# Near-Nature Ecological Technique for Pier Scour Countermeasure in a Submerged Overfall
## 3 Experimentation and Methodology  
### Experimental Setup  
All the experiments in this study were conducted in a flume with a length of 15 m and a width of 1 m (Figure 6). The flume was made of a steel frame with 0.8 m high transparent reinforced glass on the side wall. A local deepen area with a length of 2 m, a width of 1 m, and a depth of 0.4 m was designed 7 m downstream of the entrance of the flume for fully developing the scour hole. The hunched platform was 4.4 m in length and was used to keep the flow steady. The height difference between the top of the platform and the flume bed downstream was 0.08 m. A circular pier with a diameter of 0.08 m (\(b\)) was placed 0.75 m (\(L_{d}\)) downstream of the hunched platform. The ratio of the channel width and pier's diameter is larger than 10; thus, the side wall effect can be ignored [21]. A 0.0.73 m high gate was designed at the end of the flume to elevate the water table. All flow from upstream was stored in a tank at the tail of the flume. The water was then pumped from the tank to the upstream entrance and reentered the flume after being filtered.  
Figure 5: Illustration of CKAW’s protective effect. (**a**) Without the installation of CKAW; (**b**) With the installation of CKAW.  
Figure 6: Setup of the experiment flume.  
The laboratory test was performed in clear-water and live-bed conditions. As shown in Figure 6, sand was placed in the deeper area. At the rest of the bottom, sand with 0.025 cm depth was filled to maintain the same roughness of the downstream flume bed and prevent discontinuous roughness of flume bed from inducing scour. No sediment supplement from upstream was used in this experiment work. The threshold of sediment motion can be designed according to the filled sand and the hydraulic boundary condition of the experiment flume [35; 42; 43]. The live bed in the experiment was filled with sand, of which the median diameter (D\({}_{50}\)) of sediment was 0.46 mm, the geometric standard deviation (\(\sigma_{g}\)) was 1.69, and the specific gravity specific (Gs) was 2.64. When the ratio of the pier's diameter and the median diameter is larger than 50, the local depth of the pier scour is not influenced by sediment size [44]. Although the maximum depth of the pier scour cannot neglect the armor effect and the viscous effect, the relative change in the depth of the pier scour due to the CKAW is not affected in this study [21; 43; 45; 46]. For the consideration of the hydraulic boundary conditions, the flow depth and flow velocity were designed to reach the threshold of sediment motion in the clear-water condition. The discharge in the flume was 0.0247 m\({}^{3}\)/s. The upstream flow depth (\(h_{u}\)) at the hundred overall platform was 0.051 m, and the average velocity (\(V_{u}\)) was 0.484 m/s. The flow depth (\(h_{d}\)) and average velocity (\(V_{d}\)) after overall were 0.125 m and 0.198 m/s, respectively. The Froude number of flow was 0.685 and 0.179 before and after overall, respectively. [PERSON] and [PERSON] (1988) suggested that the critical mean approach flow velocity (\(V_{c}\)) can be estimated as 0.329 m/s using  
\[\frac{V_{c}}{V_{sc}}\ =\ 5.75\ \times\ \log\left[5.53\ \times\ \frac{h_{d}}{D_{50}} \right], \tag{5}\]  
where \(V_{sc}\) is the shear velocity when sediment is used, and it can be determined as 0.018 m/s by using the Shields diagram [47]. The flow intensity of downstream (\(V_{d}/V_{c}\)) was maintained at 0.6. It is greater than 0.5, thus satisfying the occurrence condition of scour in clear water [44]. The aforementioned conditions in this study are summarized in Table 2.