PREDICTION OF COMPLEX PIER SCOUR USING GENE EXPRESSION PROGRAMMING REZA MOHAMMADPOUR (1) & AMINUDDIN AB GHANI (2) (1) Department of civil engineering, Estahban Branch, Islamic Azad University, Estahban Iran reza564@gmail.com (2) River Engineering and Urban Drainage Research Centre (REDAC), Universiti Sains Malaysia, Engineering Campus, Nibong Tebal, Penang, Malaysia Redac02@usm.my ABSTRACT Accurate estimation of scour depth at complex pier leads to an economic and correct design for level of foundation. Due to technical and economic reasons, complex piers (pier including foundation) are more general for bridge design. In this study, two robust techniques, artificial neural networks (ANNs) and Gene Expression Programming (GEP) were employed for prediction of scour depth at complex piers. A wide range of dataset was collected from present study as well as literature, the clear water condition was chosen for experimental tests. The result shows that the scour depth at complex piers is a function of pier diameter (b c ) and foundation level (Y). The RBF (Radial Base Function) network with R 2 =0.945 and RMSE =0.031 provides better prediction in comparison with previous equations and the GEP techniques (R 2 =0.811and RMSE=0.263). A formula was developed using GEP to predict local scour at complex piers. Although, accuracy of the RBF is higher than GEP, the GEP based formula is more useful for practical purposes and can be easily employed to predict the depth of scour at complex piers. This paper highlights that the mentioned techniques can be successfully used for accurate estimation of local scour and lead to the protection of bridge piers. Keywords: Neural Network; GEP; Local scour; Complex piers scour; Erosion. 1 INTRODUCTION Accurate estimation of scour depth at bridge pier is one of the major concerns during a flood hazard. Bridge piers are usually constructed on the foundation (pile cap) with or without the group of piles (complex piers). Due to technical and economic reasons, complex piers are more general for bridge design (Ataie- Ashtiani et al., 2010; Sheppard and Glasser, 2004). Generally, the foundation of complex piers can be found below the initial bed. Moreno et al. (2016) showed that scour around pile groups is caused by two types of mechanisms: (1) those causing local scour at individual piles, i.e., downflow, horseshoe vortex, wake vortices, and bow wave; and (2) those attributable to the interaction of the different piles, i.e., scour reinforcement, sheltering, wake vortex interaction, and compressed horseshoe vortices (Figure 1). Figure 1. Flow structure around complex piers (Moreno et al., 2016). Proceedings of the 37th IAHR World Congress August 13 – 18, 2017, Kuala Lumpur, Malaysia ©2017, IAHR. Used with permission / ISSN 1562-6865 (Online) - ISSN 1063-7710 (Print) 1635