Accretion and erosion for different coastal types (2022)

This article describes the accretion and erosion for different coastal types resulting from a coastal structure. The coastal structure in this example is a large port with an extension greater than the width of the surf zone, but the structure could also be a set of tidal inlet jetties or a long groyne. The coastal erosion for three different types of ports is also described in the article Port breakwaters and coastal erosion.

Contents

  • 1 Introduction
  • 2 Accretion and erosion in the case of small to moderate wave incidence angles
    • 2.1 Initial situation
    • 2.2 Development of erosion and accretion
      • 2.2.1 Bypass development
      • 2.2.2 Importance of layout
  • 3 Accretion and erosion in the case of large wave incidence angles
    • 3.1 Description of this situation
  • 4 Related articles
  • 5 References

Introduction

The accretion and erosion of a sedimentary coast relates to the angle of incidence of prevailing waves at the depth contour where waves start breaking (this angle between incident wave front and breaker depth contour is usually denoted αb). Based on this angle, it is possible to distinguish between 5 main types of coasts (for a more detailed description, see the article Classification of sandy coastlines).

  1. Type 1: Perpendicular wave approach, angle of incidence close to zero
  2. Type 2: Nearly perpendicular wave approach, angle of incidence 1o - 10o, net transport small to moderate
  3. Type 3:Moderate oblique wave approach, angle of incidence 10o - 50o, large net transport
  4. Type 4:Very oblique wave approach, angle of incidence 50o - 85o, large net transport
  5. Type 5:Nearly coast-parallel wave approach, angle of incidence >85o, net transport near zero

This classification has been subdivided according to the wave exposure as follows:

  • P:Protected, the “once per year event” having [math]H_{12h/y} \lt 1 m [/math]
  • M:Moderately exposed, the “once per year event” having [math]1 m \lt H_{12h/y} \lt 3 m [/math]
  • E:Exposed, the “once per year event” having [math]H_{12h/y} \gt 3 m [/math]

[math]H_{12h/y}[/math] is the significant wave height that is exceeded only 12 hours per year. The next sections describe the accretion and erosion due to the construction of a port for type 2-4 M/E coasts.

(Video) How Coastal Erosion Works

Accretion and erosion in the case of small to moderate wave incidence angles

We consider type2/type3 coasts with an E-W directed shoreline and a net eastward littoral drift rate (LDR) of 5, which is composed by an eastward LDR of 10 and a westward LDR of 5 (the LDR is presented here without any unit, specific numbers are used to illustrate the principles only). Prevailing waves from the NW and secondary waves from NE, as shown in Fig. 1. below:

Fig. 1. Schematic shoreline development, morphological development and net littoral drift budgets for a port at a coast with a slightly oblique resulting wave attack.

Initial situation

Initially, there will be an eastward LDR of 10 close to the port on the updrift west side of the port, as this area is sheltered from the easterly waves by the structure. There will be no westward LDR-component in this sheltered area. Outside the lee zone westward of the structure, there will be a net eastward LDR of 5. This means that the transition section between these two areas will receive 5, but 10 will leave this section, which means a deficit of 5 in supply to this local area. The transition area will therefore initially be exposed to a sediment deficit of 5, whereas the area close to the structure will receive 10. This will cause initial erosion as well as sand accumulation on the updrift side of the structure. However, considering the entire updrift side as one unit, this unit will receive a surplus of 5 until bypassing of sediment starts.

(Video) How the Coast Works

Close to the structure on the lee east side, there will be a westward LDR of 5, as this area is sheltered from the westerly waves by the structure. This will result in a short accumulation of sand immediately east of the port. Outside the lee zone east of the port, there will be a net eastward LDR of 5. Initially no sediment will bypass the port. The area east of the port will consequently, considered as one unit, have a deficit of 5. This is the so-called lee side erosion. However, there will be an area in the transition zone close to the port which will have a deficit of 10, but this is only temporary, as the local reversed transport towards the structure will cease when the local coastline has adjusted to the conditions.

Development of erosion and accretion

The above sediment budget is applicable for the “initial” situation immediately after the construction of the port. Initial is a relative concept. The duration of the initial period depends on the magnitude of the port and on the area and volume of the sheltered areas compared to the littoral drift rates. The sediment budgets for the initial situation, as well as for a situation when the bypass of sediments has started, are both presented in the same figure.

The development of accretion and erosion on the updrift and downdrift sides of the port is sketched in the figure. As long as the transport is completely blocked by the port, the accumulation will take place as a seaward movement of the coastline adjacent to the breakwater parallel with the direction of the coastline of zero transport, i.e. perpendicular to the direction of the resulting waves.

Bypass development

When the bypass starts, a bar will build up in front of the entrance, and the accreting coastline will gradually turn towards the original direction concurrently with a gradual increase in the bypass. This bypass causes a gradual increase in the sedimentation of the port entrance and/or the navigation channel. The part of the bypassing material, which is not trapped in the entrance, will be transported past the port, building a shoal at the lee side of the port. The downdrift shoreline will suffer from erosion until this shoal reaches the shore. Even then, the downdrift shore will not receive the same amount of material as it originally received from the updrift shore, as this would require that the accreting coastline attained an orientation parallel with the original coastline. This would require a sand filet of infinite length, which is not possible. Furthermore, it would require that there was no loss of sand in connection with the bypass of the port, which is also unrealistic. This explains why the downdrift shoreline will forever suffer from erosion as a result of the port construction, or another similar coastal structure, unless shore nourishment/bypass is introduced.

This situation is thus characterised by a long slowly developing sand filet at the updrift side of the port and the formation of a fairly short narrow shoal downdrift of the port, as well as shoreline erosion relatively close to the port along the downdrift shoreline. However, there will, in most cases, also be a very short accretion zone immediately leeward of the port. Sedimentation in the entrance will develop slowly. It is worth noting that as soon as a coastal structure of an extension comparable to the width of the surf zone has been built along such a shoreline, the downdrift shoreline will forever suffer from erosion.

Importance of layout

In addition to the phenomena described above, a non-optimal layout of the protective structures can result in additional trapping of sand. This typically happens in the sheltered area generated by port layouts, which consist of a main breakwater overlapping a secondary breakwater. This kind of layout will act as a sediment trap which is filled at the rate of the maintenance dredging. This will cause additional lee side erosion depending on where the sand is deposited.

Accretion and erosion in the case of large wave incidence angles

When the angle of incidence of the resulting waves is larger than 50º, the shoreline development and corresponding morphological changes are quite different from the situation described above, see Fig. 2. below.

(Video) Basic SCIENCE | Coastal Processes and Its Effects

Fig. 2. Upper: Relation between transport and angle of incidence. Lower: schematic shoreline development and morphological development for a port at a coastlinewith very oblique wave attack.

Description of this situation

The wave incidence angle at the breaker depth contour (which is assumed approximately parallel to the original shoreline) is denoted as [math]\alpha_2[/math], which corresponds to the transport [math]Q_2[/math], see figure 2, upper part. There is, however, another smaller angle of incidence [math]\alpha_1[/math] which gives the same transport, [math]Q_2= Q_1[/math] (this is because littoral drift depends on the wave incidence angle [math]\alpha_b \;[/math] as [math]\; \sin 2 \alpha_b \;[/math], see Littoral drift and shoreline modelling or Shallow-water wave theory). This means that the shoreline in the accumulation area updrift of the port will immediately switch to the position corresponding to the angle of incidence [math]\alpha_1[/math]. This provides a very minor accretion, which will very quickly develop into a situation with full bypass equal to [math]Q_2[/math], and the corresponding build-up of a bar past the entrance.

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The bypassing sand will, due to the very oblique wave attack, develop into a bypass shoal nearly parallel with the coastline, i.e. a very long shoal.

This situation is thus characterised by a short and quickly developing accretion zone and a fairly long, slowly developing bypass shoal downdrift of the port. Another effect is a gentle shoreline erosion over a fairly long distance from the port along the downdrift shoreline. Sedimentation in the entrance will develop quickly.

Related articles

  • Dealing with coastal erosion describes two types of erosion, dune erosion and structural erosion, and how to deal with these two different types of erosion.

Articles about structural erosion and the presence of structures:

  • Hard coastal protection structures
  • Port breakwaters and coastal erosion

Articles on littoral drift:

  • Littoral drift and shoreline modelling
  • Shallow-water wave theory
  • Coastal Hydrodynamics And Transport Processes

For more information on different types of coastlines, see:

  • Characteristics of sedimentary shores
  • Classification of sandy coastlines

References

Mangor, K., Drønen, N. K., Kaergaard, K.H. and Kristensen, N.E. 2017. Shoreline management guidelines. DHI https://www.dhigroup.com/marine-water/ebook-shoreline-management-guidelines.

The main author of this article is Mangor, Karsten
Please note that others may also have edited the contents of this article.
Citation: Mangor, Karsten (2022): Accretion and erosion for different coastal types. Available from http://www.coastalwiki.org/wiki/Accretion_and_erosion_for_different_coastal_types [accessed on 4-09-2022]
  • For other articles by this author see Category:Articles by Mangor, Karsten
  • For an overview of contributions by this author see Special:Contributions/Karsten
(Video) 10GG1 - Coasts & River Landforms (Miss Lilley)

FAQs

What are the 4 types of coastal erosion? ›

The material that is deposited by constructive waves can most often be seen by the creation of beaches. Destructive waves erode through four main processes; Hydraulic Action, Compression, Abrasion and Attrition.

What are the 5 types of coastal erosion? ›

There are five main processes which cause coastal erosion. These are corrasion, abrasion, hydraulic action, attrition and corrosion/solution. Corrasion is when waves pick up beach material (e.g. pebbles) and hurl them at the base of a cliff.

What are 3 types of coastal erosion? ›

Erosion
  • Hydraulic action - this is the sheer power of the waves as they smash against the cliff. ...
  • Abrasion - this is when pebbles grind along a rock platform, much like sandpaper. ...
  • Attrition - this is when rocks that the sea is carrying knock against each other.

What is the difference between erosion and accretion? ›

“Accretion” is the term which applies to the gradual increase or acquisition of land by the action of natural forces washing up sand, soil or silt from the water course or seashore. The opposite of accretion, “erosion” is the gradual washing away of land along the shoreline.

How many types of coastal erosion are there? ›

There are two types of coastal erosion; rapid-onset hazard erosion occurs at a time scale of days to weeks, whereas slow-onset hazard erosion occurs at a time scale of decades to centuries. Both of these are caused by natural processes.

What are the 4 types of erosion and how do they work? ›

The four main types of river erosion are abrasion, attrition, hydraulic action and solution. Abrasion is the process of sediments wearing down the bedrock and the banks. Attrition is the collision between sediment particles that break into smaller and more rounded pebbles.

What are the examples of coastal erosion? ›

There are four main processes of coastal erosion. These are corrasion, abrasion, hydraulic action and attrition. Corrasion is when destructive waves pick up beach material (e.g. pebbles) and hurl them at the base of a cliff.

What are the three major coastal processes? ›

The three principle marine processes that influence coasts are erosion, transportation and deposition.

What are the main causes of coastal erosion? ›

Coastal erosion is typically caused by various natural forces. These include ocean waves, wind, rainfall, as well as coastal storms.

What is meant by coastal erosion? ›

Coastal erosion is the process by which local sea level rise, strong wave action, and coastal flooding wear down or carry away rocks, soils, and/or sands along the coast.

What is coastal erosion in geography? ›

Coastal erosion is the wearing away and breaking up of rock along the coast. Destructive waves erode the coastline in a number of ways: Hydraulic action: Air may become trapped in joints and cracks on a cliff face. When a wave breaks, the trapped air is compressed which weakens the cliff and causes erosion.

What causes coastal accretion? ›

Accretion begins with sand deposition on shore, from the movement of the waves, tides and longshore current. Some sand is then dried out by the wind and sunlight, allowing it to be blown to other areas of the beach by the prevailing winds. Sand is the major component in accretion/erosion cycles.

What is coastal accretion in geography? ›

Coastal plain landscapes form in low-energy environments where deposition > erosion, so they experience a net accumulation of sediment. ​They form through coastal accretion (a continuous net deposition of sediment.) This comes from: offshore sources (transported by waves, tides or current)

What causes beach accretion? ›

Wind blowing across the beach can move finer grained particles, shaping, and, in some cases, eroding the beach. Wind- blown sediment will continue to move until it reaches a barrier, such as vegetation, which reduces the wind speed and causes sediment to be deposited. It is this accretion that forms dunes.

What are the types of erosion? ›

The main forms of erosion are:
  • surface erosion.
  • fluvial erosion.
  • mass-movement erosion.
  • streambank erosion.
Nov 24, 2008

What are the two main types of coastal management? ›

There are two types of coastal management techniques, hard engineering, and soft engineering.

What are the 4 processes of coastal transportation? ›

Traction - large material is rolled along the sea floor. Saltation - beach material is bounced along the sea floor. Suspension - beach material is suspended and carried by the waves. Solution - material is disolved and carried by the water.

What coastal landforms are made by erosion? ›

Landforms of coastal erosion include cliffs, wave-cut platforms, caves, arches, stacks, stumps, and headlands, amongst others.

Is coastal erosion a natural process? ›

Coastal erosion is a natural phenomenon that can be exacerbated by human activities and by storms and tsunamis. In Modules 2 and 3, we briefly touched on the natural processes of coastal erosion. We discussed the role of sediment movement and deposition in the evolution of coasts.

Why coastal erosion is a problem? ›

Coastal erosion is a growing problem internationally because of sea-level rise driven by climate change, and increasing population and development in coastal areas.

What are the natural causes and effects of coastal erosion? ›

Natural processes that cause coastal erosion include the actions of winds, waves, and currents, while human influences include the construction of seawalls, groins, and jet ties, navigation of inlets and dredging, and other interruptions of physical processes (FEMA, 1997).

What is coastal erosion and how it can be prevented? ›

Present beach erosion prevention methods include sand dunes, vegetation, seawalls, sandbags, and sand fences. Based on the research conducted, it is evident that new ways to prevent erosion must be obtained. Each way that is currently used has extensive negative effects on beaches and their natural tendencies.

How does coastal erosion affect the environment? ›

For ecosystems, erosion translates into habitat loss as coastal wetlands deteriorate. The plants and wildlife that depend on these ecosystems are negatively impacted by the effects of erosion. Economically, loss of these ecosystems leaves coastal areas more vulnerable to damages from tropical storms and storm surges.

What are the 4 processes of coastal transportation? ›

Traction - large material is rolled along the sea floor. Saltation - beach material is bounced along the sea floor. Suspension - beach material is suspended and carried by the waves. Solution - material is disolved and carried by the water.

What are the examples of coastal erosion? ›

There are four main processes of coastal erosion. These are corrasion, abrasion, hydraulic action and attrition. Corrasion is when destructive waves pick up beach material (e.g. pebbles) and hurl them at the base of a cliff.

What are the 3 coastal processes? ›

The three principle marine processes that influence coasts are erosion, transportation and deposition.

What are the main causes of coastal erosion? ›

Coastal erosion is typically caused by various natural forces. These include ocean waves, wind, rainfall, as well as coastal storms.

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