Modelling complex flood flow evolution in the middle Yellow River basin, China

Flood routing processes in the middle Yellow River basin are complex since they consist of three types of flood: bidirectional, convergent and divergent flood flows between the main channel and its tributaries. We propose three computation schemes to simulate the complex flood routing: a simple scheme for a single main channel with no tributary or backwater, an improved schemes for convergent or divergent flow at the confluence, and an improved scheme for bidirectional flow. The schemes are examined by analyzing seven historical flood events and three scenarios of flood routing in the middle Yellow River basin. The model was calibrated and validated based on the simulation of three different types of flood. As compared with the observed hydrographs, the results show that the model is able to simulate flood routing processes efficiently for the study river (with Nash-Sutcliffe indices falling in the range 0.75-0.91). The model demonstrates that flood routing during historical flood events (1962-2003) in the middle Yellow River was altered under boundary condition changes. The shape of the hydrographs changed from high and thin to low and wide, which was accompanied by a delayed occurrence and extended duration of peak flow after the 1960s. Moreover, these trends were intensified after the early 1980s. Backwater resulted from divergent flows and bidirectional flood flows. An analysis of combined boundary conditions shows that flood wave volume has the strongest impact on flood duration, peak discharge and water level, the time of occurrence of peak discharge, and the magnitude of backwater. River bed slope has the second strongest impact on flood duration and the magnitude of backwater. Channel roughness has the second strongest impact on peak discharge and water level.