Various factors affect how quickly coastlines retreat. Lithology has a particularly significant effect on how coastlines change. Retreat is also a consequence of human, physical, and sub-aerial factors, further influenced by short and long-term factors. It is important to understand coastal retreat and the factors that influence it, then we can see how coastal retreat can be managed to avoid adverse effects on both society and the environment.
Coastal retreat describes how quickly a coastline is moving inland.
Human factors affecting coastal retreat
Inevitably human activity contributes to coastal erosion and retreat, usually by interfering with the sediment cells. This is through coastal management approaches, human activity, and economic and social issues.
Fig. 1 - Crumbling Cliff Edge, New Milton, Wikimedia
Coastal management
Coastal management in some form or other is needed to prevent the erosion of the coastline. There are two management approaches to this, hard engineering and soft engineering. Generally, a soft engineering approach is considered more natural, inexpensive, long-term, and sustainable. On the other hand, hard engineering defences are artificial, noticeable, more expensive and have a shorter life span. Although both have advantages and disadvantages, it is acknowledged that a hard engineering approach tends to shift problems experienced at one location to an alternative one.
For more information on these management approaches, please refer to the explanations on Managing Coastlines and Engineering Management Approaches
Human activity
Human activity can be through dredging, where the construction industry dredges up sand and gravel. This increases the ocean's depth, increasing the chances of destructive waves because they maintain their energy further towards the coast. Then there is the building of dams, which interfere with the sediment cells and cause erosion elsewhere.
Economic and social issues
This involves the economic and social issues influenced by coastal retreat. An example is the Holderness coastline in the UK. Here a holiday park lost over 400 holiday homes through coastal retreat. This had a direct effect on the operating ability of the park but also a knock-on effect on the local community as individuals lost out both financially and socially.
Fig. 2 - Coastal erosion in Holderness, Paul Anderson/Wikimedia Commons, CC BY-SA 2.0
Managed retreat case study: The Nile delta
The Nile delta is situated in northern Egypt, where the Nile spreads out and drains into the Mediterranean sea. It is 160Km in length. It covers 240Km of the Mediterranean coastline from west to east, making it one of the largest river deltas. It is a marine arcuate delta, fan-shaped. About 95% of Egypt's population lives along the course of the Nile, and in the delta region, and the Nile is their primary source of water.
Situation
The delta region is exceptionally fertile. Before constructing the Aswan dam, heavy rains in the Ethiopian highlands would send nutrient-rich silt down the Nile into the delta area. However, this was too inconsistent; one year, the water supply would be poor, resulting in weak soil and therefore a reduced agricultural yield, while the following year could bring severe flooding, destroying farmland, buildings, and road. In addition, the sand and mud in the delta region are naturally salty.
Before the dam, annual flooding would wash the salt into the Mediterranean. Now that the flooding has been stopped, the natural salt and salt added from artificial fertilisers are beginning to build up and seep into the groundwater. The increase in salt in the soil has created two issues. Firstly, salty soil produces salty groundwater, which is not suitable for drinking or irrigation as it's harmful to humans and plants. Secondly, most of Egypt's ancient monuments are built of limestone; the salt is causing the limestone to decay quicker than expected.
Aswan High Dam Project
The Aswan High Dam took ten years to build, completed on 21st July 1970. It's situated 10 miles south of Aswan. It is 3,600 meters long and 111 meters high, and cost over $1 billion to construct.
The dam produces 175 megawatts of electricity via hydroelectricity, giving power to villages that didn't previously have it. To control the flooding and regulate agricultural production, farmers now undertake three growing cycles a year. More people are currently employed in agriculture. Lake Nasser also acts as a water storage facility. The dam improved navigation along the Nile benefiting tourism and fishing, with a new fishing industry springing up around Lake Nasser. Salt production also increased, with 27 million tonnes a year exported.
Fig. 3 - The Aswan Dam as seen from above, NASA/ Wikimedia Commons, Public Domain
The ‘dam’ problem
The dam's construction flooded large areas and rehoused over 100,000 people. Although many archaeological sites were relocated, many were submerged. There has been an increase in the number of people infected with a disease called schistosomiasis, or bilharzia. It is caused by a parasite carried in snails that bores itself into the skin of humans, and this parasite flourishes in standing water. The flooding of the Nile had previously prevented this as bilharzia requires slow-moving or standing water.
The dam is also blamed for the increase in soil erosion further down the river due to the loss of sediment; with the land retreating by approximately 100 yards in places causing the delta to shrink. The coastal region is eroding and retreating by up to 575 feet per year due to wave action, causing some of the coastal lagoons to increase in salinity levels as their connection to the sea increases. Changes in the water table near the dam are causing salinity, with too much sediment now collecting in Lake Nasser.
Future prospects
So what does the future hold? Rising sea levels are causing Egypt's Mediterranean coastline to be flooded, with a 30cm rise in sea levels predicted by 2025, flooding 200 square km of land. The flooding will submerge the ancient city of Alexandria. Most crops will be destroyed, turning the delta into a salty wasteland. Some farmers are importing sand to build flood defences; most will be forced off the land, triggering a massive food shortage in the region unless farmers can change to cultivating especially adapted plants which are able to flourish in salty conditions.
Within Egypt, there is a massive effort to switch cropping activities to aquaculture. Existing cropping patterns are being changed, planting crops more tolerant to salinity closer to the coast. The drainage water from agriculture is being reused, minimising the required water. Existing engineering coastal structures in the low lying areas along the coast are being strengthened, and an early warning system for flooding is being developed. Other strategies include the development of coastal protection policies and the introduction of ICZM's.
Physical factors affecting coastal retreat
Alongside subaerial processes geological and marine factors combine, contributing to erosion. This inevitably influences coastal retreat. Porosity is a major factor. Water flowing through a rock structure will weaken it by removing any sediment which holds the rocks together. Permeable rocks will permit water to flow through them, e.g. sandstone. Impermeable rocks such as marble do not allow water to pass through them.
Geological factors
| Lithology | Geological structure |
| Definition | Rock-type which influences how rocks erode at different rates. | The arrangement of rocks in three dimensions. |
| Types | Igneous – for example, granite, basalt and dolerite, all erode very slowly, with an erosion rate of less the 1mm a year. | Strata – this concerns the different layers of rocks and how they relate to each other. |
| Metamorphic – for example, slate, schist and marble, erode slowly with an erosion rate of 1mm - 10cm a year. | Faulting – this concerns the presence of significant fractures which may have moved rocks from their original location. |
| Example | Granite – for example, sandstone, limestone and shale, unlike the other rock types, have a moderate to fast erosion rate of between 10cm to several meters a year. | Deformation involves the degree to which those rocks have been deformed either through tilting or folding by tectonic activity. |
Marine factors
Waves are a critical element along any coastline as they directly influence erosion, transportation and deposition, all of which affect coastal retreat by a few processes.
- Erosion – softer rocks are easily eroded by actions of waves through hydraulic action, abrasion, attrition and corrosion.
- Transportation – longshore drift is a crucial element in sediment movement along the coast, allowing erosion to commence.
- Deposition – sediment is deposited when the waves no longer have sufficient energy to carry it.
- Wave fetch – The more significant the fetch, the larger the waves will be, more powerful.
- Wave type – Destructive waves are more influential in retreat than constructive waves.
Subaerial processes
Subaerial processes occur on land and, therefore, above water levels. The main examples of subaerial processes are weathering and mass movement. You may not have initially thought of these processes, but they work together to influence coastal retreat. This happens because the result of weathering and mass movement is that the area in question is more susceptible to erosion. Why? Because weathering and mass movement destabilise the ground, which means that less erosional force is required in order for erosion to take place. Subaerial processes will be more influential if the rocks are made of less resistant materials such as shale and clay. Rocks can be weathered by chemical, biological or mechanical breakdown of rocks into smaller fragments; which, includes the effect of surface runoff and mass movement. Mass movement is the movement of material downslope under the influence of gravity by various methods such as landslides, slumps and rockfalls. Surface runoff is where water runs down the surface of the rocks and causes erosion; this is more prominent during heavier downpours.
Short and long term factors of coastal retreat
Rates of recession constantly vary, influenced by different factors, both short- and longer-term, such as:
| Process | Explanation |
| Wind direction | Retreat is more advanced when the wind comes towards the shoreline. |
| Seasons | Storms are more likely to occur in the winter months due to the contrast in temperate and pressure. |
| Weather systems | Depressions have stronger winds resulting in rapid rates of recession. |
| Storms | Storm surges with destructive waves; consequently, retreat rates are more advanced. |
| Tides | Rapid erosion occurs during a high tide. |
Managed retreat
Managed retreat (also known as managed realignment) is where the shoreline is allowed to move inland instead of holding the line with structural engineering.
It is a relatively new approach to coastal management where it is acknowledged that to “hold the line” in many places is not going to work. So how can this be done? One way is through the controlled flooding of a low-lying coastal area, creating a wetland area, or salt marsh. However, this is usually only an option where the site is of low value and at a high risk of erosion.
The advantages of managed retreat
- A cheaper option compared to paying for a hard engineering option such as a sea wall.
- Creates a natural sanctuary for wildlife.
- Salt marshes are diverse ecosystems encouraging numerous species.
- It provides a natural defence against erosion and flooding.
- The initial cost may be high, but long term savings are likely.
- It's more sustainable.
Disadvantages of managed retreat
- The loss of land to the sea.
- Compensation cost – approximately £5,000-£10,000 per hectare.
- Lack of understanding causes controversy.
- Relocation costs may be applicable in some cases.
Managed Retreat Diagram
The diagram below outlines the consequences of doing nothing, holding the line, and managed retreat.
Fig. 4 - The five generic policy options
When considering managed retreat, it must be considered whether or not the area is of high value. That is, how valuable the land is vs. how much engineering options might cost. In some cases, it is best to go with managed retreat in order to mitigate the risk of coastal erosion without investing a large amount of money to repair the coastline.
Coastal Retreat Geography - Key takeaways
- Coastal retreat is how quickly the coastline is moving inland.
- Retreat is a consequence of human, physical, and sub-aerial factors, further influenced by short and long-term factors.
- Coastal retreat is an example of coastal risk, given its potential to impact society and the environment.
- Geological and marine factors combine, contributing to erosion, which inevitably influences the coastal retreat
- Managed retreat also known as managed realignment is where the shoreline is allowed to move inland instead of holding the line with structural engineering.
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