If the height of the hydraulic gradient line above the floor of thickness t is h and the specific gravity of the material of the floor is G, the minimum thickness t of the floor down stream of the crestwall, is given by the equation

While deciding the alignment of a proposed canal taking off from a river at B, three alignments of approximately equal lengths are available. These cross a drainages at C_{1}, C_{2} and C_{3} where drainage bed levels C_{1} > C_{2} > C_{3}, you will provide at site C_{3}