How to Maintain a Portable Weir Installation

Although usually installed for short-term flow studies, portable 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 (H_max) 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 H_max of the weir crest.

The Weir Installation

• Verify that the crest of the weir is at least 2 H_max 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 H_max above the maximum downstream water level.
• The maximum water surface through the weir crest must be at least 2 H_max 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 H_max 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.