The commissioners shall not be confined to the points of location, commencement, routes, or termini of the drains, reservoirs, dams, canals, ditches, pumps, or other work, or the number, extent, or size of the same, as proposed by the petitioners, but shall locate, design, lay out, and plan the same in the manner that they determine is best to promote the public welfare and to benefit the lands of the parties interested with the least damage and greatest benefit to all lands affected thereby. All alterations or deviations in the design plans of the irrigation works shall have the final approval of the department.