La. Stat. tit. 38 § 1904
In any case where it becomes necessary, or it is desired, by the governing authority of any gravity drainage district or sub-drainage district to exercise the authority granted by R.S. 38:1903, the governing authority shall first cause to be made, by its supervising engineer or other competent engineer, an estimate of the work already done on the system of drainage which has been adopted, and if it be found by the estimate that eighty per cent, or more, of the system has been completed, the governing authority shall authorize, by proper ordinance, the creation of an additional indebtedness and shall levy an additional ad valorem or acreage tax or forced contribution, and fund the tax into bonds, for the purpose of perfecting and completing the system of gravity drainage, or of paying any indebtedness incurred or to be incurred in completing the system, not to exceed, however, twenty per cent of the total cost of the system of drainage. All other or further procedure for this purpose, shall be as provided by law for the creation of indebtedness, the levying of taxes and forced contributions and the funding of taxes into bonds by gravity districts or gravity sub-drainage districts for gravity drainage purposes, except that it shall not be necessary to submit the proposition to the vote of the property taxpayers.
La. R.S. § 38:1904