CHAPTER 10 - CURRENT METERS

13. Computing Discharge

The velocity-area principle is used to compute discharge from current-meter data. Total discharge is determined by summation of partial discharges. A partial discharge is the product of an average point or vertical line velocity and its meaningfully associated partial area, expressed as:

The total discharge is then:

The simple average, the midsection method, and Simpson's parabolic rule applied to both the depth and average vertical line velocity values will be discussed using figure 10-16. This figure shows: (a) where the boundary is broken up into inflecting straight line sections and (b) where the boundary is smoothly curved.

 Figure 10-16a -- Calculation of discharge using the midsection method.
 Figure 10-16b -- Calculation of discharge using Simpson's parabolic rule method.

The measured and computed variables are as follows:

q = the discharge in cubic feet per second (ft3/s) for a partial area
Q = total discharge
= the mean velocity associated with the partial area
a = partial area of total cross section
L1, L2,... Ln= distance to vertical measurement locations in feet from an initial point to vertical station
L = the distance in feet between consecutive vertical measurement stations
,,... = the respective mean velocities in feet per second at vertical measurement stations
D1, D2,... Dn= the water depths in feet at verticals
n = the number of verticals related to the partial area