CFD Modeling of Supercritical Flows in a Flood Control Channel

Abstract

Complex free surface flows in flood control hydraulic structures have traditionally been analyzed using physical models, but CFD modeling offers a less expensive and more flexible tool for evaluating hydraulic performance. This paper presents a CFD modeling approach using the volume of fluid (VOF) model for free surface to predict wave actions in a flood control channel. This CFD modeling approach was applied to evaluate proposed improvements in a 4,000 ft supercritical flow flood control channel on Pole Creek in Fillmore, California with velocities above 30 fps at design capacity. These improvement alternatives involved the use of an asymmetrical compound channel, transitions in channel widths and cross section shape in the vicinity of a highway bridge, a railway bridge and the access ramp. Cross waves and flow disturbances exist in these areas, which raised concerns regarding hydraulic performance of the channel. In this study, four CFD models – each for a section of the flood control channel – were developed using FLUENT software. These models were integrated together to predict water surface profiles and the locations and magnitudes of cross waves and local flow disturbances at changes in the channel shape, access ramps, and channel transition to the bridge. Combined with HEC-RAS modeling, changes to the initial design concepts were recommended to reduce wave heights and optimize capacity. A design water surface profile in the channel was developed that accounts for the magnitude of expected waves and flow disturbances varying significantly along the length of the channel. CFD modeling demonstrated that the recommended design could provide adequate capacity to convey the 100-year design flow.