The calculation of the discharge from the velocity and depth measurements can be made in several ways. Two of these are illustrated in Fig. 7.18. In the mean section method, averages of the mean velocities in the verticals and of the depths at the boundaries of a section sub-division are taken and multiplied by the width of the sub-division, or segment, r> ^ (v <-1 -V< )(d<-1 - d< hu u \
Mean section method bi-i b bi-i b
Mid-section method bi bi+i
Mid-section method bi bi+i b b
Fig. 7.18 Calculating discharge.
b b where bi is the distance of the measuring point (i) from a bank datum and there are n sub-areas. In the mid-section method, the mean velocity and depth measured at a subdivision point are multiplied by the segment width measured between the mid-points of neighbouring segments:
with n being the number of measured verticals and sub-areas. In the mid-section calculation, some flow is omitted at the edges of the cross-section, and therefore the first and last verticals should be sited as near to the banks as possible.
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