River junction design for urban flood control: A case study

The Yuen Long Bypass Floodway is designed as the floodway to collect flows from the Sham Chung and the San Hui Channels, and also to serve as a diversion channel of the Yuen Long Main Nullah. Tests with a 1:50 undistorted Froude scale model were conducted to study water stages and flow characteristics in the floodway, and to investigate optimal design arrangements at the channel junctions and transitions. Special emphasis is placed on the complicated flow interactions at channel junctions, which cannot be resolved by I-D mathematical models for unsteady free surface flows that are in commercial use. Five schemes were compared; the results indicate that the best choice can enhance the flood control capacity from that of a one in 10-year to a one in 200-year flood. The study serves as a good example of the complementary roles played by numerical and physical models in the solution of hydraulic engineering problems.