The governing body of cities of the first class having a population of 25,000 or more shall have power to build floodgates in sewers that empty through dikes built by the city for the purpose of preventing water in flood time from flooding portions of the city protected by such dikes; and to build intercepting sewers for the purpose of conducting the sewage to a central point during flood time, and to erect a pumping plant for the purpose of forcing such sewage over the dike to its usual destination.