# 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
Deformation changes are detected by ultra-weak fiber Bragg grating which is written in the optical fiber. Since temperature changes at pier at an elevation of around 10 meters below water are slow and small, the central wavelength of the fiber is only affected by the altered strains, i.e., deformations of the fiber. Differences in central wavelength changes give scour length. Some innovative researchers used innovative technology, i.e., smart rocks for scour monitoring. [51; 55]. Based on the theory of the magnetic field, developed algorithms were able to find the locations of the rocks, which were rolled down into scour holes. The proposed system for \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Monitoring Type**} & \multirow{2}{*}{**Study Reference**} & \multirow{2}{*}{**Numerical Method and Sensor Technology**} & **Presence of Experimental** \\  & & & **Cases /Field Tests** \\ \hline \multirow{8}{*}{Direct} & [42] & Mode Shape Ratio & None \\ \cline{2-3}  & [43] & Vibration energy harvesting device & Yes \\ \cline{2-3}  & [44] & Hilbert Huang Transform & Yes \\ \cline{2-3}  & [45] & Fiber Optic Sensors & Yes \\ \cline{2-3}  & [46] & Eigen frequency & None \\ \cline{2-3}  & [47] & Frequency Domain Decomposition & None \\ \cline{2-3}  & [23] & Decentralized modal analysis & Yes \\ \cline{2-3}  & [48] & Frequency analysis of piezoelectric rod sensors & Yes \\ \cline{2-3}  & [50] & Unmanned Aerial Vehicle using smart rocks & Yes \\ \cline{2-3}  & [31] & Smart probes & \\ \cline{2-3}  & [52] & instrumented with electromagnetic sensors & Yes \\ \cline{2-3}  & [53] & Micro energy harvesters & None \\ \cline{2-3}  & [54] &
\begin{tabular}{c} Horizontally-displaced mode shapes and changes \\ in dynamic flexibility \\ \end{tabular} & Yes \\ \cline{2-3}  & [55] & Unmanned Aerial Vehicle-based smart rock & Yes \\ \hline \multirow{8}{*}{Indirect} & [56] & \multirow{2}{*}{Wavelet transformation} & \multirow{2}{*}{None} \\  & [37] & & \\ \cline{1-1} \cline{3-3}  & [57] & & \\ \cline{1-1} \cline{3-3}  & [18] & Eigen frequency & None \\ \cline{1-1} \cline{3-3}  & [19] & Closed-form mode shape derivation & Yes \\ \hline \hline \end{tabular}
\end{table} 
Table 1: List of monitoring-based scour detection studies. scour monitoring of other studies [31; 52] had a probe with an integrated electromagnetic sensor that identifies different dielectric permittivity values of the soil that surround the foundation of the bridge. Using the frequency of the oscillators implemented in the sensors, measuring the capacitance which only changes with the dialectic property of the soil around them was proven possible [33]. The monitoring system of [53] on the other hand, consisted of MEMS harvesters to detect pier scour, which gathered energy from environmental energy sources to produce electricity. Further expiation of the authors about these micro energy harvesters revealed that depending on the frequency change induced by the oscillation of the electrode, having the rent as output was possible and the frequency was detected via a small wireless apparatus. There are many various transformation methods to decompose signals gathered from different sensors. Fourier transform is one of the most common signals processing methods in a frequency domain analysis. Fourier integrals containing the Fourier transform represent non-periodic (arbitrarily changing) excitations [58]. Direct Fourier transform equals the o multiplication of the Fourier transform and complex frequency response function. To obtain a response in the time domain, inverse Fourier transform is applied. Fourier transform is best applied to stationary and transient signals since it results in average frequency [59]. Hilbert spectrum is also an amplitude in the frequency-time domain; however, it is used for processing non-stationary signals as well. [50]. [60] indicates one of the most significant features of the Hilbert transform--it could detect frequency response nonlinear effects.