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UM200002 ISSUE A REV 00
User Manual
4 Theory of Operation
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 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 ideal case, let us assume the rectangular channel profile, with constant flow velocity at all points, as in Figure 1 Simple
Channel Diagram.
Figure 1 Simple Channel Diagram
The discharge can be calculated according to the formula:
Q = V * A
Where,
•
Q is discharge (in m
3
/s)
•
V is flow velocity (in m/s)
•
and A is cross-section area (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 2 Flow velocity in a typical
cross-section.
Figure 2 Flow velocity in a typical cross-section
In order to determine the discharge in a realistic channel, the area must be precisely measured by measuring water depths
at a series of points across the stream and multiplying by the width of the stream within each segment represented by the
depth measurement. The mean cross-section flow velocity needs to be determined from measured surface flow velocity.
Studies performed by USGS reveal that, typically, the mean velocity is 80-95% of the surface velocity, the average being 85%
Knowing non-rectangular area of the stream cross-section, and knowing the surface flow velocity, the following formula can
be used:
Q = k * V * A