JPH0745726B2 - Revetment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of use] The present invention relates to a revetment for conserving or recovering a sea cliff.

The coast is slowly changing according to its environmental conditions,
In some cases, the changes can destroy the coastal environment and create hazards in the neighborhood. Especially on the coast where there is no sandy beach and the cliff is directly facing the sea, the cliff is scraped off by the erosion action and continues to recede, and the land is gradually taken away, and roads and structures close to the coast are removed. It poses a risk of collapse and even destroys the excellent scenery of the coast.

More specifically, according to the present invention, there is a reef having a wave-dissipating action in the sea area facing the erosion cliff, of the cliffs being destroyed and thus continuously retreating due to the erosion action. Regarding sea cliffs, it particularly relates to revetments that are intended to be preserved or restored while maintaining the scenery.

[Prior art] The revetment works that are currently being carried out in various places mainly arrange various concrete blocks such as abandoned blocks and tetrapods in front of a sea cliff and arrange them by the wave-dissipating action of those concrete blocks. , Is designed to protect sea cliffs from strong waves.

The above concrete blocks include those having various shapes and those colored like rocks. Also, there are various sizes.

In addition, a breakwater is constructed parallel to the coast in the sea at a predetermined distance from the coast, and the coast is protected by these, and at the same time, a jetty is provided on both sides of the coast corresponding to the opening of the breakwater. There is also proposed a revetment work in which boulder-like wave breaking material is arranged without providing (Japanese Patent Laid-Open No. 47-15942).

In addition, on relatively flat beaches such as beaches where sand has been lost, there are cases where seawall protection work was performed by simply adding sand to the beach.

[Problems to be Solved by the Invention] According to the seawall construction for stacking the concrete blocks in the sea as described above, especially at low tide, most of the concrete blocks are exposed, which causes a problem of damaging the landscape of the coast. It has been pointed out that there is. As mentioned above, there are cases where blocks are colored to resemble rocks or are slightly modified in shape, but it is also unnatural and far from maintaining the landscape. I have to say. Therefore, it is the actual situation that it is difficult to adopt it in a tourist area for that reason alone.

In addition, since the seawall construction work using concrete blocks as described above does not have the effect of drawing sand around the sea cliff, it is not possible to expect a buffering function against waves due to sand as described below. On the contrary, even some existing sand may be washed away on the back side of the concrete block masonry, resulting in the exposure of continental gravel.

Furthermore, there is a problem that concrete blocks are not immediately compatible with the sea.

The second-mentioned breakwater and revetment using cobblestone breaking material also constitute a breakwater, similar to the above, which does not match the natural landscape, while the backside of the breakwater is the base. However, there is a serious drawback in that the problem of its collapse may occur sooner or later due to the scouring.

Furthermore, in the above revetment work, due to the existence of the opening of the breakwater,
The flow velocity of the flowing water returning in the meantime becomes very high, and the boulder-shaped wave breaking materials are arranged as described above in order to prevent the receding due to the erosion of the shoreline corresponding part caused by that. However, depending on their size, they will roll down into the sea and will not be useful for coastline conservation. Some of this suggestion is 20 cm to 70
Some of them have a diameter of cm, but even if they are so large, they will still flow out if the sea becomes rough.

Of course, it is acknowledged that the larger ones, the longer it takes to roll down. However, the power of the waves is unexpectedly large, so even if it is as high as a human head, it will fall into the sea in no time.

As mentioned above, if they are buried in sand, they will not easily drop into the sea under normal conditions, but if the sea eventually becomes rough due to a typhoon, they will be washed away and fall into the sea. It will be.

This mechanism will be explained. First, gravel, etc., where parts that are stacked at an angle that is greater than the slope of the coast, quickly roll down under normal conditions. Especially for large rocks, it will take some time, but it will fall when the sea is rough.

On the other hand, first of all, there are times when migrating sand (drifting sand) gathers on the coast many times during the year. At that time, migrating sand, which is fine sand, gathers on the gravel etc. arranged on the shore, and gradually When it comes to filling gaps such as gravel, even general gravel will not easily roll down under normal conditions. However, when the sand further accumulates and the thickness of the sand accumulated in the gap exceeds a certain thickness (the thickness varies depending on the diameter of the sand,
(The smaller the diameter, the thinner it is.) As a result of the loss of penetration into the sand in the sea, almost all of the waves coming back will turn back, increasing the equivalent flow velocity on the land side and both sides of each gravel, removing the sand at that position and returning. Will be gone. Therefore, each gravel gradually becomes floating from the sand. And such floating gravel will be easily moved by the waves, and will reciprocate along the shore by the reciprocation of the waves, but due to the large amount of downward movement, it will gradually move into the sea and eventually be lost. It is a thing. Rock masses that are slightly larger than gravel are basically the same except that the movements are slightly smaller.

The process by which the gravel and the like float up from the sand due to the receding wave is that when a person stands on the beach where the waves reciprocate on the beach, the returning waves cause the sand on the land side and both sides of the foot to be deprived. It is the same.

However, when the sea is rough, as mentioned above, it is easily washed away even when it is buried in migratory sand.

In addition, the surface of gravel is not always flat like sand, and irregularities often occur, so in some cases a slightly strong wave may cause some of the gravel near the surface to fly. To the base of the cliff with or before the waves, causing the formation of sea erosion depressions.

However, even gravel and slightly larger rocks will eventually fall into the sea. It just takes a while if it is large. Also, beautiful sandy beaches cannot be formed by these large rocks.

In addition, if the seawall construction, which is the last-mentioned method of simply putting sand into the shore, is convenient for the scenery, it is not possible to secure the anchorage of the sand, and if the sea becomes rough due to a typhoon, it will immediately flow out. The sand is gone. The reality is that it will last almost all summer.

[Concept of Solving Problems] According to the present inventors' observation of the coast for many years, the erosion cliffs on the Iwahama coast are not only subjected to the erosion action caused by waves but also rocks and continental gravel scattered around the cliffs. As a result, the base of the cliff is repeatedly hit with a wave, such as a pickaxe or a hammer, and is scraped off to form a wave depression. It has also been observed that the rock that has been scraped off is gradually crushed and turns into crushed stone and sand over the long term.

On the coast where the water depth is deep, the energy of the waves also works greatly, and due to the action of the reflected waves, the above sand does not remain on the shoreline and is carried offshore and deposited on the seabed, or depending on the topography, it moves and setstles elsewhere. To be observed. Loss of sand on the above sea cliffs, and further sea erosion, while on the coast where sand has settled,
Sea erosion is prevented by the buffering action of the beach.

However, if the above-mentioned sand can be artificially deposited and settled on the rocky beach, it is possible to obtain a buffering effect against waves and prevent sea corrosion.

By the way, in recent years rivers have been improved and dams have become upstream, so the risk of floods has decreased, but along with this, the absolute amount of sediment carried to the sea by rivers has also decreased, and as a result, the ability to supplement sand has also increased. It has come to lower.

The beach sand is composed of a mixture of fixed sand (ground sand) and migratory sand (drifting sand) with different properties, and it constantly moves and accumulates, but fixed sand with a tendency to settle does not have a natural metabolism. As long as it is, it is thought that it will eventually become extra fine migratory sand and will flow out of the continental shelf into the trench.

Therefore, we have come to think that it is necessary to artificially form a sandy beach to protect the coast at this time, but depending on the conditions of the coast, it is formed by utilizing the sand generation and wave-dissipating action of weathered granite. Or, it came to the idea that it is necessary to directly insert and artificially deposit sand having a fixing property that matches the conditions of the coast to form a beach.

By the way, from the results of many years of observation of the coast by the present inventor, it is usual that, even on the coast, even a considerably large rock mass eventually rolls down to the seabed, and they never reach their original positions. It does not come back. On the other hand, sand moves back and forth between the seabed and the coast, and it has come to be considered that selecting a more appropriate one among them is the most suitable for conservation of the coast.

As described above, the present invention is intended for the beach under the latter condition.

[Structure for Solving Problems] The gist of the structure of the present invention is to provide a seawall cliff for a seawall cliff construction in which a rock reef having a wave-dissipating action exists in a sea area facing the seawall. In the sea area facing
It is a revetment work that forms artificial sandy beach by adding coarser sand that has a larger specific gravity than the sand naturally existing in the sea area.

The sand is poured so as to completely fill the wave depression of the sea cliff and further fill the circumference of the reef.

Coarse sand having a large specific gravity is preferable for the above-mentioned sand for forming the artificial beach. This is roughly the case for sand with a diameter of 1.5 mm to 2 mm. However, the material is also important, and quartz sand is particularly preferable because it has a large specific gravity. Specifically, the coarse sand of the Takahagi Sandstone Group, which is quartz and has a large specific gravity, is extremely effective as the fixed sand.

It should be noted that the particle size should not be made as large as possible in order to improve the fixing property. When it goes beyond the range of coarse sand and reaches gravel, as mentioned above, the fixing property is lost. If you add the reason here, the distance that gravel is pulled back when returning is longer than the distance that is pushed up by the gathering waves.

Of course, while only gravel is present on the shore, waves coming in penetrate the space between gravel, so unless there is something protruding from the top surface, the amount of water on the returning wave will decrease, and gravel will be easily removed. However, as mentioned above, when the migratory sand fills the gaps between the gravel and the permeability is lost, the total amount of waves coming back is returned and the sand on both sides of each gravel and the land side is washed away. Then, it will float and will roll down with the returning wave. Moreover, since the falling gravel cannot rise and roll more than the amount of falling by the gathering waves as described above, it gradually rolls into the sea.

However, as mentioned above, the gravel buried in the sand will also flow out together with the sand if the sea is a little rough.

On the other hand, since the sand beach on which coarse sand is deposited is permeable, the amount of water of the returning wave is slightly smaller than that of the coming wave, and the action of first carrying away the sand from the beach is decreased. Even if the sea is a little rough and the sand is carried away, if the size is about the size of coarse sand, it will not be uniform at each time, but it will be pushed up the same distance or more by the waves coming up. Will result. However, the sand that has flowed out then returns to its original position. And the sand is left there again. This is partly because, as mentioned above, the waves that come in are often stronger than the waves that return due to the fact that the beach has permeability.

By the way, in some cases, the coarse sand contains some rocks of Takahagi layer sandstone and weathered granite, and these rocks may be buried in the beach.

In short, spillage on the coast is inevitable even if the size is a little large, and what is needed is that the spill is relatively strong against spillage and easily returns to the shore even if it spills. To have. And coarse sand is the best from this point of view.

Note that the fine migratory sand accumulated in the harbor, etc. cannot be used due to its poor fixability.

[Operation] Since the present invention is configured as described above, since the sea cliff is protected by the sand beach and is not directly exposed to the waves, the sea erosion stops.

Further, when the beach is formed in this way and the sand is settled on the coast, the sea surface of the coast is kept calm and the environment becomes more likely to accumulate the sand.

That is, in addition to the permeability of the beach, the reflected wave becomes weaker by returning to the gentle slope.

The energy of the waves gathered due to the gentle inclination of the beach gradually decreases while moving over a long distance and finally retreats, but due to the gentle inclination of the beach, the retreating speed is extremely gentle compared to before the formation of the beach. Will be things. The waves coming further,
While moving along the beach of coarse sand generated as described above, because the sand is coarse sand, the reflected wave that penetrates very well into the sand and recedes has a smaller amount of water than when gathering. Will be done. It is speculated that the water that penetrates into the sand leads to the sea in the sand.

However, the amount of sand carried by the coming waves is less than the amount returned from the beach, so to speak, the sand is called sand, and the sand filling the offshore reef is near the coast again. I will come back to and replenish the sand.

On the other hand, it is also possible that the rocks off the coast will be exposed again, and seaweed resources such as wakame will be generated.

In terms of landscape, this revetment works to throw sand into the sea area facing the sea cliff to form a sandy beach, so it is possible to adapt to the natural landscape and improve the landscape in some cases.

In the long run, it may not be possible to expect natural replenishment of the sand, and in some cases it may be necessary to reload the sand.

[Embodiment] An embodiment of the present invention will be described below with reference to the drawings.

This was carried out in a secret state on a part of the coast (4-3655 Hidakacho, Hitachi City, Hitachi City, Ibaraki Prefecture), which is the private land of the present inventor.

In July 1983, as shown in FIG. 2, coarse sand of the Takahagi Group sandstone group was thrown into the front of the sea cliff 1 up to the height of the wave depression 2. The sand was poured from the sea shelf 3 to the surrounding areas of the reefs 4, 4, .... 5 is the input sand.

Then, as of July 1985, the above sand has completely settled, and when observing the beach, a new layer of sand is found. This new sand deposit layer, as described above, keeps the coastal sea surface calm due to the formation of the above-mentioned beaches, and as a result, the power of the reflected waves weakens and the offshore areas carried by the waves are removed. It is considered that the sand was left behind and was generated.

During this period, the waves did not directly hit the sea cliff 1, so the sea erosion naturally stopped.

Also, in terms of the landscape, sand was injected into the front of the sea cliff 1 to form a beach, but this is different from the presence of concrete blocks such as tetrapods, and it is said to be suitable for the natural landscape. Is recognized.

[Effects of the Invention] According to the present invention, it is possible to protect sea cliffs while utilizing the activities of nature without damaging the landscape. Further, since the sand used in the present invention satisfies the above conditions, there is also an advantage that the construction can be carried out at a low cost without problems as compared with the conventional revetment work using the tetrapod and other concrete blocks.

[Brief description of drawings]

The drawings show an embodiment of the present invention. FIG. 1 is a schematic sectional explanatory view showing a state of a coast before construction, and FIG. 2 is a schematic sectional explanatory diagram of a coast immediately after construction. 1 ... Sea cliff, 2 ... Wave erosion, 3 ... Sea shelf, 4 ... Reef, 5 ...
Input sand.

Claims (1)

[Claims] 1. A revetment work for a sea cliff in which a reef having a wave-dissipating effect is present in a sea area facing the sea cliff, in which the sea area facing the sea cliff is more than the sand naturally present in the sea area. A revetment with a large specific gravity and the use of coarse sand to form artificial sandy beaches.