The total amount payable in each year to the general reserve fund shall be not less than the sum of the rates percent known as the normal contribution rate and the accrued liability contribution rate of the total compensation earnable by all members during the year, and the aggregate payment by the city shall be sufficient when combined with the amount in the fund to provide the retirement allowances and other benefits payable out of the fund during the then current year. The city may contribute at any time from bond issue or other available funds an amount equal to the unfunded accrued liability as certified by the actuary in which event no further accrued liability contribution will be required or may contribute any lesser amount which will be used to proportionately reduce future accrued liability contributions.