Notwithstanding any other provision contained in the Special Assessment Investigation, Limitation and Majority Protest Act of 1931 to the contrary, in the event the district board shall order the work contained in the report prepared by the district engineer pursuant to Chapter 5 (commencing with Section 56050) of this part to be done in whole or in part pursuant to the provisions of the Improvement Act of 1911 or the Improvement Bond Act of 1915, the provisions of the said Special Assessment Investigation, Limitation and Majority Protest Act of 1931 shall not be applicable to such work if the district board so declared by resolution duly adopted by four-fifths vote after a finding that said work is necessary for the public welfare, which finding shall not be made by the district board if, prior to the conclusion of the hearing on said report as provided in Chapter 5, the owners of more than 20 percent of the total assessed value of the taxable property within the district file written protests against the doing of said work.