Near-Nature Ecological Technique for Pier Scour Countermeasure in a Submerged Overfall

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A new near-nature ecological technique was proposed to protect against local scour induced by drop structures and bridge piers, and its protective effective was investigated through a series of experimental tests. According to the experimental result, this study suggests the potential application and optimal assignment of the proposed method.

Abstract

This paper proposes a near-nature ecological technique, which can consist of a wide range of materials, to protect against pier scouring. The proposed technique can involve the use of many long strips that behave like water weeds. This paper studied a protection method against pier scour by using long strips in a submerged overfall, particularly for a pier located at the maximum depth area of overfall scour. The length and size of the strips were chosen as factors to study their protective effect. Our results showed that this approach slowed the flow velocity between the installed strips and bed. The sediment in flow might accordingly move slowly or even settle down. Thus, the pier could be kept stable and safe by the installation of those strips. Experimental results show that the protective effect is more efficient when the strip length is closer to the pier and has a small diameter. Moreover, the maximum protective effect reached 45.5% scour reduction. Varied lengths provided different protective effects against overfall scour. These findings show that this near-nature ecological technique could be a good and economical solution for pier protection in submerged overfalls.

1. Introduction

Overfall often takes place at the downstream of a grade-control structure, dam, or reservoir, and scours the riverbed, thus posing a risk to the stability of local structures (i.e., bridge piers) [1,2,3,4]. The investigation of local scour has been researched for many years. According to Schoklitsch (1932) [5], the maximum scour depth induced by drop structures and bridge piers is related to water table differences between its downstream and upstream, discharge per unit width, tailwater depth, and median diameter of riverbed sediment. The above pioneering work generated much scientific and engineering interest in the following years. Varied empirical formulas have been proposed for the estimation of local scour due to drop structures and bridge piers under different conditions of structure types, approach flow, and bed material [6,7,8,9,10,11,12,13,14,15]. Such contributions are important to effectively control overfall-induced scour and diminish the scour around piers, an area that is influenced by scour hole.

To prevent local scour below drop structures and at bridge piers, construction work for protection and/or stilling basin is generally conducted in engineering practice to resist scour and/or diminish overfall energy [16]. The frequently applied protective construction involves the use of rectangular concrete block, irregular block, and ding-shaped block to resist scour and energy dissipation [17]. A flow-altering device can be another option to minimize scouring through flow adjustment [16], such as by mitigating the down-flow motion by using a collar plate near the base of piers, decreasing the flow velocity by using a series of surface guide panels around the pier, and decreasing the eddy near the pier by using counter-eddy bottom panels [18,19,20,21,22]. Among the engineering methods for natural river management, near-nature ecological engineering has been attracting attention, especially in developed and developing countries [23,24,25]. This study aims to develop a near-nature ecological structure to control the local scour induced by drop structures and bridge piers.

The use of geosynthetic components to control erosion and sediment materials has been considered a more economic and effective solution than costly concrete structures in some circumstances since the late 1950s [26,27]. Generally, geosynthetic components range from temporary work such as hydraulic mulch geofibers, plastic erosion control meshes and netting, erosion control blankets and reinforcement mats, geotextiles, and fabric-formed revetments [25,28]. Their applications were introduced and discussed in past studies [29,30,31,32]. With the awareness of the need for environment protection nowadays, geosynthetic materials need to be less disruptive to the environment. Many naturally occurring textile products can be used to solve geo-environmental engineering problems. For example, southeast and southwest Nigeria have an abundance of vegetation from which soft fibers such as flax, hemp, jute, and coir are obtained [33]. Many of these natural fibers, such as malvaceae and penduculata, are used by farmers in these regions to stabilize soil and control erosion.

To solve river erosion, the authors have conducted many field investigations and found that the sediment protected by hard man-made concrete structure sometimes performs worse than sediment protected by water weeds growing in the river. The effect of vegetation on the change of flow features, i.e., flow velocity, flow turbulence, wave dissipation, and drag force, has been studied widely [34,35,36,37,38,39]. Due to the effect of vegetation, the flow-induced bed shear stress can be reduced so that the capacity of sediment motion can be weakened [35,36,37,38,39,40,41]. Therefore, the authors introduced a new way to control sediment and erosion in a river. This paper proposes a new technique composed of easily obtained materials and installation method aimed at controlling erosion and sediment movement in the river. This simple technique uses long strips that behave like natural water weeds to provide shelter on the riverbed. The protective effect on a bridge pier against pier- and submerged overfall-induced scour is demonstrated with strips of varying lengths and sizes, proving the effectiveness of the proposed technique.

In this study, we propose a new near-nature ecological structure called Cheng Kung artificial water weeds (CKAW) to control the local scour induced by drop structures and bridge piers. The concept of artificial water weeds is introduced in Section 2. A series of laboratory tests was conducted to investigate the protective effect of artificial water weeds on local scour. The implementation of the experiment and the estimation of the protective effect are illustrated in Section 3. Then, the protective effect of artificial water weeds is discussed and shown in Section 4. The potential application and optimal assignment of artificial water weeds for local scour control are presented in the final section.

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

$$R_d = \frac{n{b}_a}{B},$$

(1)

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 weeds 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:

• Parts of strip are cut with different length L_a for tests.
• The cross-section of each strip is measured.
• A smooth and moist glass is used as an experiment platform (Figure 3).
• The strip is submerged into water and then placed on the platform after it absorbs water fully.
• 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.
• The bent width, L_w, is measured after the bent arc of the strips stabilizes.
• The relationship between L_w/L_a and L_a/$\sqrt{A}$ is plotted for each test (Figure 4).

Strips made with four different diameters of the circular cross-section and materials, listed in

Table 1. Flexibility index of strips made of different materials and sizes.

Material Composition and Application	Diameter Da (mm)	$\mathit{\alpha }$	$\mathit{\beta }$	Correlation Coefficient R2	Flexibility Index Fi
Cotton dressmaking	0.1	2.5276	−0.01305	0.9226	248
Cotton dressmaking	1.0	1.4780	−0.05840	0.9212	46
Nylon craft use	1.5	3.2063	−0.01618	0.9950	211
Silk and satin Chinese knots	2.0	1.7109	−0.02562	0.9789	110

, were tested. The relationship between the ratio of bent width L_w and the length of strip L_a, (L_w/L_a) and L_a/$\sqrt{A}$ is shown in Table 1 and plotted in Figure 4. L_w/L_a and L_a/$\sqrt{A}$ show an exponential relationship in Equation (2), and the correlation coefficient is more than 0.9. This equation represents the degree of the bent arc along with the change in length while the strip is made of a certain material.

$$\frac{L_w}{L_a} = \alpha \exp \left(\beta \frac{L_a}{\sqrt{A}}\right),$$

(2)

Figure 4 shows that when L_w/L_a = 0.1, an obvious change of L_a/$\sqrt{A}$ occurs. Therefore, we define the flexibility index as L_a/$\sqrt{A}$ at L_w/L_a = 0.1, as shown in Equation (3).

$$F_i = \frac{L_a}{\sqrt{A}} \left(if \frac{L_w}{L_a} = 0.1\right),$$

(3)

This index is a characteristic of material property. When F_i is smaller, it represents that the strip is softer; for a perfect rigid rod, as L_w/L_a = 0.1 is required, a very large L_a is needed, and F_i is defined as infinite at this moment.

For flexibility of a flow arranging strip with length L in real, F_p represents the flexibility parameter of arranging strips placed on site. F_p denotes the ratio of L/$\sqrt{A}$ to F_i defined in Equation (4). When the value of F_p is large, then it means the arranging strips are soft. The perfect rigid strips have a value of F_p = 0.

$$F_p = \left(L/\sqrt{A}\right)/F_i.$$

(4)

The hydraulic characteristic of CKAW’s protective effect is explained in Figure 5. When flow passes through a submerged overfall, the velocity can be divided into horizontal and vertical components. V_x and V_z represent the flow velocity in the original condition, and V_px and V_pz represent the flow velocity in a riverbed sheltered by CKAW. Assuming that the resultant velocity in the front fringe of horizontal platform in the protected and unprotected conditions is the same, the only change in the flow velocity is the direction of the component. After passing through the strips, the flow tends to transfer to x component and shows V_px > V_x and V_pz < V_z. The flume bed is sheltered from scour because the scouring energy is diminished, thus reducing the scour depth.

In restraining overfall-induced scour, the main factor is to control the downward flow velocity V_y and restrain pier-induced scour. Thus, controlling V_x is a main factor. A greater contraction effect of pier induces, a larger V_x and local scour at the pier will occur. In this paper, the topography of the flume bed was measured after the local scour reached an equilibrium state. The interaction between scour depth (d_s) and CKAW is discussed in detail in the following sections.

3. Experimentation and Methodology

3.1. 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.

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 (D₅₀) of sediment was 0.46 mm, the geometric standard deviation (σ_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 m³/s. The upstream flow depth (h_u) at the hunched overfall 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 overfall were 0.125 m and 0.198 m/s, respectively. The Froude number of flow was 0.685 and 0.179 before and after overfall, respectively. Melville and Sutherland (1988) suggested that the critical mean approach flow velocity (V_c) can be estimated as 0.329 m/s using

$$\frac{V_c}{V_{*c}} = 5.75 \times \log \left[5.53 \times \frac{h_d}{{\mathrm{D}}_{50}}\right],$$

(5)

where V_*c 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. Experiment conditions.

Discharge	Water Depth Upstream	Water Depth Downstream	Velocity Upstream	Velocity Downstream	Flume Bed Difference 1	Water Table Difference 1
m3/s	m	m	m/s	m/s	m	m
0.0247	0.051	0.125	0.484	0.198	0.08	0.006

¹ The difference is between upstream and downstream.

.

3.2. Experimental Procedure

The implementation of the experiments can be separated into two parts. First, the installation of CKAW was set according to the experimental design. Second, the topography of riverbed was measured after the local scour reached an equilibrium state. The detailed procedures of the experiment are illustrated as follows.

This paper investigated the effect of CKAW on protecting against scour from bridge piers and discussed the influence of different sizes and lengths of the strips in detail. The rest of the variables were kept the same for a convenient comparison. Generally speaking, the strip size influences its swinging motion and ability to protect the flume bed. The strip length affects the protection range and extent. The strips can be made of nylon, which is the same material as Chinese knots, with different sizes and larger density than water. CKAW were installed from the front fringe of the platform in one layer with a close arrangement (R_d = 1) in this test, as shown in the right side of Figure 6. The material composition of CKAW and the experiment conditions are listed in

Table 3. Composition of CKAW and study cases.

Cases	0	1	2	3	4	5	6	7
Shape of strip 1	N/A	C	C	C	R	C	C	C
Diameter of strips Da (mm)	N/A	0.1	1	2	ba = 25 bs = 0.25	1	1	1
Density of strips (g/cm3)	N/A	1.141	1.141	1.120	1.063	1.141	1.141	1.141
Flexibility index Fi	N/A	248	46	110	69	46	46	46
Length of strips L (cm)	N/A	25	25	25	25	37.5	50	75
L/$\sqrt{A}$	N/A	2821	282	141	100	423	564	846
Flexibility parameter Fp	N/A	11.4	6.1	1.3	1.4	9.2	12.2	18.3
2L/Ld	N/A	1/3	1/3	1/3	1/3	1/2	2/3	1
Composition and application of the material	N/A	Cotton dress	Cotton dress	Silk & satin Chinese knots	Cotton ribbon	Cotton dress	Cotton dress	Cotton dress

¹ C means circular; R means rectangular. ² L_d = Distance from hunched platform fringe to the maximum scour depth.

. The protective effect of pier scour due to the size of the strips was investigated by using four different sizes of strips as Cases 1, 2, 3, and 4. In Cases 1 to 3, the cross-section of single strip has a circular shape, and D_a = 0.1, 1, and 2 mm, respectively. In Case 4, the cross-section of a single strip has a rectangular shape (b_a = 25 mm and b_s = 0.25 mm). The protective effect against pier scour due to the length of strips was studied by using four different lengths of strips as Cases 2, 5, 6, and 7 (i.e., L/L_d = 1/3, 1/2, 2/3, and 1). The flexibility parameter of CKAW is a function of the strips’ size and length, as shown in Equation (4); thus, the protective effect due to the flexibility parameter can be understood from these seven experimental tests. To clarify the effect of CKAW against scour, a contrast test without CKAW was performed and named Case 0. The parameters of the total running cases are listed in Table 3.

A 24 h preliminary test without CKAW (Case 0) was implemented to obtain the equilibrium scour depth in clear-water conditions. The formation of the scour hole can take a long time to reach an equilibrium state in clear-water conditions [48]. In this study, a variation of scour depth (d_s) < 0.05 b in 24 h is a criterion for the judgment of the equilibrium condition [48]. As shown in Figure 7, the evolution of scour depth in front of the pier in Case 0 is kept at a fixed value after 5 h runtime. The protective effect of CKAW can be estimated using the measurement of scour depth at the fifth hour. Therefore, the runtime for each test is set at the fifth hour in this paper, and the measured scour depth in front of the pier was used to investigate the protective effect of CKAW.

The topography was measured after the pier scour reached an equilibrium state, and the protective effect of CKAW is discussed based on the experimental data. After the pier scour reached an equilibrium state, the water in the channel was leaked out from the experiment flume slowly without disturbing the sand bed. With help of three digital single-lens reflex cameras installed and fixed above the experiment flume, the topography of the sand bed can be obtained using post-processing software from the synchronized photographs. The measured topography of Case 0 in the equilibrium condition is depicted in Figure 8. The local scour was triggered around the pier, and the spatial distribution of scour depth was symmetrical. The maximum scour depth in Case 0 was found in front of the pier, and the measured value (d_sn) was 11 cm. This study measured the topography of all experimental cases after the pier scour reached an equilibrium state, and then the maximum scour depth at pier (d_s) can be obtained. Therefore, the protective effect due to CKAW can be estimated by

$$P_s = \left(\frac{d_{sn} - d_s}{d_s}\right) \times 100\%,$$

(6)

where P_s is the protective effect of CKAW. The water surface above the region of CKAW was also measured after the pier scour reached an equilibrium state, and the effect on water surface due to CKAW is discussed. Above the experiment flume, several movable probes were set up to detect the water surface. After the pier scour reached an equilibrium state, the water surface can be measured by these water level gauges. The measured water surface of Case 0 is plotted in Figure 9. The zero of coordinate X was given between the hunched platform and sand bed. The half length of surface wave (${\pi }_{ln}$) and the magnitude of surface wave (${\pi }_{mn}$) can be obtained as 10.5 and 3.4 cm, respectively. In the same way, this research can record the water surface of all experimental cases, and then the half length of the surface wave (${\pi }_l$) and the magnitude of surface wave (${\pi }_m$) can be found. The protective effect of CKAW on the water surface can be evaluated using the following formulas:

$$P_l = \left(\frac{{\pi }_{ln} - {\pi }_l}{{\pi }_l}\right) \times 100\%,$$

(7)

$$P_m = \left(\frac{{\pi }_{mn} - {\pi }_m}{{\pi }_m}\right) \times 100\%,$$

(8)

where P_l is the protective effect on the half length of surface wave due to CKAW, and P_m is the protective effect on the magnitude of surface wave due to CKAW.

4. Results and Discussions

4.1. 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 × 0.25 mm were applied with a length of 25 cm, which was one-third of the distance from the overfall 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).

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. Protective effect with different size of strips.

Group	Case 0	Case 1	Case 2	Case 3	Case 4
Strip size (mm)	N/A	0.1	1	2	ba = 25 bs = 0.25
Flexibility index Fi		248	46	110	69
Flexibility parameter Fp		11.4	6.1	1.3	1.4
Maximum scour depth at pier ds (cm)	−11.0	−8.0	−9.1	−10.5	−10.4
Protective effect of scour depth at pier Ps (%)	--	27.3	17.3	4.6	5.5
Half length of surface wave (cm)	10.5	9.5	9.5	9.0	9.0
Protective effect of surface wave’s half length Pl (%)	--	9.5	9.5	14.3	13.3
Magnitude of surface wave (cm)	3.4	2.1	2.7	2.4	3.4
Protective effect of surface wave’s magnitude Pm (%)	--	38.2	20.6	29.4	0

. 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.

Figure 12 shows the history of scour depth in front of the pier. The scour depth changed unstably without CKAW because while the overfall flow carried a greater amount of sediment than the one being washed away at the pier, deposition took place; otherwise, scour took place. While the submerged overfall was in an unstable flow condition, the sediment supplement was also unstable. When CKAW was applied, the scouring history was more stable. This finding implies a stable development of overfall-induced scour on the flume bed. When circular strips with different diameters were used, a smaller diameter corresponded to reduced scour. The scour depth in the cases with CKAW was greater than that in the case without CKAW before 100 min. However, the scour depth development reached a stable state in the cases with CKAW but kept increasing in the case without CKAW after that. Eventually, the differences could be observed. The scour depth with CKAW installation showed a larger value in the first 100 min because the flow velocity was transferred into V_x. Therefore, V_x was increased but V_z was decreased due to CKAW. The increased V_x induced a larger scour around the pier at the beginning until the scour hole reached a certain depth, and eventually, CKAW was able to slow down the velocity close to the flume bed.

For the case of rectangular arranging strips, its scour depth history was smaller than that of the others in the first 140 min. This finding indicates that a stronger protective effect was provided by the wider strips that transfer downward flow velocity better, thus providing better protection on the flume bed in the first 140 min. However, when the rectangular strips tangled up, which occurs easily with this type of shape, the scour depth started to increase intensively and reached a similar depth as in the case without CKAW. This finding shows that when CKAW strips tangle up, the scour depth increased because of the reduced protective effect.

The installation of CKAW was able to stabilize the vibration of the water surface, which was induced by the submerged overfall after the flow passed through the hunched platform, thereby stabilizing the sediment transport. The water table profile is shown in Figure 13, where the difference between peak and bottom was taken as the magnitude of water surface change to represent the scale of water surface vibration. This figure shows that the greatest vibration occurred in the case without CKAW, and the greatest reduction was found in Case 1, where the thinnest strips were applied, achieving a reduction of 1.3 cm or 38.2%. The wave moved downstream in Case 3, having a similar shape as the one without CKAW in Case 4. The half wave length in the case without CKAW was 1–1.5 cm longer than the one with CKAW. This finding proves that the wave length and magnitude reduced due to CKAW. All the results are listed in Table 4.

4.2. 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 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. Protective effect with different lengths of strips.

Group	Case 0	Case 2	Case 5	Case 6	Case 7
Length of strip (cm)	N/A	25	37.5	50	75
Flexibility index Fi		46	46	46	46
Flexibility parameter Fp		6.1	9.2	12.2	18.3
Maximum scour depth at pier ds (cm)	−11.0	−9.1	−7.3	−7.1	−6.0
Protective effect of scour depth at pier Ps (%)	--	17.3	33.6	35.5	45.5
Half length of surface wave (cm)	10.5	9.5	13.5	11.5	11.0
Protective effect of surface wave’s half length Pl (%)	--	9.5	−28.6	−9.5	−4.8
Magnitude of surface wave (cm)	3.4	2.7	2.4	2.1	1.7
Protective effect of surface wave’s magnitude Pm (%)	--	20.6	29.4	38.2	50

. 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 overfall, 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° 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 effect when the strip length covers half the distance to the pier location. Figure 17 shows the change in water surface with different strip lengths. The magnitude reduced when the strip length increased. In the case of L/L_d = 1, the reduced percentage of magnitude was up to 50%, as listed in Table 5, showing the influence of CKAW on stabilizing the water surface. When L/L_d exceeded 0.5, the wave transferred further downstream than in the case without CKAW.

4.3. Protective Effect Due to Flexibility of CKAW

As mentioned in Section 2, the swinging part of CKAW can arrange and weaken the approaching flow, thus reducing the overfall- and pier-induced scour depth. The material’s flexibility is a response to the size and length of CKAW. Through this experimental study, we found that the flexibility parameter of CKAW is an important index for the protective effect of scour depth and the stabilizing capability against approaching flow. On the basis of the experimental results in Table 4 and Table 5, Figure 18 depicts the relationship between flexibility parameter (F_p) and protective effect (P_s) on the scour depth. A linear relationship was found, with a correlation coefficient of 0.92, as shown in Equation (9).

$$p_s = 2.72F_p,$$

(9)

This equation presents that the flexibility parameter has a strong influence on the protective effect; a large flexibility parameter corresponds to increased protective effect. The flexibility parameter can be estimated by using Equation (5). This formula can be used to estimate whether any new synthetic fiber is suitable for CKAW.

5. Conclusions

Through a series of experiments, this study proposes a new technique called CKAW, which protects a bridge pier located in a submerged overfall flow from being scoured. CKAW involves a simple installation method and easily obtained materials, i.e., cotton, nylon, and silk. The flow condition considered in this paper includes two types of scour around the pier: one is bridge pier-induced local scour due to the contraction of flow, and the other is overfall-induced scour due to downward velocity. The dominant velocity component in pier-induced scour is mainly horizontal component V_x, and that in overfall-induced scour is mainly vertical component V_z. CKAW shows good protective performance in transforming the vertical velocity component into a horizontal one, therefore protecting the flume bed well in this test. Circular CKAW worked better than a flat rectangular CKAW, avoiding the tangling problem that easily occurs in the latter. According to a series of laboratory tests in clear-water and live-bed conditions, this study also shows that a thinner strip with a length (L) that reaches the pier location (L_d) performed better. According to the experimental tests in this study, the best setup conditions were found to be D_a = 1 mm with a circular shape and L/L_d = 1. The best protective effect of scour depth at pier was up to 45.5%. Moreover, CKAW with a larger flexibility parameter (F_p) performed better in protecting the flume bed, and the best setup condition was found to be F_p = 18.3 in this experimental study. The durability of the protection due to CKAW performed well without any damage in the entire process of the experiments.

This paper presents preliminary research about a near-nature ecological technique for pier scour countermeasure. From a series of laboratory tests, the protective effects of the near-nature ecological technique against pier scouring can be estimated according to the geometric features, i.e., D_a, L/L_d, and F_p. However, the performance of the proposed near-nature ecological technique in field application has not been investigated completely. In future studies, the experimental design is expected to be similar to the condition of the field site, i.e., a sand supplement from the upstream side of the flume, the durability of the protection, and the performance on the curve channel. Besides the investigation of the influence of geometry features, the influence of hydraulic and sediment features will be considered. In order to improve the laboratory test, the experimental design will reduce the effect of armor and viscosity, and the recorded data should also include the velocity field.