Ala. Code § 33-2-10

Immediately upon the completion of each unit of development along its inland waterway system acquired or erected pursuant to this article, the director of the Alabama State Docks Department shall ascertain the cost of such unit, which cost shall be entered in the accounts kept by the State Docks Department as a charge against capital account for that unit. After the first 12 months from the completion of each unit pursuant to this article, one half of one percent of the cost thereof, not exceeding one half of the gross income thereof during each month, shall at the end of the month be credited to an account designated “sinking fund” and shall be used from time to time only for the purpose of paying interest charges and of retiring, by repurchase, bonds issued pursuant to this article at not more than par or by paying them off as the same mature, until all bonds issued pursuant to this article shall have been retired; provided, that whenever the sinking fund thus created shall exceed the bonds outstanding plus interest charges maturing within the next 24 months, then the surplus shall be available for use in future development and improvements pursuant to this article.