Application of a soft computing technique in predicting the percentage of shear force carried by walls in a rectangular channel with non-homogeneous roughness

Boundary shear stress is an important parameter in steady, fully developed open channel flow. Trustworthy prediction of boundary shear distribution in open channel flow is very difficult in many critical engineering problems, such as sedimentation, channel design and energy loss calculation. The boundary shear stress distribution in rectangular channels with smooth or rough boundaries has been investigated by many researchers, such as Knight (1981), Knight et al. (1984) and Bilgil (2005). Due to difficulties in measuring shear stress distribution in channels by direct and indirect methods, a number of studies have been extended in order to calculate this by different approaches. Some studies have focused on numerical or analytical means of predicting shear stress distribution (Berlamont et al. 2003; Guo & Julien 2005; Yu & Tan 2007; Ansari et al. 2011; Bonakdari et al. 2015; Sheikh & Bonakdari 2015).

Nowadays, soft computing methods of forecasting various phenomena in all fields are widely exploited. Alternatively, soft computing techniques, such as artificial neural networks (ANNs), evolutionary computation, fuzzy logic, genetic programming (GP) and gene-expression programming, have been successfully applied for water engineering problems in recent years (Azamathulla & Jarrett 2013; Ebtehaj & Bonakdari 2014; Zaji & Bonakdari 2015; Azamathulla 2015). Boundary shear force carried by walls is a major parameter related to mean shear stress. Only a few studies have addressed predicting shear stress in channels using soft computing methods. Cobaner et al. (2010) investigated the ability of the ANN approach to predict the percentage of shear force acting on walls (%SF_w) in smooth open channels and ducts. The authors indicated that the ANN model's function was better than previously obtained equations of other researchers. In this study, the capability of GP and genetic artificial algorithm (GAA) models in predicting the %SF_w in rectangular channels with non-homogeneous roughness is investigated. Consequently, two models are developed and the best one is selected. The function of the best model is also compared with equations obtained by Knight (1981), Knight et al. (1984, 1994) and Seckin et al. (2006) for rectangular channels, and an equation proposed by Rhodes & Knight (1994) for smooth ducts.

GP was introduced by Koza (1994) and is among the more practical genetic algorithm (GA) applications. The GP method is very similar to the GA, except that the chromosomes are in fact computer programs. GP begins with a random initial population consisting of certain computer programs. Each computer program is run to evaluate the cost of each chromosome. The cost function is calculated using fitness functions. Thus, by sorting out the initial population costs, and performing crossover, mutation and elite GA processes, a new population is achieved. The main objective of GP is to find a computer program that can accurately predict shear force using the given input variables.

Three major variables affect GP performance: (i) selecting the appropriate input variables; (ii) selecting the functions that GP is permitted to use in the computer programs (functions are arithmetic operators such as + and − or mathematical functions such as exp, sin, power and logical functions like OR and NOT; in this study, four different function combinations are tested); and (iii) selecting a suitable fitness function (a fitness function is used to evaluate the efficiency of each individual, and selecting the appropriate fitness function directly affects the model's performance). The mean squared error (MSE) and mean absolute error (MAE) statistics methods are herein regarded as fitness functions.

Upon selecting the best input combination for each model, the best fitness function was selected. In the GP model, input combination (2) and the default function were considered and two fitness functions MSE and MAE were tested. The results indicated that for the GP model, the MAE fitness function with RMSE of 0.0521 outperformed the MSE fitness function with RMSE of 0.053. However, the MSE fitness function for the GAA model with RMSE of 0.0791 outperformed MAE for the GAA model with RMSE of 0.0873.

The final step in modeling with the GP and GAA methods differs for each method. In selecting the best GP model, four basic arithmetic operators and the mathematical functions were employed. When input combination (2) and MAE as the fitness function were used, four different structures were chosen to identify the best GP model. The principal range of the investigated functions is:

The results signify that function F2 with RMSE of 0.0515 performs superior to functions F1, F3, and F4 with RMSE of 0.0521, 0.0629 and 0.0699, respectively.

The efficiency of soft computing methods in predicting the %SF_w in rectangular channels with rough boundaries was investigated in this study. GP and GAA models were developed in three steps. First, the effective parameters were selected and different input combinations were tested to select the optimum combinations. Then the best fitness and transfer functions were studied and the superior one was accepted for both models. Finally, after extending the two models, their ability to predict the percentage of shear force was examined. According to performance results, the GP model predicted %SF_w more accurately than the GAA model. The results of the proposed model were also compared with five equations presented by other researchers. The GP model predicted %SF_w with lower error (RMSE = 0.0515) and higher accuracy than the equations by Knight (1981), Knight et al. (1984, 1994), Seckin et al. (2006) and Rhodes & Knight (1994) with RMSE of 0.0642, 0.2413, 0.2327, 0.2424 and 0.2063 respectively. The equations proposed for smooth rectangular channels and ducts predicted overestimated %SF_w values. It can be deducted that the relations for smooth boundaries do not yield more accurate results for estimating %SF_w in channels with rough boundaries except for high aspect ratio (when B/h > 5). The equation obtained by Knight (1981) produced the most appropriate results after the best GP model, as the equation was also presented for rectangular channels with rough boundaries.