Gauging site selection and current meter use

Factors for selecting a stream gauging site

• Straight reach: about 5–10 times the river width upstream and downstream, with parallel flow lines.
• Stable channel: bed and banks free from scour, deposition and shifting, so the stage–discharge relation stays constant.
• Well-defined control (rock bar, rapids or a weir) downstream, giving a unique stage–discharge relation.
• No backwater effect from confluences, dams or tides; no eddies or dead water.
• Flow contained in one channel at all stages, without overflow at floods.
• Adequate depth and velocity for the current meter in the dry season.
• Easy access in all seasons and safe working (bridge, cableway).
• Away from tributaries, so the measured flow represents the site of interest (e.g. near the intake for a hydropower project).

Measuring velocity with a current meter

A current meter (cup type or propeller type) has a rotor whose speed of rotation $N$ is proportional to water velocity. Its calibration equation is $V = aN + b$.

Procedure:

1. Stretch a tag line across the river and divide the width into 15–25 segments (each carrying ≤ 10% of flow).
2. At each vertical, measure the depth $y$ with a sounding rod or weight.
3. Lower the meter (wading rod for shallow water; cable, weight and cableway or bridge for deep water) to the required depth:
   • Shallow (< 0.6–1 m): one reading at 0.6y from the surface.
   • Deeper: two readings at 0.2y and 0.8y; mean $V = (V_{0.2} + V_{0.8})/2$.
4. Allow the meter to settle, then count the revolutions over a fixed time (40–100 s). $N$ = revolutions/time.
5. Convert to velocity with the rating equation, e.g. $V = 0.51N + 0.03$.
6. Compute discharge by the mid-section or mean-section method: $Q = \sum b_i y_i V_i$.

  tag line  0   1   2   3   4   5
  ~~~~~~~~~~|~~~|~~~|~~~|~~~|~~~|~~~~
   \        o   o   o   o   o      /  o = meter at 0.6y
    \_______o___o___o___o_________/      or 0.2y & 0.8y