D.2.2 Measuring from a Bridge

If a watercourse can not be waded, discharge measurements can be made from a bridge or cableway (Section D.2.3). Measurement cross sections under bridges are often satisfactory for current meter measurements, but cableway sections are usually superior.

Either a bridge rod (Figure D-8) or a cable/weight suspension system can be used to position the current meter in the watercourse.

Figure D-8. Bridge rod in use on downstream side of bridge.

The cable supporting the sounding weight and current meter may be suspended from a handline (Figures D-9 and D-10) or one of several models of sounding reels. Handlines and sounding reels mounted on bridge boards (Figure D-11) are especially useful when carrying out measurements from logging or farm bridges with no guard rails; however, these assemblies are limited to sounding weights less than 22 kg. Three- or four-wheeled bridge cranes (Figure D-12) can accommodate sounding weights of up to 77 kg, depending on the model of sounding reel, but can be used only on bridges with strong guard rails.

The discharge measurement can be made from either the upstream or downstream side of the bridge. Make this decision independently for each bridge, according to the advantages and disadvantages in each case. Also consider the physical conditions at the bridge, such as location of the walkway, traffic hazards, and accumulation of trash on pilings or piers.

The advantages of measuring from the upstream side of the bridge are:

1. Hydraulic characteristics on the upstream side of bridges are usually more favourable.
2. Drift material can be avoided more easily because it can be seen it coming downstream. With downstream measuring, an assistant may be needed to watch for floating debris.
3. The channel bed at the upstream side of the bridge is not likely to be scoured as badly as the downstream side.

Figure D-9. Price 622AA meter mounted on a 1-ft. hanger, with a 30-lb. Columbus weight.

Figure D-10. Handline-suspended meter and Columbus weight. Just over 1 m of sounding cable unwound from reel. Tags 0.5 m and 1.0 m above centre of meter.

Figure D-11. Sounding reel/bridge board mounted on a boat.

Figure D-12. Three-wheeled bridge crane.

The advantages of using the down stream side of the bridge are:

1. Bridge rods are less likely to suffer damage from bending over the edge of the bridge if caught by the current and/or debris.
2. The vertical angle of a cable-suspended current meter is more easily observed.
3. Bridge abutments and piers can straighten flow lines in some cases.
4. If the bridge is angled across the channel, a single horizontal correction for angular flow can be applied to the measured discharge.