Engineering Hydrology Lab Notes
Course Information
Subject: ENCE 306 – Engineering Hydrology
Year/Part: III Year I Part
Description: Lab report covering Determination of Discharge by Velocity Area Method using a current meter
Determination of Discharge by Velocity Area Method
1. Objective
To calculate the discharge flowing through a given cross section.
2. Scope
Measurement of discharge in streams and rivers is a practical problem faced by engineers. Interestingly, it is the only part of the hydrologic cycle that can be measured accurately. Discharge measurement in streams forms an important branch of hydrometry. Knowledge of discharge in streams aids us in the design of various structures such as dams, bridges, etc. However, continuous measurement of stream discharge is very difficult and costly.
3. Theory
A current meter can be used to measure stream velocity. It consists of a rotating element which rotates due to the reaction of the stream current. The current meter is placed at a depth of 0.6d from the surface to measure the average velocity. Based on the number of rotations of the cups around the vertical axis, the stream velocity is calculated as:
Where,
V = Stream velocity (m/s)
Ns = Number of revolutions per second
a and b = Constants specified by the manufacturer
(a = 0.7002 b = 0.0004)
The area velocity method is employed to calculate the discharge across a cross section. For this, the given cross section is divided into a number of smaller sub-sections as shown in the figure above. The average velocity of each cross section is calculated. For deep sections (not less than 2 ft), the two-point method is preferred. It consists of measuring the velocity at 0.2 and then at 0.8 of the depth from the water surface and using the average of the two measurements. High accuracy is obtainable with this method. Discharge is then given by:
4. Apparatus Required
-
Current Meter
Fig: Velocity measurement using current meter in a flume - Flume
5. Procedure
- The pump was started to initiate flow in the flume. After steady flow was attained, the current meter was set up.
- The time mode was selected at the control display, and the time for a certain number of revolutions was noted.
- Likewise, in the revolution mode, the frequency or number of revolutions for a given time was recorded.
- The procedure was repeated for 3 sets of observations.
6. Observation and Calculation
Width of Flume (b) = 30 cm = 0.3 m
Depth of flow (d) = 0.287 m
Area = b × d = 0.3 × 0.287 = 0.0861 m²
| Exp. No. | Mode | t (sec) | No. of Revolutions | Rev/s (Ns) | V = aNs + b (m/s) | Q (m³/s) |
|---|---|---|---|---|---|---|
| 1 | T (Time) | 20 | 10 | 0.500 | 0.350 | 0.030 |
| R (Revolution) | 15 | 7 | 0.466 | 0.326 | 0.028 | |
| 2 | T (Time) | 28 | 15 | 0.536 | 0.375 | 0.032 |
| R (Revolution) | 30 | 15 | 0.500 | 0.350 | 0.030 | |
| 3 | T (Time) | 39 | 20 | 0.512 | 0.359 | 0.031 |
| R (Revolution) | 45 | 24 | 0.533 | 0.373 | 0.032 | |
| Average | 0.355 | 0.030 | ||||
7. Result
The average discharge calculated through the given cross-section by the velocity area method is Qavg = 30 Lps (0.030 m³/s).
8. Discussion and Conclusion
The experiment was successfully conducted to determine the discharge of the flume using the velocity area method. The cross-sectional area of the flow was calculated to be 0.0861 m². Using a current meter, the velocity of the flow was measured across three distinct trials in both “Time” and “Revolution” modes. The calculated average velocity was found to be 0.355 m/s, yielding an average calculated discharge of 0.030 m³/s, which is equivalent to 30 liters per second (Lps). Slight variations observed in the individual discharge values (ranging from 0.028 to 0.032 m³/s) can be attributed to minor fluctuations in the flume’s flow rate during the experiment or slight observational discrepancies. Overall, the velocity area method using a current meter proved to be a reliable and highly effective approach for measuring open-channel discharge.
9. Precautions
- It must be ensured that the flow in the flume is completely steady before taking any readings.
- The current meter should be placed accurately at the specified depth (0.6d for single-point average velocity) and its axis should be kept strictly parallel to the direction of flow.
- The rotating cups of the current meter must be checked to ensure they are free from any debris or obstruction that could hinder rotation.
- Sufficient time should be allowed for the current meter to stabilize after being submerged in the water before starting the timer or counting revolutions.
- Readings from the control display unit should be observed carefully to avoid any logging errors.
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