Study on the Separate Characteristics of Oil Spill Driven by Combined Water and Wind in Braided Rivers

In general, there are many bifurcations in natural rivers. When the oil spilled accident occurred at the upstream of bifurcation and was not controlled in time, the pollution scale would be expanded and the treatment would become harder as the oil slick entered into braided river driven by combined water and wind. To Study the separate law at the bifurcation in rivers, a two-dimensional numerical simulation model of oil spilled was established by oil particle model theory on the basis of two-dimensional hydrodynamic model. The quantitative relationship between the oil separate ratio and the flux ratio, velocity ratio, width ratio was obtained driven by water. The wind effect on the proportion of oil separate was also discussed based on the oil spilled numerical simulation. This study will provide technical support for forecast and response of oil pollution in braided rivers.


Introduction
The simulation model of sudden oil spill pollution incident is an important content of oil slick emergency control and disposal system construction. The rapid quantitative formula of oil slick based on control parameters can predict the migration and expansion process of oil slick in water and provide decision support for emergency disposal.
In recent years, sudden water pollution incidents have occurred in rivers, lakes, water diversion projects and other water areas in China, which have seriously affected the water environment of the waters and the drinking water supply along the river. It has become the current focus of attention in response to sudden water pollution incidents. For this, domestic scholars have focused on the research of water hydrodynamic characteristics, engineering regulation, pollutant transport and diffusion characteristics, pollution disposal technologies and devices (Lian, et al., 2006;Lian, et al., 2013;Gu, et al., 2011;Tao, et al., 2013). Sudden oil pollution is one of the main types of sudden water pollution in rivers. For sudden oil pollution incidents in river systems, realizing the characteristics of oil pollution and identifying the range of oil pollution is the basis for rapid and efficient pollution disposal. The earliest proposed oil slick center-of-gravity trajectory combined with the oil slick expansion empirical formula method, the equivalent ellipse of the oil spill area is given by extending the empirical formula, and the oil pollution range is obtained after being added to the oil-oil center drift position. But it only applies to open water areas such as the ocean and estuary. The oil slick dynamics model method has operational limitations, because it assumes that the oil slick is a continuum during the change and movement process, and can not consider the oil slick fracture process.
Based on the Lagrangian theoretical system, the oil particle model discretizes the oil spill into a large number of oil particles. The oil slick expansion is simulated by the random movement of oil particles, and the calculation efficiency is high (Zhang, et al., 2008), which has become the mainstream method for the current study on oil spill numerical simulation. REINALDO (Reinaldo, et al., 1999) designed a fourthorder Runge-Kutta method to calculate oil particles to improve the calculation accuracy of river oil spill simulation. M.SAYED (M.SAYED, et al., 2008) developed a simple and easy oil spill simulation model based on oil particle model theory. providing decision support for oil pollution disposal in the St. Lawrence River.
In China, relevant scholars (Jiang, 2007;Zhang, et al., 2010;Zhao, et al., 2012) take the tidal reach of huangpu river, the Yangtze river, the han river and the Three-Gorges reservoir as examples, and use the oil particle model to carry out the oil spill numerical simulation study of sudden oil spill accident, and the research results lay a foundation for the emergency response and disposal of oil spill pollution in multiple types of waters.
At present, the river oil spill simulation is mostly based on the typical section of the river, and there is a lack of the research results on the oil spill separate characteristics of the braided rivers. Based on this, the paper has developed the study on the separate characteristics of oil spill driven by combined water and wind in braided rivers. Based on the two-dimensional hydrodynamic model of braided river, a numerical simulation model is established by using the theory of oil particle model.
Firstly, the influence of oil spill expansion time on oil flow shunt is analyzed, and then the oil separate ratio under different flow, velocity and braided width is studied, and the influence of wind on oil spill separate characteristics is also analyzed. The research results are expected to provide technical support for the prediction of the impact range of oil spills in the braided river and the disposal of oil pollution.

Two-Dimensional flow equation
in which, η = + h dis total water depth；d is static water depth；η is water level；u、v is the velocity of x、y direction, respectively；T xx 、T yy 、T xy is the horizontal viscous stress term； bx τ 、 by τ is the shear force of the bed surface； u 、 v is the average velocity in the direction of depth.

the migration and expansion simulation model of oil spill
(1) Expantion After the spilled oil enters the water, the spread area of oil slick with the change of time can be calculated with modified Fay's gravity-viscous formula： 4 ，is the spread area of oil slick，m 2 ； R oil is the radius of oil slick，m； α K is the coefficient，s -1

。
(2) Advection Transport and Turbulent Diffusion Oil slick transport refers to the translatory movement of oil particles under the influence of wind, surface current, and waves in the surrounding waters. The formula for calculating the drifting velocity of the oil slick is: in which ， ( ) The new position of the i th oil particle moving from moment n to moment n+1 is: in which, X and Y are the coordinates of oil particles.

Setting of research object and simulation parameter
As shown in Fig.1, the study on the oil spill separate characteristics of the braided river was carried out with the example of oblique branch and vertical branch. (b) vertical Fig.1 The forms of braided rivers The numerical simulation of oil spill only considers the physical change process of expansion and drift, and ignores the chemical change process of oil spill evaporation, dissolution and emulsification. The main parameters of the model are shown in table 1. By changing the boundary conditions of the two-dimensional hydrodynamic numerical simulation model, the hydrodynamic data under different diverging conditions are obtained as shown in table 2.

The influence of oil slick expansion time on the oil separate ratio
As shown in Fig. 2, the flow would be separated in the channel branches. The distribution of surface separate width determines the amount of oil entering the channel branches (Cao, et al., 2003). The oil slick expansion time is the primary factor influencing the oil distribution within the braided river width. Based on the condition of scenario 3, by changing the distance between the oil spill point with the branch inlet, the influence of the oil slick expansion time on the oil distribution rules is simulated. Simulation results is shown in Table 3. Time revised coefficient is defined as the ratio of the oil separate ratio of each operating scenario to the oil separate ratio of the scenario 3-6.
It can be seen from Table 3 that as the oil slick expansion time increased, the amount of separated oil increase gradually; when the oil slick expansion time is greater than 1600s, the amount of separated oil tends to be stable. When t ≤1600s，the relationship between the time revised coefficient of the oil separate ratio and the oil slick expansion time is shown in Fig.3.
When the oil slick expansion time t T < ， the relationship expression between the time revised coefficient of the oil separate ratio and the oil slick expansion time is as: Where a 、 b 、 T are related to the width of the main channel.
The value of the parameter when the main channel width is 50m: The value of the parameter when the main channel width is 30m:

Study on the separate characteristics of oil driven by water flow
The influence of water flow on oil separate ratio is mainly reflected in the separate width. Literature (Yang, et al., 2006) studies have shown that the surface split width increases with the increase of the separate ratio 1 0 / η = V V . Under different hydrodynamic conditions, the larger V1 is, the larger s B is, and the larger the oil volume W1 flow into the braided river is. The flow rate Q1 and width 1 B of the braided river will have a significant effect on the split velocity V1.
Based on the scenario 1~13，the oil spill point is set at the central position of the river at 2km upstream of the braided inlet (the same as scenario 3-6 in table 3

Study on the separate characteristics of oil spill driven by combined water and wind
The influence of wind on the proportion of oil separation is reflected in changing the oil distribution within the width of flow split. Based on the flow condition of scenario 3, the oil spill point is set at the central location 2km away from the upstream of the branch inlet. The simulation results of oil separate are shown in table 5. It can be seen from the simulation results in table 5 that the influence of wind on oil separate ratio is related to the size of wind speed, wind direction and the acting time of wind: (1) Comparing the scenarios of 3-12~3-14, it can be known that when the wind and the main river flow are in the same direction, and the influence of wind speed on the oil separate ratio can be neglected; Comparing the scenarios of 3-15~3-17, it shows when the wind deflects towards the distributary reach., the amount of oil volume in the braided river increases as the wind speed increases.
(2) Comparing the critical scenarios of 3-18~3-23 which the separate oil amount are zero, it can be seen that when the wind field action time is the same (scenarios of 3-18~3-21), the components of the vertical speed of the wind speed are the same, as shown in Fig. 5. When the greater the wind speed, the shorter the time required for the wind field (scenarios of 3-21~3-23).

Conclusion
In this paper, a widely used oil particle model is adopted to study the separate characteristics of oil slick in braided river. The research results can provide the basis for the prediction of the oil spill range and the disposal of oil spill in the braided river. The specific research conclusions are as follows: (1) Under the driving of water flow, the longer the oil film expansion time, the more adequate the oil slick expansion; Under the condition of oil slick sufficiently expansion, the oil separate ratio oil K is linear with increases with the increase of wind speed; When the wind deviates from the braided river, oil K decreases with the increase of wind speed; When the component perpendicular to the shore of the wind are the same, the wind has the same influence on the oil separate ratio.
Beijing Municipal Natural Science Foundation (8181001). The authors acknowledge the assistance of anonymous reviewers.