JP2006193981A - Littoral transport control structure - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce the local deposit of sand by reducing the flow-out of sand from an opening of a littoral transport control structure and a rise of a water level on a shore side to create a coastline reduced in local influence. <P>SOLUTION: An artificial reef 1 is provided on a rubble mound constructed on the sea bottom along a coastline to form an angle of 25 to 50 degrees for the coastline. By arranging it obliquely, the substantial width of cross section is increased, the transmission coefficient of waves is reduced, the energy of ocean waves going toward the shore side by crossing a shore structure is reduced, the size of an apparent opening 2 between artificial reefs in the direction of the progression of ocean waves becomes almost zero, and the velocity of the flow of a return stream is reduced to control drifting sand. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a sand drift control structure that prevents coastal erosion caused by waves, suppresses the occurrence of sand drift, and preserves the coast.

  Coastal erosion due to waves is a major problem. In order to prevent erosion, coastal structures such as offshore dikes or artificial reefs are installed on the eroded coast, and the wave energy is attenuated and calmed down, and the sand drifts offshore. The coast is preserved by preventing runoff.

Patent Document 1 (Japanese Patent Laid-Open No. 2-88810), Patent Document 2 (Patent Document 1) which scoured seabed sand in a region where waves were attenuated by installing such a coastal structure and intended to prevent erosion. JP-A-9-125336).
JP-A-2-88810 JP-A-9-125336

  Coastal structures such as breakwaters, breakwaters, artificial reefs, and jetty for coastal protection are not installed separately, as shown in Fig. 3 (1) and Fig. 4 (1). It is common to install a plurality of coastal structures parallel or perpendicular to the coastline to be preserved.

In the conventional arrangement method in which the sand drift control structure is installed parallel to the coastline, the sand drift control structure is installed at intervals, so that the sand is extremely close to the center of the back of the structure due to the presence of the opening. The shape of the coastline has changed greatly as it accumulates and sand flows out from the opening toward the sea.
Submerged structures such as artificial reefs are less susceptible to erosion by waves and terrain deformation due to scouring, but the protective effect of the coastline is smaller than that of the offshore levee. It was necessary to make it larger and it was expensive.

  When it is installed offshore, such as a breakwater, local sedimentation is generated behind the coastal structure due to the movement of the seabed sand and drift sand due to the waves, but beyond the coastal structure. On the other hand, when the water flow toward the shore is directed toward the opening between the structures and goes from the shore to the offshore, it becomes a rapid flow and scouring and erosion around the shore structure occurs, and sand and drift sand on the sea floor It flows out to the offshore side, and the coastal landform is in a state where a dragonfly is generated as shown in FIG.

In the case of a jetty installed perpendicular to the coastline, coastal drift sand accumulates on the lower side of the coastal structure and erodes the upper side, resulting in a sawtooth coastline as shown in Fig. 4 (2). Landscape problems arise.
Considering the use of the coastline, it is desirable to reduce the local topographical deformation and generate sediment and advance the coastline offshore while maintaining the current shape.
The present invention reduces the outflow of sand from the opening of the coastal structure that controls the drift sand and the rise of the water level on the shore side, and also generates a dent-lined coastline due to local accumulation of sand or deposition We propose an effective arrangement of sand drift control structures to create a coastline with minimal local impact.

In order to solve these problems, effective sand drift control can be achieved by installing multiple coastal structures diagonally to the coast and reducing or eliminating the apparent opening in the direction of wave travel It is to make.
Since waves act on the sand drift control structure from an oblique direction, the apparent top end width (size of the structure) increases, and the top end width of the structure may be reduced.

  As shown in Table 1, when the relationship between the incident angle (θ) of the wave on the artificial leaf and the wave extinction performance when the top width of the artificial leaf was 50 m was tested, It was found that the wave-disposition rate was reduced and the wave-dissipating effect was improved by using the arrangement.

  As shown in Fig. 2 (1), when an artificial reef is installed parallel to the coast, the cross-sectional width (L) of the artificial reef is the width of the wave, but it is placed obliquely. As shown in FIG. 2 (2), it is considered that the cross-sectional width of the structure becomes relatively large with respect to the direction of the dominant wave. In other words, it suggests the possibility that the cross-sectional width of the structure can be reduced by arranging it at an angle with respect to the wave direction.

In the conventional method of placing sand drift control structures, coastal drift sand can hardly be blocked by the coast structure, but it has a length component perpendicular to the coast by installing the coast structure diagonally. Therefore, the coastal current can be blocked and the same effect as the jetty can be expected, so the influence of coastal sand drift can be reduced.
Moreover, the wave refraction effect by the coastal structure and the influence of the oblique arrangement suppress the rise of the water level on the shore side and increase the return flow, thereby increasing the wave extinction efficiency.

  Furthermore, since the apparent size (B ′) of the opening 2 between the sand drift control structures as seen from the direction of the action of the waves becomes almost zero, the sand outflow from the opening 2 is reduced. Therefore, the formation of a saw-toothed coastline is prevented and the coastline can be made gentle.

As shown in Fig. 1, on a rubble mound constructed with an artificial leaf 1 made of concrete with a crest width of 30m and a length of 60m along the coastline, at an angle of 25 to 50 degrees with respect to the coastline. Installed. The water depth at the top of the artificial reef was 1.5 m at low tide.
By arranging them diagonally, as shown in Table 1, the wave transmission rate is lowered, and the energy of the waves going to the shore side beyond the coastal structure is considerably reduced. In addition, by arranging it obliquely, the apparent size (B ′) of the opening 2 between the coastal structures in the traveling direction of the waves becomes almost zero, and the flow velocity of the return flow can be reduced. Sand drift can be controlled and scouring and erosion around artificial reefs can be prevented.

Since the waves act on the sand drift control structure from an angle, the apparent top edge width becomes large, and when the same wave extinction efficiency is set compared to the case where the transverse wave acts, the top width of the structure can be reduced. Cost can be reduced.
Since there is no apparent opening of the sand drift control structure seen from the direction of the action of the waves, it is possible to reduce the outflow of sand from the installation opening, and therefore it is possible to prevent the formation of a saw-toothed coastline, The coastline can be advanced while maintaining the shape of the current coastline, and the beach landscape can be maintained well.

The top view which shows the arrangement | sequence state of the coast structure of this invention. Explanatory drawing of the effect of this invention. The top view which shows the arrangement | sequence of the conventional breakwater. The top view which shows the arrangement | sequence of the conventional jetty.

Explanation of symbols

1 Coastal structure 2 Opening 3 Coast

Claims (3)

Sand drift control structure with multiple coastal structures installed at an angle to the coastline. 2. The sand drift control structure according to claim 1, wherein the apparent opening of the coast structure with respect to the wave direction is substantially zero. 3. A sand drift control structure according to claim 1, wherein the coast structure is either a breakwater or an artificial reef.

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