International Journal of Agriculture and Crop Sciences. Available online at www.ijagcs.com IJACS/2012/4-21/1565-1572 ISSN 2227-670X ©2012 IJACS Journal
Numerical Study of the behavior Algorithm of pollutants movement in open channel hydraulics with compound cross-sections Roozbeh Aghamajidi1, Mohammad Heidarnejad1, * 1. Department of Irrigation, Science and Research Branch, Islamic Azad University (IAU), Khouzestan, Iran *
Corresponding author’s email:
[email protected]
ABSTRACT: Generally, knowledge and awareness of the way how two flows influence each other and the changes of one matter in a different liquid are among the significant issues in hydraulics. It is of great importance in hydraulics to know and identify the behaviors and distribution patterns of materials to study the likely happenings. This study attempted to investigate the spread and distribution patterns of pollutants in open channels in compound cross-sections (such as Crosssections of Torrent) by means of pure and applied hydraulic methods. To this end, calculation were conducted through applying the classic equation of Computed hydraulics (one – dimensional model materials distribution in liquids).Then, the Shiono and Knight two – dimensional model was used to determine the hydraulic parameters of flow in the Ackeres hypothetical river by the software. Afterward, these parameters were applied to the one – dimensional transfer equation to investigate the materials density changes in the river for both spatial and temporal dimensions. Results revealed that if we apply the effect of flow between the channel and the Torrent section, the longitudinal propagation coefficient decreases because the amount of flow and its effective strength fall. Therefore, the determined density reaches the target location in the channel at a longer time period and a longer movement and the temporal distribution for transfer from the point of departure will be obtained. Keywords: Torrent, Computer simulation, Distribution, Mike11, Compound Channel INTRODUCTION Almost water reaches the ground through rainfalls. The first source to pollute water is the atmosphere which encompasses dust and different gases. The water from rainfalls or glaciers meltdown flows on the ground or penetrates the lower layers of soil and mixes with a variety of materials and as such loses its quality. Natural water, which is mostly (H2O) in its chemical compound, is abundant on the earth; 98.7 percent of the earth is covered by oceans, natural ice, rivers and lakes. Salty and fresh ground waters also account for 1.2 percent of the total waters. The population explosion and development of industries have led to more demands for water. Throughout history, there have been increasing needs to water for drinking, health, fishing, agriculture, recreation, shipping, industrial products, cooling plants which run by fossil and atomic fuels. The highest demand for water in terms of quantity (such as watering and cooling factories) has the fewest limitations in terms of quality. Drinking water resources and specialized industrial producers may have the most intricate demands for water quality but their needs to quantity are limited. In addition to this typical water usage, there are many other human activities which indirectly impact on water bodies. Rivers are the most important drinking water Resources for humans. In the past, social, economical and political developments greatly depended on the access to and distribution of water in rivers. By studying changes in river discharges throughout a period of time, human interference in a rivers water regime can be detected in a way that depleting the river water results in the reduction of water flowing out of the river and the loss of quality downstream. Because of all these factors, investigating quantitative and qualitative elements of river waters seems necessary. Pollution transfer in rivers is mainly important longitudinally. That is why the majority of studies with this respect and the models developed for this purpose are one- dimensional. Because the hydraulic conditions of flow and the geometry of the channel cross section play an important role in the pollutants transfer, this study aims to examine this issue, especially for compound cross- sections in which the geometry greatly influences the flow hydraulics. The difference between flow
Intl J Agri Crop Sci. Vol., 4 (21), 1565-1572, 2012
velocity on the main cross section and the flood plain caused by depth variation and differences in roughness coefficients of these cross sections is trivial. Therefore, this triggers shear tension and energy loss on the boundary of the main and the flood flows which not only influences the whole flow, but also it can influence the small flows on the main and the flood cross section. The term compound cross-section refers to waterways which have floodplain. The degree of the impact of the interaction between the main channel flow and floodplain is represented in the relative depth parameter H which is defined as the proportion of water depth in the floodplain to water depth in the main channel because computing the propagation coefficient requires the hydraulic flow parameters and pollution transfer is studied in compound cross-sections. Therefore, these parameters should be predicted in the compound cross-sections through the present methods. Moreover, the transfer part in the one dimensional transfer- diffusion includes the flow velocity. Thus, to solve the propagation equation, it is necessary to predict the cross-distribution of velocity in channels with compound Cross-sections. The earliest investigations by Silin (1964) and Zelzeniakov (1971) confirmed the presence of vortexes and their effect on the velocity and discharge of flows in floodplains (Abril, 2002). British researchers studied compound cross-sections on rigid beds at two levels (Ayyoubzadeh, 1994). The first level included the study of compound channels with fixed Wall and beds and direct paths and at the second level they studied meandering channels with fixed wall and beds (Myers., et al, 2001). Ackers, (1992-1993) illustrated all the parameters influencing the degree of the conflict between the main channel flow and the floodplain flow in a dimensionless parameter called coherence and as a onedimensional model. Myers et al (2001) examined the effect of wall roughness on the flow velocity and discharge in compound channels. They concluded that the cross-section analysis method can better predict the compound channel discharge. Moreover, this method yields acceptable results when it is utilized for experimental data for flat floodplains. Haidera et al (2002) put forth the method for predicting the total discharge in rivers with compound cross-sections and rigid wall. This method investigates compound channels at two states; first, when the relative depth equals zero and, second, when the water level in the channel reaches the maximum flood level. Atabay., et al (2002) scrutinized the effect of relative width in the discharge- scale equation in compound channels and came up with the total discharge - scale equation for all FCF channel positions with the same roughness coefficients and different relative widths. As far as pollution is concerned, after Streator and Philips (1925) formula predicting the amount of dissolved oxygen in rivers, various models for the quality of rivers and reservoirs have emerged and evolved according to which numerous numerical and mathematical models have been developed. A lot of studies have also been carried out regarding longitudinal propagation coefficient after Taylors (1954) studies on flows in pipes ( Zheng., et al, 1995) MATERIALS AND METHODS To conduct this study, that hypothetical Ackers river whose main cross-section is 15 meters wide at the bottom, the side slopes side slop of the main channel and the floodplain are 1:1, the Manning roughness coefficient is n0=0.03 and the Manning roughness coefficient is nf=0.06. Based on Abril’s (2002) studies about finding the flow direction on compound cross-section, the waterway slope is S0=0.003 and the studied river length is considered to be 20 Kilometers. Shiono and Night (1998) proposed a two-dimensional model for cross changes of the average velocity at the depth and the walls shear tension on different cross-section including the compound Trapezoidal crosssection. This method was developed based on flow stability and uniformity conditions and the principle equations dominating the flow in terms of Navir-Stoks equations (Ayyoubzadeh, 1997; Ayyoubzadeh and Zahiri, 2003). Shiono and Night suggested the following equation to determine the flow velocity in areas with a stable depth(Ayyoubzadeh and Zahiri, 2003, 2004, 2009): 1) u =
A1 e w + A 2 e − w +
8g S0H f
1/ 2
And the following equation for areas with an instable depth or side slope (0
