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How to Maintain a Weir Installation

Although simple devices, weirs do require periodic maintenance to provide accurate flow measurement.

Upstream CHANNEL 

  • The approach channel should be straight and of a uniform cross-section for at least 15-20 times the maximum anticipated head (Hmax) upstream of the weir.

  • The approaching flow must be sub-critical (Fr<1), tranquil, and well distributed across the channel.

  • The flow should not be turbulent, surging, or unbalanced.

  • The upstream channel should be inspected for debris, vegetative growth, or other changes that may affect the velocity profile or produce local increases in velocity.

  • The approach velocity should not exceed 0.5 ft/s [0.15 m/s].

  • The weir pool should be free of debris or sediments that raise the floor of the pool to within 2 Hmax of the weir crest.

THE WEIR INSTALLATION

  • Verify that the crest of the weir is at least 2 Hmax above the weir pool floor.

  • Verify that the crest is horizontally and vertically level.

  • Verify that the crest is centered in the flow stream.

  • Verify that the crest is at least 2 Hmax above the maximum downstream water level.

  • The maximum water surface through the weir crest must be at least 2 Hmax from the channel sidewalls.

  • Clean the crest of any debris, ragging, vegetative growth, etc.

  • Inspect the weir crest for nicks, burrs, rust, ice, or any other condition that affects flow over it.

  • Verify that the weir channel banks do not show signs of erosion or bypassing of flow past the weir.

  • Verify that the point of measurement is at least 3-5 Hmax upstream of the weir itself.

  • Verify that the zero elevation at the point of measurement is the same as the bottom of the weir crest.

Downstream CHANNEL

  • Inspect the downstream channel for signs of submergence.

  • Inspect the downstream channel for signs of scour – reinforcing the channel as necessary.

  • Remove any debris, growth, etc. which may impede flow through the downstream channel.