Any city improvement estimated to cost more than $15,000 shall be performed under contract with sealed bid and accompanying performance bond, except where such improvement is authorized by the Council to be made directly by a city department in conformity with detailed plans, specifications and estimates. All such contracts for more than $15,000 shall be awarded to the lowest responsible bidder after such public notice and competition as may be prescribed by ordinance or resolution provided that all bids may be rejected for cause, and new bids may be sought in accordance with the original procedure. Alterations in contracts affecting the scope of work, time requirements, cost or any other substantive provisions affecting cost or the city's interests, shall be approved by the Council after considering the recommendations of the City Manager.