HKDSE Geography/E1/Reclamation Materials
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In a previous chapter, we've looked at how reclamation changes the landscape of Hong Kong. Now we'll take one step backwards, and look at the extraction of reclamation materials in Hong Kong. There are three types of reclamation materials: Rock fill, marine sand fill (soft fill) and public fill.
Rock fill comes from weathered rock in granitic landscapes. The rocks are excavated from quarries in sloping ground, then mixed with cement to form rock fill.
Pros and Cons
- Rock fill comes in very large quantity.
- Its extraction is very costly.
- The landscape is permanently scarred as all vegetation and the top layer of rocks and regolith in the area is removed to create a barren surface. If the land is further eroded by rain, rills, gullies and a badland landscape will be formed. This reduces the quality of life of nearby areas. This is known as visual pollution.
- Rock fill potentially causes landslides. As slopes are cut for quarrying, the barren land will be exposed to the rain, and the infiltration rate will be quicker. The pore water pressure will increase, so the internal cohesion of slope particles will decrease, and the friction will also decrease. This increases the landslide risk.
- Other types of pollution are produced in the process of extracting rock fill. The machinery produces noise pollution, which disturbs nearby residences. The process of extraction will release dust into the nearby air, causing air pollution. Water pollution is also serious: As the soil has lost its protective vegetation layer, surface run-off will be sped up and rainsplash will occur. The run-off may carry the washed-off topsoil to nearby bodies of water such as rivers and lakes, reducing their capacity. This is silting/sedimentation (which has been discussed in M2). When heavy, persistent rain occurs, the ability of these bodies to hold water will decrease and the chance of overflowing will increase, inducing floods.
- The local ecosystem is destroyed. As the vegetation is removed, the animals also lose their shelter and food source, and will be forced to migrate elsewhere. It is difficult if not impossible for plant succession to occur in the area because the topsoil is also removed, revealing the harder rock layer below. It is difficult for tree roots to extend into such layers and thrive in such an environment.
Rock fill is usually used in large public works projects as a huge amount of fill is needed for construction. It used to be the predominant source of fill for Hong Kong, but extraction activities have been discontinued because of exorbitant costs of extraction.
Marine Sand Fill
Marine sand fill refers to sand extracted from the seabed using trailing suction hopper dredgers, which, as their name suggests, dredge the seabed. In Hong Kong, deposits of alluvium on seabeds are a common source.
Pros and Cons
- Large quantity
- As the seabed is dredged, the deeper harbour can facilitate navigation.
- As the seabed is dredged, the effect of silting on the depth of the sea floor will be mitigated. Macau is an example.
- If the reclamation site is close to the marine borrow area, the transport costs incurred are much lower than public fill.
- Ecosystem destruction: Marine ecosystems are destroyed when the seabed is dredged. The habitats of animals who lived on the seabed will face total destruction, and their lives will also be disturbed by the dredgers. In particular, dredging destroys coral reefs, which are important habitats for many marine animals, particularly fish.
- Visibility: Dredging increases the suspeneded sediment content. The turbidity of the sand will increase, lowering visibility. The ability of sea creatures to find food is reduced, and eventually, marine life will die of starvation.
- Toxicity: Sometimes, there is a layer of mud (or sludge) which covers the sand. It must be removed and deposited elsewhere. This will further increase the turbidity of the area where the sludge is deposited, disturbing more aquatic life.
- The layer of sludge may also be heavily polluted by industrial and domestic sewage if the marine borrow area is close in proximity to nearby urban or industrial areas. It may be teeming with organic chemicals and heavy metals. When the sludge is removed, the toxicity of the seawater will increase, potentially poisoning aquatic life. Penny's Bay is an example.
- If heavy metals may accumulate in marine life near the dumping and dredging sites, they will further accumulate along the food chain and finally reach humans, threatening public health. In recent years, toxic seafood has become a major concern.
The use of marine sand fill is most common in Hong Kong. Apart from local marine borrow areas, sand fill is also exported from the Zhu Jiang Delta.
As marine sand fill is the commonest fill in Hong Kong, much has been done by the Civil Engineering and Development Department to reduce its impact.
- Monitoring: The CEDD has set up the Marine Fill Committee, which oversees the extraction of marine fill reserves to ensure sustainability.
- Extracting the right way: The Environmental Impact Assessment (EIA) Ordinance requires that extractors apply for environmental permits before designating marine borrow areas. They must ensure that the impact of dredging will be minimised and the environmental costs will be compensated.
- Extracting in the right place: Not all viable deposits of sand fill are exploited, for two main reasons
- Some areas are ecologically valuable with precious marine resources and they are left untouched for ecological conservation. Inner and Outer Deep Bay and Sai Kung are two examples.
- Marine fill resources offshore of Tuen Mun and Tsing Yi are covered with thick layers of contaminated sludge, so dredging is prohibited.
- Not extracting at all: Where possible, marine fill is not used at all. It is used as public fill.
Public fill is derived from inert construction and demolition materials. 'Inert' means that hey will not decompose. They include asphalt, building debris (such as bricks), broken rock and concrete.
Pros and Cons
- It conserves natural resources.
- The cost is lower.
- Waste is reduced, which in turn eases the landfill saturation problem.
- On-site sorting is required, which is a cumbersome and costly process.
- It is unreliable and limited in availability
Because of its limited availablity (only used in construction and demolition sites), it is used whenever possible, but it is far from the predominant fill type.