**4. Field installation and monitoring**

The artificial installation of assembled boulders can be applied to eliminate discontinuities due to drop structures. In order to maintain the balance between energy dissipation during floods and the migration of aquatic animals under normal conditions, consecutively assembled boulders were installed with 74.4 m in width of a channelized weir with a 2.3 m drop. The weir was located in the Ohmu River, Hokuto city, Yamanashi Prefecture, Japan (**Figure 19**). Every year, many boulders, including ones 0.7 m in size, are transformed during large floods; strength in the structure is therefore required. In order to install consecutively assembled boulders near an apron behind the channelized weir, the downward slope was set to 1/8. The size of the boulders was set to about 0.7 m to ensure water depth between the assembled boulders (**Figure 20**). During floods, the main stream was formed along the water surface downstream of assembled boulders, and the flow velocity above the boulders could be reduced by shape resistance from the assembled boulders. As shown in **Figure 21**, considering the water width of the main stream under normal conditions (in this case, 10 m in width)

**Figure 19.** *Consecutively assembled boulders.*

**Figure 20.** *Structure of assembled boulders.*

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

**Figure 21.** *Aerated main flow and shallow water flow.*

**Figure 22.** *Shallow water flow region.*

**Figure 23.** *Transported rocks during floods.*

the bed level at the upstream end of the consecutively assembled boulders was adjusted to be 0.20 m lower than both sides of the main stream. The river bed downstream of the consecutively assembled boulders was adjusted by installing rocks in order to form a subcritical flow during floods. As shown in **Figure 22**, a shallow water flow was formed on both sides of the main stream. The flow condition can be confirmed from the physical model (**Figure 23**). After the installation of the consecutively assembled

boulders, it was confirmed that aquatic organisms at various stages of growth that are native to the river could migrate upstream through the channelized weir. Moreover, the main flow was located along the water surface during floods; there was thus no local scouring.
