Weir Plate Installation Instructions To ensure a successful weir plate installation, it is important to keep the following points in mind:

1) The approach channel should be straight and of uniform cross-section for a distance of 15 to 20 times the maximum anticipated water depth (head) (Hmax) upstream of the weir.

2) The approaching flow should be sub-critical, tranquil, and uniformly distributed across the

channel. The flow should not be turbulent, surging, unbalanced, or possessing a poorly distributed velocity pattern.

3) The weir should be set so that it is centered in the flow stream.

4) The crest of the weir must be set at least two to three (2-3) times the maximum anticipated water

depth (head) (Hmax) above the channel floor.

5) The crest of the weir should be at least two (2) times the maximum anticipated water depth (head) (Hmax) from the channel sidewalls.

6) The crest of the weir should be set a minimum of 2 inches (5 cm) above the downstream level.

7) The weir must be installed level from front-to-back and from side-to-side. No deviation from these

conditions is allowed.

8) The weir must not move out of its intended final position during installation. The upstream / downstream banks and channel floor should be sufficiently tamped so as to secure the weir and to ensure that flow does not bypass the structure.

Figure 1: Point of measurement location for thin-plate / sharp-crested weir The correct point of measurement is three to four (3-4) times the maximum anticipated water depth (head) (Hmax) upstream of the weir. Measurements closer than this will be affected by drawdown as the flow approaches the weir.

3-5 HmaxChannel floor

Hmax

Weir Pool

Point of measurement

Crest elevation

Weir

Nappe

Crest

Downstream Pool

2-3 Hmaxminimum

The zero elevation at the point of measurement is the same as the bottom of the crest of the weir – which is the same as the surface water level once flow over the weir has ceased. It is important to mark this location accurately at the time of the initial installation of the weir, as a failure to do so will make future head / level readings difficult. It is not advisable to simply measure from the surface of the water to the floor of the channel as upstream sedimentation may have occurred since the weir was installed and the channel floor profile may be different at the point of measurement and at the weir itself. In earthen channels, localized scouring may occur downstream of the weir plate. Scouring can lead to flow bypassing the weir plate or the weir plate shifting out of position. As a rule of thumb, downstream riprap protection should extend not less than 4 times the maximum normal depth of the downstream channel, nor less than 5 feet [1.50 m]. Design details of channel reinforcement can be found in Water Measurement with Flumes and Weirs, ISBN 90-70754-55-X.