Coastal sediment supply

Coastal sediment supply

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Coastal sediment supply is the transport of sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

 to the beach
Beach
A beach is a geological landform along the shoreline of an ocean, sea, lake or river. It usually consists of loose particles which are often composed of rock, such as sand, gravel, shingle, pebbles or cobblestones...

 environment by both fluvial
Fluvial
Fluvial is used in geography and Earth science to refer to the processes associated with rivers and streams and the deposits and landforms created by them...

 and aeolian
Aeolian
Aeolian or Eolian may refer to:* things related to Aeolus, the Greek God of wind or the patriarch of Greeks of Aeolia* Aeolian harp, a harp that is played by the wind* Aeolian processes, wind generated geologic processes...

 transport. While aeolian transport plays a role in the overall sedimentary budget
Sedimentary budget
Sedimentary budgets are a coastal management tool used to analyse and describe the different sediment inputs and outputs on the coasts, which is used to predict morphological change in any particular coastline over time...

 for the coastal environment, it is paled in comparison to the fluvial supply which makes up 95% of sediment entering the ocean. When sediment reaches the coast it is then entrained by longshore drift
Longshore drift
Longshore drift consists of the transportation of sediments along a coast at an angle to the shoreline, which is dependent on prevailing wind direction, swash and backwash. This process occurs in the littoral zone, and in or within close proximity to the surf zone...

 and littoral cells until it is accreted upon the beach or dunes.

While it is acknowledged that storm systems are the driver behind coastal erosion
Coastal erosion
Coastal erosion is the wearing away of land and the removal of beach or dune sediments by wave action, tidal currents, wave currents, or drainage...

. There is a general consensus that human activity, mainly dam and reservoir impoundments on rivers are the cause of indirect human related coastal erosion, along with other local scale effects such as: land use change, irrigation
Irrigation
Irrigation may be defined as the science of artificial application of water to the land or soil. It is used to assist in the growing of agricultural crops, maintenance of landscapes, and revegetation of disturbed soils in dry areas and during periods of inadequate rainfall...

, gravel extraction and river re-alignment.

Rate of supply


Worldwide, rivers discharge approximately 35x103 km3 of freshwater into the ocean annually. Transported in this freshwater is 15 to 20 x 109 tons of sediment. This sediment load is not proportionally distributed across the worlds rivers, with Asian and Oceanic regions being among those most significantly affected by changing sediment regimes, as they account for 75% of this global sediment budget.

These changing rates of supply/replenishment from the fluvial environment are a dominant factor in controlling the rate of coastal erosion. While sediment supply is actually increasing, due to increased erosion rates, the supply of this sediment to the coastal environment is decreasing.

Factors that influence sediment supply


Fluvial systems are key elements for operating Earth surface change because they convey most of the global fluxes of water and sediment from land to oceans. Human activities can affect the discharge of water and sediment from a river to the coastal environment in many ways. Deforestation
Deforestation
Deforestation is the removal of a forest or stand of trees where the land is thereafter converted to a nonforest use. Examples of deforestation include conversion of forestland to farms, ranches, or urban use....

 and agriculture
Agriculture
Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...

, as well as urbanization
Urbanization
Urbanization, urbanisation or urban drift is the physical growth of urban areas as a result of global change. The United Nations projected that half of the world's population would live in urban areas at the end of 2008....

 can increase the erosion of a river basin by as much as an order of magnitude. Freshly exposed soil is much less likely to resist erosion by rainfall or moving water, especially in areas where land is often used for agriculture and precipitation is high.

Since the 1950s the number of dams in the world has increased more than sevenfold. The creation of reservoir
Reservoir
A reservoir , artificial lake or dam is used to store water.Reservoirs may be created in river valleys by the construction of a dam or may be built by excavation in the ground or by conventional construction techniques such as brickwork or cast concrete.The term reservoir may also be used to...

s has significantly reduced the sediment yield of many rivers as sediment that was entrained in the flow is stopped by a manmade barrier and the energy required to transport the material is lost, the sediment is no longer entrained as there is no flow or the flow is to slow.

Removal of water for irrigation reduces river flow and also plays its part in reducing the sediment carrying capacity of a river. Agricultural and farming practices often require intensive irrigation systems to achieve appropriate production levels. This creates a high demand on waterways as water is diverted and used to irrigate crops and pasture. This decreases the flow rate within the river system, which in turn lowers the sediment carrying capacity of the river as the energy within is less. Sediment is deposited along the reach of the river and takes much longer to reach the coastal zone if at all.

Effects on the coast


This changing coastal sediment supply regime leads to predominantly erosional outcomes but it is a case by case, river by river scenario. There are areas where the coastal sediment flux has increased and accretion of the shoreline has been evident. The effects of changing sediment flux can be very localised but pronounced, below are examples within the Bohai Sea
Bohai Sea
Bohai Sea , also known as Bohai Gulf, Bohai, or Bo Hai, is the innermost gulf of the Yellow Sea on the coast of Northeastern and North China. It is approximately 78,000 km2 Bohai Sea , also known as Bohai Gulf, Bohai, or Bo Hai, is the innermost gulf of the Yellow Sea on the coast of...

. The Yellow River
Yellow River
The Yellow River or Huang He, formerly known as the Hwang Ho, is the second-longest river in China and the sixth-longest in the world at the estimated length of . Originating in the Bayan Har Mountains in Qinghai Province in western China, it flows through nine provinces of China and empties into...

 delta is undergoing an accretion phase while 200 km across the sea the Laun River area has experienced increased erosion rates along the coastline as a result of the changing sediment input.

Accretion


The Yellow River transports an order of magnitude more sediment than it did prior to widespread cultivation of the loess plateau
Loess Plateau
The Loess Plateau , also known as the Huangtu Plateau, is a plateau that covers an area of some 640,000 km² in the upper and middle reaches of China's Yellow River. Loess is the name for the silty sediment that has been deposited by wind storms on the plateau over the ages...

s in northern China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

, about 2400 years ago. One implication of this large increase in the sediment discharge of Asian rivers has been the increased shoreward accretion of some deltaic areas over the past several millennia. The Yellow River Delta has accreted 100s of km2 over the past few decades. Building the coastline at a rate greater than sea level rise and depositing sediment faster than erosional processes can remove it. Mankind in Asia is occupying land areas that may not have existed if not for the increased upstream erosion and delivery of the sediment to the coastal environment.

Erosion


Coastal erosion/retreat has wide-ranging implications for human habitat. With 45% of the world’s population living within 100 km of a coastline. The decline of sediment supply to the coast generally results in increased rates of erosion as there is no nourishment of the beach profile. Over time as storm systems ‘attack’ the coastline removing sediment to offshore bars or from the system altogether a real issue develops in how to slow the advancing ocean and protect coastal developments.

Qinhuangadao coast


Since the completion of construction of two large dams on the Luan River
Luan River
The Luan River is a river in China. It flows northwards from its source in the province of Hebei into the province of Inner Mongolia, and then flows southeast back into Hebei to its mouth on the Bohai Sea. Its length is about 600 km. One subsidiary of the Luan River is the Yixun He, which...

 in 1979, its annual sediment delivery to the Bohai Sea has been cut from 20.2 million tons to an annual sediment flux of only 1.9 million tons. This sharp reduction in sediment discharge has been accompanied by phase shift of the Luan River Delta into para-abandonment. This drop in sediment yield has inevitably altered the longshore transport equilibrium in the local coastal area. With prevailing northward winds, shoreline retreat rates north of the Luan River have increased from 1 m/y to 3.7 m/y over twenty year period since dam construction, rapidly changing the morphology and beach profile via positive feedback until a new equilibrium was reached, at which the present retreat is back at its original 1 m/y. With a longshore drift equilibrium was reached within a new state.

Mississippi Delta


The effects of changing sediment flux were a definite compounding factor in the havoc brought upon New Orleans in 2005 by Hurricane Katrina
Hurricane Katrina
Hurricane Katrina of the 2005 Atlantic hurricane season was a powerful Atlantic hurricane. It is the costliest natural disaster, as well as one of the five deadliest hurricanes, in the history of the United States. Among recorded Atlantic hurricanes, it was the sixth strongest overall...

. The Mississippi River Delta
Mississippi River Delta
The Mississippi River Delta is the modern area of land built up by alluvium deposited by the Mississippi River as it slows down and enters the Gulf of Mexico...

 is slowly sinking due natural compaction and its sediment supply from up river being greatly diminished by up to 50%, due to dam construction in the Mississippi Basin. The loss of sediment supply has resulted in subsidence of the delta and wetland reduction. Fresh sediment is not deposited at a rate fast enough for vegetation to grow on as sea level rises. The brackish conditions that are essential for vegetation growth on the delta are no longer. These coastal wetlands are vital for the diverse wildlife habitat that lives among them, and for protecting developed areas from storm surges, like the one experienced during Katrina.

Beach nourishment


This is a short term fix for a long term problem. It treats the immediate symptoms of coastal erosion by dumping of sand either just off shore or on the berm itself, It can be a costly and time consuming process as it requires upkeep on regular bases. While it may look aesthetically pleasing it does not fix the problem at hand, the beach has lost its source of natural nourishment. Artificial nourishment is an appropriate short term solution on small scale restoration projects.

Management


Humans are simultaneously increasing the availability of sediment for fluvial transportation through activities and practices that increase soil erosion, and decreasing this flux to the coastal environment through retention of sediment behind reservoir walls and decreasing river discharge rates. The net result is a global reduction in the overall sediment flux. This impact on coastal erosion will be further accelerated as the sea level rises, which is anticipated because of climate change. Given the modern levels of fluvial sediment loads, over 100 billion tons of sediment have been sequestered behind human-made reservoirs. This gives reason for the effects of changes in the upstream fluvial environment to be included into the Integrated coastal zone management
Integrated coastal zone management
Integrated coastal zone management or Integrated coastal management is a process for the management of the coast using an integrated approach, regarding all aspects of the coastal zone, including geographical and political boundaries, in an attempt to achieve sustainability.This concept was born...

 framework. Planners and decision makers must consider what the impacts of upstream development will have on the coastal environment. The management of the coastal zone must account for both the effect of human activities as well as the impacts resulting from corresponding changes in the coastal zone.

See also

  • Human impacts on coasts
  • Sea level rise
    Current sea level rise
    Current sea level rise potentially impacts human populations and the wider natural environment . Global average sea level rose at an average rate of around 1.8 mm per year over 1961 to 2003 and at an average rate of about 3.1 mm per year from 1993 to 2003...

  • Coastal management
    Coastal management
    In some jurisdictions the terms sea defense and coastal protection are used to mean, respectively, defense against flooding and erosion...

  • Beach nourishment
    Beach nourishment
    Beach nourishment— also referred to as beach replenishment—describes a process by which sediment lost through longshore drift or erosion is replaced from sources outside of the eroding beach...