**1. Introduction**

In natural rivers, imbrication and assembled rocks can be observed. Imbrication is an overlapping arrangement of boulders which are different from the assembled rocks. In particular, in imbrication (**Figure 1**), collisions between boulders or driftwood during floods improve the ease of transport. On the other hand, naturally assembled rocks (**Figure 2**) can be stabilized against many floods, and they contain a stable habitat space for several kinds of aquatic animals under normal conditions. Moreover, the space may be useful as a refuge area during floods because of the flow velocity, including the turbulence is always low due to the formation of a seepage flow.

**Figure 1.** *Imbrication of boulders.*

**Figure 2.** *Naturally assembled rocks.*

There is a regular assembly method for forming the assembled rocks that makes sense in nature. Still, it has not been focused on as a technical method in civil engineering [1]. The mechanism of imbrication and its relationship with the habitat have been studied from the perspective of geomorphology and ecology. According to Hassan [2], the macroform is defined as transverse ribs of rubble in the direction of flow, and stone cells of rubble in a circle. The microform is defined as a stone cluster which is an accumulation of gravel of different sizes around a large piece of gravel; imbrication is a folding of gravel pieces along the direction of flow. Some researchers have further classified stone clusters into several forms (e.g. Strom et al. [3]). The formation of macroforms and stone clusters has been reported using field surveys to carefully record the arrangement of stones that have been moved and those that have not (Church et al. [4], Lamarre and Roy [5], Wittenberg and Newson [6]). The developmental process of clusters has been examined in laboratory experiments with simpler conditions (Papanicolaou et al. [7]). Strom et al. [3] studied the shape characteristics of a large number of samples from five different types of stone clusters in the field. Once imbrication and various types of stone clusters are formed, they are less likely to move with the flow than stone and gravel on their own. From the perspective of sediment hydraulics, the effects of microforms on flow resistance and sediment volume have been investigated by Hassan and Reid [8], as well as by Strom et al. [9]. From the perspective of river ecology, studies have been conducted focusing on the function of flow refuge against benthic disturbance (Biggs et al. [10]). Accordingly, imbrication

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

**Figure 3.** *Structure of assembled boulders.*

and stone clusters can be formed artificially by arranging stones, and this process may be developed into a simple method for contributing to the improvement of the river bed environment in small and medium-sized rivers. However, these approaches were not applied for large floods, because the stability of the assembled boulders was not examined for a wide range of discharges. Recently, the authors focused on artificially assembled boulders, as shown in **Figure 3**, as naturally assembled rocks from the point of view of energy dissipation during floods and on the possibility of upstream migration under normal conditions [11, 12]. Based on field measurements, Rickenmann and Recking [13] as well as Hey [14], for example, investigated the flow resistance in rough river beds. Still, the stability of assembled rocks during floods and the formation of multiple flows under normal conditions were not found to be associated.

This chapter presents findings from practical and experimental research on the hydraulics passing through artificially assembled boulders. The focus was on consecutively assembled boulders installed at the drop structure. In order to clarify the flow conditions around the assembled boulders and the possibility of upstream migration, the hydraulics of consecutively assembled boulders were investigated experimentally based on three different downward slopes and discharges as the first stage of the research project on assembled boulders. A shallow water flow was formed under normal conditions as an upstream migration route if consecutively assembled boulders with transverse mild slopes were installed. Moreover, a surface jet flow passing over the assembled boulders was always formed, even during floods. Then, the structure of the consecutively assembled boulders was stable, even if the assembled boulders were installed without fixing the bottom part. The consecutively assembled boulders were practically installed in a check dam, introducing a flow in the assembled boulders, whose details could not be covered by the experiments, and stability in the assembled boulders during flooding. These results can help to provide practical applications in installing assembled boulders for fish passage, energy dissipation for low head structures, ground sills, protruding stones, and fishing reefs.
