Calculate the cross-sectional area of the pipe if this is not already provided in the problem statement. Use basic geometric area equations depending on the cross-section's shape, which may be circular, rectangular or trapezoidal. The cross-sectional area is known as the channel shape perpendicular to the flow. Equations that may be used are:
Area of a circle = pi x radius x radius
Area of a rectangle = length x width
Area of a trapezoid = .5 x height x (length1 + length2)
Multiply the cross-sectional area by the flow velocity, provided in unit of length over unit time squared. If the cross-sectional area and flow are truly perpendicular, the flow rate angle is zero degrees. The value you just calculated is the hydraulic flow.
Multiply the value from step 2 by cosine of theta, where theta is the angle of flow between the area and flow direction. Use theta only when you are unable to calculate the exact perpendicular cross-sectional area in step 1.