# 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
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Study Ref.** & **Device Type** & \begin{tabular}{c} **Sensing** \\ **Mechanism** \\ \end{tabular} & \begin{tabular}{c} **Signal** \\ **Processing** \\ \end{tabular} & \begin{tabular}{c} **The Target** \\ **Property of** \\ **Signal** \\ **Processing** \\ \end{tabular} & \
\begin{tabular}{c} **Scour Validation** \\ **Tests** \\ \end{tabular} &
\begin{tabular}{c} **Laboratory or** \\ **Field Tests** \\ \end{tabular} & **Target Property** \\ \hline
[45] & \begin{tabular}{c} Unconstrained \\ distributed fiber \\ optic sensors \\ \end{tabular} & \begin{tabular}{c} Ultra-weak fiber \\ Bragg grating \\ \end{tabular} & \begin{tabular}{c} Empirical \\ formula \\ \end{tabular} & \begin{tabular}{c} Central \\ wavelengths \\ \end{tabular} & \
\begin{tabular}{c} Detecting \\ different signals \\ of set of fibers \\ embedded in \\ sand and end fiber \\ fibers freely \\ in water \\ \end{tabular} &
\begin{tabular}{c} Standard \\ deviation value \\ higher than zero \\ and location \\ \end{tabular} \\ \hline
[44] & \begin{tabular}{c} Velocity sensors, \\ inclincometer, \\ wireless \\ transmitter, \\ and camera \\ \end{tabular} & \begin{tabular}{c} Higher \\ transform and \\ sensors \\ \end{tabular} & \begin{tabular}{c} Industrial \\ empirical mode \\ decomp. \\ \end{tabular} & \begin{tabular}{c} Single-pier \\ instant \\ frequencies \\ \end{tabular} & \
\begin{tabular}{c} Caisson-type \\ laboratory \\ sour test \\ \end{tabular} &
\begin{tabular}{c} Rigid body \\ motion \\ \end{tabular} \\ \hline
[48] & \begin{tabular}{c} Piezoelectric \\ Polymer Film \\ \end{tabular} & \begin{tabular}{c} Wavelet packet \\ transform and \\ Polymer Film \\ \end{tabular} & \begin{tabular}{c} Instant \\ transport and \\ transport \\ \end{tabular} & \
\begin{tabular}{c} Flume test \\ frequency of \\ the rod \\ \end{tabular} &
\begin{tabular}{c} Test with \\ different pier \\ cross-sections \\ \end{tabular} & \\
[49] & \begin{tabular}{c} Rod sensor \\ \end{tabular} & \begin{tabular}{c} Piezoelectric \\ Polymer Film \\ \end{tabular} & \begin{tabular}{c} Fast fourier \\ transform \\ transform \\ \end{tabular} & \
\begin{tabular}{c} Instant natural \\ frequency of \\ the rod \\ \end{tabular} &
\begin{tabular}{c} Clamped to a \\ laboratory bench \\ Planted in sand \\ implemented in \\ \end{tabular} & None & \\
[50] & & & & & & \\
[51] & & & & & & \\ \hline
[51] & \begin{tabular}{c} Direction- \\ Unknown and 1 \\ Direction- \\ Known \\ smart rocks \\ \end{tabular} & \begin{tabular}{c} Ambient \\ magnetic field \\ \end{tabular} & \begin{tabular}{c} Theory of \\ magnetic field induced by \\ smart rocks \\ \end{tabular} & \begin{tabular}{c} Distribution of \\ the magnetic \\ itself \\ \end{tabular} & \begin{tabular}{c} Field validation \\ tests \\ \end{tabular} & \
\begin{tabular}{c} Tests on the \\ upstream side of \\ a pier \\ \end{tabular} &
\begin{tabular}{c} Localize the \\ position or track \\ the move of the \\ smart rock \\ \end{tabular} \\ \hline
[52] & \begin{tabular}{c} E.magnetic \\ sensors \\ the soil \\ \end{tabular} & \begin{tabular}{c} Changes in the \\ feature of \\ the soil \\ \end{tabular} & \begin{tabular}{c} The reflection \\ feature of \\ e.g.magnetic \\ waves \\ \end{tabular} & \begin{tabular}{c} The porosity of \\ the soil \\ \end{tabular} & \
\begin{tabular}{c} Static’scour \\ scintillations \\ Real-time open \\ channel \\ flume tests \\ \end{tabular} & Not provided &
\begin{tabular}{c} Scour depth \\ variation \\ \end{tabular} \\ \hline
[55] & \begin{tabular}{c} Unmanned \\ Aerial Vehicle \\ -based smart \\ rock positioning \\ system \\ \end{tabular} & \begin{tabular}{c} 3-axis \\ magnetometer \\ locale smart \\ \end{tabular} & \begin{tabular}{c} Algorithm to \\ locate smart \\ rocks using \\ presence and \\ magnetic \\ intensities \\ \end{tabular} & \begin{tabular}{c} Magnetometer \\ measuring \\ magnetic fields \\ before and after \\ the smart rock \\ has been \\ deployed \\ \end{tabular} & \
\begin{tabular}{c} 1-44W \\ Koolvidous \\ Creek Bridge \\ \end{tabular} &
\begin{tabular}{c} Depth of \\ scour, Le, \\ vertical move of \\ the rock \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table}