700-SDI-RADAR-300W-Man Rev 2 30 May 2022
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Part# 21370
CALCULATING DISCHARGE FROM FLOW
VELOCITY
The 300W flow meter measures flow velocity at the water surface. This measurement can be used to
calculate actual discharge – the total volume of water that passes through a channel cross-section in
a specific period of time. Discharge measurement is important for a wide variety of purposes
including flood and pollution control, irrigation, watercourse regulations and broadly as an input
data for dimensioning of almost any new structure on the open channel flows.
Discharge is calculated by multiplying mean flow velocity and the channel cross section area. The
cross-section area is the area of the slice in the water column made perpendicular to the flow
direction.
For an ideal case, let us assume the rectangular channel profile, with constant flow velocity at all
points, as in Figure 7-1.
Figure 6-1: Simple Channel Diagram
The discharge can be calculated according to the formula:
𝑄𝑄
=
𝑉𝑉
∗
𝐴𝐴
Where:
Q is discharge (for example in m
3
/s),
V is flow velocity (for example in m/s), and
A is cross-section area (for example in m
2
).
For real-world measurements it is important to understand that the velocity of the moving water
varies both across the stream channel and from the surface to the bottom of the stream due to
friction, as in figure 7-2.
Figure 6-2: Flow Velocity in a Typical Cross-section
