No use shall be permitted in a combining FW district which will adversely increase the water surface elevation or otherwise adversely change the flow characteristics of the one hundred (100) year flood at any cross section of the floodway. The effect of any use on said flow characteristics and water surface elevation shall be determined by the county flood control and water conservation district. Consideration of the effects of a proposed use shall be based upon a reasonable assumption that there will be an equal degree of encroachment extending for the entire reach of the stream on both sides thereof in which the proposed use is permitted by the underlying district. (Prior gen. code § 8-44.6)