If for any of the causes hereinbefore recited, or for any other cause, a sum of money greater than the proceeds of sale of the drainage bonds shall become necessary to complete the drainage system, and the board of viewers shall determine that the amount to be raised is greater than can be realized from the collection of one annual assessment upon the lands in the drainage project without imposing an undue burden upon the lands, or if it is advisable or necessary to raise the money more expeditiously, then and under such conditions additional bonds may be issued in such aggregate sum as may be necessary.