All assessment drains that have been constructed in any district, except township drains, shall be under the charge of the water resource board and it shall be the duty of the board to keep those drains open and in good repair. It shall be the mandatory duty of the board, within the limits of available funds, to clean out and repair any assessment drain when requested to do so by petition of the affected landowners having fifty percent or more of the possible votes, as determined according to section 61-16.1-20.