مجله آب و خاک (Feb 2016)
Assessment of Estimation Methods ForStage-Discharge Rating Curve in Rippled Bed Rivers
Abstract
Introduction: Interactionbetweenwater flow characteristics andthe bed erodibilityplays an important role in sediment transport process. In order to reach stability, rivers with deposition or bottom erosion make a different bed form in the riverbed. One way to identify thebehavior of therivers is to study the structure and formation of bed forms within them. Ripples are the smallest of the bed forms. The longitudinal cross section of ripples are usually not symmetrical. The upstream face is long and has a gentle slope, and the downstream face is short and steep. The height of ripples is usually between 0.5 cm and 2 cm; the height ripple is not more than 5 cm. The wave lengths normally do not exceed 30cm, and they are usually within the range of 1 cm to 15 cm. Their occurrence is the result of the unstable viscous layer near the boundary. They can form in both shallow and deep water.With an increase of the flow velocity, the plan form of the ripples gradually develops form straight line to curves and then to a pattern like fish scales, symmetrical or unsymmetrical, as shown in Fig 1. Figure1-The patterndevelopment oftheripple Raudkivi (1966) was the first person that, the flow structure over ripples was investigated experimentally.Hethenestablishseveraldifferent conditionsonthemovingsandbedinanlaboratorychannelconsisted of a rectangular cross-section with base width of 70cm, wasable toform arow ofripples , he wassucceed toform arow ofripples.JafariMianaei and Keshavarzi(2008),studied the turbulentflow betweentwoartificialripples for investigate the change of kinetic energyandshearstress on overripples. The stage- discharge rating curve is one of the most important tools in the hydraulic studies. In alluvial rivers,bed rippled are formed and significantly affect the stage- discharge rating curve. In this research, the effects of two different type of ripples (parallel and flakeshape) onthe hydraulic characteristicsof flow were experimentally studied in a flume located at the hydraulic laboratory ofShahrekordUniversity, Iran. Bass (1993) [reported in Joep (1999)], determined an empirical relation between median grain size, D50, and equilibrium ripple length, l: L=75.4 (logD50)+197 Eq.(1) Where l and D50 are both given in millimeters. Raudkivi (1997) [reported in Joep (1999)], proposed another empirical relation to estimate the ripple length that D50 is given in millimeters: L=245(D500.35) Eq. (2) Flemming (1988) [reported in Joep (1999)], derived an empirical relation between mean ripple length and ripple height based on a large dataset: hm= 0.0677l 0.8098 Eq.(3) Where hm is the mean ripple height (m) and l is the mean ripple length (m). Ikeda S. and Asaeda (1983) investigated the characteristics of flow over ripples. They found that there are separation areas and vortices at lee of ripples and maximum turbulent diffusion occurs in these areas. Materials and Methods: In this research, the effects of two different type of ripples onthe hydraulic characteristics of flow were experimentally studied in a flume located at the hydraulic laboratory of ShahrekordUniversity, Iran. The flume has the dimensions of 0.4 m wide and depth and 12 m long. Generally 48 tests variety slopes of 0.0005 to 0.003 and discharges of 10 to 40 lit/s, were conducted. Velocity and the shear stress were measured by using an Acoustic Doppler Velocimeter (ADV). Two different types of ripples (parallel and flake ripples) were used. The stage- discharge rating curve was then estimated in different ways, such as Einstein - Barbarvsa, shen and White et al. Results and Discussion: In order to investigateresult of the tests, were usedst atistical methods.White method as amaximum valueofα, RMSE, and average absolute error than other methods. Einstein method offitting the discharge under estimated. Evaluation of stage- discharge rating curve methods based on the obtained results from this research showed that Shen method had the highest accuracy for developing the stage- discharge rating curve than other methods. It also showed that theShenmethod was much accuracy by the parallel shape of ripplebed form compared with the flake shape. Conclusion: Evaluation of stage- discharge rating curve methods based on the obtained results from this research showed that Shen method had the highest accuracy for developing the stage- discharge rating curve than other methods. It also showed that theShenmethod was much accuracy by the parallel shape of ripplebedform compared with theflakeshape. The results of the estimation of statistical parameters such as root mean square error (RMSE) and mean percent error for these cases indicated that the Shen method is associated with the lowest RMSE and error percentage. Therefore it estimates the stage- discharge rating curve more accurately than other methods. Furthermore in the case of parallel and flake ripple bed forms correlation coefficient was obtained as 0.87 and 0.43 respectively. This indicates that the Shen method is more accurate for the parallel shape of rippled bed forms than the flake shape ones.
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