Human causes of coastal erosion
Although there are many natural causes of coastal erosion, most of the causes affecting coastal communities are due to human intervention in the transport processes along the coastlines and/or reductions in the supply of sand to the shorelines.
Contents
Inteference by Coastal Structures
Coastal structures interfering with the littoral transport are the most common cause of coastal erosion. The presence of the structure has a series of effects:
- Trapping of sand on the upstream side of the structure takes sand out of the sediment budget, thus causing shore erosion along adjacent shorelines. Mostly, of course, on the lee side, but large structures may also cause (initial) erosion on the upstream side.
- Loss of sand to deep water
- Trapping of sand in entrance channels and outer harbours.
The structures, which may cause this type of erosion, are:
- Groynes and similar structures perpendicular to the shore
- Ports
- Inlet jetties at tidal inlets and river mouths
- Detached breakwaters
The accumulation and erosion patterns adjacent to coastal structures depend among other things on:
- The type of coastline, i.e. the wave climate and the orientation of the shoreline
- The extent of the structure relative to the width of the surf-zone
- The detailed shape of the coastal structure
The typical impact on the coastal processes and related shore erosion problems for different types of structures will be discussed briefly in the following. A more comprehensive description of the structures and their function is given in Shore protection.
Groynes and similar structures perpendicular to the shore
Groynes are normally built perpendicular to the shoreline with the purpose of protecting a section of the shoreline by blocking (part) of the littoral transport, whereby sand is accumulated on the upstream side of the groyne. However, the trapping of the sand causes a deficit in the littoral drift budget, and this kind of coast protection is always associated with corresponding erosion on the lee side of the structure. In other words, a groyne just shifts the erosion problem to the downstream area. This is the reason that groynes are often built in long series along the shoreline, a so-called groyne field. In an attempt to compensate for the effect of the upstream groyne(s), new downstream groynes were built, which shifted the lee side erosion problem even further downstream.
The efficiency of a groyne field as a protection measure depends on the length of the groynes relative to the width of the littoral zone and the number and spacing of the groynes. The more efficient the groyne field is protecting the shoreline within the groyne field, the more lee side erosion will be experienced downstream.
These effects of groynes were not fully understood and realised at the beginning of the last century when most major groyne fields were constructed. Nowadays, this mechanism is understood and can be modelled and therefore groynes can be designed to fulfil their purpose optimally.
Apart from being beneficial to erosion impacts, groynes do not add to the beauty of the landscape, and they generate dangerous rip currents.
Examples of coastline development for different types of groyne schemes for different types of coast are presented in Groynes.
Ports
The primary purpose of a port is to provide safe mooring and navigation for the calling vessels but when built on the shoreline it interferes with the littoral drift budget and the results are sedimentation and shoreline impact.
Like a groyne, the port acts as a blockage of the littoral transport, as it causes trapping of sand on the upstream side in the form of an accumulating sand filet, and the possible bypass causes sedimentation at the entrance. The sedimentation requires maintenance dredging and deposition of the dredged sand. The result is a deficit in the littoral drift budget, which causes lee side erosion along the adjacent shoreline. A port must, consequently, minimise sedimentation and coastal impact. Attention has not always been paid to these requirements. The result is that many ports trap large amounts of sand and suffer from severe sedimentation.
The principal shoreline development on littoral transport coasts with slightly oblique wave approach and very oblique wave approach, coast types 2-3M/E and 4M/E respectively, is discussed in Accumulation and erosion for different coastal types
Inlet jetties at tidal inlets and river mouths
Tidal inlets and river mouths are often by nature shallow and variable in location, which makes them unsuitable for navigation. In order to improve navigation conditions and, to some extent, flushing conditions, many tidal inlets and river inlets have regulated mouths. The regulation may consist of jetties, possibly combined with maintenance dredging programmes. If the tidal inlets and the river mouths are located on littoral transport shorelines, they are often in a natural equilibrium with respect to bypassing of the littoral drift, which normally occurs on a shallow bar across the inlet. If the inlet/mouth is upgraded to accommodate navigation, this bar is normally cut off by the jetties or dredged.
For the above reasons, regulated inlets are normally obstructions to the littoral transport which means upstream sand accumulation along the upstream jetty, loss of sand due to sedimentation in the deepened channel and the associated maintenance dredging. All in all, regulated inlets will very often cause lee side erosion problems. Modern legislation requiring mitigation measures, such as artificial sand bypass does not always work ideally. At many such locations the mitigation measures have never been introduced or severely delayed.
In conclusion, past and present regulations of tidal inlets and river mouths are responsible for major erosion along many coastlines throughout the world.
Detached breakwaters
Detached breakwaters are used as shore and coast protection measures. In general terms, a detached breakwater is a coast-parallel structure located inside or close to the surf-zone. As this subject is too extensive for this page, please see Detached breakwaters for a detailed discussion on the subject.
Passive Coastal Protection Structures
Other types of coastal protection that do not protrude into the sea will, however, also cause increased coastal erosion. Seawalls and revetments are typically constructed along coastal sections to provide protection of the coast.
An eroding shore/coast supplies material to the littoral transport budget if the erosion is allowed to continue. When the erosion is stopped at certain sections by the construction of seawalls or revetments, the supply of sand from this section of the shoreline to the sediment budget along the adjacent sections of shorelines will stop, whereby these adjacent shorelines will be exposed to increased erosion. The active trapping structures, such as groynes and breakwaters, will also act in this way in addition to their more direct coastal impact as discussed above.
The erosion of high cliffs often appears to be very drastic, which is why they have, in many cases, been the first to be protected in an area. However, before their protection they were the main suppliers of sediments to the littoral cell in question. Consequently, their protection leads to increased erosion at adjacent lower sections of the coastline. The result is that the erosion has been shifted to less resistant areas resulting in higher area losses per year.
Erosion of Crescent-Shaped Bays
In areas where the coastal landscape is formed by an interaction between rocky headlands and a littoral transport regime, the shoreline configuration will often be in the form of crescent-shaped or spiral-shaped bays, see Fig. 1.
The form and stability of these bays mainly depend on two factors -
- The wave climate, which is considered stable
- The supply of sand to the bay from the upstream bay, QE, and from a possible river, QR
The overall transport mechanisms in a crescent-shaped or spiral-shaped bay can be characterised as follows. The supply of sand from the upstream bay QB will pass the headland and cross the bay via a bar. If, as shown in the figure below, a river also contributes QR to the littoral budget, this material will be transported downdrift into the bay, partly along the shoreline and partly onto the bar. These transport processes are fairly complicated and 2-dimentional in nature, but they result in the supply of QB + QR to the straight downdrift section of the crescent-shaped shoreline of the bay. The direction of this straight section is given by the wave climate and the actual sum QS1 = QB + QR according to the transport correlation between incident wave direction α1 and the transport QS1, which is also shown in Fig. 1. above.
The shape of the crescent-shaped bay is stable, apart from seasonal variations, as long as the supply of material to the bay QS1 = QB + QR is not changed. However, if the supply of material to the bay is reduced, typically by changes to the upstream bay or changes to the river, the overall shape of the bay will also change, as the direction of the straight section will adjust to the new sum Q2, where Q2 < Q1, leading to erosion in the entire bay, as sketched in Fig. 1.
This means that changes in one bay will gradually penetrate into the downstream bays, so crescent-shaped bays, although they appear fairly stable, are actually very sensitive to changes in the supply of sand.
River Regulation Works and Sand Mining in Rivers
A decrease in the supply of sediments to a shoreline, due to the regulation of rivers, which previously supplied material to the shoreline, is a very common cause of coastal erosion. The river regulation works can be the construction of dams for power production and irrigation purposes, or the deepening of navigation channels and sand mining, but all of them cause less supply of sediment to the shoreline. Perhaps the best-known example of this is the trapping of the sediments of the Nile River by the construction of the High Aswan Dam in the 1960´s, see figure below.
The promontory propagated until 1909 and then began to erode. The reasons for the erosion of 42 m/year during the period 1909-1971 were mainly a reduction in the river discharge and the construction of the Low Aswan Dam, whereas the drastically increased erosion rate of 129 m/year after 1971 was caused by the construction of the High Aswan Dam.
Sand mining in rivers is a major cause of coastal erosion in many countries, such as Sri Lanka. The supply of sand to the coast Qcoa from a river depends of many parameters and there is no simple correlation between sand mining and decrease in supply to the coast. However, in general there are five components in the sediment balance for a degrading river section, refer the illustration in Fig. 3. on sediment balance for a river segment.
The components are:
Sources
- Sand supply from the catchment, Qcat
- Degradation of the river bed, Qbed
- Bank erosion, Qban
Sinks
- Sand extraction (sand mining), Qmin
- Sand discharge to the coast, Qcoa
The following equation must be fulfilled for a given segment of the river:
Qcat + Qbed + Qban = Qmin + Qcoa
Impacts of sand mining
Sand mining in a river lowers its bottom, causes bank erosion and reduces the supply of sand to the coast. Dependent of the circumstances sand mining in a river may cause a drastic or moderate drop in supply to the coast. Many rivers consist of a steep upper part, the mountain part, and a gently sloping lower part, where the river crosses the coastal plain. Sand mining in the two parts of the river has very different impacts on the supply of sand to the coast. River morphology modelling for Sri Lankan rivers has shown the following pattern [1]:
- Sand extraction in the upper part of the river causes a parallel translation (lowering) of the bed and of the water surface in the river, hence no changes in the sediment transport capacity. Thus the sand extraction in the upper part of the river is almost entirely balanced by local bed degradation, and has hardly any immediate impact on the supply of sand to the coast.
- Sand mining in the landward lower part of the river (far away from the mouth) will cause a local lowering of the river bed, but not a lowering of the water surface. This will cause a local decrease in the flow velocity and in the sediment transport capacity. This depression in the river bed will gradually be filled in from upstream supply, however dependent of the ratio between mining and catchment supply. The depression in the river bed will gradually travel towards the coast, however it may take many years before the impact, in the form of reduced sediment supply to the coast, is felt at the coast. But when the impact finally reaches the coast there may be an accumulated deficit in available river bed material corresponding to several decades sediment supply from the catchment. This means that an immediate halt in the sand mining will have hardly any remedial effect on the supply of sand to the coast, as the entire river bed has to be rebuild before the original supply is re-established. In the case of the Kelani River in Sri Lanka, it was calculated that it will take more than 70 years for the original river bed and the supply to the coast to recover following a complete stop in the sand mining
- Sand mining close to the river mouth will cause a local lowering of the river bed and an immediate decrease in the supply of sand to the coast. Halt of the sand mining in this situation will relative quickly cause recover of the supply of sand to the coast.
The impact on the supply of sand to the coast caused by sand mining in rivers has been discussed above. However there are other severe impacts along the river basin, which also have to be taken into consideration in order to find an overall sustainable solution. An integrated approach taking into account all the impacts has to be applied. This requires close collaboration between river authorities and coastal authorities. The main river related impacts are the following:
- Tide penetrates deeper into rivers and estuaries, which may cause increased saline intrusion in river estuaries, especially during the dry season, causing troubles for water intakes and for irrigation and causing changes to the estuarine habitats
- Increased flooding origination from the sea
- Lowering of the beds of the rivers also cause lowering of the water level in the rivers, which affects the ground water table in the flood plains. This may have impact on the agriculture especially during the dry period. This also causes problems for intakes to older irrigation schemes as they are now above the water level in the river
- Reduction of flood levels for small and medium sized floods as the deepened river bed and the low water level provides more volume between the river banks. However, the extreme floods will still spread over the flood plains. The absence of regular "small" inundations is likely to enhance man's encroachment on to the flood plain, thus causing increased "flooding problems" when the real large floods occur.
Wake from Fast Ferries
The special wake generated by fast ferries is characterised by a series of approximately 10 relatively low waves (Hs in the order of magnitude maximum 1 m), but relatively long waves. These wake waves are very similar to swell waves and they are exposed to considerable shoaling when approaching the coast. They often break as plunging breakers.
If a fast ferry navigates through protected waters, the wake waves are very different from the natural waves along the navigation route. The wake waves caused by fast ferries may influence the coastal conditions in the following ways:
- by higher wave uprush than that produced by normal waves
- by changing the coastal morphological processes in the area. This can result in erosion as well as the formation of a large beach berm
- by breaking unexpectedly and violently, the waves can be dangerous for small dinghies and for bathers
A precondition for approval of a new fast ferry route is therefore to perform an environmental impact assessment study. This will often result in navigation restrictions for certain parts of the route. An example of the impact of such waves is presented in figure. Note the violent breaking, turbid water and rip currents.
Sand and Coral Mining, and Maintenance Dredging
The mining of sand and gravel along beaches and in the surf-zone will cause erosion by depleting the shore of its sediment resources. In connection with maintenance dredging of tidal inlets, harbours, and navigation channels, sand is very often lost from the littoral budget because the sand, unless otherwise regulated by legislation, is normally dumped at deep water. Coral mining and other means of spoiling the protective coral reefs, for example, fishing by the use of explosives or pollution, will also cause coastal erosion and beach degradation. The protective function of the reef disappears and the production of carbonate sand stops.
Concluding Remarks
In conclusion, it can be seen that nearly every type of development and coastal protection along a littoral shoreline or along rivers will result in erosion of the adjacent shores and coasts.
An overview over sediment sources and losses to the coastal zone is presented in Fig. 5. below.
References
- ↑ Mangor, K., 2002. Shoreline Management, Background Document for the second revision of the Coastal Zone Management Plan, Sri Lanka, 2002. Performed under the Coastal Resources Management Program, Sri Lanka. ADB TA No. 3477 SRI
Further reading
- Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.
Please note that others may also have edited the contents of this article.
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