# Scour Detection with Monitoring Methods and Machine Learning Algorithms--A Critical Review
## 2 Conventional Monitoring-Based and Machine Learning-Based Methods to Identify Scour
### Direct Monitoring-Based Studies

Table 2: Properties of devices used in direct monitoring studies. rocks induced the distribution of the electromagnetic field and their locations were detected through an unmanned aerial vehicle, equipped with a locating algorithm. [45] preferred a spiral shape since this geometry brings the sensing points of the fiber optic sensor closer and to obtain a higher spatial resolution. Adjusting the number of rings around the piers changes the scour monitoring range along the length. Fibers are supported by the pier only in two points so that they can freely deform and so central wavelength differences can be obtained clearly. Studies that are not tabulated in Table 2 are those having a finite element model to represent the bridge and piers. [43], [47], [17][46], [31], [53], and [54] represented the bridge superstructure (deck and beams) with Euler-Bernoulli beams. Springs were assumed for the piers and the foundations beneath them. Different spring stiffnesses corresponded to different stages of scour. In their studies, [23] and [42] also included vehicles in their finite element models, i.e., vehicle-bridge interaction models. Vehicles were modeled as rigid masses and were connected with springs and dampers. [31], [43], [54], and [23] also verified their numerical approaches with experiments. The scour indicators in the research of [43], [42] and [14], and [53] were the frequency shifts of the bridge. While [63] used differences in the first mode shape amplitudes at pier locations, [17] and [54] chose mode shape changes. [54] specifically focused on horizontally-displaced mode shapes of a frame-type bridge. [23] set fitting curves to the mode shapes and used root mean square (RMS) differences between healthy and damaged mode shapes to indicate scour. [48], [49], and [50] detected scour through changes in the fundamental frequencies of the polymer rods. [52] used the increasing permittivity of the soil to identify scour; as scour developed, the sensors' permittivity suddenly increased. Higher apparent permittivity was related to the reduced soil density of the re-deposited sediment around the sensor. The outcomes of the direct monitoring studies were summarized here. [43] numerically proved that the frequencies decreased due to a reduction in foundation stiffness in two piers (mimicking 24.5% and 44.9% scour cases). In their experimental setup, cantilever-based piezoelectric energy harvesting devices were used and three scour scenarios were adopted. One device attached to the pier was successfully the detecting frequency shift in that pier. When the device was attached to a healthy pier, it could detect the scour of another damaged pier. For a better performance of the method, they suggested performing an initial modal analysis of the bridge with accelerometers since this way it would be possible to monitor obtained modal frequencies in the frequency domain of the device's voltage. In the research of [17], the normalized mode shape of a two-span integral-type concrete bridge changed by around 50%, with a 20% frequency difference for scour depth of 5 m. They noted that their approach might alleviate the known frequency-associated problems because the method is based on using the same sensors as the ones used for gathering the frequency. According to [46], abutments and piers had local vibration modes and the corresponding frequencies were only sensitive to scour related with the adjacent element. In their experiment, sensors were represented by virtual node points and located on two abutments and one pier. The method successfully identified the local frequencies and the location of the scour. However, they indicated that experimental validation through a field study was needed. [42] generated research for a two-span integral-type bridge. For 5 m of scour depth, due to central pier scour, the mode shape ratios of two points on the pier and deck changed by around 50% over the center of the foundation.