Nothing in RCW 85.05.510 through 85.05.550 shall be construed as repealing or modifying any act or statute now in force pertaining to diking districts, but the rights and remedies hereby granted shall be deemed cumulative as to the districts to which RCW 85.05.510 through 85.05.550 is limited. RCW 85.05.510 through 85.05.550 shall apply to districts heretofore or hereafter organized and to property owners' petitions heretofore or hereafter filed; provided that the decision of the board of dike commissioners of a district to which RCW 85.05.510 through 85.05.550 applies to issue bonds of a district under existing law or under RCW 85.05.510 through 85.05.550, shall be conclusive of such election.