Coastal erosion occurs when sediments from upstream fail to replenish shore sediments. The latter are constantly eroded by wave action and, thus, coast lines undergo dramatic change. Reduction of sediments is primarily due to the construction of structures for hydrological modifications and diversions, in particular dams. The effect of dams on the relative ecology of an area is a subject which has been studied in detail. This is due to the dramatic consequences seen in all rivers once large-scale dams have been constructed. Given below is a detailed account of the effects of dams on aquatic habitats.
Dams are constructed for various reasons and very few are built for a single purpose. Agriculture may be viewed as a primary factor, however, as it is responsible for the majority of water abstracted from reservoirs created by dam construction. The effect of damming a waterway is to effectively halt the transport of sediment from upstream of the dam as it settles within the reservoir. This sediment accumulation has a negative effect on the reservoir as over time it dramatically reduces the volume of water that can be stored, thereby reducing the dam’s effectiveness. The ‘flushing’ of sediments from the dam sometimes controls this but can result in a sudden flush of anoxic water and sediments with detrimental results as they smother remaining fertile soils downstream.
Downstream of the dam, lands which were once replenished by fertile silts from upstream, lose fertility and organic matter in topsoil can be reduced. This can increase erosion and sedimentation into the river. However, due to the reduced volume and rate of flow of the river these sediments are not carried as far into the coastal zone as before. Instead they settle in slack areas of waterways and deltas, increasing the need for dredging. It has been estimated that following dam construction there is 2 – 16 times more erosion than was originally estimated and that immediately downstream sediment yields increase by 50% per decade. This effect is due to the decreasing stability of soils immediately downstream of dams through the lack of replenishment of fertile soils.
Silts from upstream not only physically replenish delta areas, but also the fertility of the marine environment, providing nutrition for the basis of many food webs, particularly in relatively nutrient poor seas such as the Mediterranean. These nutrients provide fertilizer for marine plant growth, which, together with organic matter and detritus from land, provide food for fish and shrimps. Dam construction has reduced this source of nutrition and has had deleterious effects on coastal fisheries as their stocks have undergone dramatic reduction.
Lack of sediment replenishment increases the vulnerability of the foreshore areas of delta to wave erosion. Sediments eroded by wave action are now being deposited further upstream as the slower moving rivers no longer have the power to carry them out to the delta front. Such erosion can occur at an alarming rate – 250 metres per annum has been reported from areas of the Nile Delta. As the front erodes it threatens areas of human settlement and other construction. The encroachment of seawater into coastal agricultural land, previously buffered by fresh water, increases soil salinity and decreases fertility. Losses to coastal agriculture of around 10% per annum have been quoted.
Increased sedimentation poses a serious health hazard to human inhabitants of estuaries and coasts as the sediments accumulate toxins, such as heavy metals. This problem is increased as areas downstream of the dam, which were once unsuitable for habitation due to relatively frequent flooding, are now occupied by people attracted by the opportunity of being able to exploit the remaining fertile soils and relatively flat land. The road building, drainage, construction and sanitation works that follow human settlement contribute significantly to the erosion of the area.
Sustainable Coastal Development
The Great Lakes coastal areas have never been in greater demand. A steady growth of permanent and seasonal residents near the Great Lakes has produced a substantial increase in the use of coastal areas for economic and recreational purposes. Consequently, industrial, residential and recreational developments put pressure on coastal shoreline areas that lead to potential chronic problems, including increased beach erosion, water quality degradation, brownfields, and habitat loss in coastal areas including nearby waterways and bodies of water.
The increased pressure to support a high quality of life for growing coastal populations raises concerns about long-term adverse impacts on the Michigan coastline. Coastal development managers should recognize several over-arching principles, including: the coastline is truly a finite resource; coastal ecosystems are dynamic and complex; and intensity of coastal development can have a significant impact on the sustainability of the coastal ecosystem.
Michigan land use decisions are made at the local level where there is little understanding of the complexity of the entire coastline. Michigan Sea Grant has the ability to provide information across jurisdictional boundaries and create partnerships and collaborations to assist coastal communities in creating sustainable development plans.
Contribute to the overall understanding of the impact of coastal development and provide information and technical assistance to promote and facilitate sustainable coastal communities.
1. Facilitate sustainable coastal community forums in both urban and rural settings, involving a broad diversity of local government officials, and private sector and citizen stakeholders to develop successful sustainable community development models.
2. Continue to lead and support on-going efforts to improve the quality of the Detroit River and other community waterfronts, including brownfield redevelopment.
3. Educate property owners, public and private developers, financial institutions, and insurance agencies on environmentally appropriate coastal erosion control techniques. This techniques
MALAYSIA: Malaysia Awakens to Erosions Threat
Along Its Extensive Coast
Coastal erosion along Malaysia’s nearly 5000 km coastline is reaching crisis proportions even without sea level rise and is the most serious climate change impact facing the nation. Inundation and increased flooding also pose severe threats particularly to the dominant coastal activity – agriculture.
A majority of the population lives in the coastal zone which can be divided into two major types: sandy beaches, and silt and clay mud flats, fringed by mangroves. The major towns, ports, large agriculture and aquaculture projects of Malaysia’s coasts contribute significantly to the nation’s economic development and the physical and economic impact for the whole nation of a greenhouse-induced sea level rise could be devastating.
The country is separated into two regions: Peninsular Malaysia and, 640 km across the South China Sea, the States of Sarawak and Sabah, known as East Malaysia, on northern Borneo Island. Sarawak’s coast has long, straight beaches and mangroves; Sabah is more rugged, with offshore islands and coral reefs.
Because of awakening concern, coastal erosion is being studied on a national basis. A control plan has been formulated, with a conscious effort toward mitigation. Besides natural processes, beach erosion occurs when upstream dams trap sediment. But numerous dams are projected for the future because of the government’s aim to provide the population with adequate and safe water. Watershed development, especially dam construction, has a strong bearing on the stability of the coast, especially near river mouths. The cost to mitigate coastal erosion in 47 critically eroding areas is estimated at US $85 million.
The stronger waves accompanying a rise in sea level will exacerbate the wearing away of the beaches. In some places the average rate of retreat of the shoreline is 1- 8 meters per year. Bunds and dikes, which also compensate for anticipated ground settlement, are now the standard approach for protection. But as the sea rises the waves will overtop these. Dunes offer some security but on developed coasts these have been artificially flattened or removed.
The other major threat – besides erosion – if the seas rose would be inundation and flooding. About 12 percent of the area of Peninsular Malaysia, where the western low plains of muddy sediment are home to 2.5 million people, is flood prone. Erosion of mud coasts is poorly understood, and improved understanding in the near future is not considered likely.
Along the sandy beaches and bays of the east coast of Peninsular Malaysia, more than 200,000 people and another 12,600 km2 are in danger of floods. The causes are a combination of excessive precipitation and runoff, inadequate river capacity, flood plain encroachment, blocking of river mouths, and high tide. Tidal flooding, is now largely prevented by extensive coastal and river bunds which are important to polder development in this area. More than 150 river systems in the country are used for inland navigation, transportation, irrigation, water supply and hydropower. Accentuated by high tides, river flooding is a frequent source of harm to people, property and public facilities, totaling an estimated annual damage of $38 million.
The wetlands, primarily the mangrove forests, are important for firewood and charcoal, as habitats for aquatic life and food and for prawn, and for protection from erosion. More than half the nation’s annual fishery income is related to the mangroves, and the livelihoods of 300,000 fishermen depend on them. Sea level rise is not yet perceived as a direct threat, but “it is highly probable that the mangrove buffer may vanish altogether,” say the authors, “exposing the previously protected coastal development to direct wave attack.” Wetlands are considered wastelands by developers. Clearance and conversion to agriculture or aquaculture, industrialization, urbanization, and pollution all menace the mangrove forests.
Of the various threats to the coasts – and some may together affect the same area – tidal submergence is likely to come first, then wetland loss, and finally coastal erosion. Flooding is a nuisance unless it is prolonged. Areas can easily be protected from inundation. Wetland loss, while small in terms of area, would have serious economic repercussions, and reclamation could cause the mangrove firests of Peninsular Malaysia to disappear by the year 2000.
Among possible responses, the authors feel a wait-and-see attitude is untenable given the “overwhelming scientific evidence that points to a rise in sea level and the potentially large costs of inaction.” Given “the long lead time required to transform a policy into reality,” they continue, “makes it prudent to begin” to incorporate adaptation measures into development and land use planning.