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26.Preliminary Study on the Impact of Flow Rate and Sediment Load to the Geometry Formation of Meanders(Noor Safwan Muhamad)Pp 181-190 | Sediment | Erosion

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    KONAKA 2013   181 Preliminary Study on the Impact of Flow Rate and Sediment Load to the Geometry Formation of Meanders in a Meandering River  Noor Safwan Muhamad  Junaidah Ariffin  Zulhafizal Othman   ABSTRACT This study demonstrates the development of meanders with respect to different flow rate and sediment load. The knowledge on meanders is important, not only because it changes the geometry of the river but it also can cause threats to the people and properties. With more than 150 rivers throughout Malaysia, the knowledge on meander behavior is vital to avoid problems associated with poor planning of the development at riverside. Eight sets of experiments have been conducted using Armfield S12 MKII Advanced Hydrology Study System at the Faculty of Engineering, UNISEL Bestari Jaya Campus. The experiments was conducted under controlled environment with flow rate and sediment load as the variable parameters while the other  factors that can affect meander growth and migration are kept constant. The changes in cross section were measured and 3D surface was generated using Surfer 9 to analyze the effect of variation in flow rate and  sediment load to the erosion, transportation and deposition processes that involved in meander meander  growth and migration. The relationship between flow rate and sediment load with the migration and growth  pattern of meanders are discussed well in this paper. This study is meant to be continued further to come out with suitable equations in predicting meander migration. Key words: meandering; river sinuosity; erosion; sediment transport capacity   Introduction Growth and migration of meanders not only change the river geometry but also posed threats to the people and structure. It may continuously erode the bank thus affect the properties and human activities along the floodplain area. In some scenarios, the deposition of eroded sediments can reduce the river carrying capacity and cause flooding, which normally occur at the river bend. Problem to maintain the stability of roads and  bridges may also become a concern when the meanders migrate downstream over time as the erosion scours its foundation. In order to reduce the possibility of flood occurrence, one of the widely used mitigation measures is to come out with channel improvement by straightening, widening and deepening the river or by reducing its surface roughness. However, it is well understood that the meanders are natural phenomenon that act as the energy dissipater for river flow thus disturbing it might affect the natural characteristic of the stream. To ensure that the channel migration did not cause losses to the human being and to avoid channel modification from affecting the ecosystem negatively, good understanding on meander growth and migration are crucial. Acknowledging the importance of meanders, large numbers of studies has been conducted to explore the causes and impact of river meandering, for example it is now accepted that secondary circulation can cause meanders as it erodes one river bank and deposit the sediments at the toe of the others. However, until now there is no established method to predict river migration, it is dynamic and there are many factors that can influence the process. Even though many studies were conducted worldwide, in Malaysia, there is still no research available focusing in meander migration. Therefore, this study will focus on the fundamental effect of two governing factors which are flow rate and sediment load to the geometry formation of meanders. The experiment was conducted using Armfield S12 MKII Advanced Hydrology Study System at the Faculty of Engineering, UNISEL Bestari Jaya Campus. Further investigations are required to correlate the growth pattern and migration of meanders with respect to different value of flow rate and sediment load.    KONAKA 2013   182 Background There are several literatures discussed about meander growth and migration, some of them are using flume test while other studies based on field data from real rivers. Meander growth can be defined as the change in meander dimensions over time, for example its radius of curvature, amplitude and width. Whereas, meander shift is the displacement of the bend in either downstream or upstream direction, most of the time it will shift downstream. The growth of meanders involves changes in its dimension. As the meander become larger, its amplitude and width increases. The same goes to the radius of curvature of the bend. To do research on meanders, knowledge on river geometry is crucial. Usually channel bends are treated as arcs; the parameters to describe meander geometry are shown in Figure 1. Figure 1 : Parameters Defining Meander Geometry (Briaud et al., 2007) The definitions of these parameters are as follows: A = meander amplitude W = channel width M = channel migration distance R = radius of curvature = = bend angle θ   = relative angle (0≤ θ   ≤ Ø) within each bend  t = time Since 1970s, study on meander migration has become the interest of the researchers in river engineering field. Soil-water interaction and geometry are widely studied in various ways to come out with the best prediction of meanders. These approaches can be divided into three categories: i.   Time-sequence maps and extrapolation ii.   Fundamental modeling iii.   Empirical equations Generally, the findings are not successful due to the complex characteristics of the river geometry and the dynamic of the flow. For the time-sequence map and extrapolation method, the researchers usually gather the series of topographic map and aerial photographs throughout the study period. These maps and aerial photographs can be obtained from local libraries and even websites such as the Google Earth. The method is quite simple and the study is based on a full-scale observation of real data, however, the disadvantages of this method are the maps and photographs might be limited to certain developed areas, and the assumption that the future flow and soil conditions will remain the same, which is not suitable due to development. Fundamental modeling consists of erosion process modeling at the interface between soil and water. This will then be projected by using future hydrographs. Fundamental modeling can mimic the actual  phenomenon at a specific site, and it can be utilized to model erosion at the particle level. However the disadvantages of using this method is that it requires site-specific measurement of soil properties, which is complicated and make it unique to that site only (Wang, 2006).    KONAKA 2013   183 An example of the empirical equation method of Keady and Priest (1977) for the estimation of migration rate correlates the downstream migration to the free surface slope of the river and meander amplitude as shown in Equation 1. The limitation of this formula is that it did not consider the rainfall that is one of the governing factors for flow condition. Rainfall can affect the flood peak and it is well understood that flood can result to a rapid and significant meander migration. Hooke (1980) equation predicts bank erosion rate using regression analysis. Equation 2 shows the formula proposed by Hooke which considers the area of catchment for different scenarios with respect to surface runoff, precipitation, infiltration and evaporation rates which are functions of location, climate and type of soil. Interest on meanders can be seen in the 21 st  century where there are increasing numbers of research  papers published. For the field-based study, the researchers are looking at the interaction between the rivers flow with the evolution of plan form and bed morphology over long run (Harrison et al., 2011; Hooke, 2008; Guneralp and Rhoads, 2009, 2010). While for the laboratory-based study, the researchers generally investigating on the flow and sediment transport behaviour in curved channels (Peakall et al., 2007; Termini, 2009 and Braudrick et al.,2009). Studies on real rivers usually are not preferred, as it require years of observation, and involved many unpredictable factors that make it difficult to forecast the future rates of meander migration. Nevertheless, most of the researchers agreed that the migration rates would slow down as the channel approaches a state of near stability. Other than the field and laboratory based investigation, some researchers preferred the theoretical and numerical modeling of meander morphodynamics (Luchiet al., 2011; Bolla et al., 2009 and Crosato, 2009) which involve complex modeling of the parameters. In 21 st  century, the researchers also show interest on submarine meandering, where they are focusing on the effect of turbidity current to the growth and migration of meanders and also the interchannel-sedimentation patterns (Amos et al., 2010; Parsons et al., 2010 and Abad et al., 2011). Zhang and Shen (2008) and Fu et al. (2009) also involved with the numerical simulation to investigate the growth and migration of meanders. They established a 3D simulation that was based on finite volume method. Although the correlation between natural and computed channel deformation can be achieved after the calibration of numerical models, it is not necessarily suitable to be applied to another river due to the varieties of influential factors. Rapid interest in meander migration can be seen where a lot of research has been conducted worldwide. This involved both the study based on field and laboratory data, which look at the effect of flow (Equation 1) (Equation 2)    KONAKA 2013   184  pattern and sediment transport to the meanders evolution. Generally, for a meandering channel, the centreline (not the thalweg) follow sine-generated curves, its Froude number is small, the flow is turbulent and the channel ratio of width over depth is large. The variation of deflection angle,   along the channel centreline, l c  and the channel sinuosity,   can be expressed as shown in Equation 3 and 4 (Dai, 2008). Using this equation, Dai and Tang (2010) come out with a mathematical model to simulate and  predict the migration and expansion of meander loops. From their findings, it shows that for small  o  (0<  o <=30 0 ), migration of meanders is the main deformation that will occur in a meandering channel while for large  o  (100 0 <  o <138 0 ), the migration and expansion will reduce. They also suggest that more numerical and experimental investigation should be done in order to validate the multiplier function of the migration and expansion found by them. M ethodology   A.   Equipment Setup The main equipment used for this study is the Armfield S12 MKII Advanced Hydrology Study System. It is equipped with a water storage tank and pumping system where the flow can be set between 0 to 2.75 L/min. For the platform, the 2 m long x 1 m wide x 0.2 m deep stainless steel tank is also connected to the dual linked jacking system so that the slope can be varied between 0 to 5 %. B.   Experimental Works Eight sets of experiments were conducted on a 2000 mm long, 40 mm wide and 20 mm deep channel, molded in the Armfield S12 MKII Advanced Hydrology Study System. The details of the parameters and apparatus used can be described as follows: i.   Size of sediments - Uniform sand was used to ensure homogeneity of the material. The median grain size, D 50  for the sediments used in this study is 0.80 mm. ii.   Flow rate  –   The submersible pump used in this study able to cater the flow between 0 to 2.75 L/min. From the preliminary study, no bed movement can be seen for the flow below 2.00 L/min. Thus, four different flow rates were chosen as the variable parameter for this study which is 2.00, 2.25, 2.50 and 2.75 L/min. iii.   Slope  –   The channel slope is important to maintain a constant depth of water, based on the literature, meander develops at a less steep area thus the initial slope was set at 0.5 % for the entire tests. iv.   Sediment load  –   The sediment load is one of the variable parameter for this study where two conditions were provided, there is either no sediment load or gradual sediment load. The material used is the same with the bed material where the median grain size, D 50  is 0.8 mm. The flow rate and sediment load for all the eight sets of experiment conducted are shown in Table 1. (Equation 3) (Equation 4)
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