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 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 wave action. On silty and muddy coast composed of fine particles, sedimentation is mainly completed by tidal current.

(1) coastal erosion landforms

The erosion in the coastal zone is called marine erosion, and the various forms of coastal erosion mainly caused by seawater dynamic factors are called marine erosion landforms.

The wave water directly washes the coastline, which is called erosion. Because the waves hit the coast with huge energy when they reach the shore, the huge pressure of the water itself, the compressed air in rock cracks and joints have caused strong damage to the coast, and this force can reach tens of tons per square meter. The erosion of the seabed caused by the gravel carried by the sea water with the wave reciprocating motion is called abrasion. In the reciprocating motion of the wave forward and backward, the sea water carries gravel, mud sand and rock fragments eroded on the coast, which grinds the seabed bedrock and speeds up the coastal erosion. The coastal erosion caused by the dissolution of coastal bedrock by seawater is called dissolution, and the dissolution ability of seawater to rocks and minerals is stronger than that of fresh water, especially on the coast composed of easily soluble rocks such as carbonate rocks, which is more destructive to the coast and can form a unique dissolution platform.

The main dynamic factors for shaping 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 tropics and subtropics are eroded by abundant surface water and strong chemical weathering. In addition, they are also restricted by the anti-corrosion ability of the rocks that make up the coastal areas. The rocky coast with dense and hard structure has strong anti-corrosion ability, but due to the development of cracks and joints, many marine arches, cliffs, caves and columns are formed (Figure 7-19, Figure 7-2). The soft rocky coast has poor anti-corrosion ability, and the sea cliff retreats quickly, which is easy to form a sea erosion platform. The 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 and sea level change. At the same time, the sea erosion landforms shaped by waves are magnificent and often turned into tourist attractions.

Figure 7-19 Sea Cliff and Sea Pillar-Shilaoren, Qingdao

Figure 7-2 Sea Cave, Lingshan Island, Qingdao

Debris eroded from the coast is carried by waves below the front edge of the sea 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 the profile will be washed away if it exceeds this value.

(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 accumulate and form various marine landforms.

The movement of coastal mud and sand is mainly influenced by waves and gravity, and the mud and sand in coastal areas are moving in different forms under their combined 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 surface) is on the same straight line, and the activated mud and sand will produce back-and-forth movement towards the shore and the sea, which is called the lateral movement of mud and sand. When the wave direction (wave ray) is oblique to the coastline, the tangential component of wave force and gravity along the slope is not on the same line, and the route of the started mud and sand moving to the shore is different from that of rolling to the sea along the beach slope. The mud and sand not only move laterally, but also move along the coast in the direction of the resultant force of wave force and gravity, which is called the longitudinal movement of mud and sand. In most cases, lateral movement is combined with longitudinal movement.

1. lateral movement of mud and sand and accumulation landform

(1) lateral movement of mud and sand

under the condition that waves are introduced into the coast vertically, mud and sand in the coastal zone will move ashore and offshore under the action of waves and gravity. When the wave force exceeds gravity, the mud and sand move to the shore; On the other hand, it produces offshore movement. If they are equal, the mud and sand will swing back and forth, resulting in no net displacement in a wave period. On the beach profile, mud and sand particles only oscillate back and forth without net displacement. The connecting line at this point is called "neutral line" or "balance line". Neutral line is a theoretical concept to understand the complex 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, mud and sand 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, the coastline moves towards the sea due to the continuous accumulation of sediments near the coastline, and the beach slope becomes steeper. In the process of mud and sand moving up, the influence of gravity is increasing, which gradually offsets the upward thrust of waves until the materials above the neutral line only move back and forth in the same place and no longer move to the shore. The bank slope below the neutral line, with the erosion zone moving up and down, the underwater accumulation terrace is also moving up and widening, making the profile more gentle, and the influence of gravity becomes less and less when the mud and sand move down, and gradually they 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 joined together, and the whole bank slope section forms an upper concave curve, and the particles at any point on the section oscillate without net displacement. This upper concave section is the marine balance section.

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

in the process of shaping the balance profile, the fluctuating bottom flow and the starting velocity of sand play a major role, and the balance profile will change with the change of wave parameters. With the change of waves, the coastal profile can undergo development cycles with different sizes. A storm wave lasting for several days can scour the coast for more than one year or even many years, so rare storm waves are 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 scoured and transformed for millions of times, and the sediments have been transported and sorted back and forth for countless times. Therefore, the sand in coastal zone is well sorted.

the shaping of the balanced profile depends not only on the strength of waves, but also on the particle size of the debris that constitutes the profile. If the sediment on the coastal profile is coarse particles such as gravel, waves must be strongly deformed to make it move. However, the difference between the wave bottom velocity and the sea velocity of the strongly deformed waves is very large, and the coarse particles must be on a 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 be moved when the wave bottom velocity is very small. At this time, the difference between the wave bottom velocity to the shore and to the sea is small, and the fine-grained sediments can reach a balance when the slope is gentle, so the slope of the profile composed of fine-grained sediments is relatively gentle (Figure 7-21).

Figure 7-21 Balance Profile of Different Mud and Sand Particle Sizes

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

the distribution of silt in coastal zone depends not only on the particle size, but also on the relative density of particles. On the underwater bank slope, the zone where waves act strongly is also the zone where heavy minerals are enriched, 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 the underwater bank slope is relatively high, but obviously decreases toward the sea. 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 zone with water depth of 1 ~ 15m.

(2) landforms formed by lateral movement of mud and sand

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

1) underwater terrace. There is an erosion zone above and below the neutral line, and the mud and sand in the erosion zone below the neutral line keep moving towards the sea, and some of them are accumulated at the foot of the underwater bank slope, which becomes an integral part of the underwater bank slope. This is the 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 slightly parallel to the coast without exposing the sea surface. Shallow water waves are partially broken in the water depth equivalent to 1-2 wave heights, and the overturned water body at the peak strongly erodes the seabed, and the lifted water body drives a large amount of mud and sand. Some of these mud and sand are carried to the coast by the surf, while most of them are accumulated on the sea side of the breaking point, forming an underwater sand dam. After the wave is partially broken, all kinds of wave elements decrease, continue to advance to the coast, and break again in the water depth equivalent to 1 ~ 2 wave heights, and so on until it is completely broken to form a surf 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 usually only one or two underwater sand dams on the steep slope coast composed of coarse particles. Waves scour the front slope of underwater sand dam and deposit mud and sand behind the dam, resulting in asymmetry on both sides of the sand dam, with a gentle seaward slope and a steep landward slope.

seasonal wind and waves change the position of wave breaking point, which can cause the migration of underwater sand dams. In the stormy season, the underwater sand dam moves deep, and in the stormy season, the underwater sand dam moves shallow. The underwater sand dam moves to the shore and is constantly raised. When the sea level drops rapidly, it can gradually emerge from the water surface and become a long island-like accumulation sand dam isolated from the coast, that is, an offshore dike. Although there is still controversy about the transformation of underwater sand dams into offshore dikes, underwater sand dams in the Gulf of Mexico did emerge from the sea under the action of storm waves.

3) Offshore breakwaters (offshore sand breakwaters) and lagoons. Offshore breakwater is a sand breakwater that is higher than the sea surface at a certain distance offshore, which is mainly the product of the action of surf. Before the mud and sand contained in the surf flow reach the waterline, it forms a dike-like accumulation body exposed above the water surface 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. Offshore breakwater relatively separates the seawater on the inland side of the breakwater from the outside, forming a semi-closed shallow water area, which is called lagoon. Its wave action is weak, and the sediments are mostly fine-grained sediments. It should be pointed out that apart from the lateral transportation and accumulation of mud and sand, there are different views on its causes. For example, one view thinks that it is formed by the longitudinal movement of mud and sand, and the other thinks that it is the result of sea level rise flooding the original accumulation terrain.

4) the beach. Above the neutral line, the mud and sand in the erosion zone move to the shore to accumulate under the action of the inflow of the surf, forming a water accumulation terrace, that is, the beach. Beach is a sandy accumulation formed by the action of surf and connected with land, 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 surf.

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

if the land side of the beach is restricted by sea cliffs, coastal deposits or artificial buildings, a beach with a single slope inclined to the sea is developed, which is called a beach with an incomplete section. Because there is no sufficient activity space for the surf in the upper part of the beach, most of the inflow water participates in the ebb current, and the materials brought with it are accumulated in the lower part of the beach, so the sandy beach section is often wide and concave. However, in the gravel beach, due to the massive infiltration of the inflow water, the ebb flow speed decreases rapidly, and the substances brought by the inflow stop on the beach, and the beach profile is convex (Figure 7-22).

Figure 7-22 Gravel Beach in Qingdao Green Island Bay

2. Vertical movement of mud and sand and accumulation landform

(1) Vertical movement of mud and sand

In nature, it is very rare that the wave propagation direction is completely perpendicular to the coast. In most cases, the wave crest line has a certain angle with the coastline, which makes the wave generate a component parallel to the coast and makes the mud and sand move along the coast. When the wave direction line intersects the coastline obliquely, the particles will move along the direction of the resultant force of waves and gravity. After a wave period, the moving direction of mud and sand particles on the underwater bank slope will always deviate from the original wave direction. At the neutral zone, the silt particles only do longitudinal displacement parallel to the coast; Below the neutral zone, the sediment particles move longitudinally and move down offshore; The neutral zone is on the shore slope, and the mud and sand particles move up to the shore in addition to the coastal displacement.

On the beach, the coastal movement of particles is the most easily observed phenomenon, which has already attracted people's attention. When the waves cross the coast obliquely, after the waves are broken, the particles move in the direction of upwelling, and then move downward along the beach surface under the action of reflux and gravity. In a wave period, the particle movement route is toothed 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 the beach move along the coast. The velocity of particles moving along the coast not only depends on the wave strength, particle size and seabed slope, but also has a close relationship with the intersection angle between waves and the coast. Field observation shows that the optimal incident angle of waves is also related to seabed slope.

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

The change of mud-sand 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 mud and sand that waves can carry through a section in unit time, which is the expression of sand-carrying capacity of waves; Strength refers to the actual amount of mud and sand transported by waves through a section in unit time, which is the expression of sand carrying capacity of waves; Saturation is the ratio of the intensity of mud-sand flow to its capacity.

when the mud-sand flow is saturated, the full capacity of waves is consumed by the migration of mud and sand. If the mud-sand flow is not saturated, part of the wave energy can be used to erode the coast or underwater bank slope, so the erosion phenomenon is a sign of the mud-sand flow being unsaturated. When the capacity of saturated sediment flow decreases, the wave energy is not enough to carry all the sediment, and accumulation will occur. Shoreline is the reason for the decrease of sediment flow capacity.