Cascading Flow System for Urban Drainage Design

A stormwater drainage system is an important element in the urban infrastructure network. Under the conventional concept, an urban drainage system is designed to collect and pass the extreme storm event when sizing channels, street gutters, and storm drains. An extreme-event drainage system is often designed to target the runoff peak flow reduction, but not the runoff volume. Under the concept of low-impact development (LID), an urban drainage system shall be designed to mimic the nature flow system that conveys the full spectrum of runoff flows. In this study, it is suggested that the full spectrum of runoff flows should be divided into three categories: micro (up to a 6-month event), minor (up to a 10-year event), and major (up to a 100-year event). A 3M Cascading Flow System is proposed using three preselected storm events—micro, minor, and major—to design an urban waterway to be a three-layer flow corridor. In practice, porous devices are placed at the upstream of the entrance into the drainage system to capture the initial runoff volume up to the micro event for on-site runoff disposal, streets are utilized to safely collect and quickly remove minor to major storm events within the allowable gutter depths, and a detention basin is installed at the system exit to reduce the peak flows to the predevelopment condition. In this study, it is proposed that the flow-frequency curve should be used as the basis to quantity the effectiveness of a drainage system. The concept of a 3M Cascading Flow System provides a guide to conduct the regional drainage planning that can be further refined for details at the final design stage.