# Near-Nature Ecological Technique for Pier Scour Countermeasure in a Submerged Overfall
## 4 Results and Discussions
### Protective Effect Due to Length of Strips of CKAW  
This section examines the protective effect due to 4 different lengths of strip, namely, one-third, one-half, two-third, and one times the location of maximum scour depth (\(L_{d}\) = 75 cm) (cf. Cases 2, 5, 6, and 7 in Table 3). The size of the strip in Case 2 (\(D_{a}\) = 1 mm for circular strips) was used. The erosion and deposition distribution of the flume bed is shown in Figure 14. The beds deformed upstream were similar to each other, showing that the influence from the length was not significant upstream. The downstream scour reduced with longer strips. Therefore, CKAW had a greater protective effect when the length of strips reached the maximum scour depth. Figure 14: Sand bed changes due to different lengths of strips (the erosion and deposition contour is 1 cm; a negative value means erosion). (**a**) Case 2: \(D_{a}\) = 1 mm, \(L\) = 25 cm; (**b**) Case 5: \(D_{a}\) = 1 mm, \(L\) = 37.5 cm; (**c**) Case 6: \(D_{a}\) = 1 mm, \(L\) = 50 cm; (**d**) Case 7: \(D_{a}\) = 1 mm, \(L\) = 75 cm. Figure 13: Response of water surface due to different strip sizes. Figure 15 shows the change in the flume centerline with strips of different lengths. With longer strips, the scour in front of the pier was reduced. When the strips reached the pier's location, the scour occurred only at the pier's surrounding area, which was different from the other cases. For cases with lengths of \(L_{d}=1/2\) and \(L_{d}=2/3\), the scour distribution and the scour depth were similar to each other. The protective effect achieved with different lengths of strips is listed in Table 5. When the length of arranging strips reached the location of the pier (Case 7), the best protective effect (\(P_{s}\)) was achieved, being equal to 45.5%. The reason for this condition was thought to be that the velocity component in \(x\) direction \(V_{x}\), which induced scour near the pier due to the contraction effect, was mainly dominated by the pier after the length of the strips reached a certain value. In other words, the CKAW capacity in transforming the velocity from vertical to horizontal component did not increase with the length without limitation, but reached a certain value instead. Overall, the CKAW provided good protection for the flume bed from the location of the overall, in which a major velocity component was in the \(z\) direction \(V_{z}\), when the strip length was close to the maximum scour depth. Eventually, the scour depth reduced. Figure 16 shows the time history of flume bed changes at \(0^{\circ}\) point of the pier location. In the first 5 min, scour depth developed rapidly in the case with and without CKAW. Afterward, the protective effect was gradually observed. The cases with longer strips showed a slower development of scour depth, reaching the minimum scour depth eventually. As mentioned previously, the development of the scour hole was more stable in the case with CKAW. Once the length of the strip exceeded \(0.5\)\(L_{d}\), the developing history of the scour hole was not deeper than the case without CKAW, thus implying a significant protective  
Figure 15: Change of centerline with CKAW with different lengths of strips.