No permit shall be issued for excavating of land within a drainagecourse, or on any lands within 100 feet, or more than 100 feet when so designated on the Zoning Map, of the center line of such drainagecourse, except upon approval, in writing, by the City Engineer that such excavating will not divert water from the established channel, will not cause flooding of lands outside such drainagecourse, will not cause any erosion, and will not otherwise impair the design and character of such drainagecourse.