The use of complex piers to build bridges has become widespread. In this study the experimental data was obtained to determine scour hole’s dimension and its changes versus pile cap elevation. Five models with different geometry were used in experiments under clear water condition and thresholds velocity. The models were made with an appropriate scale from real piers of bridges. A 3D illustration of the model for each experiment was drawn. The length, width and depth of the scour hole coordinates were measured in three directions. The results showed that similar to the scour depth, the scour length and width, apart from pier geometry, are significantly depended on the pile cap elevation. It was found that the variation in scour hole dimensions can be categorized into two cases as before and after pile cap undercutting. Moreover, the maximum length and width of the scour hole at each model were derived. ARTICLE HISTORY Received 14 July 2020 Accepted 17 July 2020 KEYWORDS Bridge; sediment; river; scour; complex pier 1. Introduction The local scouring phenomenon is one of the most common causes for bridges failure. Scour-related failure of bridges in streambeds involves interacting geotechnical and hydraulic processes (Ng et al. 2015). Finding an accurate method to predict the formation, development and final form of the scour hole is of great importance in designing hydraulic structures (Amini et al. 2011). In recent years, executive and economic issues in the construction of bridges have drawn engineer’s attention to complex piers. These piers are made from a column that is located on a pile cap and held by a pile group. Many studies have been focused on finding a relationship for calculating the scour depth in complex piers and their components using experimental data (Jones and Sheppard 2000). Amini et al. (2012) proposed relationships for determining the scour depth in the pile group. To esti- mate the scour at the complex and simple piers, the HEC-18 is one of the well-known methods which was presented by the Colorado State University (CSU) and proposed by the American Highway Administration, FHWA, for many years (Arneson et al. 2012). The other commonly used methods for computing the scour depth in piers with non-uniform geometries was presented by the Florida State Department of Transportation (FDOT). Superposition method is used in both of these methods. A method for scouring at complex piers that is based on real data, predicts reasonable scour depth (Jannaty, Eghbalzadeh, and Hosseini 2016) was pro- posed by Amini and Mohammad (2017). Based on the experimental simulation, Moreno, Maia, and Couto (2016)
