Coastal topography

Wave action is the main driving force of coastal erosion and accumulation, and the shaping of coastal landforms mainly occurs during storm waves, and the wind and waves under normal weather conditions only play a continuous role in modifying coastal landforms. The influence of tide on bedrock, gravel and sandy coast is realized by changing the wave action. On silty and muddy coasts composed of fine particles, sedimentation is mainly completed by tidal current.

(A) coastal erosion landform

Coastal erosion is called marine erosion, and various forms of coastal erosion mainly caused by seawater dynamic factors are called marine erosion landforms.

Wave water directly washes the coastline, which is called erosion. Because the waves hit the coast with great energy when they reach the shore, the huge pressure of water itself and the compressed air in rock cracks and joints have a strong destructive effect on the coast, which can reach dozens of tons per square meter. The erosion of the seabed by the gravel carried by the seawater with the reciprocating motion of waves is called abrasion. In the reciprocating motion before and after the waves, seawater carries the gravel, mud and rock fragments eroded by the coast, grinds the bedrock on the seabed, and accelerates the erosion of the coast. Coastal erosion caused by seawater's dissolution of coastal bedrock is called dissolution. Seawater's dissolution ability of rocks and minerals is stronger than that of fresh water, especially on the coast composed of soluble rocks such as carbonate rocks, which is more destructive to the coast and can form a unique dissolution platform.

The main dynamic factors that shape coastal erosion landforms are waves and tidal currents, but the coastal areas in high latitudes are also eroded by freezing, while the coastal areas in tropical and subtropical regions are eroded by abundant surface water and strong chemical weathering. In addition, they are also limited by the corrosion resistance of rocks that make up coastal areas. The rocky coast with dense and hard structure has strong corrosion resistance, but due to the development of cracks and joints, many sea arches, cliffs, caves and columns are formed (Figure 7- 19, Figure 7-20). Soft rock coast has poor anti-corrosion ability, and the sea cliff retreats quickly, which is easy to form a sea erosion platform. Limestone coast has a unique cellular marine erosion landform under seawater dissolution. Marine erosion landform is usually regarded as one of the signs to distinguish regional tectonic movement from sea level change. At the same time, the sea erosion landform shaped by waves is very spectacular and often becomes a tourist attraction.

Figure 7- 19 Cliff Pillar-Shilaoren in Qingdao

Figure 7-20 Shihai Cave in Lingshan Island, Qingdao

Debris eroded by the coast is carried by the waves below the front edge of the marine erosion platform and accumulated in the depth of the bank slope, forming an underwater accumulation terrace.

Under the action of waves, the coastal profile has different shapes at different stages of the development of bedrock coast, so the intensity and distribution of wave energy on the coast are also different. The long-term action of waves on the bedrock coast can finally make it reach a balanced state, at which time the profile is no longer transformed by wave action, and the shaping of the balanced profile of marine erosion is completed. After the formation of the marine erosion balance profile, the wave energy corresponding to any point on the profile is at the critical value, and beyond this value, the profile will be washed away.

(2) coastal accumulation landform

Loose materials entering the coastal zone move under the action of waves and currents. When the power is weakened or the movement is blocked, they will pile up and form various marine landforms.

The movement of coastal sediment is mainly influenced by waves and gravity, and the sediment in coastal areas moves in different forms under their joint action. When the wave direction (wave ray) is perpendicular to the coastline, the projection of the wave force and the direction line of gravity on the underwater bank slope or beach surface (tangential component of gravity along the slope) is on the same straight line, and the activated sediment will move back and forth to the shore and the sea surface, which is called the lateral movement of sediment. When the wave direction (wave ray) is oblique to the coastline, the tangential components of wave force and gravity along the slope are not on the same line, and the route of starting sediment moving to the shore is different from that of rolling to the sea along the beach slope. Sediment not only moves laterally, but also moves along the coast in the direction of the resultant force of wave force and gravity, which is called longitudinal movement of sediment. In most cases, lateral movement is combined with longitudinal movement.

1. lateral movement of sediment and accumulation landform

(1) lateral movement of sediment

When waves are introduced vertically into the coast, the sediment in the coastal area will move ashore and offshore under the action of waves and gravity. When the wave force exceeds gravity, the sediment moves to the shore; On the other hand, it produces offshore movement. If they are equal, mud and sand will swing back and forth, resulting in no net displacement in a wave period. On the beach profile, the sediment particles just oscillate back and forth, and there is no net displacement. The line connecting this point is called "neutral line" or "balance line". The neutral line is a theoretical concept to understand the complicated coastal sediment movement, which has methodological significance and is difficult to determine its exact position in nature. Because the coastal zone is affected by many conditions, such as waves, seabed slope and sediment particle size, the neutral line actually has a certain width range, so it is called the neutral zone, and its position is equivalent to the middle section of the underwater bank slope profile.

On the bank slope above the neutral line, due to the continuous accumulation of sediment near the coastline, the coastline moves to the sea, the beach slope becomes steeper, and the influence of gravity increases during the upward movement of sediment, gradually offsetting the upward thrust of waves until the substances above the neutral line only move back and forth in place and no longer move to the shore. Along with the up-and-down movement of the erosion zone, the underwater accumulation terrace on the bank slope below the midline also moves up and widens, making the profile more gentle, and the influence of gravity becomes less and less when the sediment moves down, and gradually it can only oscillate back and forth in the same place. Finally, the upper and lower neutral zones of the bank slope are widened and finally connected together, and the whole bank slope section forms a concave curve, and the particles at any point on the section oscillate without net displacement. This concave part is the ocean balance part.

Similar to the concept of neutral line, the equilibrium profile is also derived under various assumptions. In nature, these conditions often change. Balanced profile is the state that coastal profile development tries to achieve, while other factors constantly destroy it. Therefore, the equilibrium profile can only be used as the basis of theoretical thinking in studying complex coastal processes, and cannot be regarded as a stable state.

In the process of forming the equilibrium profile, the fluctuating bottom flow and the incipient velocity of sediment play a major role, and the equilibrium profile will change with the change of wave parameters. With the change of waves, the coastal profile can undergo development cycles of different sizes. A storm surge lasting for several days can scour the coast for more than a year or even years, so the rare storm surge is of great significance in the process of coastal development. Due to the cycles with different lengths in the development of coastal profile, the formation of any coastal sand body has been washed and reformed for millions of times, and the sediments have also been transported and sorted back and forth for countless times. Therefore, the sand in the coastal zone is well sorted.

The shape of the balanced section depends not only on the intensity of waves, but also on the particle size of the fragments that make up the section. If the sediment on the coastal profile is coarse particles, such as gravel, waves must be strongly deformed to make it move. However, the bottom velocity of the strong deformation wave is quite different from the sea speed, and the coarse particles must be on the steep slope to achieve dynamic balance. Therefore, the slope of the profile composed of coarse particles is steep. On the contrary, if the coastal profile is composed of fine-grained sediments, it can move when the wave bottom velocity is very small. At this time, the velocity difference between the bottom of the wave and the sea is small, and the fine sediment can reach a balance when the slope is slow, so the slope of the profile composed of fine sediment is relatively slow (Figure 7-2 1).

Fig. 7-2 1 balanced profile of different silt particle sizes

Sediments in the coastal zone are usually composed of particles with different particle sizes. Because coarse particles are transported to the shore and fine particles are transported to the sea, under the long-term action of waves, particles with different particle sizes are in their respective equilibrium positions. In this way, on the profile, the sediment particles gradually become thinner from the shore to the seaside, and the slope of the profile gradually becomes slower. Therefore, the distribution of coastal sediment from coarse to fine is the inevitable result of wave action.

The distribution of sediment in the coastal zone depends not only on the particle size, but also on the relative density of particles. On the underwater bank slope, the zone with strong wave action is also the zone with rich heavy minerals, and heavy minerals are often distributed in different zones according to different relative densities. In the southern part of Shandong Peninsula, the content of heavy minerals in the upper part of underwater bank slope is relatively high, but it decreases obviously towards the ocean. Among them, ilmenite and other relatively dense minerals are mainly distributed within the 5m isobath, while amphibole and epidote are mainly distributed outside the 5m isobath, especially in the water depth zone of 10 ~ 15m.

(2) Geomorphology formed by lateral movement of sediment

The accumulation landforms formed by the lateral movement of sediment include underwater accumulation terraces, beaches, underwater sand dams and offshore breakwaters.

1) underwater accumulation terrace. There are erosion zones above and below the neutral line, and the sediment in the erosion zone below the neutral line keeps moving into the sea, and some of it accumulates at the foot of the underwater bank slope and becomes an integral part of the underwater bank slope. This is an underwater accumulation terrace. On the steep coast composed of coarse-grained materials, underwater accumulation terraces are developed.

2) Underwater sand dam. Underwater sand dam refers to a long and narrow accumulation landform that is not exposed to the sea surface and slightly parallel to the coast. Shallow water waves are partially broken at the water depth equivalent to 1 ~ 2 wave height, and the overturned water body at the peak strongly washes the seabed. The uplifted water body drives a large amount of sediment, some of which are brought to the coast by the broken waves, and most of them are accumulated on the sea side of the breaking point, forming underwater sand dams. After the wave is partially broken, various wave elements decrease, continue to push to the coast, break again at the water depth equivalent to 1 ~ 2 wave height, and so on until it is completely broken, forming a broken wave flow. On the gentle slope coast composed of fine particles, there can be many underwater sand dams, and their scale and spacing gradually decrease towards the coast. There are only 1 ~ 2 underwater sand dams composed of coarse particles on the steep coast. Waves scour the front slope of the sand dam under water and deposit sediment behind the dam, resulting in asymmetry on both sides of the sand dam, gentle seaward slope and steep landward slope.

Seasonal wind and waves change the position of wave breaking points, which can cause the migration of underwater sand bars. In storm season, underwater sand bar moves deeply, and in storm season, underwater sand bar moves shallowly. The underwater sand dam moves to the shore and keeps rising. When the sea level drops rapidly, it can gradually surface and become a long island-like accumulation sand bar isolated from the coast, that is, an offshore dike. Although it is still controversial to transform underwater sand dams into offshore dams, underwater sand dams in the Gulf of Mexico did emerge from the sea under the action of wind and waves.

3) Offshore breakwaters (offshore sand breakwaters) and lagoons. Offshore breakwater is a sandy breakwater with a certain distance higher than the sea surface, which is mainly the product of wave action. Before reaching the waterline, the sediment contained in the wave breaking flow forms a dike-like accumulation body exposed to the water at a certain position, and its main components are gravel, sand, shells and their mixtures, depending on the degree of wave action and material supply conditions. The offshore breakwater relatively isolates the seawater on the inland side of the breakwater from the outside world, forming a semi-closed shallow water area called a lagoon. Its wave action is weak, and the sediments are mostly fine-grained sediments. It should be pointed out that apart from the lateral transport and accumulation of sediment, there are different views on its causes. For example, one view thinks that it is formed by the vertical movement of sediment, and the other view thinks that it is the result of sea level rise flooding the original accumulation terrain.

4) the beach. Above the neutral line, the sediment in the erosion zone moves and accumulates to the shore under the inflow of broken waves, forming a water terrace, that is, the beach. Beach is a sandy accumulation body connected with land formed by the action of waves, which is widely developed on the gentle coast. The shape of the beach is closely related to the ratio of inlet and outlet velocity caused by breaking waves.

If there is free space on the land side of the beach, the inflow of waves can flow over the top of the beach to the land slope, so the outflow is very weak, forming a double-slope beach, which is called a beach with complete section. Its cross section is convex, which is called beach ridge or coastal dike. On the open coast, there are usually several coastal dikes parallel to the coastline.

If the land-facing side of the beach is limited by sea cliffs, coastal sediments or artificial buildings, a beach with a single slope inclined to the sea is called a beach with incomplete profile. Because the breaking waves in the upper part of the beach don't have enough room to move, most of the incoming water takes part in ebb tide, and the resulting materials accumulate in the lower part of the beach, so the section of sandy beach is often wide and concave. In the gravel beach, due to the massive infiltration of incoming water, the ebb tide velocity decreases rapidly, and the substances brought by incoming water stay on the beach, and the beach surface profile is convex (Figure 7-22).

Figure 7-22 Gravel Beach in Qingdao Green Island Bay

2. Vertical movement of sediment and accumulation landform

(1) longitudinal movement of sediment

In nature, it is very rare that the wave propagation direction is completely perpendicular to the coast. In most cases, the crest line has a certain angle with the coastline, which makes the wave produce a component parallel to the coast and makes the sediment move along the coast. When the wave direction line intersects the coastline obliquely, the particles will move in the direction of the combined force of wave and gravity. After a wave period, the movement direction of sediment particles on the underwater bank slope will always deviate from the original wave direction. In the neutral zone, sediment particles only make longitudinal displacement parallel to the coast; Below the neutral zone, sediment particles move longitudinally and offshore; The neutral zone is on the bank slope, and the sediment particles not only move to the shore, but also move upward to the shore.

On the beach, the movement of particles along the coast is the most easily observed phenomenon, which has attracted people's attention. When the waves cross the coast obliquely, after the waves are broken, the particles move in the upwelling direction, and then move downward along the beach surface under the action of reflux and gravity. In a wave period, the particle movement route is zigzag and carries a certain distance along the coast, so that when the wave intersects the coastline obliquely, the particles on the underwater bank slope and beach move along the coast. The velocity of particles moving along the coast depends not only on the wave intensity, particle size and seabed slope, but also on the included angle between waves and the coast. Field observation shows that the optimal incident angle of waves is also related to the slope of the seabed.

There is often a lot of sediment movement in the coastal zone. Although the wave direction often changes under the action of wind, the movement direction of sediment in a year is roughly the same, and the quantity is relatively stable. We call the phenomenon that coastal sediment groups move in an average direction for a long time under the action of waves as wave field sediment flow. Its direction is usually consistent with the strong winds and huge waves prevailing in this area. If the longitudinal movement of sediment is a short-term local phenomenon and a temporary coastal hydrodynamic process, then the sediment flow is the long-term average state of this process.

The change of sediment flow plays an important role in coastal development and sand body formation, and its characteristics can be described by the following elements. Capacity refers to the maximum amount of sediment that waves can carry through a section in unit time, and it is the expression of the sand-carrying capacity of waves; Strength refers to the actual amount of sediment transported by waves through a section in unit time, which is the expression of wave sediment carrying capacity; Saturation is the ratio of the intensity of sediment flow to its capacity.

When the sediment flow is saturated, the full capacity of waves is consumed by the migration of sediment. If the sediment flow is not saturated, part of the wave energy can be used to scour the coast or underwater bank slope, so the scouring phenomenon is a sign of the unsaturated sediment flow. When the saturated sediment flow decreases, the wave energy is not enough to carry all the sediment, and accumulation will occur. The reason for the decline of sediment transport capacity is the turning of coastline and the shielding of offshore barrier.

(2) Geomorphology formed by vertical movement of sediment

It is assumed that the incident angle of waves is the best angle for sediment to move along the straight bank, and the sediment flow is saturated. If the conditions change, resulting in a decline in capacity, the sediment carried will be partially deposited, forming coastal sand bodies. Sand bodies formed by sediment flow include concave bank filling, convex bank accumulation, barrier covering and wave energy attenuation in the bay.

1) concave bank filling. As shown in Figure 7-23, when waves act on the AB coastline at the optimal incident angle of 45, alluvial logistics outside the AB coastline will be formed. If this kind of alluvial logistics is saturated or unsaturated, it will change the incident angle when it enters BC coastline, which will reduce the capacity of alluvial logistics, make it saturated or supersaturated, and accumulate at the concave shore. Generally speaking, the beach accumulation between the two headlands of the coast is formed in this way.

Figure 7-23 concave bank filling

2) Sand nozzle accumulation on the convex dike. Like the concave bank, the convex bank changes the incident angle of waves, reduces the capacity or increases the intensity of alluvial material flow, resulting in accumulation. On the convex bank, this accumulation extends into the sea with the turning point of the coastline as the fulcrum, forming a sand nozzle or free sand body (Figure 7-24). During the development of this sand body, with the change of seasons or other reasons, the sand nozzle will be silted and washed from time to time, forming a complex shape. If this kind of alluvial logistics develops from a certain estuary, when the flood season comes, it will carry a lot of sediment, and its intensity will increase, which will make the alluvial logistics supersaturated, which will increase the deposition of the original sand mouth. If the dry season comes, the alluvial logistics will carry a small amount of sediment and be in an unsaturated state, which will erode the sand mouth. Repeatedly, the shape of the sand mouth will become so complicated that a lagoon or swamp will form inside the sand mouth.

Figure 7-24 sand body accumulation on convex bank

3) Barrier coverage. Due to the barrier function of the island, a wave shadow zone is formed between the island and the coast. After the alluvial material enters the wave shadow zone, it may accumulate in the form of sand mouth due to the decrease of energy, and finally the island and the coast may be connected to form a sand island (Figure 7-25). The sand island connected with this island can develop in both directions. That is to say, from the wave shadow area of the island to the shore, a sand mouth is developed at the same time, which is connected with the sand mouth developed on the shore to form the island-linked sand island, which may have lagoons and of course more complicated island-linked sand island. If the width and depth of the strait behind the island are not large, the sand mouth may develop into an island dam connected with the island (Figure 7-26). The bank slope accumulation caused by breakwater is similar to this.

Figure 7-25 Liandao Sand Island

4) The wave energy in the bay is reduced. In the long and narrow bay, due to wave refraction, wave energy decreases, the ability of wave to carry sediment decreases, and the ability of sediment flow decreases, and finally it reaches supersaturation, and some sediment accumulates to form sand bodies. In nature, the sand bodies on both sides of the bay often face each other and finally form a bay dam (Figure 7-27). According to the parts where it is formed, it can be called Bay Mouth Dam and Bay Middle Dam. A bay separated by a bay is called a lagoon. On the tidal coast, the entrance and exit of tidal current often make the relative sand bodies unable to communicate and preserve tidal channels.

Figure 7-26 Dalian Island Dam in Dabijiashan, Jinzhou

Figure 7-27 Formation of Bay Sand Mouth

3. Coastal landform under tidal action

(1) Influence of Tidal Fluctuation on Bedrock and Sandy Coast

The erosion of bedrock coast and the transportation and accumulation of debris in sandy coast are mainly completed by wave action, and the tide can strengthen or weaken the wave action through periodic sea level fluctuation. In the tidal-free sea area, the position of breaking waves is relatively stable, the wave energy is concentrated and the scouring intensity is high; On the tidal coast, tidal fluctuation makes the position of breaking waves move up and down in the intertidal zone, and the coastal geomorphological characteristics are related to tidal range.

On the beach under the action of waves, the tidal action will make the beach change periodically. Sandy beach has great permeability, and the groundwater level of the beach rises and falls with the tidal sea surface, but lags behind the tidal sea surface. At high tide, the rising speed of groundwater level lags behind the rising speed of sea surface, and seawater supplements groundwater, which makes the inflow caused by waves penetrate into the beach in large quantities, and the recession is weakened, which leads to the upward migration of beach gravel and the increase of beach slope. At low tide, the groundwater level falls behind the sea surface, and the groundwater is discharged from the beach, which strengthens the ebb tide of the waves, and the gravel of the beach moves to the lower part, and the slope tends to ease. Similarly, the scouring and silting of the beach also changes with the ebb and flow of the tide for half a month. When the tidal range increases, the gravel at the bottom of the beach moves upward; When the tidal range decreases, the gravel in the upper part of the beach moves to the lower part.

(2) The deposition of tidal current on the muddy intertidal shoal.

Although the disturbance effect of tidal current on seabed sediment is far less than that of waves, the migration effect of tidal current on suspended sediment is incomparable to that of waves. In the intertidal shoal, tidal current plays an important role in the transport and accumulation of sediments.

In the broad and gentle muddy intertidal shoal, the distribution of sediment particles changes from coarse to fine from the sea to the land, which is just the opposite to the beach, which may be caused by many reasons. For example, under the constant action of tidal current, particles move to the shore until the later tidal current speed is too small to move particles any more. In addition, the tidal current velocity is greater than the ebb current velocity, which makes the lateral differentiation of sediment trend and particle size in intertidal shoals more significant. When Pusme (196 1) studied the beaches in the Netherlands, he found that the distributary period at high tide was 2 hours longer than that at low tide, which was enough to make the suspended solids deposit on the high tide beach, while at low tide, the distributary period was less than 1 hour, and the suspended solids were not completely deposited, but were transported to the shore by high tide, which also made the suspended solids deposit on the low tide beach. In addition, the wave energy near the low tide line is large, and the sediment is easily picked up with the high tide and transported to the shore.

(3) Evolution and geomorphological characteristics of silt and muddy coast.

The formation and development of muddy coast need a large number of fine-grained sediments, and its evolution depends on the source of fine-grained substances. If the source of sediment is sufficient, it can form silting dikes; If the source of sediment is cut off, the coast will be eroded and even become a sandy coast.

On the silted silt dike, the shoals in intertidal zone are continuously silted and pushed to the sea, and the original shoals are gradually separated from the action of seawater, forming wetlands first and then marine plains. When the source of sediment is cut off, the muddy silt dike is quickly washed back, and the biological crustaceans left in the sediment are washed away by the waves that washed the shallows, and then piled up on the shore by violent waves, forming shell ridges or shells. Shell accumulation is a sign that silty silt dikes are washed away, and its landform is the basis for judging the coastal scouring speed. On the strongly washed shore, shells can not be piled up stably, and often form a shell beach with low accumulation and flaky distribution, while on the slowly washed shore, shells can be piled up stably into dikes. Shell dikes appear on the low muddy marine plain, representing the position of the coastline at that time (Figure 7-28).