In determining the relative values of such lands, including improvements thereon, the assessed valuation of the same for general tax purposes last equalized shall be construed to be prima facie correct: PROVIDED, That nothing herein contained shall be construed to prevent the fixing of values where none are shown on the general tax roll or the revision of such values on the general tax roll in any instance where in the sole judgment of the revising officers for the district the value for general tax purposes is manifestly and grossly erroneous in its relation to value of like property in the district similarly situated: AND PROVIDED FURTHER, That in any instance where any tract of land is protected or partially protected from floods and is financially supporting the works affording such protection the revising officers for the district shall take the value of such existing flood protection into consideration and give such land equitable credit therefor.