Flow Rate Calculation for Run of River (ROR)

Flow rate is defined as the volume of liquid move at certain amount of time. Calculating the flow rate for a run-of-river system manually (without sensors) involves using physical measurements and basic hydrodynamic principles. These ind of calculations are made for the small and mini hydro power plants which is running with no dam facility to store water. Here’s a straightforward way to estimate the flow rate:

1. Calculate Flow Rate Using the Velocity-Area Method

• This approach involves measuring the cross-sectional area of the river and the velocity of the flow.

Step-by-Step Process:

Step 1: Measure the Cross-Sectional Area of the River

• Width (W): Measure the width of the river using a measuring tape across the point where you want to measure flow.
• Depth (D): Take depth measurements at regular intervals across the river width, then find the average depth.

$$\text{ Average Depth (D)}=\frac{\text{Sum of depth measurements/ }}{\text{Number of measurements}}$$

• Cross-sectional Area (A): Multiply the average depth by the river width.

$$A=W\times D$$

Step 2: Measure the Flow Velocity

• To measure the water velocity manually:

  1. Place a floating object (like a small, visible leaf or a piece of wood) at the start of a known distance along the river.
  2. Measure the time it takes for the object to float across this distance.
  3. Calculate the velocity by dividing the distance traveled by the time taken.
  4. $$\text{Velocity (V)}=\frac{\text{Distance Traveled/ }}{\text{Time}}$$

• Tip: Perform this measurement several times and take the average velocity to improve accuracy.

Step 3: Calculate the Flow Rate (Q)

• Finally, multiply the cross-sectional area (A) by the average velocity (V) to get the flow rate.

$$\mathrm{Q }= A\times V$$

where:

• $Q$ = Flow rate (cubic meters per second or m³/s)
• $A$ = Cross-sectional area (square meters or m²)
• $V$ = Velocity (meters per second or m/s)

Example Calculation

• Suppose:
  • Width of the river $W = 10$ meters
  • Depth measurements are 1.5, 1.7, 1.8, 1.6, and 1.5 meters, so the average depth $D = 1.62$ meters.
  • Distance for floating object to travel = 20 meters
  • Time taken for object to travel 20 meters = 15 seconds
• Cross-sectional Area $A = 10 \times 1.62 = 16.2 \, \text{m}^2$
• Velocity $V = \frac{20}{15} = 1.33 \, \text{m/s}$
• Flow Rate $Q = 16.2 \times 1.33 = 21.55 \, \text{m}^3/\text{s}$

Additional Tips:

• For accuracy, measure at multiple sections along the river and average the results.
• Repeat measurements under different flow conditions (e.g., after rainfall or during dry spells) for seasonal data.

Click here for online calculator for "Flow Rate calculations for Run of River"