Model Law Reserch On Rainfall Intensity In Spray Atomization Due To Flood Discharge

Affected by continuously changing wall surface and air friction, the high-speed water flow is strongly turbulent and aerated, when high dam discharging flood water. Small part of water body gradually loses stability, disengages from main flow and smashes into water drops. Then, the aerated jet flow fall down and collide into waterbody downstream. The spray water caused by collision, as well as water drops, splash and sprinkle in the downstream and form huge intensity rainfall. Some of tiny water drops float in the air and come into being fog flow.The practice shows, the flood spray atomization appeared in large water conservancy project have much more threat and damage on dam architecture and shore slope downstream than conventional nature rainfall, for its tremendous intensity and enormous scope. As the present research result, the influences on project by flood spray atomization are nearly negative. The huge rainfall and strong fog bring by it always result in the collapse of shore slope (such as Er'Tan project in China), shutdown of power generation for transformer problem by strong fog (such as HuangLongTan dam in China), traffic halt of road approach to dam and plant (such as DongFeng project in China),and influence on daily work and life of people near the downstream of dam, some of them are forced to move to other places ( such as ZaXi dam in China) .With more and more high dams built in narrow valley in the west of China and more cases emerged , that flood spray atomization affecting the operation of dam and plant, this phenomena began to gain wide recognition by academia, as an engineering and environmental issue. In China, the realated researches began in 1980s and the issues of flood spray atomization were listed in the top 7 engineering hydraulics problems. The flood atomization in Er'Tan plant and Three Gorges project are studied in this period and achieve some results.At present, researches on flood atomization are basically carried on in three ways, namely prototype survey, physical model test and numerical analysis. Among of them, physical model test gained much more attention for the intuitionistic result and quantificational description on this issue. Nevertheless, the model law problem is controversial in academia.According to the result of prototype observation, the flood atomization brings rainfall and fog flow. Howere, as the limit of model scale, it is very difficult to simulate the fog flow in prototype. Or even it can be, the precision hardly be ensured for a mass of influence factors. Based on this, the present physical model is limited in research on the intensity of rainfall(mm/s) .According to Froude's simulated rule, the scale of rainfall intensity (Rr) and the scale of physical model should obey the following relationship: Rr = Lr0.5, but result of test differ from this.In this paper, the model law on rainfall intensity caused by water spray is further explored through the prototype experiment and model research, and the new fitting formula on that is given by the raindrop spectrum analysis.The reserch method is to integrate the prototype survey with a series of corresponding model tests.Firstly, the prototype survey tests on rainfall intensity of J dam were made in 2003. Then a series of model tests with different scales were carried on the basis of the prototype survey data. After compareing prototype survey data with model test data, the model law of rainfall intensity was researched and the fitting formula was presented.Through the prototype and model research on rainfall in flood spray atomization, it is shown that the intensity of rainfall has no direct relationship between the prototype and model values. Otherwise, on the basis of raindrop spectrum analysis, it is discovered there has two different riandrops which demonstrate different contribution to the rainfall intensity. And this lead to a new relationship between prototype rainfall intensity and model one. After a series of models research, the new model law on rainfall intensity are presented in this paper. And with this model law, flood spray atomization of GouPiTan dam is researched by physical model and the reseach results guided the slope defence design in this project.