---
title: "Current meter use by flow characteristics"
subject: "Engineering Hydrology"
chapter: 4
key: "semester_5/engineering_hydrology:current-meter-use-flow-chars"
source: https://www.bctengineer.com/q/semester_5/engineering_hydrology/current-meter-use-flow-chars
site: BCT Engineering
---

# Current meter use by flow characteristics

Engineering Hydrology · Chapter 4: Surface Runoff

- Asked in: 2074 Asoj, 4 marks
- Video: [Stream gauging in hydrology | Stream gauging methods](https://www.youtube.com/watch?v=JIlZzd7ZmOU) — Civil Engineer
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/engineering_hydrology/current-meter-use-flow-chars
- Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-4

## Question

Describe about the use of current meter according to flow characteristics of channel.

## Short answer

Measurement depth dictates method: one-point at 0.6d for shallow, two-point at 0.2 and 0.8d for moderate, three or five-point for deep. Unreliable in very low/high velocity, sediment, weeds; then use floats.

## Answer

A **current meter** (cup type Price meter, or propeller type) measures velocity from the rate of rotation of its rotor: $v = aN + b$. The way it is used depends on the depth and flow condition of the channel.

#### According to depth (points of velocity measurement in a vertical)
| Depth | Method | Mean velocity |
|---|---|---|
| Very shallow (< 0.6 m) | One point at 0.6 d | $v_m = v_{0.6}$ |
| Moderate (0.6 to 3 m) | Two points at 0.2 d and 0.8 d | $v_m = (v_{0.2}+v_{0.8})/2$ |
| Moderate to deep | Three points at 0.2, 0.6, 0.8 d | $v_m = (v_{0.2}+2v_{0.6}+v_{0.8})/4$ |
| Deep and variable | Five points (surface, 0.2, 0.6, 0.8 d, bed) | $v_m = \frac{1}{10}(v_s + 3v_{0.2} + 3v_{0.6} + 2v_{0.8} + v_b)$ |

#### According to flow characteristics
- **Shallow, small streams:** the engineer wades with a rod-mounted meter (pigmy or Price AA) held at 0.6 d.
- **Wide and deep rivers:** the meter is suspended from a bridge, cableway or boat with a weight (sounding weight) on a cable; the weight is chosen to keep the cable nearly vertical.
- **Very low velocity (< 0.15 m/s)** or **very high turbulent flood:** a current meter is unreliable, so floats or other methods are used.
- **Sediment-laden or weedy flow:** weeds and silt can foul the rotor, so it is cleaned and checked before each use.
- **Uniform flow:** velocity is measured at 20 to 30 verticals spaced so that no strip carries more than 5 to 10 % of the discharge.
- **Time of observation:** at least 40 to 60 s at each point, repeated when the velocity fluctuates.
- The meter must be rated (calibrated) in a towing tank to give $v = aN+b$, and held pointing into the flow, away from the observer's body.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. For very shallow depth (< 0.6 m), which method is used?
   A. Surface only
   B. Two points at 0.2 d and 0.8 d
   C. Five points
   D. One point at 0.6 d
   Answer: D. A single reading at 0.6 d gives the mean velocity. (Key point: Shallow depth method)

2. For moderate depth, mean velocity with two points is
   A. (v0.2 + v0.8)/2
   B. (v0.2 + 2 v0.8)/3
   C. v0.6
   D. v0.2 x v0.8
   Answer: A. The mean of the 0.2 d and 0.8 d velocities. (Key point: Two-point mean)

3. When is a current meter unreliable so floats are used?
   A. Moderate velocity
   B. Very low velocity (< 0.15 m/s) or very turbulent flood
   C. Uniform flow
   D. Shallow wading streams
   Answer: B. The rotor does not respond reliably at very low velocity or in violent floods. (Key point: Unsuitable conditions)

## Related in Engineering Hydrology

- [Stream flow from current meter data](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Acurrent-meter-0-035-0-74n) — Engineering Hydrology, chapter 4, asked 2×
- [Stream flow from current meter, V=0.045+0.76N](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Acurrent-meter-0-045-0-76n) — Engineering Hydrology, chapter 4, asked 2×
- [Current meter discharge, rating equation](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Acurrent-meter-0-53n) — Engineering Hydrology, chapter 4
- [Mid-section method discharge from current meter data](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Amidsection-0-3n-0-004) — Engineering Hydrology, chapter 4
- [Discharge from multi-point current meter data](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Acurrent-meter-two-point-0-3n) — Engineering Hydrology, chapter 4
- [River discharge from two-point current meter data](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Acurrent-meter-0-02ns) — Engineering Hydrology, chapter 4

## Related in other subjects

- [Stream discharge from current meter revolutions (V = 0.51N + 0.03)](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Astream-gauging-revolutions-0-51) — Hydro Power, chapter 2, asked 2×
- [Stream discharge from current meter revolutions (v = 0.55N + 0.04)](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Astream-gauging-revolutions-0-55) — Hydro Power, chapter 2, asked 2×
- [Gauging site selection and current meter use](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Astream-gauging-site-current-meter) — Hydro Power, chapter 2
- [Speech characteristics; formant and channel vocoder](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspeech-formant-channel-vocoder) — Wireless Communication, chapter 6