Site Location

Where to Install a Weir Channel

Proper site selection is vital to the success or failure of any weir channel installation. The need for a sizeable upstream weir pool and a drop after the weir can make it challenging to find just the right site. 

Upstream of the Weir Channel (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_a upstream of the flume. 

Weir Channel Location

• The crest (flow edge) of the weir in the Weir Channel 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.
• This distance should extend at least 20 times H_max (per Weir Pool Sizing above).
• The weir must be able to 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 Channel

• 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 so that flow.