The formation mechanism of mountain torrents under the known rainstorm process

As for the basic informationon about the formation mechanism of mountain torrents under the known rainstorm process, including the mechanism of runoff and sediment production on the slope, the movement mechanism of river flow and sediment in the mountain river and the formation of mountain torrents, researchers has made the following new progress:

The inducer, inducing mechanism, the critical condition of mountain torrents

Heavy rainfall is the main driving force for mountain torrents. Topography, landforms, vegetation and land use also play an important role in the formation of mountain torrents. The formation of mountain torrents is closely related to rainfall, rainfall intensity and rainfall history. High intensity concentrated rainfall is the main cause of mountain torrents. The occurrence of landslides and debris flows is closely related to the rainfall intensity in the early period and the short diachronic rainfall. There are three main causes, that is, the water infiltration and accumulation, which destroy the stability of the accumulation body, the mixing of the landslide soil and water, and water bursting banks. The study of critical conditions for mountain torrents includes the threshold and uncertainty of surface runoff, critical rainfall or rainfall intensity applied widely in the United States. The research methods include statistical analysis of historical data, hydrological history data and unit hydrologic process based on GIS technology and remote sensing technology.

Basic laws of flow and sediment dynamics in mountain rivers

Mountain torrents often induce active sediment movements. There is a substantial interaction between the sediment movement and the mountain torrents. The existing research involves the prototype observation and analysis of the process of water and sediment and the study of the basic transport law of the sediment. The main theory and formula of sediment motion mechanics are generally established for the constant uniform flow on the gentle slope, but the mountain torrents are usually unsteady and non-uniform flows on the steep slope. The sediment model is different from the actual observation result, and the main reason may be to calculate the motion of the sediment in the unsteady flow by the formula obtained under the condition of constant flow.

At present, the study of resistance to overland flow has already been quite extensive at home and abroad, but there are still the following problems.

1)The slope is limited to the gentle slope, which is generally less than 10 degrees.

2)Although the research about resistance to overland flow begins to carry on the experimental study of the rough moving bed, the data is limited, which affects the comprehensive analysis of its influencing factors and the further quantitative relationship.

3)Due to the lack of quantitative relation of resistance to overland flow with power process, overland flow and hydrodynamics models of watershed inviariably use the empirical parameters of open-channel flow (such as Manning coefficient), which causes the great uncertainty of the confluence of overland flows and the prediction of floods in mountain areas. So far, some progress has been made in the study of sediment transport under the action of weak unsteady flow, but it is still far from clear understanding of the law of sediment movement in mountain rivers under the action of strong unsteady and rapid flow. It is necessary to carry out systematic and in-depth study, which is of great value for understanding and forecasting the process of mountain torrents evolution.

Simulation and prediction of mountain torrents

As for the prediction of mountain torrents’ formation and evolution under different regional conditions, different experiences or mathematical models have been developed and established. The range and scale of the mountain torrents are comprehended by exercising mountain torrents' formation and evolution process. Generally, a statistical model for mountain flood formation is established by considering rainfall and terrain factors.

In recent years, large-scale meteorological models have been used to simulate rainfall processes and integrate with the flash flood formation models.Such as the high accuracy regional atmospheric model system (RAMS) and the synchronous satellite imagery simulation of rainfall, rainstorm unit processes and thermodynamic process models.

Among them, the simulation results of meteorological models are more accurate on large scales, while dose worse on small and medium scales. The prediction of the formation and evolution of mountain torrents is generally based on the hydrological, meteorological and runoff models which predict the movement process of mountain torrents in space, including the empirical statistical model, the distributed hydrological model based on the motion wave and diffusion wave (or its variant form and approximation), as well as the application of one dimensional hydrodynamic model.

In particular, the roll wave formed on steep slopes has also been studied, and a one-dimensional hydrodynamic model for mountain rivers has been developed.

In recent years, China has made some progress in flood forecasting. With the aid of distributed mechanism model and digital basin platform which developed the flood forecasting system, the simulation model of integration of runoff and sediment, slope collapse and river transport, are integrated to predict the runoff process, water sediment confluence and evolution process on regional scale.

Risk analysis method of mountain torrents

The study of flood hazard risk has been paid more and more attention, including the theoretical framework of flood warning system, the application of GIS technology in mountain torrents defense and the risk map, the basic framework of flood risk assessment and management, the study of flood control decision risk analysis based on flood forecast information; the mathematical model of flood loss, flood risk management model based on GIS, flood risk assessment models based on various new theories and new methods.

At present, the risk study of mountain torrents is moving from the analysis of history and status to the combination of prediction and research, from individual analysis to combination of individual and regional research, from qualitative analysis to quantitative evaluation, from single element analysis to comprehensive factor evaluation, from traditional survey and manual mapping to GIS Technology as the core of the model evaluation, computer graphics and network technology. However, the current definition of tprrential flood disaster’s risk has not been unified. The analysis of flood hazard risk is limited to a single subject, and there is no systematic theoretical system for the analysis of flood hazard risk. There is no unified standard for the content and method of flood hazard risk analysis.

Mountain torrents disaster is a kind of special flood disasters and there are not so many studies. Risk analysis of torrential flood disaster is a complex giant system. From the point of view of system theory, the mountain flood disaster system includes 4 subsystems, which are its natural environment, disaster causing factor, disaster bearing body, and disaster situation, and each subsystem contains a number of sub subsystems. For example, the disaster’s natural environment includes secondary subsystems such as space, time and humanistic and social background. And space can also be divided into atmospheric environment, hydrometeorological environment and underlying surface environment, which are mixed together through material circulation, energy conversion and information transfer to form the drive, feedback and development of the system as well as the changing mechanism.

Mountain torrents disaster system is a complex nonlinear system with multi-level structure, especially the randomness and dynamics of the system and the correlation among subsystems.

In addition, there are many uncertainties in various factors of torrential flood disaster system, such as randomness, fuzziness, grey character and chaos. It is of great practical significance for researchers to systematically study these problems to effectively prevent mountain torrents disasters.