JP7575047B2 - Scour prevention structure - Google Patents

Description

The present invention relates to a scour prevention structure.

Agricultural intake weirs are often installed in the mid-upper reaches of rivers to allow natural water supply to fields. In rivers, the riverbed is often excavated downstream of the agricultural intake weir for the purposes of extracting construction aggregate, preventing overflowing of the levees, and preventing levee breaches. In addition, downstream of the agricultural intake weir, the riverbed is prone to scouring due to the flow of water, leading to further lowering of the riverbed. As a result, the lowering of the riverbed in the downstream area can spread to just below the agricultural intake weir in the upstream area, which can lead to the washing away of bed protection works and damage to the weir. For this reason, there is a demand for technology to prevent the lowering of the riverbed just below or around structures such as agricultural intake weirs.

Known techniques for preventing riverbed lowering include increasing the number of bed protection blocks, expanding the installation area of bed protection blocks, connecting bed protection blocks, making bed protection blocks larger, expanding the installation area of weir aprons, and installing sub-dams downstream of weirs. Non-Patent Document 1 describes a technique for laying a mat under revetment blocks. Non-Patent Document 2 describes a technique for laying a filter material on the bottom of a bed protection work. Patent Documents 1 to 3 also describe revetment structures that combine a net-like body with weights such as concrete blocks.

Japanese Patent Application Publication No. 11-264128 JP 2001-193036 A JP 2002-173926 A

Tatsuhiko Uchida et al., Collection of Papers on River Technology, Vol. 10, 2004 Hiroshi Kawaguchi et al., Journal of Hydraulic Engineering, Vol. 48, 2004

However, the above-mentioned technologies have the following problems. Technologies that increase the number of bed protection blocks, expand the installation range of bed protection blocks, connect bed protection blocks, and make the bed protection blocks larger cannot prevent the bed protection works from tilting or being washed away due to lowering of the riverbed directly below them, and cannot prevent damage to the agricultural water intake weir. Furthermore, technologies that expand the installation range of the weir apron require large-scale renovations, which increases construction costs, and there is a risk that scouring will progress directly below the weir apron, causing it to be damaged. Furthermore, technologies that install secondary dams require costs and effort similar to those required for building a new weir, and as the riverbed lowers, there is a higher risk of the secondary dam being damaged, making it necessary to add additional secondary dams.

In addition, the technology described in Non-Patent Document 1 requires a dry construction method in which the riverbed is dried before laying the mat, which causes problems with workability. In addition, if roll-shaped mats are laid on top of each other without being dried, gaps may form between the mats due to riverbed fluctuations during or after construction, which may lead to scouring. Furthermore, if cracks appear in the mat after construction, it is time-consuming to repair them by relaying the roll mat or by drying it and repairing it with glue. Furthermore, in the technology described in Non-Patent Document 2, as the riverbed lowers, the filter material is washed away directly below the bed protection work, losing its bed protection effect. In addition, the technologies described in Patent Documents 1 to 3 are technologies aimed at bank protection that take into consideration the natural environment, such as vegetation, and are unable to prevent riverbed scouring.

The present invention was made to solve the problems of the prior art described above, and its purpose is to prevent scouring of riverbeds and waterway bottoms.

In order to solve the above problems, one aspect of the present invention is a scouring prevention structure that prevents scouring of riverbeds and waterway bottoms, and is configured by laying a net-like body that prevents the passage of sediment from the riverbed or waterway bottom on the riverbed or waterway bottom and prevents it from floating up from the riverbed or waterway bottom.

According to one aspect of the present invention, scouring of riverbeds and waterway bottoms can be prevented.

FIG. 1 is a schematic top view showing a scour prevention structure according to a first embodiment of the present invention. 1 is a schematic side view showing a scour prevention structure according to a first embodiment of the present invention. FIG. FIG. 6 is a schematic top view showing a scour prevention structure according to a second embodiment of the present invention. FIG. 11 is a schematic side view showing a scour prevention structure according to a third embodiment of the present invention. FIG. 11 is a schematic top view showing a scour prevention structure according to a fourth embodiment of the present invention. FIG. 2 is a diagram showing a state before water passes through the scour prevention structure of the embodiment. FIG. 11 is a diagram showing the state of the scour prevention structure of the embodiment after water has passed through it.

[Embodiment 1]
A scour prevention structure according to an embodiment of the present invention is a scour prevention structure for preventing scouring on a riverbed and a waterway bottom, in which a net-like body for preventing the passage of sediment on the riverbed or waterway bottom is laid on the riverbed or waterway bottom while preventing it from floating up from the riverbed or waterway bottom.

The scouring prevention structure according to one embodiment of the present invention is laid on the riverbed or the bottom of a waterway. The riverbed on which the scouring prevention structure is laid includes the riverbed and the slope of the river. The waterway bottom on which the scouring prevention structure is laid includes the bottom and slope of various waterways such as agricultural drainage channels and dam spillways. When laying the scouring prevention structure on the riverbed, the area is not particularly limited, and it may be laid adjacent to the downstream side of a structure installed in the river. Examples of such structures include weirs such as agricultural water intake weirs, sluice gates, drop structures, etc.

Scour prevention structures consist of a net-like body that is laid in a way that prevents it from floating up from the riverbed or waterway bottom on which it is laid. "Floating up from the riverbed or waterway bottom" means that the net-like body moves from the riverbed or waterway bottom toward the water surface due to the buoyancy of the water in the river or waterway.

The scouring prevention structure according to this embodiment will be described with reference to Figures 1 and 2. Figure 1 is a schematic top view showing the scouring prevention structure according to the first embodiment of the present invention, and Figure 2 is a schematic side view showing the scouring prevention structure according to the first embodiment of the present invention. As shown in Figures 1 and 2, the scouring prevention structure 10 is a structure in which a net-like body 1 is laid on the riverbed or waterway bottom together with a weight 2. In this embodiment, the scouring prevention structure 10 is a structure in which a weight 2 is provided on the net-like body 1. Therefore, the weight 2 prevents the net-like body 1 from floating up from the riverbed or waterway. The number, position, size, weight, shape, etc. of the weight 2 provided on the net-like body 1 are not particularly limited, and it is sufficient to set it so that the net-like body 1 can be prevented from floating up from the riverbed or waterway.

The net-like body 1 prevents the passage of sediment on the riverbed or waterway bottom. In other words, the net-like body 1 has a structure that allows water to pass through its mesh, but prevents the passage of sediment. Therefore, the net-like body 1 only needs to have a mesh width that prevents the passage of sediment on the riverbed or waterway bottom. The mesh width that prevents the passage of sediment can be determined according to the particle size distribution of the sediment on the riverbed or waterway bottom. When the particle size near the riverbed or waterway bottom is large, the net-like body may become a filter material with a wide mesh width. In such cases, a configuration using a net-like body with a mesh width larger than the average particle size of the sediment on the riverbed or waterway bottom will also have the effect of preventing scouring, and can be included in the scope of the present invention.

The net-like body 1 may have a mesh width equal to or smaller than the average grain size of the sediment present in the riverbed or waterway in which the net-like body 1 is laid. Even if fine grain sediment is present in the riverbed or waterway, sediment larger than the average grain size may block the mesh of the net-like body 1, forming a filter material and preventing the passage of the sediment. Here, the average grain size of the sediment means the grain size at an integrated value of 60% in the grain size distribution determined by a sieving method. In addition, the net-like body 1 may be made of multiple overlapping nets with mesh widths larger than the average grain size, thereby preventing the passage of sediment.

The net-like body 1 also has the flexibility to follow the gradient and unevenness of the riverbed and waterway. In other words, even if the riverbed and waterway bottom have a gradient or unevenness, the net-like body 1 bends along the gradient or unevenness and does not float up from the riverbed and waterway bottom. Materials for such net-like body 1 include, for example, resin, natural fibers, chemical fibers, steel fibers, etc. Furthermore, it is preferable that the net-like body 1 is made of a water-resistant material or has been treated to be water-resistant.

The net-like body 1 is formed by, for example, weaving resin strings, fiber strings, string-like nonwoven fabric, etc. into a lattice or honeycomb shape. Also, if there is no problem with strength, a commercially available net can be used as the net-like body 1. The scouring prevention structure 10 prevents scouring of the riverbed or waterway bottom by preventing the outflow of soil located under the net-like body 1, so the size of the net-like body 1 can be designed according to the size of the area where scouring should be prevented. Note that if the slope of the riverbed or waterway bottom to be laid is large, the net-like body 1 may be made larger so that the scouring prevention structure 10 can be laid over a wider area.

The sinkers 2 prevent the net-like body 1 from floating up from the riverbed or waterway bottom, and are, for example, natural stones, processed stones, imitation rocks, imitation stones, concrete blocks, wood, mats, filter materials, metals, etc. The sinkers 2 may be connected to each other. The sinkers 2 may also be fixed to the riverbed, riverbank, waterway bottom, waterway side bank, etc. with anchors, etc. The net-like body 1 and the sinkers 2 may be connected.

The scouring prevention structure is a net-like body that prevents the passage of sediment on the riverbed or waterway bottom, laid on the riverbed or waterway bottom while preventing it from floating up from the riverbed or waterway bottom. The net-like body prevents sediment from flowing out, and can prevent scouring of the riverbed and waterway bottom. This also prevents the riverbed and waterway bottom from lowering. Furthermore, the scouring prevention structure is easy to install, as it does not require the riverbed or waterway bottom to be dried before installation.

Furthermore, the scouring prevention structure can prevent not only scouring in the upstream area of a river caused by the lowering of the riverbed in the downstream area, but also scouring (exposed jet scouring) that occurs directly below the weir apron due to high-speed discharge from the weir gate. In addition, by laying the scouring prevention structure adjacent to the downstream side of a structure such as a weir installed on the riverbed or waterway bottom, it can also prevent damage to the weir and other structures caused by the lowering of the riverbed and waterway bottom.

[Embodiment 2]
A second embodiment of the present invention will be described below. For ease of explanation, the same reference numerals will be given to members having the same functions as those described in the above embodiment, and the description thereof will not be repeated.

Figure 3 is a schematic top view showing a scouring prevention structure according to embodiment 2 of the present invention. As shown in Figure 3, the scouring prevention structure 20 is a structure in which a net-like body 1 and a sinker 2 are connected. In the scouring prevention structure 20, the net-like body 1 is provided so as to surround the periphery of the sinker 2. The net-like body 1 and the sinker 2 may be connected by a rope or the like, or the net-like body 1 and the sinker 2 may both be fixed to the riverbed or the bottom of the waterway. The sinker 2 prevents the net-like body 1 from floating up from the riverbed or the bottom of the waterway. The number of sinkers 2 relative to the size of the net-like body 1, the connection position between the net-like body 1 and the sinker 2, the size, weight, shape, etc. of the sinker 2 are not particularly limited, and may be set so as to prevent the net-like body 1 from floating up from the riverbed or the bottom of the waterway.

The scouring prevention structure 20 uses the net-like body 1 to prevent the outflow of soil and sand, and can prevent scouring of the riverbed and the bottom of the waterway. Furthermore, even if the net-like body 1 needs to be replaced due to damage or deterioration of the net-like body 1, the scouring prevention structure 20 can be repaired simply by disconnecting the net-like body 1 from the sinker 2, or by replacing the connection between the net-like body 1 and the sinker 2, including the damaged area. Therefore, the maintenance and management of the scouring prevention structure is easy.

[Embodiment 3]
A third embodiment of the present invention will be described below. For ease of explanation, the same reference numerals will be given to members having the same functions as those described in the above embodiment, and the description thereof will not be repeated.

Figure 4 is a schematic side view showing a scouring prevention structure according to embodiment 3 of the present invention. As shown in Figure 4, the scouring prevention structure 30 is a net-like body 1 connected to a sinker 2. In the scouring prevention structure 30, adjacent sinkers 2 are connected via the net-like body 1. Therefore, the sinker 2 prevents the net-like body 1 from floating up from the riverbed or waterway bottom. The number of sinkers 2, the size of the net-like body 1 connecting the sinkers 2, the connection position between the net-like body 1 and the sinker 2, the size, weight, shape, etc. of the sinker are not particularly limited, and may be set so as to prevent the net-like body 1 from floating up from the riverbed or waterway bottom. In addition, as a modified example of the scouring prevention structure 30, a structure in which the net-like body 1 is laid over the entire area where the scouring prevention structure 30 is laid, and the sinker 2 is connected only to its end, can be mentioned.

The scouring prevention structure 30 uses the net-like body 1 to prevent the outflow of soil and sand, and can prevent scouring of the riverbed and the bottom of the waterway. Furthermore, even when the net-like body 1 needs to be replaced due to damage or deterioration, the scouring prevention structure 30 does not require the movement of the sinker 2, and it is sufficient to simply disconnect the replacement net-like body 1 from the sinker 2, or to move only the sinker 2 connected to the replacement net-like body 1. Therefore, maintenance and management of the scouring prevention structure 30 is easy.

[Embodiment 4]
A fourth embodiment of the present invention will be described below. FIG. 5 is a schematic top view showing a scouring prevention structure 40 according to the fourth embodiment of the present invention. As shown in FIG. 5, the scouring prevention structure 40 is composed of a net-like body 41. The net-like body 41 is formed of a material having a specific gravity of more than 1. The specific gravity of the net-like body 41 refers not only to the specific gravity of the woven string alone but also to the specific gravity of the entire members constituting the net-like body 41. This prevents the net-like body 41 from floating up from the riverbed or waterway bottom. That is, the scouring prevention structure 40 does not require a weight, and the weight of the net-like body 41 alone prevents the net-like body 41 from floating up from the riverbed or waterway bottom. The net-like body 41 has the same configuration as the net-like body 1, except that the net-like body 41 is formed of a material having a specific gravity of more than 1. The scouring prevention structure 40 may use the weight 2 used in the above-mentioned embodiment in combination with the net-like body 41.

The scouring prevention structure 40 uses the net-like body 41 to prevent the outflow of soil and sand, and can prevent scouring of the riverbed and the bottom of the waterway. Furthermore, the scouring prevention structure 40 is easy to install, since it is only necessary to lay the net-like body 41. Furthermore, the scouring prevention structure 40 is easy to maintain.

[Example]
A scouring prevention structure according to an embodiment of the present invention was subjected to a hydraulic model experiment. A 1/70.5 scale, Froude similar, distortion-free model was used as the hydraulic model. The following description of this embodiment is based on on-site converted values.

Figure 6 shows the state of the scour prevention structure before water was passed through it. A model of an acrylic weir apron was fixed to the upstream end of the model waterway (left end in Figure 6), and water was passed at a high flow rate from the weir apron model side to the downstream (right side in Figure 6). The combinations of flow rate per unit width and flow rate at this time covered almost the entire range expected for an intake weir where downstream riverbed lowering is a problem, and the water passing time was also set to the equivalent of the longest expected flood. That is, the flow rate per unit width was 3.3, 6.6, 10, 20, ^{and 30 m3} /s/m, and for these flow rates per unit width, the flow rate was changed from the equivalent of discharge from a fixed weir with a weir height of 3 m, the equivalent of discharge from a gate-type movable weir with a dam-top depth of 10 m, to the equivalent of discharge with the gate fully open. Water was passed for 1/4 day for each of the above combinations of flow rate per unit width and flow rate. The water flow was gradually reduced from maximum flow rate, then stopped, then increased again, reached maximum flow rate, reduced again, and stopped again. The total number of days for which water was allowed to flow was seven days.

The topography downstream of the apron was set to a drop of 3.5 m from the top of the apron to the downstream riverbed model, a gradient of the downstream riverbed of 1/150, and an average grain size of the riverbed of 6.1 cm. These drop and average grain size are standard values for intake weirs where downstream riverbed lowering is an issue, but the gradient is at its maximum, making it a condition where scouring directly below the apron is likely to be significant due to discharge from the weir.

The slope of the bed just below the apron was 1/12, which was the maximum assumed value, and this was also set as a condition that was likely to cause large scouring. The length of this bed was 49 m, and the weight of the bed block and the weight per unit volume of the bed block were set as standard conditions of 3.6 t and 2,350 kg/ ^m3 , respectively. Meanwhile, the shape of the block was a rectangular parallelepiped with a height of 0.7 m, a width of 1.5 m, and a length in the flow direction of 1.5 m. A polyethylene net model with a mesh width of 1.8 cm that does not allow sediment to pass through was laid on the bottom of this bed to prevent scouring. That is, the bed blocks were connected to each other so that they would not be scattered by the flowing water and would continue to function as a weight to prevent the net from floating up. The play in the connection was set to 28 cm to ensure sufficient flexibility of the bed so that a gap would not be generated between the net and the block and the block would not lose its function as a weight.

Figure 7 shows the state of the scouring prevention structure after water was passed through it. As shown in Figure 7, no scouring occurred directly below the weir apron model even after water was passed through it, and the slope of the bed model was generally maintained at the initial state. No scouring occurred on the bed protection structure with the scouring prevention structure, and scouring occurred on the riverbed directly below it. However, the scouring depth was shallower than the value when flowing water was allowed to fall freely from a head of 3.5 m, and no additional damage was caused to the revetment or levee. This scouring shape is such that the maximum scouring point occurs downstream when the flow rate increases, and the maximum scouring point moves upstream to just below the bed protection when the flow rate decreases, and it stabilizes over time by simply repeating this process with increases and decreases in flow rate. The downstream end of the scouring prevention structure almost reaches the maximum scouring point that moves to just below the bed protection structure when the flow rate is stopped, and when the flow rate increases, it is backfilled as the maximum scouring point moves downstream. Therefore, even if the flow rate increases and decreases and water passes through the net, the net prevents the sediment from flowing downstream from the bottom of the bed protection structure. Since the soil at the bottom of the bed protection does not flow out, scouring of the bed protection area does not occur, the original bed protection shape is maintained, and the topography directly below the apron is stable.

In other words, the scouring prevention structure according to one embodiment of the present invention prevented the outflow of sediment from the bottom of the weir apron model, and the weir apron model did not collapse or break. This example shows that the scouring prevention structure according to the present invention can prevent scouring for a long time even under repeated severe conditions such as during a major flood. Note that the head and grain size in this example are standard, but even if these are values that make scouring more likely to occur, if the distance at which the scouring prevention structure is laid is adjusted so that the depth of the maximum scouring point corresponding to the head and grain size is reached, the topography directly below the apron will be stable as in this example. In other words, it is important that the mesh width of the net of the scouring prevention structure is large enough to prevent the passage of sediment, that the net does not float up, and that it is laid to a sufficient distance.

The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. The technical scope of the present invention also includes embodiments obtained by appropriately combining the technical means disclosed in different embodiments.

1, 41 Net-like body 2 Sinker 10, 20, 30, 40 Scour prevention structure

Claims (4)

A scouring prevention structure that prevents scouring on the riverbed and the bottom of a waterway, which is laid adjacent to the downstream side of a structure installed on the riverbed or the bottom of a waterway,
A net-like body that prevents the passage of sediment on the riverbed or waterway bottom is laid on the riverbed or waterway bottom to prevent it from floating up from the riverbed or waterway bottom.
The net-like body has a mesh size equal to or smaller than the average grain size of soil and sand present in the riverbed or waterway to be laid,
Here, the average particle size of the soil and sand means the particle size at an integrated value of 60% in the particle size distribution obtained by a sieving method,
The net-like body is formed of a material having a specific gravity of greater than 1,
The net-like body is laid on the riverbed or waterway bottom together with weights or blocks ,
The net-like body is arranged to surround the periphery of the sinker or block , thereby forming a scouring prevention structure. The scour prevention structure according to claim 1, in which the net-like body and the weight or block are connected. The scour prevention structure according to claim 2 , wherein the weights or blocks are connected to each other. The scour prevention structure according to claim 1 , wherein the net-like body is formed by overlapping a plurality of the net-like bodies.

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