Site Selection

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 (H_max).
  • 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 H_max 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 H_max upstream of the flume.

Weir Location

• The crest (flow edge) of the weir must be able to be set 2-3 times H_max 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 H_max should be at least 2 times H_max from the sidewall of the channel (on each side).
• This distance should extend at least 20 times the maximum anticipated head (H_max) (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.