When the proposed improvements increase on-site runoff, and the subsequent discharge exceeds the hydraulic capacity of the downstream drainage system, as determined by the Public Works Department, on-site storm water detention facilities will be required to regulate the release rate of the runoff. The Public Works Department may waive detention facility requirements when the developer makes satisfactory arrangements to improve or to provide a downstream drainage system of adequate hydraulic capacity for the peak rates of discharge to the system, including discharge from the developer's site, to a point downstream where the rate of total runoff from the site is ten percent (10%) or less than the total runoff rate conveyed by the downstream system measured at the time of peak discharge. The Public Works Department may also permit a combination of downstream improvements and on-site detention that provides the same level of control.