A non-Federal interest may carry out operation and maintenance activities for an authorized navigation project, subject to the condition that the non-Federal interest complies with all Federal laws and regulations applicable to such operation and maintenance activities, and may receive credit for the costs incurred by the non-Federal interest in carrying out such activities towards the share of construction costs of that non-Federal interest for another element of the same project or another authorized navigation project, except that in no instance may such credit exceed 20 percent of the total costs associated with construction of the general navigation features of the project for which such credit may be applied pursuant to this section.