# Scour Detection with Monitoring Methods and Machine Learning Algorithms--A Critical Review
## 2 Conventional Monitoring-Based and Machine Learning-Based Methods to Identify Scour
### Methods, Properties, and Main Outcomes of Studies  
#### 2.1.1 Cluster 1--Conventional Monitoring-Based Approaches to Detect Scour  
Table 1 provides the type of monitoring, numerical approach, and dynamic structural property addressed, and the presence of experimental/field data in the studies. Descriptions of direct and direct monitoring systems, technical information about the sensors, and analysis methods used in the studies in Table 1 are explained in this section. Scour monitoring could be classified under direct and indirect methods [38]. In direct monitoring, structural parts of the bridge are equipped with sensors. The sensors are fixed while in indirect monitoring, the vehicle passing over the structure is instrumented with measurement devices, i.e., sensors are moving. Indirect or so-called drive-by monitoring utilizes the responses of the sensors on the vehicles to extrapolate the condition of the bridge [38]. These responses (measurements) could be accelerations, displacements, or speed. There are some challenges of direct monitoring as well as its major advantages of ensuring improvement in public safety early risk detection and minimizing downtime [39]. There are some disadvantages. Direct monitoring is not suitable for all applications, the whole scheme is costly, and there are some undesired implications such as high maintenance costs. This was where indirect monitoring became more advantageous since it was found to be more economical, independent from the features of the structural system to be assessed, and provided maintenance-friendly measurements. The indirect monitoring technique is also known as drive-by monitoring and enables scour identification without any traffic interruptions to the railway or highway operations on the bridge. In direct monitoring, signals are obtained from structure form input data, while signals of single run or multiple vehicle batches are products of indirect monitoring. To extract the dynamic properties of the bridge, data were processed with numerical approaches. Utilizing displacements of a beam which were found from a vehicle-bridge interaction finite element model was aimed to calculate the distribution of the flexural rigidities throughout the length retrospectively [40]. They also sought to estimate the damage level of the bridge. For detecting damage, it is also possible to eliminate the dependence on reference signals from a healthy bridge. Another study focused on monitoring analyzed the difference between right and left accelerations of the train (i.e., roll component) which occurred in the presence of deformation, to prove the imbalance in the bridge behavior [41]. The studies listed in Table 1 use numerous sensors. Accelerometers were located at the mid-spans and piers of the bridges. Signals from the superstructure of the bridge were collected using velocity sensors to reveal the dynamic features of scour [55]. Their experimental setup consisted of inclinometers, a camera, a level transmitter, and a wireless transmitter as well. As the flow continued, the velocity sensors sensed vibrations and the inclinometer showed the angle of tilt during scour. Besides accelerometers and velocity sensors, piezoelectric rods, electromagnetic sensors, micro-electro-mechanical systems (MEMS), vibrational power generators, optic sensors (image tracking), and permittivity change of the soil were utilized for the detection of scour. When the water flowed, the buried piezoelectric rods were subjected to hydrodynamic effects resulting in vibrations that created voltage in the rods [48]. A frequency domain analysis of the voltage-time history of the rod will result in the length. As sediment moves away, the length of the rod increases and this is felt through the change in the frequency. The difference between the initial and instantaneous length will give scour depth. Moreover, unconstrained distributed fiber optic sensors are one of the recent technologies used for investigating scour. In the research of [45], fibers deform freely under water flow.