**3. Experimental results**

The clarification of flow condition, water depth, and velocity on consecutively assembled boulders under normal conditions might help to clarify the upstream migration route of aquatic animals. Experimental investigation of flow condition, the water surface profiles, bed shape, and velocity field yields is helpful for hydraulic design on the consecutively assembled boulders. The important points are summarized as follows:


### **3.1 Flow condition under normal conditions**

The flow conditions on the consecutively assembled boulders are shown in **Figures 6**–**8**. Under normal conditions, the main flow concentrated at the lowest part of the consecutively assembled boulders. On the other side, a shallow water flow on the assembled boulders was formed as a small cascade at each combination of assembled boulders. Moreover, pools with a low velocity were formed in the gaps between the boulders.

In the case of a 1/25 downward slope and a transverse slope around 1/8, even when the discharge in the prototype changes from 0.5 to 3.0 m3 /s, multi-aquatic animals (e.g. swimming fish, benthic fish, crustaceans, and shells) may migrate upstream in the shallow water flow region (**Figure 6**).

In the case of a 1/12.5 downward slope and a transverse slope around 1/11, the flow condition for the upstream migration route was formed up to the discharge in the prototype 5.3 m3 /s in the shallow water flow region (**Figure 7**). In this case,

### **Figure 6.**

*Flow conditions for a 1/25 downward slope and a transverse slope around 1/8. (a) Flow condition for Case 4. (b) Flow condition for Case 2.*

### **Figure 7.**

*Flow conditions for a 1/12.5 downward slope and a 1/11 transverse slope. (a) Flow condition for Case 1. (b) Flow condition for Case 2.*

*The Efficacy of Artificially Assembled Boulder Installations in Improving Migration Routes… DOI: http://dx.doi.org/10.5772/intechopen.105198*

**Figure 8.**

*Flow conditions for a 1/8.5 downward slope and a 1/11 transverse slope. (a) Flow condition for Case 1. (b) Flow condition for Case 2.*

since the cross-section of consecutively assembled boulders was symmetrical (8 m in width in the prototype), the upper discharge was 2.65 m3 /s for a half section 4 m in width.

In the case of a 1/8.5 downward slope and a transverse slope around 1/11, the flow condition for the upstream migration route was formed up to the discharge in the prototype 2.0 m3 /s in the shallow water flow region (**Figure 8**).

In the three different downward slopes in the flow direction, the main stream passing over the consecutively assembled boulders was always formed along the water surface, and enabling aquatic animals to find the upstream migration route easily.

### **3.2 Velocity fields under normal conditions**

**Figures 9**–**11** show the velocity profiles on the assembled boulders at cross-sections for three different downward slopes. Here, up is the mean velocity above the boulders in the flow direction, Xp is the streamwise direction coordinate from the upstream end of the consecutively assembled boulders, yp is the transverse direction coordinate from the right side wall, and Yp is the transverse direction coordinate from the center line of the symmetric cross-section. Moreover, these values are expressed according to the prototype scale. The velocity distribution depends on the gradient in the flow direction and the discharge, except for the upstream migration route of multi-aquatic animals. In the shallow water flow region, the flow velocity above the assembled boulders varied from 0 to 2.2 m/s on the prototype scale. The flow velocities between the assembled boulders might be further reduced, although they might be difficult to measure on a 1/10 scale model.

**Figures 12**–**14** show the relationship between evaluated depth [12] and mean velocity for three different downward slopes in the flow direction. The data in these figures were recorded in a shallow water flow region. As shown in these figures, similar results were obtained, although there were variations. Accordingly, the flow condition in a shallow water flow region might be less sensitive to the downward slopes and variations in discharges under normal conditions. These results reveal that multiple flow with various water depths and velocities with low turbulence was formed in the shallow water flow region by installing assembled boulders on a transverse inclined slope. Furthermore, aquatic animals might migrate upstream amid various downward slopes and discharges because the shallow water flow is formed for three different downward slopes (1/8.5, 1/12.5, and 1/25).

### **Figure 9.**

*Velocity profiles for a 1/25 downward slope and a transverse slope around 1/8. (a) Velocity profiles for Case 2. (b) Velocity profiles for Case 3.*

### **Figure 10.**

*Velocity profiles for a 1.12.5 downward slope and a 1/11 transverse slope. (a) Velocity profiles for Case 1. (b) Velocity profiles for Case 2.*

### **Figure 11.**

*Velocity profiles for a 1/8.5 downward slope and a 1/11 transverse slope. (a) Velocity profiles for Case 1. (b) Velocity profiles for Case 2.*

**Figure 15** shows the velocity distributions in the assembled boulders under both normal and flooding conditions. The values of the velocity are expressed in the prototype. This experiment was an additional experiment with a 1/4 scale model; the velocity was measured using a two-dimensional electric magnetic current meter of type I with a diameter of 3 mm (sampling time: 30 s; sampling frequency: 50 ms). As shown in **Figure 15**, the mean velocity up and the standard deviation up were always low in the assembled boulders under both the normal and flooding conditions.

## *The Efficacy of Artificially Assembled Boulder Installations in Improving Migration Routes… DOI: http://dx.doi.org/10.5772/intechopen.105198*

**Figure 12.** *Relationship between evaluated depth and mean velocity for the 1/8.5 slope.*

### **Figure 13.**

*Relationship between evaluated depth and mean velocity for the 1/12.5 slope.*

**Figure 14.** *Relationship between evaluated depth and mean velocity for the 1/25 slope.*

As the flow concentrated toward the lowest part of the assembled boulders, the velocity reached its maximum value. The main flow was always located near the water surface. **Figure 16** shows the maximum velocity decay downstream of the consecutively assembled boulders for downward slopes of 1/12.5 and 1/8.5.

As shown in these figures, the main flow continued all the way downstream to the lowest part of the assembled boulders, and the maximum velocity downstream of the shallow water flow region was lower than 1 m/s. From these results,

**Figure 15.**

*Velocity distribution in assembled boulders (1/4 scale model, 1/10 downward slope). (a) Normal conditions (hcp = 0.166 m). (b) Flooding conditions (hcp = 0.453 m).*

**Figure 16.** *Maximum velocity decay downstream of consecutively assembled boulders. (a) 12.5 slope. (b) 1/8.5 slope.*

fish and crustaceans could easily migrate upstream through the consecutively assembled boulders.

### **3.3 Velocity fields during floods**

**Figures 17** and **18** show the streamwise change in the maximum velocity around the assembled boulders amidst a large flood. The maximum velocity was defined at each vertical measurement section. In the prototype, the critical flow depth was set to about 1.1–1.5 m, and the maximum velocity reached 6.0 and 6.5 m/s for the 1/8.5 and 1/12.5 slopes respectively. In the experiments shown in **Figures 6**–**8**, the boulders were assembled only on each step, and the material for fixing was not used. The velocity near the boulders reached 4.0–5.0 m/s during floods, but the assembled boulders were not destroyed at all. In the case of the imbrication of boulders in a gravel river (**Figure 1**), the stacked boulders might be fragile during large floods because the boulders might not support each other against the fluid force. Regarding the maximum velocity decay downstream of the consecutively assembled boulders, as the main flow was located near the water surface, it was possible to prevent bed erosion. Moreover, the maximum velocity decay changed transversely. If the consecutively assembled boulders were arranged symmetrically, it was possible to prevent side bank erosion.

To evaluate the flow resistance of the assembled boulders in a flood flow, the depth of the water in a pseudo-uniform flow was evaluated according to the shape of the assembled boulders and the water surface profile. The friction coefficient was evaluated using the Darcy–Weisbach equation; it was found to vary with the slope in the downstream direction. For the 1/8.5 slope, the friction coefficient was 0.180, and *The Efficacy of Artificially Assembled Boulder Installations in Improving Migration Routes… DOI: http://dx.doi.org/10.5772/intechopen.105198*

**Figure 17.**

*Streamwise change in velocity for Case 5 (qp = 5.75 m2 /s) under the 1/12.5 slope. Vd = averaged velocity at downstream subcritical flow (Xp = 46 m).*

**Figure 18.**

*Streamwise change in velocity for Case 4 (qp = 3.61 m2 /s) under the 1/8.5 slope. Vd = averaged velocity at downstream subcritical flow (Xp = 46 m).*

for the 1/12.5 slope, it was 0.252. The measured results obtained by Rickenmann and Recking [13] were converted into a friction coefficient corresponding to the relative roughness height, which is shown in Eq. (1).

$$\sqrt{\frac{1}{f}} = 2.821 \left( \frac{d}{D\_{\text{s4}}} \right)^{0.697} 0.646 < \frac{d}{D\_{\text{s4}}} < 5.07 \tag{1}$$

where d is the flow depth, and D84 is used as characteristic grain size (following Ferguson [16]).

If the value of D84 is evaluated from the friction factor and the flow depth, D84 = 0.204 m for the 1/8.5 slope and D84 = 0.445 m for the 1/12.5 slope respectively. Accordingly, the equivalent roughness height, which contributes directly to a flow

resistance, might become smaller as the downward slope increases. The estimation of the friction coefficient can be applied to the stable structure as in the case of the assembled boulders.
