JP3015678B2 - Beach nourishment method and beach nourishment structure - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a beach nourishment method for preventing coastal erosion or actively expanding a sandy beach area and a very shallow sea area, and a beach nourishment structure used therefor.

[0002]

2. Description of the Related Art On an oceanic sandy beach, a change in the shape of the sea bottom occurs near the shoreline and in a shallow sea due to partial erosion and accumulation of gravel. Fluid motion interferes with the sandy beach terrain and is caused by transport of gravel trapped in the turbulent turbulent flow of hoisting waves as drifting sand. The movement of drifting sand is more likely to occur as the waves increase, but natural sandy shores are fundamentally shores, which naturally recover with a time-dependent rhythm based on the degree of daily wind waves even if a large seabed shape change occurs. It has a maintenance equilibrium mechanism.

In recent years, the coastal erosion area in Japan has been reported to be 45 km ² in 15 years immediately before the change of the Heisei era,
In the case of sandy beaches that have been balanced by the supply of sand by rivers, it has been described that coastal erosion is occurring due to a decrease in the supply.

In the past, shore erosion countermeasures have often used piers and detached shores, and in recent years, structures such as artificial capes have been planned.

[0005]

However, although these structures have the function of preventing the outflow of sand drift, they cannot restore the eroded sandy beach to the old state. In addition, there is a problem that enormous cost is required for installation of the structure.

[0006] On the other hand, there is a growing nationwide need for beaches, which are the core of marine recreation. For this reason, the expansion of sandy beaches on existing sandy shores, the stabilization of the topography of shallow seas (being shallow), and excessive There is a demand for suppression of waves.

The present invention has been made in view of the above circumstances, and an object of the present invention is to supply new gravel to an eroded sandy beach and restore it to an old state. An object of the present invention is to provide a beach nourishment method and a beach nourishment structure that realizes a new equilibrium by realizing a new balance by expanding the topography, stabilizing the topography of the shallow sea area, and suppressing excessive waves.

[0008]

According to the present invention, there is provided a beach nourishment method comprising: a plurality of rods each having a plate-like brim; and a plurality of connected bodies that connect the rods at their ends. , A triangle or 4 with a rod as a side and a connected body as a vertex
A beach nourishment structure consisting of an underwater truss that is continuously developed by combining the basic structures assembled to form a rectangular shape continuously from the water movement limit of the sand drift on the coast to nourish the sea to the deeper sea floor It is characterized by doing.

The above-mentioned beach nourishment structure may be such that only the offshore end is fixed to the seabed, may be provided alone, or may be a strip-shaped beach nourishment structure extending along the seabed toward the shore. Alternatively, a plurality of rows may be provided at a distance from each other in a direction substantially perpendicular to the traveling direction of the waves. The beach nourishment structure of the present invention has a large number of rods each having a plate-shaped brim and a number of connected bodies that connect the rods at their ends, each having a triangular shape having the rods as sides and the connected bodies as vertices. A beach nourishment structure consisting of an underwater truss formed by continuously combining base structures assembled to form a quadrilateral, which is installed from near the movement limit water depth of the sand drift on the beach to be nourished to a deeper sea bottom. It is characterized by having.

[0010] The moving depth limit of sand transport refers to the water depth that defines the lower limit of the occurrence of coastal sand transport, or the limit water depth within a range where remarkable topographical changes are observed on sandy beaches.

[0011] The transport limit water depth of drifting sand can be determined based on survey data, but can basically be determined by a predetermined formula given wave and sediment conditions.

[0012] In addition, the surface movement (the state in which the surface sand is swept collectively in the direction of the waves) at the movement depth limit of drifting sand.
There is a critical water depth and a critical water depth of complete movement (a state in which significant movement is seen as the water depth change clearly appears).
In the present specification, the two are not particularly distinguished from each other, and are simply referred to as the transport limit water depth of drifting sand.

[0013]

According to the present invention, the beach nourishment structure installed from the vicinity of the movement limit water depth of the drifting sand to the deeper sea bottom can resist scouring and bottom sweeping due to waves. Exciting eddy turbulence to roll up gravel at a position deeper than the critical water depth, and then move and deposit the gravel on the shore side by flow toward the shore to create a normal coastal equilibrium. Can be. In particular, in the case of a large wave, a turbulent bed entrained by a large amount of sedimentary gravel rides on the countercurrent caused by the waves, resulting in large-scale shore-to-shore transport including new gravel that is not in conventional equilibrium.

As described above, according to the present invention, by setting up the beach nourishment structure, the natural normal sandy shore process area is expanded, and the adjacent gravel layer deeper than the drift limit water depth of drifting sand is actively shored. The erosion of sandy shores can be restored to the old state due to the reduced supply of gravel from rivers, and the existing sandy shores can be expanded and shallow water It is possible to stabilize the terrain at a shallow depth. Further, the beach nourishment structure of the present invention has a wave-dissipating function of reducing the energy of waves by turbulence, thereby suppressing excessive waves and calming the waves, thereby reducing the normal seashore. To achieve a new balance.

Further, the beach nourishment structure of the present invention can be easily installed compared to conventional heavy structures such as piers and piers, and the cost required for installation can be extremely low. The economical advantage is also great.

The oceanic sandy beach is classified into a stormy shore where the seafloor topography changes greatly due to the action of waves and a normal shore where the change is not so large.
By applying the present invention to a stormy shore, it is also possible to change the stormy shore into a normal shore.

[0017]

Embodiments of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 is a plan view showing the basic structure of a beach nourishment structure according to one embodiment of the present invention. As shown in the figure, the basic structure of the beach nourishment structure according to the present embodiment has three rods 1 and three spherical connecting bodies 2 connecting the rods 1 at ends.
Are underwater trusses assembled to form an equilateral triangle having the rod 1 as a side and the connecting body 2 as a vertex. The underwater truss is described in detail in Japanese Patent Publication No. 52-3487 by the present applicant. The basic structure may be a square with the rod 1 as one side.

The connecting body 2 is provided with twelve connection holes (not shown) on its surface, and an end portion of the rod 1 is provided with a connection part (not shown) connected to the connection hole. The rod 1 is
It is composed of a tube that resists a compressive force and a tension rod that resists tension that extends in the tube in the axial direction, and is configured such that the length can be adjusted when connecting with the connecting body 2. . The configuration of the rod 1 and the connecting body 2 is disclosed in detail in Japanese Patent Application Laid-Open No. 6-136727 by the present applicant.

At the center of the rod 1 in the axial direction, for example, a circular plate-shaped brim 3 is mounted orthogonal to the axis. By providing the brim 3, the turbulent flow excitation function as a beach nourishment structure can be controlled. Brim 3 is rod 1
The member is not limited to a circular shape, but may be a polygon such as a quadrangle or a hexagon, or may have a large number of holes. Although only one brim 3 may be provided for one rod 1 as shown in the figure, a large one is provided at the center to enhance the turbulent flow excitation function, and a relatively small one is provided on both sides thereof. May be provided one by one. Such a configuration is disclosed in detail in Japanese Patent Publication No. 58-26443 (wave-dissipating structure using underwater truss) by the present applicant. Also, brim 3 is rod 1
The brim 3 may be elastically attached to the rod 1. The elastic attachment of the brim to the rod is based on the applicant's
No. 0530 is disclosed in detail.

FIG. 2 is a perspective view showing a schematic configuration of a beach nourishment structure according to an embodiment of the present invention, and FIG. 3 is an enlarged perspective view showing a main part thereof.

As shown in FIG. 2, the beach nourishment structure 10 according to the present embodiment has a base 11 extending in a belt shape substantially parallel to the shoreline of the gravel.
And a plurality of rows of extending portions 12 extending in a band shape (comb shape) from the base portion 11 toward the shore along the sea bottom at a predetermined interval from each other. Note that the extending portion 12 may be provided as a single unit.

As shown in FIG. 3, the extending portion 12 is formed by continuously combining the basic structure with the brim 3 shown in FIG. 1, while the base portion 11 is attached with the brim 3 as shown in FIG. The basic structure in which the rod 1 not connected and the connector 2 are assembled to form an equilateral triangle is continuously combined. In addition, the base 11 is provided with an anchor 13 and the base 11 is fixed to the seabed via the anchor 13, whereas the extension 12 is simply placed on the seabed. is there. For this reason, the extension part 12 descends in accordance with the decrease in the gravel on the seabed portion on which the extension part 12 is placed, and always keeps in contact with the seabed.

Next, the operation of the beach nourishment structure 10 of the present embodiment configured as described above will be described. FIG. 4 is a diagram schematically showing the operation of the beach nourishment structure 10, and FIG. 5 is a diagram showing a change in the shape of the seabed cross section of a sandy beach as an example in which the beach nourishment structure 10 is installed.

As shown in FIG. 5, on this sandy beach, the amount of change in ground height at a portion shallower than 4 m is generally large, but a change in the cross-sectional shape at a portion deeper than 4 m is almost observed except during a storm. Absent. Judging from such a change in the cross section of the seabed, it is judged that the movement limit water depth of the drift on the sandy beach is about 4 m to 10 m.

Therefore, as shown in FIG.
Shall be installed on the sandy sea floor adjacent to a location deeper than the drifting depth of 4 m. When the beach nourishment structure 10 is installed, the bottom sweep of the waves mainly interferes with the extending portion 12 of the beach nourishment structure 10, thereby exciting a turbulent vortex field and rolling up the seabed gravel. The gravel that is rolled up and held in the turbulent turbulent flow is transported in the shore direction mainly by the countercurrent flow between the extending portions 12 and is deposited on the shallow sea portion. Due to the accumulation of gravel at a position deeper than the drift depth of sand drift, the sandy beach expands and the topography of the shallow sea is stabilized to a shallow depth.

Further, the waves consume kinetic energy when interfering with the beach nourishment structure 10 and being turbulently sulfurized. Therefore, the waves on the sea surface closer to the shore than at the position where the beach nourishment structure is installed become gentler, No excessive rip currents will occur. This wave-reducing effect is an important factor for equilibrium stability. In this way, the sandy shore on which the beach nourishment structure 10 is installed becomes a sandy shore extremely suitable for a beach.

FIG. 6 shows another embodiment of the beach nourishment structure of the present invention. The illustrated beach nourishment structure 10A is a brim 3 shown in FIG.
It is a part that is constructed from the basic structure attached to the entire surface. With such a configuration, the rate of turbulence of the waves increases, and a large amount of gravel can be supplied in a short period of time.

Although the embodiments of the present invention have been described above, the present invention is not limited to the specific embodiments of the embodiments, and various changes can be made.

For example, although the beach nourishment structure of each of the above-described embodiments is constructed by combining the basic structures shown in FIG. 1 two-dimensionally, these may be combined three-dimensionally.

[Brief description of the drawings]

FIG. 1 is a plan view showing a basic structure of a beach nourishment structure according to one embodiment of the present invention.

FIG. 2 is a perspective view showing an overall configuration of a beach nourishment structure according to one embodiment of the present invention.

3 is an enlarged perspective view showing a main part of the beach nourishment structure shown in FIG. 2;

FIG. 4 is a view schematically showing the operation of the beach nourishment structure shown in FIG. 2;

FIG. 5 is a diagram showing a change in shape of the seabed cross section of a sandy beach on which the beach nourishment structure shown in FIG. 2 is installed.

FIG. 6 is a perspective view showing the overall structure of a beach nourishment structure according to another embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Rod 2 Connected body 3 Brim 10, 10A Nourishment structure 13 Anchor part

Claims (2)

(57) [Claims] 1. A large number of rods each having a plate-shaped brim and a large number of connected bodies that connect the rods at ends thereof form a triangle or a quadrangle having the rods as sides and the connected bodies as vertices. A nourishment structure consisting of an underwater truss composed of a continuous combination of base structures assembled in such a way that the A beach nourishment method characterized by installation. 2. A large number of rods each having a plate-shaped brim and a large number of connecting bodies that connect the rods at their ends to form a triangle or a quadrangle having the rods as sides and the connecting bodies as vertices. A beach nourishment structure consisting of an underwater truss formed by continuously assembling a base structure assembled as a whole in a planar manner. A beach nourishment structure that is installed in a plane along a surface.