# Near-Nature Ecological Technique for Pier Scour Countermeasure in a Submerged Overfall
## 4 Results and Discussions  
### Protective Effect Due to the Size of Strips of CKAW  
Circular strips with diameters of 0.1, 1, and 2 mm and rectangular strips with dimensions of 25 mm \(\times\) 0.25 mm were applied with a length of 25 cm, which was one-third of the distance from the overall platform to the bridge pier to clarify the protective effect of the strip size. The change in the flume bed due to the protective effect of CKAW is shown in Figure 10a-d. In the case of the thinnest strip (\(D_{a}\) = 0.1 mm) in Figure 10a, the change extent of the flume bed was less than that in the other cases, and the range and size of scour hole were reduced compared with the case without CKAW. A comparison between the protective effect of the circular strips shows that the larger strips had less protective effect; almost no protective effect was found in the case of \(D_{a}\) = 2 mm (Figure 10c). The rectangular strip did not provide a good protective effect either (Figure 10d). Figure 10: Sand bed changes due to different sizes of strips (the erosion and deposition contour is 1 cm; a negative value means erosion). **(a)** Case 1: \(D_{a}\) = 0.1 mm, \(L\) = 25 cm; **(b)** Case 2: \(D_{a}\) = 1 mm, \(L\) = 25 cm; **(c)** Case 3: \(D_{a}\) = 2 mm, \(L\) = 25 cm; **(d)** Case 4: rectangular shape (25 mm width, 0.25 mm thickness), \(L\) = 25 cm. Figure 9: Water surface of Case 0 (without CKAW) after the pier scour reached an equilibrium state. The protective effect due to the size of strips of CKAW is discussed based on the topography of Cases 0, 1, 2, 3, and 4. The difference of maximum scour depth at the pier (\(d_{s}\)) between ones with and without CKAW is listed in Table 4. With the use of Equation (6), we learn that the best protective effect was achieved with strips that have the smallest diameter (Case 1), in which the protective effect (\(P_{s}\)) was 27.3%. This finding shows that CKAW protects the flume bed from being scoured by overfall and pier. When the diameter of the strips increased, the protective effect decreased, thus indicating that smaller strips better protected the flume bed from scour. This result occurred because when the thin strips were with more numbers than in other cases, they could slow the downward flow velocity more efficiently by the swinging strips and change it into horizontal flow downstream, as shown in Figure 5.  
Figure 11 shows the changes in the centerline of the flume bed due to erosion and deposition. The scour depth of the flume bed around the bridge pier decreased after CKAW was applied. Case 1 shows an apparent difference from the others on the landforms in the front of the pier. Significant scour was found in the tail of the arranging strips; after that, a significant deposition was found almost close to the original flume bed. Case 1 shows the largest deposition at the back of the pier, indicating a weak sediment transport capacity near the bridge in Case 1 due to the protective effect. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Group** & **Case 0** & **Case 1** & **Case 2** & **Case 3** & **Case 4** \\ \hline Strip size (mm) & N/A & 0.1 & 1 & 2 & \(\begin{array}{c}b_{a}=25\\ b_{s}=0.25\end{array}\) \\ \hline Flexibility index \(F_{i}\) & & 248 & 46 & 110 & 69 \\ \hline Flexibility parameter \(F_{p}\) & & 11.4 & 6.1 & 1.3 & 1.4 \\ \hline Maximum scour depth at pier \(d_{s}\) (cm) & \(-11.0\) & \(-8.0\) & \(-9.1\) & \(-10.5\) & \(-10.4\) \\ \hline Protective effect of scour depth at pier \(P_{s}\) (\%) & – & 27.3 & 17.3 & 4.6 & 5.5 \\ \hline Half length of surface wave (cm) & 10.5 & 9.5 & 9.5 & 9.0 & 9.0 \\ \hline Protective effect of surface wave’s half length \(P_{l}\) (\%) & – & 9.5 & 9.5 & 14.3 & 13.3 \\ \hline Magnitude of surface wave (cm) & 3.4 & 2.1 & 2.7 & 2.4 & 3.4 \\ \hline Protective effect of surface wave’s magnitude \(P_{m}\) (\%) & – & 38.2 & 20.6 & 29.4 & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Protective effect with different size of strips.