B.2.4 Selecting the Metering Section

The location of the metering section may vary with changes in stage. The best location for measuring high and medium flows from a cableway, boat, or bridge may not be acceptable for low water wading. Ideally, once the metering sections for low, medium, and high flow measurements have been selected, they should not be changed.

1. The metering section should be perpendicular to the general direction of flow. (A procedure for determining angle of flow corrections is described in Special Techniques, "Correcting for Angular (Oblique) Flow".)
2. The metering section should be located where the bed and banks of the watercourse are straight and uniform:
• Upstream - for a distance of approximately five times the width of the metering section.
• Downstream - for a distance of approximately twice the width of the metering section.
3. The channel bed at the metering section should be as uniform as possible. It should be free from vegetation, immovable rocks, and obstructions such as bridge piers.
4. The metering section normally must accommodate 20 subsections of uniform discharge. This factor should be considered during site selection.
5. The spacing of verticals along the metering section is not usually uniform. Where the water is shallow and/or slow moving, the spacing will be greater than where the water is deep and swift. Spacing depends largely on the following factors:
• Overall width of the watercourse.
• Unevenness of the channel bed.
• Variation in velocity across the channel.
6. If the verticals are properly spaced, then the discharge in any one of the subsections should be about 5% of the total discharge, and may not exceed 10% for the Provincial Class A Standard.
7. The spacing of each vertical may be referenced to a permanent initial point on the shore. The initial point should be well defined, usually by an iron pin driven into the ground above the high water mark. And, the initial point in turn should be referenced to another permanent feature near the metering section or gauging station. (These data are often required for detailed studies long after the gauging station has been established or discontinued.) This is particularly important where it is desirable to define channel erosion or deposition. As changes are usually determined by creating a series of cross-section plots, an iron pin is usually set above the high water mark at the far end of the cross section. The distance between the two points is used to maintain the horizontal vertical ratio in successive computer-generated plots.