The powers granted by sections 16-6,106 to 16-6,109 are independent of and in addition to all other grants of powers on the same or related subjects but may be exercised jointly with or supplemented by the powers granted by existing state law, including, but not limited to, sections 16-667 to 16-672.11, 16-680, 16-683, 16-693, 18-401 to 18-411, 18-501 to 18-512, 19-1305, 23-320.07 to 23-320.13, and 31-501 to 31-553 and the Combined Improvement Act.