Draw balanced canal section

A balanced section is a canal section partly in cutting and partly in banking in which the earth excavated from the bed is exactly enough to build the banks, so there is no surplus spoil and no earth is borrowed. The depth of cutting at which this happens is the balancing depth.

 bank top ______                    ______
         /      \                  /      \
        /  bank  \  F.S.L ~~~~~~~/  bank  \
 G.L ===/          \            /          \===
                    \          /
                     \________/
                      bed width b

Condition: area of cutting below ground = area of the two banks above ground.

$$(b + s\,d)\,d = 2\,(t + s\,h_b)\,h_b,\qquad h_b = D - d$$

where $d$ is the depth of cutting, $D$ = full supply depth + free board, $t$ = top width of one bank and $s$ the side slope.

Example (assumed $b = 5$ m, $s = 1.5$, $t = 2$ m, $D = 1.6$ m): solving gives $d = 0.810$ m, cutting area $= 5.033$ m² and bank area $= 5.033$ m² (equal).