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Naturally assembled rocks can be found in the natural rivers with gravel beds, and assembled boulders with large-scale rocks are stable after many floods. This chapter focused on assembled rocks from the point of view of the stability of the structure during many floods and the preservation of the aquatic habitat under normal conditions. In renovated rivers, the installation of hydraulic structures may result in the degradation of the river bed, local scouring behind the structure, bank erosion due to floods, and obstacles in the aquatic migration routes of aquatic animals. A balance between the prevention of disasters and the preservation of aquatic habitats should be maintained. Artificially assembled boulders should be recommended for river engineering technology. Unfortunately, there is no information on the hydraulics of artificially assembled boulders or on the imbrication of boulders in any part of the world. This chapter presents experimental research on the hydraulics of assembled boulders under normal and flooding conditions. A shallow water flow was formed near the water side under normal conditions. The stability of the structure of assembled boulders was confirmed under flooding conditions. These findings were confirmed by field inspection.
Department of Civil Engineering, College of Science and Technology, Nihon University, Japan
Nozomi Fuchino
Department of Civil Engineering, Graduate School of Science and Technology, Nihon University, Japan
*Address all correspondence to: yasuda.youichi@nihon-u.ac.jp
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. 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 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.
Figure 1.
Imbrication of boulders.
Figure 2.
Naturally assembled rocks.
Figure 3.
Structure of assembled boulders.
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.
The experiments were conducted in the Environmental Hydraulic Laboratory of Nihon University, College of Science and Technology, in order to investigate the hydraulics passing over consecutively assembled boulders. Three different downward slopes (i.e. 1/8.5, 1/12.5, and 1/25) were tested. In the case of 1/8.5 and 1/25 slopes, as shown in Figure 4, a half model was installed in a rectangular channel with a width of 0.4 m, length of 17 m, and height of 0.6 m. For the slope of 1/12.5, a symmetric model was installed in a rectangular channel with a width of 0.8 m, length of 15 m, and height of 0.6 m. These models were constructed as 1/10 scale models, and it was assumed that the flow condition might be represented under a Froude similarity. The size of the boulders was set to around 0.6 m in the prototype. The stepped channel model was used as a base of consecutively assembled boulders (Figure 5). In order to change the water width in accordance with the discharge, a transverse stepped slope was installed at around 1/10. The assembled boulders were installed on each staircase without hardening the base, and the downstream end of the boulders was stabilized with L-shaped fittings (in the field, use stopper blocks). The experimental conditions are shown in Tables 1–3 for slopes of 1/8.5, 1/12.5, and 1/25, respectively. Here, B is the channel width, hc is the critical depth, Q is the discharge, and W.L.d is the downstream water level based on the lowest bottom of the stepped channel at the downstream end of the consecutively assembled boulders. Moreover, the subscript.
Figure 4.
Half model of consecutively assembled boulders. (a) Half model with 1/25 slope. (b) Half model with 1/8.5 slope.
Figure 5.
Symmetric model of consecutively assembled boulders (1/12.5 slope). (a) Stepped channel as a base. (b) Consecutively assembled boulders.
Case
Q (m3/s)
Qp (m3/s)
hc (m)
hcp (m)
W.L.d (m)
W.L.dp (m)
1
0.00348
1.10
0.020
0.20
0.075
0.75
2
0.00632
2.00
0.029
0.29
0.086
0.86
3
0.00949
3.00
0.039
0.39
0.097
0.97
4
0.0456
14.4
0.110
1.10
0.201
2.01
Table 1.
Experimental conditions for 1/8.5 downward slope.
Note: B = 0.4 m; transverse slope = 1/11; hc = critical flow depth (=[(Q/B)2/g]1/3).
Case
Q (m3/s)
Qp (m3/s)
hc (m)
hcp (m)
W.L.d (m)
W.L.dp (m)
1
0.0070
2.20
0.020
0.20
0.079
0.79
2
0.0111
3.52
0.027
0.27
0.089
0.89
3
0.0168
5.31
0.036
0.36
0.099
0.99
4
0.1211
38.30
0.133
1.33
0.232
2.32
5
0.1456
46.03
0.150
1.50
0.259
2.59
Table 2.
Experimental conditions for 1/12.5 downward slope.
Note: B = 0.8 m; transverse slope = 1/11; hc = critical flow depth (=[(Q/B)2/g]1/3).
Case
Q (m3/s)
Qp (m3/s)
hc (m)
hcp (m)
W.L.d (m)
W.L.dp (m)
1
0.00143
0.451
0.011
0.11
0.044
0.44
2
0.00595
1.88
0.028
0.28
0.079
0.79
3
0.0103
3.25
0.041
0.41
0.096
0.96
4
0.00312
0.986
0.018
0.18
0.066
0.66
Table 3.
Experimental conditions for 1/25 downward slope.
Note: B = 0.4 m; transverse slope = 1/6.9, 1/10, and 1/9.0 from right side; hc = [(Q/B)2/g]1/3.
"p" indicates the prototype. The downstream water level was controlled with a sluice gate located at the channel end. The velocity was measured using a propeller current meter 0.03 m in diameter [15] and the sampling time was set to 20 seconds because of steady supercritical flow. The water and bottom levels were measured using a point gauge with a 0.1 mm reading.
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:
The main flow passing over the consecutively assembled boulders is formed along the water surface without jump formation with a surface roller and it is easy for aquatic animals to find the upstream migration route.
Multi-aquatic animals are able to migrate upstream through the consecutively assembled boulders in a wide range of discharges because the formations of shallow water flow near the water side and the gap flow of the assembled boulders produce a low velocity with low turbulence. In this case, the boulders that were utilized averaged about 0.6 m in size and installed at a transverse slope of about 1/10.
The velocity and depth of the migration route were independent of the downward slope in slopes ranging from 1/8.5 to 1/25, because the flow condition among the assembled boulders was locally independent.
Concentration of the flow at the lowest part of the assembled boulders, in addition to a priming effect, can be expected for the upstream migration route.
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.
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.
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.
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, 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.
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.
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 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.
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. 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.
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.
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, 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.
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).
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 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).
1f=2.821dD840.6970.646<dD84<5.07E1
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.
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) 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.
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.
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).
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 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.
Figure 25.
Ground sill of assembled boulders.
Figure 26.
Alternative protruding assembled boulders.
Rock weirs, cross vanes, and Syvde-type weirs have been proposed as stable structures for river passage [18, 19, 20, 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.
The flow conditions, velocity fields, and stability of artificially assembled boulders, in addition to the possibility for upstream migration routes for aquatic animals, were shown experimentally in response to various changes in discharges. It was found that, at down-ward slopes of 1/8.5, 1/12.5, and 1/25, a transverse gradient about 1/10 in the cross-section created a shallow water flow and a gap flow through the assembled boulders with low time-averaged velocity and low turbulence, regardless of the degree of the downward slope, even if discharges changed under normal conditions. The experimental results showed that a shallow water flow and a gap flow of assembled boulders could allow swimming fish, benthic fish, and crustaceans to migrate upstream. The assembled boulders were constructed in such a way to support each other and resist fluid forces, confirming the stability of the assembled boulders during floods. These findings were confirmed from the field construction. In addition, the upstream migration of swimming fishes (e.g. Plecoglossus altivelis, Tribolodon hakonensis, and Oncorhynchus masou ishika-wae) and benthic fishes (e.g. Rhinogobius flumineus and Cottus pollux) were observed at sites where consecutively assembled boulders were constructed in the Ohmu river. This study supports the practical application of assembled boulder installations for river improvement. The installation of assembled boulders can be applied to improve river environments, aquatic habitats such as refugees and aquatic animal migration routes. Research on consecutively assembled boulders is still in its early stages. Practically, six sites were installed in rivers of various sizes, and biological studies have not been carefully conducted. However, it is true that a variety of aquatic animals were observed at the upstream end of the consecutively assembled boulders. Further biological surveys may be needed. In addition, a protruding assemblage of boulders, a pool-type fish passage with assembled boulders, and ground sills of assembled boulders were constructed, but limits to the application of assembled boulders may need to be examined after a systematic investigation in the future.
This study was supported by the Eel Food Culture Promotion Foundation.
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Written By
Youichi Yasuda and Nozomi Fuchino
Submitted: 15 April 2022Reviewed: 06 May 2022Published: 31 May 2022
Open access peer-reviewed chapter
