level and centered in the channel.
Downstream of the Weir
- The downstream channel should be armored (riprap) or otherwise protected so that scour does not occur.
- The downstream channel must be clear of vegetative growth or the collection of debris.