There is hereby created a fund known as the "water and electric depreciation fund." Into such fund there shall be transferred monthly, from the water and light fund, and deposited a sum equal to the depreciation chargeable against the properties from time to time constituting the water and electric light works of the city. The amount of depreciation and the amount to be transferred monthly into the fund hereby created shall be determined according to a formula heretofore or hereafter determined by the consulting engineers employed by the city. The sums so deposited into such fund shall be expended only for unusual and extraordinary repairs and replacements of the water and electric light works and for emergency expenses of such works.

(Code 1964, § 15.070)