# Scour Detection with Monitoring Methods and Machine Learning Algorithms--A Critical Review
###### Abstract  
Foundation scour is a widespread reason for the collapse of bridges worldwide. However, assessing bridges is a complex task, which requires a comprehensive understanding of the phenomenon. This literature review first presents recent scour detection techniques and approaches. Direct and indirect monitoring and machine learning algorithm-based studies are investigated in detail in the following sections. The approaches, models, characteristics of data, and other input properties are outlined. The outcomes are given with their advantages and limitations. Finally, assessments are provided at the synthesis of the research. bridge scour; scour detection; scour monitoring; machine learning algorithms +
Footnote †: journal: Medical  
detailed examination report is prepared. During underwater inspections, the human eye might fail to detect scour. Ground-penetrating radars, water-penetrating radars, SONAR, and some specific laser systems are examples of equipment-based underwater examinations. Penetrating radars in boats of floating units emit electromagnetic waves and receive echoes from the soil. In one of the latest studies in this field, the researchers mapped the bathymetric profile (water depths and sediment thicknesses) of a river bed in North Ireland and managed to detect scour holes around upstream piers using a water-penetrating radar [8]. On the other hand, another study aimed to quantify scour depths in a river in Switzerland with a gravel bed and find the relationship between the subsurface and the structure of the riverbed using a ground-penetrating radar [9]. The researchers detected a four-meter-deep scour hole that could not be detected by bathymetric (water-based) surveys. Moreover, SONAR is another technology for collecting underwater images and therefore can help detect scour holes. A recent and successful study managed to locate and measure scour depths by combining SONAR data and a deep convolutional network [10]. Furthermore, it was revealed that although it performs worse in high turbidity, green laser technology was able to detect scour around bridge piers in an economic and safe way [11]. The scour development process and how it is affected by the flow, bed material characteristics, and geometry of the scoured elements have been investigated by hydraulic model-based studies. [2] is a sample study examining the non-cohesive sediment's movement in sloping and [12] is another study where the fundamentals of the scour development process and mathematical modeling of flows around bridge piers/piles, below pipelines, and other structures such as groins, breakwaters, and seawalls were examined. According to a former study [13] the basic idea behind scour development was believed to be the horseshoe vortex system [14]. Later, the whole turbulence scheme was shown to be the most important eroding flow process; horseshoe vortex is only an element of this scheme and the whole pattern also consists of downflow and acceleration of the discharge around the sides of the pier [10]. Experimental studies have been used to test the effects of different layouts, shapes of piers, and flow parameters on scour [3; 4; 15; 16]. Scour in clear water and accumulations around hexagonal arrays of emergent circular cylinders for various solid portions and an orientation of cylinders that resulted in less scour than an individual large pile was proposed [15]. Another study measured the complex turbulent discharge for different fluid velocities, Reynold shear stresses, and the density of the horseshoe vortex [4]. There were also studies on perpendicular wall abutments [5] investigating how channel width, shape, sediment dimensions, flow depths, and abutment lengths affect pier and abutment scour. Furthermore, there have been studies that simulate scour instead of building a hydraulic model that directly represents it. To identify scour, mode shape ratio, apparent profile, Eigen frequencies, and decentralized modal analysis methods were adopted by various studies, namely [17], [18], and [19].