Accretion and erosion for different coastal types

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.

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

$H_{12h/y}$ 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.

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.

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.

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 $\alpha_2$, which corresponds to the transport $Q_2$, see figure 2, upper part. There is, however, another smaller angle of incidence $\alpha_1$ which gives the same transport, $Q_2= Q_1$ (this is because littoral drift depends on the wave incidence angle $\alpha_b \;$ as $\; \sin 2 \alpha_b \;$, 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 $\alpha_1$. This provides a very minor accretion, which will very quickly develop into a situation with full bypass equal to $Q_2$, and the corresponding build-up of a bar past the entrance.

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

Articles about structural erosion and the presence of structures:

Articles on littoral drift: