Abstract: A two-dimensional surface model with Graphics Processing Unit (GPU) acceleration is coupled with a one-dimensional channel model with SWMM to investigate the discharge capacity of interceptor ditches in the loess plateau area under extreme rainstorm events. In a case study of the southern part of Weinan City, the loess flood inflow and the ditches’ outlet flow are simulated for different rainfall conditions, and the ditches’ overflow flow rates are calculated using nonlinear fitting. The results indicate that in the plateau area, the flood peak flow varies significantly across different gullies under the great influence of two major factors－catchment area and ground slope. The volume of plateau floodwater entering the city increases rapidly with the flood return period longer than 10 years, with a growth rate of 112.1%. Ditch overflow will occur as rainfall increases up to a threshold of 70.06 mm for the flood interception ditch S2. For the return period shorter than 10 years, the ditch peak flow grows slowly; the growth is also slow under extreme rainfall conditions due to saturation. Under medium and large rainfalls, a change in gully dimensions imposes a great impact on the effect of gully discharge, and the growth rate of outlet flow from Gully No. 3 reaches 89.4% when P = 100 a. Ditch overflow into the city starts at P = 5 a in the flood interception ditches S1 and S3, and starts at P = 30 a in S2 where the overflow volume increases as rainfall increases from 1191 m3 to 128,895 m3. The study would help the design and optimization of urban flood control and drainage systems under extreme rainstorms.

Key words: urban flood control, high-external water areas, interceptor ditch, hydrodynamic model, one- and two-dimensional coupled model