Conversion of Natural Watershed to Kinematic Wave Cascading Plane

Storm water management models using the kinematic wave (KW) approach require the conversion of a natural watershed to its equivalent rectangular cascading plane. AKW cascading plane is specified by the plane’s area, width, and slope. Without on-site calibration, there is no guidance on how to translate the irregularity of a natural watershed into a rectangular shape that has an equivalent hydrologic response. As a result, the reliability of a KW model depends on an engineer’s experience. Often the use of a computer model to simulate stormwater runoff is the only option because there are not field data at the project site to use statistical methods. The need for calibration at each site defeats the purpose for using computer simulations. This long existing problem has been documented in the EPA SWMM4 User Manual, which continued into the EPA SWMM5 user guidance. In this study, the continuity and energy principles are creatively interpreted as the preservations of watershed area and vertical fall over the receiving waterway’s length. The dimensional analysis suggests that the watershed shape factor can be used to correlate the waterway length in the natural watershed to the KW plane width. A parabolic one-to-one single valued function was derived to translate the watershed shape into its equivalent KW plane width, length, and slope. After numerous tests, it is confirmed that the watershed and KW shape factors provide a consistent and stable basis for watershed geometric conversion. It is believed that the procedure outlined in this paper can be very useful to the KW surface runoff modeling to represent urban stormwater predictions when an on-site calibration is not readily available.