What Is Shore Protection?

Coastal protection projects refer to various engineering facilities that protect coastal towns, farmland, salt fields, and beaches, prevent floods from flooding, and resist the invasion and scouring of waves and currents, including seawalls, revetments and beach protection projects ^] .

Coastal protection projects refer to various engineering facilities that protect coastal towns, farmland, salt fields, and beaches, prevent floods from flooding, and resist the invasion and scouring of waves and currents, including seawalls, revetments and beach protection projects ^] .

Chinese name

Coastal Protection Engineering

Foreign name

seadefense works

Nature

Protective engineering

Function

Protect the coast

Coastal protection engineering historical development

China's coastal protection engineering has a long history and a large scale. Among them, the Qiantang River Haitang, which is resistant to typhoon storm waves and the Hangzhou Bay tidal wave, has long been famous. The Qiantang River and Haitang were built in the Han Dynasty (206-220 BC). The stone cages were filled with bamboo cages during the Five Dynasties (907-960). Later, stone ponds were built. Shitang. The famous Qiantang River fish scale big stone pond appeared in the Ming Dynasty (1368 ~ 1644), and it was improved in the Qing Dynasty (Figure 2). All fish scale ponds are constructed of strip stones, and adjacent strip stones are embedded with iron pieces. On the ground of the foreshore of Tangjiao, piles of stones are used to form a tan water. There are usually one to three lanes, which are called head tan, tan tan, and tan tan. In fact, the pond body is a bank slope protection project, and the water is a foot protection or beach protection project. The other type of stone pond is a block stone pond, which is only built on the side facing the water, and the inside is filled with stones. The sea ponds built during the Ming and Qing dynasties have experienced hundreds of years of wave invasion and scouring, and they are still partially intact and play a good role. Fish scale ponds and block stone ponds are both straight-wall and shore-wall sea ponds. In the past 40 years, slope-type sea ponds with platforms have been used, which is economical and convenient to construct and has good protection effects. There is also water in front of the pond. The banks of the Qiantang River still use traditional spur dikes to protect the beach. The head of the spur dip successfully uses a giant sunken well with a diameter of 20 meters to resist the tide. Since the 1950s, most earth dykes along the coast of China have added block rock or turf slope protection, and many beaches have adopted engineering or biological measures to prevent waves and silt.

The Netherlands, the United States, and Japan are relatively advanced in coastal protection and have extensive experience. The well-known Zuidehai project in the Netherlands has a seawall more than 30 kilometers long; the delta development project has a large cross-section, high standard, high degree of construction mechanization, hydraulic filling of the sand and soil of the embankment, and mechanical pouring of concrete or asphalt covering. The United States has systematically studied coast evolution and protection facilities for more than 50 years, and proposed a complete coast protection planning, design standards and methods, which are widely used in the world. Coastal protection projects built in Japan in the past 40 years have protected 15,000 kilometers of coastline, accounting for about half of its total length.

Coastal protection engineering structure type

The structural types of coastal engineering buildings are usually divided into 4 types: slope type, straight wall type, through-air type and floating type. The first two are traditional heavy structures made of materials such as block stones, concrete, or reinforced concrete; the latter two are light structures made of wood, steel, or reinforced concrete. Heavy structures can generally achieve better engineering benefits, but they also have a significant impact on the surrounding environment. For example, the use of traditional breakwaters can obtain relatively stable harbour waters, but it can also cause sediment deposition in the harbour and new scouring and evolution of the surrounding beaches. The light structure can reduce the impact on the surrounding environment, and also reduce the engineering cost in deeper waters, but the engineering effect and durability are often poor. Due to the complexity of the hydrological and geological conditions in the coastal zone, the layout of coastal projects and the structural types of buildings often require research and demonstration by means of indoor model tests, mathematical models, and field tests ^[2] .

Classification of Coastal Protection Engineering

Coastal protection projects include seawalls, bank protection, foot protection, and beach protection for natural bank slopes.

Coast protection engineering

In the estuary and coastal areas, in order to prevent the flooding of high tide and storm surge and the inundation of the land with the wind and waves, a type of building built on the original ground along the coast to block water is called a sea wall dyke). South of the Yangtze River in Jiangsu, China and Zhejiang are also called sea ponds. Seawall is also an important engineering facility for sea-surrounding projects.

Seawalls generally do not allow waves and currents to climb over the top of the dyke. The height of the top of the dyke is above the height of wave climbing, and there is a certain safety superelevation. The design water level and design wave element standards are generally determined based on the scale, importance, and actual economic benefits of the seawall. The top width of the sea dike is determined according to the needs of the dike body structure, transportation, flood control and construction. The layout of the sea wall should be smooth so as to avoid the strong wind and the frontal attack of the current.

The type and size of seawall sections are related to the local water depth, dynamic factors, foundation characteristics, material sources and construction conditions. Seawalls can be divided into two main types: slope banks and steep wall banks, and slope banks and steep wall banks. Combined into a dyke.

Sloped seawall

The simplest bank is a trapezoidal section with a single slope. Usually, the slope on the side of the sea uses a double cross section with different slopes at the top or bottom or a platform ( ) in the middle. The slope dyke has good wave elimination performance, strong adaptability to foundation subsidence deformation, and simple construction. However, the cross section, the amount of earthwork and the floor area are large. Slope banks are generally filled with earth. The outer slope on the side facing the sea directly bears the effects of waves and currents. Its structural stability is related to the safety of the dyke body. Wave-proof and anti-impact materials that protect the dyke body are often used, which is called artificial protection. The slope is generally 1: 1.5 to 1: 5, and the outer slope without protection is more gentle, and the specific slope depends on the soil quality. The inner slope on the side facing the sea is often protected by plants, and the slope is generally 1: 1.5 to 1: 4. The artificial protection surface includes rock, concrete block or artificial special-shaped block (see breakwater), concrete slabs and other structural types. A crushed stone filter layer or cushion layer is arranged under the protection surface to prevent the sand and soil of the dyke body from being sucked out. The protective surface ranges from the foot of the slope to the highest point on the wave. In areas with shallow water, small waves, and high beaches, plant slopes can also be used for outer slopes.

2. Steep wall sea wall

A traditional type of seawall. The wall is required to be stable under the action of waves. Steep walls or straight walls are built with block stones on the outside, and sand or sand is piled behind the walls. Steep walls can also be built with concrete blocks or caissons. The inner slope of the fill behind the steep wall is generally the same as the inner slope of the slope embankment. The steep wall embankment has a small area and a small amount of work, but the foundation stress is relatively concentrated, and the embankment has a large amount of subsidence, so a solid foundation is required. In addition, the wave pressure on steep-wall banks is also large.

In order to prevent water flow, waves sweeping the slope foot and the slope foot of the steep wall dyke, reinforcement measures are often used, such as throwing stones at the foot of the slope, throwing concrete blocks, or building stone prisms. To build a sea wall on a soft foundation, foundation treatment is also required (see sea-surrounding works).

Coast Protection Engineering

In estuaries and coastal areas, masonry reinforcement measures have been taken on the original bank slopes to prevent waves, current invasion, scouring, and bank collapse caused by earth pressure and groundwater seepage pressure.

The bank protection project is divided into two types: bank slope protection and steep wall (including straight wall). The sloping bank revetment structure and range are basically similar to slope banks. In coastal towns or on shores where both ships are needed, steep-wall banks are often constructed, using masonry stone walls, concrete gravity walls, reinforced concrete buttress-type retaining walls or sheet pile walls. In addition to the wave and water currents of the bank revetment project, the steep-wall quay walls also have to withstand the earth pressure. Because the height of the top mark of the bank protection project is flush with the ground, it will also be affected by the groundwater seepage pressure from the bank. In order to reduce the water pressure on the inside of the bank wall, drainage measures can be taken on the inside, including filling a drainage filter layer behind the bank wall and opening drainage holes in the bank wall. As far as possible, the backfill soil shall be sandy soil with good drainage performance. Reinforcement measures for bank revetments are generally the same as those for sea walls.

In addition to ensuring the stability of the bank slope, the structure, standards and layout of bank protection projects in coastal towns should also consider urban flood control, transportation, and environmental greening requirements.

Coast Protection Engineering

Engineering facilities to protect the mudflats along the beach and prevent the sand on the beach from being washed away by waves and currents. Mud or sandy beaches are lifted by waves, suspended, and transported with water, causing erosion of the beach surface. The slopes of seawalls and revetments are gradually washed away, and even the seawalls or revetments collapse. In addition to the protection of tidal flats, general beach protection projects also indirectly have the functions of dyke protection and bank protection, and have the function of promoting sediment deposition on the beach. Protective measures such as buildings (such as spur dams, shun dams, tanks, shelters, etc.), plants, and artificial beaches can be used.

Spur

The dam body is arranged in a T-shape with the shoreline, the dam root is connected with the embankment and the shore, and the dam body extends to the sea. The spur can lift the water away from the shore and intercept the drifting sand along the shore to make it silt. At the same time, it has a certain shielding effect on oblique waves. In order to protect large beaches and promote their siltation, it is often necessary to build one or more long spur dams to intercept more sediment. The Changdingba has significant picking currents, which will affect upstream and downstream, and even the other shore. When designing, it is necessary to carefully consider its plane layout and length. In order to protect the beach and protect the bank and prevent the mainstream from forcing the bank, a group of spur dams composed of multiple short spur dams are often used. Dingba group is also used in the estuary area, which can make the water flow back to the trough and brush the channel. The Dingba group protrudes not far from the shore, and the flow is relatively gentle. It will not drastically change the flow field. As long as the spacing is appropriate, there will be some siltation in the dam field. The distance between spur dams is related to the dam length and the sediment characteristics of the beach. It is generally determined based on actual experience or hydraulic model tests. There are three ways for the spur dam to be arranged orthogonally to the water flow, up or down. The picking effect is the best when it is picked up, but the scour is near the dam head; the flow is more regular when it is picked down, and the picking effect is worse. The direction of the two-way current or wave is irregular, and it is advisable to use an orthogonal arrangement. The elevation of the dam crest at the root of the spur dam is generally above the average high tide level, but not higher than the top of the adjacent or nearby seawall or bank revetment. The elevation of the dam top gradually decreases as it extends to the sea. Due to the concentrated water flow near the dam head, scouring pits often form, and anti-impact reinforcement measures need to be taken, such as using firewood to protect the bottom, throwing stones or throwing artificial blocks, using caissons or caisson structures, etc.

2. Shunba

A dam constructed approximately parallel to the shore in water at a distance from the shore. Also known as Shun'an Dam. It is used to reduce the waves and promote sediments on the bank side behind the dam. The length of the dam is determined by the required protection range and can be arranged as continuous or intermittent. In order to block coastal currents, a combination of spur dam and shun dam can also be adopted. When the dam top is higher, the wave energy can be reduced and the protection effect of the bank and beach is significant. However, at this time, the wave load is also large, and the dam body structure is required to be more stable. The top of the dam is often submerged, called Qianshun Dam, whose elevation is slightly lower than the average tide level. When the tidal level is not submerged at the high tide level, it is called a water discharge dam, and the elevation of the dam top is higher than the average high tide level.

The sections of the spur dam and the shun dam have two types: vertical and slope. The slope type is built with loose blocks, and the vertical type is built with sheet piles, wooden piles or caissons. The dam can be permeable or impervious. Light-permeable dams such as wood-pile fences, barrage dams, and net dams through which sandy water flows can be less obstructed by water flow and waves. At the same time, the load on the dam body is also small, and the degree of erosion of the dam foot slight. The caisson dam, sheet pile dam, stone masonry dam, etc. that cannot flow through the dam body are heavy impervious dams. Although rock-filled dams and block-filled dams allow part of the water flow through the dam body, the flow characteristics around the dam are similar to those of impervious dams.

3. Protect the tank and the barrier

Local beach protection measures in front of seawalls and revetments are called tank shelters or water tanks. When the beach front of the embankment has stepped relief, in order to prevent the front of the high beach from collapsing and retreating, a protective ridge is built at the steep ridge between the high and low beach (Figure 1).

The artificial sand beach is used to restore the original sand beach or throw it into a new sand beach by artificial sand filling on the coastal washed-out shore section, which is called artificial sand beach. Generally used when there are a lot of cheap sand sources nearby, or combined with dredging works. Artificial beaches are now used to build beaches. This method started in the United States and later spread to Europe and Japan. In addition, planting rice grass, mangroves or other plants on the beach can slow down the waves and promote the sedimentation. Rice grass can be planted in temperate coastal areas, and mangrove forests are suitable for subtropical and tropical beaches. In seas with abundant sand sources, sediment often accumulates into coastal dunes under the action of water currents, waves, and wind, becoming a natural barrier to protect shores and beaches. There are many natural dunes along the coast of the North Sea in Europe. In order to protect the beach and protect the shore, sand barriers are often built along the shore to promote the formation and development of dunes.