**5. Discussion**

As shown in **Figures 4** and **5**, as the consecutively assembled boulders were installed in transverse steps, it was easy for shallow water to flow to form near the water side for different discharges. Moreover, the gap flow among the assembled boulders had a low velocity with low turbulence (**Figure 15**). The flow condition confirmed these facts at the structure constructed in the Ohmu River (**Figure 22**). The width of the assembled boulders installed on site was 74 m. In this case, the experiment corresponded to the investigation with a width of 8 m, which is a partial reproduction of the original condition. With the assistance of many fishermen, a local government office, and fishery associations, it was confirmed that swimming fishes (e.g. Plecoglossus altivelis, Tribolodon hakonensis, and Oncorhynchus masou ishikawae) and benthic fishes (e.g. Rhinogobius flumineus and Cottus pollux) were able to migrate upstream on the assembled boulders for various discharges. The flow conditions and velocity fields in the assembled boulders can be observed experimentally (**Figures 15** and **24**), but they are difficult to quantitatively evaluate in detail. Therefore, it is important to understand the flow condition in the space from the field construction of assembled boulders. The experimental results shown in **Figures 12**–**14** are the most important results. Practically, the selection of a downward slope of consecutively assembled boulders is limited because the drop structure must be designed on the basis of the hydraulic design in Japan. As the relationship between the surface velocity and the evaluated depth in the shallow water flow region is independent of downward slopes (at least 1/8.5 to 1/25 slopes), the application of assembled boulders might be flexible. The main flow passing over the consecutively assembled boulders is always located near the water surface, as shown in **Figures 12**–**14**, and the velocity of the main flow changes not only in a streamwise direction but also in a transverse direction, as shown in **Figure 16**. As the main flow continues far downstream, it is easy for swimming fishes, benthic fishes, and crustaceans to find the upstream migration route. Especially if the fish passage with the assembled boulders is installed in part, the installation of the assembled boulders is effective for guidance toward the upstream migration route.

During floods, the structure of the artificially assembled boulders should be stable. The experiments with a 1/10 scale model confirmed that the artificially assembled boulders were stable during floods. In this case, the boulders were assembled on each step, and the material for fixing was not used. More than 10 assembled boulder structures were installed on-site, all of them were found to be stable even after the floods. **Figure 23** shows the transported rocks during floods. These rocks were the same size as the assembled boulders, but the consecutively assembled boulders were not destroyed. The structure of artificially assembled boulders was constructed on the basis of the naturally assembled rocks and was quite different from that caused by imbrication in the river. The artificially assembled boulders were shaped so that the boulders were stacked on the top of the downstream boulders. There were at least four *The Efficacy of Artificially Assembled Boulder Installations in Improving Migration Routes… DOI: http://dx.doi.org/10.5772/intechopen.105198*

### **Figure 24.**

*Flow conditions at assembled boulders (1/4 scale model, 1/10 downward slope). (a) Normal condition (hcp = 0.166 m). (b) Flood condition (hcp = 0.453 m).*

**Figure 25.** *Ground sill of assembled boulders.*

**Figure 26.** *Alternative protruding assembled boulders.*

points of contact for the boulders (overlapping points, points touching both sides, and one point at the bottom). The shape of the boulders was not spherical, but a flat shape is recommended for the boulders in order to be stabilized them.

From the point of view of the structure of artificially assembled boulders, as shown in **Figures 25** and **26**, the installation of assembled boulders is helpful for the improvement of river environments [17]. After many floods, the structure of the assembled boulders was still stable. Accordingly, stacked boulders can be installed

to improve river environments, refuges, and other aquatic habitats and the migration routes of aquatic animals. Limitations for the application of assembled boulders should be discussed after a systematic investigation in the near future.

Rock weirs, cross vanes, and Syvde-type weirs have been proposed as stable structures for river passage [18–21]. These structures consist of densely packed boulders. According to their design manuals and references, these kinds of boulders are not mechanically assembled to prevent them from being destroyed during large floods.
