# Near-Nature Ecological Technique for Pier Scour Countermeasure in a Submerged Overfall
## 2 Concept of Artificial Water Weeds and Its Protective Effect against Pier Scouring  
Water weeds (Figure 1) protect riverbed sediment from being picked up by flow due to a toppling effect and slow the flow velocity close to the bed. The proposed CKAW technique uses an artificial material that similarly to natural water weeds in flow. CKAW is shown in Figure 2. Figure 2a shows the response behavior of the artificial water weeds to water flow. CKAW has two major parts (Figure 2b): one is a fixed part that secure CKAW on the riverbed or structure, and the other is a swinging part that arranges flow and reduces the flow velocity close to the bed by swinging with its long and slender strip. The variables in the protective effect of CKAW include the length, shape (i.e., circular, rectangular), size, material composition, specific gravity, arrangement of water weeds, and density of water weeds. The length affects the influence area of CKAW on the flume bed, the different shapes provide different flow stabilizing effects, and the size affects how the weeds swing--slender strips correspond to better movement with the flow. The material affects the stiffness of the strips and weed swing, and the specific gravity determines whether the strips float or sink. The arrangement of the water weeds is consistent with the change in geocondition--the denser the water weeds are, the more difficult the water flow becomes through the space between strips. Among the variables, this paper discusses two major ones: length and size. The density of water weeds (\(R_{d}\)) is a controlled variable, and as shown in Figure 2b, it can be defined as  
\[R_{d}\;=\;\frac{nb_{a}}{B}, \tag{1}\]  
Figure 1: Water weeds topple due to flow. Figure 2: (a) Swinging of CKAW due to flow; (b) details of CKAW. where \(B\) represents the width of CKAW, \(n\) represents the number of strips, \(b_{a}\) is the width of the strip, and \(b_{a}=D_{a}\), \(D_{a}\) is the diameter of the strip is the strip is circular. \(R_{d}\) in this paper is \(R_{d}=1\). The flexibility of the artificial water needs plays an important factor in arranging the approaching flow and reducing its impact on the local scour. To consider water weeds as a bundle of flexible rods, the flexibility of the rod depends on the material property, size, shape, and length of the rod. For a rigid rod, the flexibility coefficient, \(f=l/EA\), can be used to represent the deflection per unit load, where \(l\) is the length of the rod, \(E\) is the Young's modulus, and \(A\) is the cross-section of the rod. In rods made of the same material, the degree of deflection depends on \(l/A\) when they are subjected to force. In the rods with the same length, a small cross-section corresponds to increased flexibility of the rod. In rods with the same size, a greater length corresponds to increased flexibility of the rod. However, for a rod made of softer material, representing its flexibility coefficient using \(f\) is difficult because this sort of material deflects greatly once a force is applied. To represent flexibility of a rod made of softer material, this paper uses a flexibility index (\(F_{i}\)), \(F_{i}=L_{a}/\sqrt{A}\), in which \(L_{a}\) represents length of the very soft rod, and \(A\) represents its size. For an extremely soft rod, such as a strip, its flexibility index, which is different from that of rigid rod, is redefined as follows to obtain a reliable value:  
1. Parts of strip are cut with different length \(L_{a}\) for tests. 2. The cross-section of each strip is measured. 3. A smooth and moist glass is used as an experiment platform (Figure 3). 4. The strip is submerged into water and then placed on the platform after it absorbs water fully. 5. A thin strip is used to roll forward from the center of the strip so that the strip deflects due to the viscosity of water and friction of the glass. 6. The bent width, \(L_{w}\), is measured after the bent arc of the strips stabilizes. 7. The relationship between \(L_{w}/L_{a}\) and \(L_{a}/\sqrt{A}\) is plotted for each test (Figure 4).