Public improvements may be made by the City Government itself or by contract. The City Council shall award all contracts for such improvements; provided that the City Council may authorize the City Manager to award such contracts not exceeding an amount to be determined by the City Council and subject to such regulations as the City Council may prescribe. A contract for public improvements in any sum exceeding the amount set by state law may be awarded only to the lowest and best responsible bidder after such notice and opportunity for competitive bidding as the City Council may prescribe. All bids may be rejected, and further notice and opportunity for competitive bidding may be given. As used herein, "Public Improvement" means any beneficial or valuable change or addition, betterment, enhancement, or amelioration of or upon any real property or interest thereon. The term does not include the direct purchase of materials, equipment or supplies by a public agency.