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.