Numerical Investigation on a Flash Flood Disaster in Streams with Confluence and Bifurcation

Abstract

On 20 August 2019, a flash flood occurred in Sanjiang Town, Sichuan, China, and caused great damage to people living there. The town lies at the junction of five streams, with streams A, B, and C combining at the town and further dividing into streams D and E. The slope of streams A, B, and C is about 3~5%, while the slope of streams D and E is around 0.3%. The Sanjiang Town actually lies in the transition from supercritical slope to subcritical slope. During the flood, huge sediments were released to streams A, B, and C, and further transported to stream E. Due to the rapid change of velocity, only few sediments deposited at the supercritical slope parts of the stream, while plenty of them sedimented at the streams with subcritical slope. In order to simulate the flood with a hydrodynamic model, a field investigation was carried out to collect high DEM (digital elevation model) data, flood marks, sediment grading, etc., after the flood. The discharge curve of the flood was also obtained by the hydrometric station near Sanjiang Town. For the inlet sediment concentrations of streams A, B, and C, we made a series of assumptions and utilized the case which best fits the flood marks to set the inlet sediment concentration. Based on these data, we adopted a depth−averaged two−dimensional hydrodynamic model coupled with a sediment transport model to simulate the flash flood accident. The results revealed that the flash flood enlargement in confluence streams is mainly induced by the inflows, and the flash flood enlargement in bifurcation streams is largely affected by the sediment deposition. The bifurcation of flows can decrease the peak discharge of each branch, but may increase the flooded area near the streams. Flow in the supercritical slope runs at a very fast velocity, and seldom deposits sediment in the steep channel. Meanwhile, most sediment is transported to the streams with flat hydraulic slopes. Due to the functioning of the reservoir, the transition region from supercritical slope to subcritical slope has a much larger probability of being submerged during the flood.

Confluence morphodynamics in mountain rivers in response to intense tributary bedload input

“…At 5% sediment concentration, deposition was minimal, occurring within 1.2 m of the confluence. When sediment concentration was 10%, intense deposition throughout the channel was observed, which is contrary to the findings of Yang et al (2022) regarding sediment deposition in supercritical channels. Since unit stream power was relatively uniform between experiments of the same discharge (Table 5), Backwater effects in the main channel (Figure 15) are evident and were amplified when the discharge ratio increased, due to the additional discharge to be accommodated at the confluence.…”

“…Tributaries in mountain regions have steep gradients originating from small catchments, variable discharges, and high rates of suspended and bedload transport during flood events (Aulitzky, 1980; Meunier, 1991). Steep channels can feature supercritical flows, especially when the relative channel roughness is low, for example, in channelized or bedrock reaches (Yang et al, 2022). Hydraulic jumps are common features in steep channels and can occur where the supercritical tributary meets the deeper, subcritical flows in the main channel, influencing the hydraulic conditions in the combined flow channel (Christodoulou, 1993; De Leo et al, 2020).…”

Impact-Based Critical Areal Rainfall for Early Flood Warning: A Case Study of Zhulong River Watershed in the Upper Reaches of the Xiong’an New Area

“…However, there are still a few aspects that need to be enhanced in future studies to further promote the timeliness and accuracy of the early flood warnings [39,40]. For instance, the incorporation of remotely sensed precipitation products and gauges to measure precipitation in finer spatial and temporal dimensions [41,42], coupled with the simulation of distributed hydrological models and the main process of hydrological statistical yearbooks to attempt to generate hourly or sub-daily discharge data [43][44][45]. On this basis, it is possible to further investigate the use of the methods based on the rising rate in flood events and real-time accumulated precipitation to determine flood warning indicators to improve the foreseeability and accuracy of flood warnings.…”

Flash Floods: Forecasting, Monitoring and Mitigation Strategies

“…In mountain basins, the sediment supply and its delivery have a significant impact on the geomorphological change in steep rivers with sediment deposition and erosion processes, thus determining the geometric boundaries where flash floods evolve. Therefore, Yang et al [3] used a depth-averaged two-dimensional hydrodynamic model to simulate a flash flood that occurred in Sanjiang Town, Sichuan, China, on 20 August 2019. Inflows and sediment deposition were the main factors that contribute to flash flood enlargement in confluence and bifurcation streams.…”