Sedimentation enhancing strategies are

environmental management Environmental resource management or environmental management is the management of the interaction and impact of human societies on the environment. It is not, as the phrase might suggest, the management of the environment itself. Environment ...

projects aiming to restore and facilitate land-building processes in

deltas A river delta is a landform, wikt:archetype#Noun, archetypically triangular, created by the deposition (geology), deposition of the sediments that are carried by the waters of a river, where the river merges with a body of slow-moving water or ...

Sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

availability and deposition are important because deltas naturally subside and therefore need sediment accumulation to maintain their elevation, particularly considering increasing rates of

sea-level rise The sea level has been rising from the end of the last ice age, which was around 20,000 years ago. Between 1901 and 2018, the average sea level rose by , with an increase of per year since the 1970s. This was faster than the sea level had e ...

Sedimentation Sedimentation is the deposition of sediments. It takes place when particles in suspension settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to th ...

enhancing strategies aim to increase sedimentation on the delta plain primarily by restoring the exchange of water and sediments between

river A river is a natural stream of fresh water that flows on land or inside Subterranean river, caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of ...

s and low-lying delta plains. Sedimentation enhancing strategies can be applied to encourage

land elevation Post-glacial rebound (also called isostatic rebound or crustal rebound) is the rise of land masses after the removal of the huge weight of ice sheets during the last glacial period, which had caused isostatic depression. Post-glacial rebound ...

gain to offset sea-level rise. Interest in sedimentation enhancing strategies has recently increased due to their ability to raise land elevation, which is important for the long-term

sustainability Sustainability is a social goal for people to co-exist on Earth over a long period of time. Definitions of this term are disputed and have varied with literature, context, and time. Sustainability usually has three dimensions (or pillars): env ...

of deltas.

Benefits of sedimentation enhancing strategies

When compared to conventional flood protection infrastructure such as embankments and

seawall A seawall (or sea wall) is a form of coastal defense constructed where the sea, and associated coastal processes, impact directly upon the landforms of the coast. The purpose of a seawall is to protect areas of human habitation, conservation, ...

s, sedimentation enhancing strategies provide various benefits. Firstly, flood protection structures can exacerbate environmental problems in deltas:

land reclamation Land reclamation, often known as reclamation, and also known as land fill (not to be confused with a waste landfill), is the process of creating new Terrestrial ecoregion, land from oceans, list of seas, seas, Stream bed, riverbeds or lake ...

and levee construction result in a loss of water storage area during peak river discharges, which may cause an increased risk of

flood A flood is an overflow of water (list of non-water floods, or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are of significant con ...

ing further downstream. Embankments also exacerbate land elevation loss due to soil drainage and hinder natural sediment accumulation. In contrast, sedimentation enhancing strategies do not cause these problems and instead address multiple issues simultaneously: they reduce

risks while simultaneously restoring

ecosystems An ecosystem (or ecological system) is a system formed by Organism, organisms in interaction with their Biophysical environment, environment. The Biotic material, biotic and abiotic components are linked together through nutrient cycles and en ...

, enhancing production (e.g.

fisheries Fishery can mean either the enterprise of raising or harvesting fish and other aquatic life or, more commonly, the site where such enterprise takes place ( a.k.a., fishing grounds). Commercial fisheries include wild fisheries and fish farm ...

) and cultural (e.g.

landscape A landscape is the visible features of an area of land, its landforms, and how they integrate with natural or human-made features, often considered in terms of their aesthetic appeal.''New Oxford American Dictionary''. A landscape includes th ...

)

ecosystem service Ecosystem services are the various benefits that humans derive from ecosystems. The interconnected living and non-living components of the natural environment offer benefits such as pollination of crops, clean air and water, decomposition of wast ...

s. Sedimentation enhancing strategies are also more flexible than conventional flood protection. Large-scale infrastructural flood defences are costly and rigid, requiring considerable investment to adapt infrastructural flood defences to changing

boundary conditions In the study of differential equations, a boundary-value problem is a differential equation subjected to constraints called boundary conditions. A solution to a boundary value problem is a solution to the differential equation which also satis ...

. Particularly considering uncertain future

scenarios In the performing arts, a scenario (, ; ; from Italian , "that which is pinned to the scenery") is a synoptical collage of an event or series of actions and events. In the ''commedia dell'arte'', it was an outline of entrances, exits, and action ...

due to

climate change Present-day climate change includes both global warming—the ongoing increase in Global surface temperature, global average temperature—and its wider effects on Earth's climate system. Climate variability and change, Climate change in ...

, sea-level rise and peak river discharges, rigid flood defences may not be the optimal choice. Sedimentation enhancing strategies are more flexible and adaptable to changing environmental conditions, which makes them more likely to perform satisfactorily under different future scenarios.

Limitations of sedimentation enhancing strategies

2014_NASA_Earth_Observatory_image_of_Pearl_River_Delta

One major obstacle to the implementation of sedimentation enhancing strategies is that they require space which may not be available, as deltas are among the most

densely populated Population density (in agriculture: Standing stock (disambiguation), standing stock or plant density) is a measurement of population per unit land area. It is mostly applied to humans, but sometimes to other living organisms too. It is a key geog ...

regions in the world.

Land-use change Land use is an umbrella term to describe what happens on a parcel of land. It concerns the benefits derived from using the land, and also the land management actions that humans carry out there. The following categories are used for land use: for ...

to make space for sedimentation enhancing strategies requires stakeholder participation, but delta inhabitants may not be willing to change land uses. Additionally, a decline in river sediment delivery due to upstream dam construction and other environmental changes in catchments caused by human activities means that less sediment is available in deltas for sedimentation enhancing strategies. The success of sedimentation enhancing strategies is highly context dependent and depends on, for example, river discharge, sediment concentration in the water, land-use in the delta, the

tidal range Tidal range is the difference in height between high tide and low tide. Tides are the rise and fall of sea levels caused by gravitational forces exerted by the Moon and Sun, by Earth's rotation and by centrifugal force caused by Earth's prog ...

, stakeholder engagement, and the financial resources of the country in which the delta is located.

Types of sedimentation enhancing strategies

River diversions

In many deltas worldwide, rivers are disconnected from delta plains by embankments or levees which constrain water bodies and prevent hydrological exchange between water and land. River diversions, designed to correct the issue of disconnection caused by hydrological engineering, are engineered structures along a river that direct water and sediments from the river into adjacent

wetland A wetland is a distinct semi-aquatic ecosystem whose groundcovers are flooded or saturated in water, either permanently, for years or decades, or only seasonally. Flooding results in oxygen-poor ( anoxic) processes taking place, especially ...

s. Diversion structures can range from simple gates to more complex siphon or pump systems. In addition to requiring the engineered structures at the point of river diversions, this strategy relies on natural land-building processes. River water loses

energy Energy () is the physical quantity, quantitative physical property, property that is transferred to a physical body, body or to a physical system, recognizable in the performance of Work (thermodynamics), work and in the form of heat and l ...

and slows down as it passes from the relatively narrow river into the wider receiving area, causing sediments to be deposited, which raises the elevation of the land and may lead to the formation of new land. 4701-TP-Fig3

Mississippi River delta, Louisiana, USA

Over the 20th century the

Mississippi delta The Mississippi Delta, also known as the Yazoo–Mississippi Delta, or simply the Delta, is the distinctive northwest section of the U.S. state of Mississippi (and portions of Arkansas and Louisiana) that lies between the Mississippi and Yazo ...

lost approximately 25% of its land. Currently, land is disappearing at a rate of almost 11,000

acre The acre ( ) is a Unit of measurement, unit of land area used in the Imperial units, British imperial and the United States customary units#Area, United States customary systems. It is traditionally defined as the area of one Chain (unit), ch ...

s per year. To combat these rapid rates of

land loss Land loss is the term typically used to refer to the conversion of coastal land to open water by natural processes and human activities. The term ''land loss'' includes coastal erosion. It is a much broader term than coastal erosion because land l ...

, the Louisiana Coastal Protection and Restoration Authority (CPRA) developed a $50 billion, 50-year plan for the Mississippi delta, a central component of which is the reintroduction of river water and sediment to the delta plain through river diversions. Engineered river diversions have previously been implemented in the Mississippi delta at Caernarvon and Davis Pond. Although these diversions were not constructed with the primary goal of building land, land growth occurred at both sites. The 2 km wide Caernarvon diversion resulted in sediment deposition of up to 42 cm in the receiving area, creating a

crevasse splay A crevasse splay is a sedimentary fluvial deposit which forms when a stream breaks its natural or artificial levees and deposits sediment on a floodplain. A breach that forms a crevasse splay deposits sediments in similar pattern to an alluvial ...

of approximately 130 km² within three months. The currently planned Mid-Barataria, Lower-Barataria and Breton diversions have been specifically designed to capture and divert sediment from the Mississippi river and deposit it in the receiving

basins Basin may refer to: Geography and geology * Depression (geology) ** Back-arc basin, a submarine feature associated with island arcs and subduction zones ** Debris basin, designed to prevent damage from debris flow ** Drainage basin (hydrology), ...

to build land.

Canal del Dique, Colombia

Canal del Dique is a 400-year-old navigation channel connecting the Rio Magdalena with the Bay of Cartagena in

Colombia Colombia, officially the Republic of Colombia, is a country primarily located in South America with Insular region of Colombia, insular regions in North America. The Colombian mainland is bordered by the Caribbean Sea to the north, Venezuel ...

. The construction of this channel increased the flow of water and sediment into the Bay of Cartagena. Sediment deposition in the

canal Canals or artificial waterways are waterways or engineered channels built for drainage management (e.g. flood control and irrigation) or for conveyancing water transport vehicles (e.g. water taxi). They carry free, calm surface ...

, connected

lake A lake is often a naturally occurring, relatively large and fixed body of water on or near the Earth's surface. It is localized in a basin or interconnected basins surrounded by dry land. Lakes lie completely on land and are separate from ...

s and

swamp A swamp is a forested wetland.Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p. Swamps are considered to be transition zones because both land and water play a role in ...

s, and in the Bay of Cartagena negatively impacted the environment. In 2013, Dutch company Royal HaskoningDHV designed a plan including two control structures on the canal. One control structure was built upstream to regulate the amount of water and sediment flowing from the Rio Magdalena into the Canal del Dique. The second control structure was built downstream of the canal at Puerto Badel to divert water and sediment toward a

mangrove A mangrove is a shrub or tree that grows mainly in coastal saline water, saline or brackish water. Mangroves grow in an equatorial climate, typically along coastlines and tidal rivers. They have particular adaptations to take in extra oxygen a ...

area west of the canal. In this way, the mangrove area is restored, land is being built, and at the same time the amount of sediment input in the Bay of Cartagena is reduced which promotes

ecological restoration Ecological restoration, or ecosystem restoration, is the process of assisting the recovery of an ecosystem that has been degraded, damaged, destroyed or transformed. It is distinct from conservation in that it attempts to retroactively repair ...

Tidal flooding of previously enclosed areas

Tidal flooding of

polder A polder () is a low-lying tract of land that forms an artificial hydrology, hydrological entity, enclosed by embankments known as levee, dikes. The three types of polder are: # Land reclamation, Land reclaimed from a body of water, such as a ...

s entails (temporarily) breaching dikes and allowing tidal water to flow into an embanked area during

high tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables ...

. Tidal water can bring in large concentrations of sediment from the

sea A sea is a large body of salt water. There are particular seas and the sea. The sea commonly refers to the ocean, the interconnected body of seawaters that spans most of Earth. Particular seas are either marginal seas, second-order section ...

into the river system, which deposit and accrete within the polder when flow velocities reduce. Tidal flooding of polders is an alternative form of coastal defence that makes use of natural tidal dynamics and associated morphological processes. During the time the polder is flooded, the area can be used for

aquaculture Aquaculture (less commonly spelled aquiculture), also known as aquafarming, is the controlled cultivation ("farming") of aquatic organisms such as fish, crustaceans, mollusks, algae and other organisms of value such as aquatic plants (e.g. Nelu ...

. We distinguish between tidal river management, implemented in the Ganges-Brahmaputra-Meghna delta,

Bangladesh Bangladesh, officially the People's Republic of Bangladesh, is a country in South Asia. It is the List of countries and dependencies by population, eighth-most populous country in the world and among the List of countries and dependencies by ...

, and exchange polders, implemented in the Rhine-Meuse delta,

the Netherlands , Terminology of the Low Countries, informally Holland, is a country in Northwestern Europe, with Caribbean Netherlands, overseas territories in the Caribbean. It is the largest of the four constituent countries of the Kingdom of the Nether ...

Ganges-Brahmaputra-Meghna delta, Bangladesh

Polders, known as beels in

Bengal Bengal ( ) is a Historical geography, historical geographical, ethnolinguistic and cultural term referring to a region in the Eastern South Asia, eastern part of the Indian subcontinent at the apex of the Bay of Bengal. The region of Benga ...

, have been constructed in Bangladesh since the 1960s. The embankments provide flood protection and initially increased

agricultural production Agriculture encompasses crop and livestock production, aquaculture, and forestry for food and non-food products. Agriculture was a key factor in the rise of sedentary human civilization, whereby farming of domesticated species created food ...

. However, together with a decrease in

water supply Water supply is the provision of water by public utilities, commercial organisations, community endeavors or by individuals, usually via a system of pumps and pipes. Public water supply systems are crucial to properly functioning societies. Th ...

due to upstream dam construction, the embankments caused an increase in riverbed sedimentation and congestion, hampering water drainage and

navigation Navigation is a field of study that focuses on the process of monitoring and controlling the motion, movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navig ...

. Another issue in Bangladesh is waterlogging, which negatively impacts the

agricultural productivity Agricultural productivity is measured as the ratio of Agriculture, agricultural outputs to inputs. While individual products are usually measured by weight, which is known as crop yield, varying products make measuring overall agricultural out ...

of the region. Tidal river management (TRM) emerged as a bottom-up, indigenous strategy to reduce waterlogging and solve river congestion problems in Bangladesh. TRM is also seen as a

climate change adaptation Climate change adaptation is the process of adjusting to the effects of climate change, both current and anticipated.IPCC, 2022Annex II: Glossary

When the water flows into embanked areas during high tide, water flow velocities reduce and sediments deposit. During low tide, water flow velocity increases again as the water is pulled back through the channels Channel, channels, channeling, etc., may refer to: Geography * Channel (geography), a landform consisting of the outline (banks) of the path of a narrow body of water. Australia * Channel Country, region of outback Australia in Queensland and pa ...

toward the sea, causing deposited riverbed sediment to erode. This increases the Drainage">drainage capacity and navigability of the channels. TRM has been implemented in five beels in the south of the Ganges-Brahmaputra-Meghna delta. The implementation of TRM by local people (Top-down and bottom-up design">bottom-up) has been particularly successful. For example, land in beel Bhaina was raised by 1.5–2 meters near the cut point in the embankment and by 0.2 meters toward the other end of the beel. Due to the success of TRM, the Bangladesh Water Development Board also formally implemented TRM in multiple beels, which has been less successful due to the top-down implementation causing conflict between locals and formal institutions. Westerschelde_bij_Rilland_01

Western Scheldt, the Netherlands

The first land reclamation efforts in the southwestern Rhine-Meuse delta in the Netherlands date back to the Middle Ages. Since then, the area has experienced multiple

storm A storm is any disturbed state of the natural environment or the atmosphere of an astronomical body. It may be marked by significant disruptions to normal conditions such as strong wind, tornadoes, hail, thunder and lightning (a thunderstor ...

s and extreme weather conditions, amongst which the flood disaster of 1953 which led to the construction of the

Delta Works The Delta Works () is a series of construction projects in the southwest of the Netherlands to protect a large area of land around the Rhine–Meuse–Scheldt delta from the sea. Constructed between 1954 and 1997, the works consist of dams, slu ...

. The construction of

dam A dam is a barrier that stops or restricts the flow of surface water or underground streams. Reservoirs created by dams not only suppress floods but also provide water for activities such as irrigation, human consumption, industrial use, aqua ...

s, locks and storm surge barriers, and the strengthening and raising of dikes in the area, initially increased flood safety. However, over time, the land behind dikes started to sink which is highly problematic in the face of sea-level rise. In the

Western Scheldt The Western Scheldt ( ), in the province of Zeeland in the southwestern Netherlands, is the estuary of the Scheldt river. This river once had several estuaries, but the others are now disconnected from the Scheldt, leaving the Westerschelde as ...

, a strategy similar to TRM has been proposed to naturally raise the land. During high tide, the Western Scheldt delivers sediment to the areas outside of the embankments. As a result, these areas naturally rise with

water level Water level, also known as gauge height or stage, is the elevation of the free surface of a sea, stream, lake or reservoir relative to a specified vertical datum. Over long distances, neglecting external forcings (such as wind), water level ten ...

s. This is illustrated by het verdronken land van Saefthinge, an area that lies outside of the embankments but has a higher elevation than other areas that are protected by embankments in Zeeland. Following this example, exchange polders, in Dutch called wisselpolders, are proposed. Exchange polders make use of natural sedimentation processes to create a

buffer Buffer may refer to: Science * Buffer gas, an inert or nonflammable gas * Buffer solution, a solution used to prevent changes in pH * Lysis buffer, in cell biology * Metal ion buffer * Mineral redox buffer, in geology Technology and engineeri ...

of elevated land along the estuary, protecting the land behind the dikes against flooding. Exchange polders can be created by breaching the seaside embankment to allow tidal water to flow into the embanked area. A second embankment on the other side of the polder stops the tidal water from flowing further land inwards. The area between the embankments would be reconnected to the Western Scheldt and should therefore gradually

silt Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension (chemistry), suspension with water. Silt usually ...

up as the tidal water slows down. Exchange polders have not been implemented yet, because the plan has been critiqued by local farmers. They question the idea of giving land back to nature as there is already a shortage of space in The Netherlands, and are afraid of increased salinisation in the area.

Creation of low energy aquatic conditions

Some sedimentation enhancing strategies focus specifically on creating low energy conditions in shallow water. Sediment deposition occurs when the water flow slows down, as the water no longer has the

to carry heavier sediment particles and so they sink. Examples of strategies that stimulate low energy conditions are semi-permeable structures made of materials such as

wood Wood is a structural tissue/material found as xylem in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulosic fibers that are strong in tension and embedded in a matrix of lignin t ...

twig A twig is a thin, often short, branch of a tree or Bush (plant), bush. The buds on the twig are an important diagnostic characteristic, as are the abscission scars where the leaves have fallen away. The color, texture, and patterning of the t ...

s and brushwood. WW_Eems-Dollard

Ems-Dollard estuary, the Netherlands and Germany

The Ems-Dollard estuary is located on the border between

and

Germany Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...

and has a high

concentration In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', '' molar concentration'', '' number concentration'', ...

. However, the silt cannot settle on the delta plains due to flood control

levee A levee ( or ), dike (American English), dyke (British English; see American and British English spelling differences#Miscellaneous spelling differences, spelling differences), embankment, floodbank, or stop bank is an elevated ridge, natural ...

s that disconnect the land from the water. Additionally, channels in the area have been widened and deepened over time for

, increasing the strength of the tidal inland flood

current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (hydr ...

and weakening the ebb current back to the

, resulting in a surplus of silt being transported from the sea into the

estuary An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime enviro ...

. Silt concentration in the Ems-Dollard estuary increased from 40 mg/L in 1954 to 80–100 mg/L currently, significantly reducing the

water quality Water quality refers to the chemical, physical, and biological characteristics of water based on the standards of its usage. It is most frequently used by reference to a set of standards against which compliance, generally achieved through tr ...

. The more silt water contains the more

turbid Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of both water clarity and wate ...

the water is, which reduces the amount of

light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...

that can penetrate the water and inhibits algae growth. Algae are

primary producers An autotroph is an organism that can convert abiotic sources of energy into energy stored in organic compounds, which can be used by other organisms. Autotrophs produce complex organic compounds (such as carbohydrates, fats, and proteins) us ...

: they use CO₂, water and light to produce

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

and food for other aquatic animals. Reduced algae growth therefore impacts oxygen and food availability for the entire food chain. Climate change climate change-induced sea-level rise may negatively impact

primary production In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through ...

and the

food chain A food chain is a linear network of links in a food web, often starting with an autotroph (such as grass or algae), also called a producer, and typically ending at an apex predator (such as grizzly bears or killer whales), detritivore (such as ...

, but may also drown the Ems-Dollard system, so pilot sedimentation projects are being executed in the estuary. The aim is to trap silt particles on kwelders, which are land areas covered with vegetation that lie outside of the embankments. This can be done by placing

willow Willows, also called sallows and osiers, of the genus ''Salix'', comprise around 350 species (plus numerous hybrids) of typically deciduous trees and shrubs, found primarily on moist soils in cold and temperate regions. Most species are known ...

groyne A groyne (in the U.S. groin) is a rigid aquatic structure built perpendicularly from an ocean shore (in coastal engineering) or a river bank, interrupting water flow and limiting the movement of sediment. It is usually made out of wood, concrete ...

s, wooden posts connected with branches, in the ground along the kwelder, slowing down the water and encouraging

sedimentation Sedimentation is the deposition of sediments. It takes place when particles in suspension settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to th ...

, which may eventually create new land. Another way in which silt sedimentation is stimulated in the Ems-Dollard estuary is by the construction of double dikes. The area in between the dikes is filled with water by a controlled

culvert A culvert is a structure that channels water past an obstacle or to a subterranean waterway. Typically embedded so as to be surrounded by soil, a culvert may be made from a pipe (fluid conveyance), pipe, reinforced concrete or other materia ...

, where silt can settle more easily due to low flow or stagnant water conditions. The settled silt can be used to make

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...

which is used to strengthen and raise dikes in the area.

Wulan delta, Indonesia

The Wulan delta is located in the Demak district, northern

Java Java is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea (a part of Pacific Ocean) to the north. With a population of 156.9 million people (including Madura) in mid 2024, proje ...

Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania, between the Indian Ocean, Indian and Pacific Ocean, Pacific oceans. Comprising over List of islands of Indonesia, 17,000 islands, including Sumatra, ...

. Northern Java's deltaic

shore A coast (coastline, shoreline, seashore) is the land next to the sea or the line that forms the boundary between the land and the ocean or a lake. Coasts are influenced by the topography of the surrounding landscape and by aquatic erosion, su ...

lines suffer from severe

coastal erosion Coastal erosion is the loss or displacement of land, or the long-term removal of sediment and rocks along the coastline due to the action of Wind wave, waves, Ocean current, currents, tides, wind-driven water, waterborne ice, or other impacts ...

. More than 3 kilometres of the Demak shoreline has already been lost to the sea. The main causes of coastal erosion are the conversion of

forests to

for coastal infrastructure, and

groundwater extraction Water extraction (also known as water withdrawal, water abstraction, and water intake) is the process of taking water from any source, either temporarily or permanently, for flood control or to obtain water for, for example, irrigation. The ex ...

causing land

subsidence Subsidence is a general term for downward vertical movement of the Earth's surface, which can be caused by both natural processes and human activities. Subsidence involves little or no horizontal movement, which distinguishes it from slope mov ...

. Mangrove restoration has been proposed as a strategy to halt coastal erosion in the district of Demak. Solely replanting mangroves in the area was not possible, because the

wave In physics, mathematics, engineering, and related fields, a wave is a propagating dynamic disturbance (change from List of types of equilibrium, equilibrium) of one or more quantities. ''Periodic waves'' oscillate repeatedly about an equilibrium ...

exposure, submersion time and sediment conditions were no longer optimal. Instead, a strategy similar to the

s in the Ems-Dollard estuary was implemented.

Semi-permeable Semipermeable membrane is a type of synthetic or biologic, polymeric membrane that allows certain molecules or ions to pass through it by osmosis. The rate of passage depends on the pressure, concentration, and temperature of the molecules or so ...

barriers were built along the Demak

coast A coast (coastline, shoreline, seashore) is the land next to the sea or the line that forms the boundary between the land and the ocean or a lake. Coasts are influenced by the topography of the surrounding landscape and by aquatic erosion, su ...

using local materials such as

bamboo Bamboos are a diverse group of mostly evergreen perennial plant, perennial flowering plants making up the subfamily (biology), subfamily Bambusoideae of the grass family Poaceae. Giant bamboos are the largest members of the grass family, in th ...

s and other brushwood. These structures let sea and river water pass through, dampen waves, capture sediment and create sheltered, low-energy conditions near the shoreline for sediment accretion. The main idea behind this strategy is that mangroves

seed In botany, a seed is a plant structure containing an embryo and stored nutrients in a protective coat called a ''testa''. More generally, the term "seed" means anything that can be Sowing, sown, which may include seed and husk or tuber. Seeds ...

s will colonise the area naturally when the shore bed level accretes high enough. Initially, the permeable structures captured considerable amounts of sediment raising bed levels behind the structures. Some locations were naturally recolonised by

s, in other locations mangroves were replanted. However, juvenile mangroves only survived in the best protected sedimentation basins. Elsewhere, they disappeared again after a few years because the bed level dropped below sea level again due to subsidence.

Wetland restoration

Coastal wetlands are

ecosystem An ecosystem (or ecological system) is a system formed by Organism, organisms in interaction with their Biophysical environment, environment. The Biotic material, biotic and abiotic components are linked together through nutrient cycles and en ...

s temporarily or permanently flooded by water. Wetland

vegetation Vegetation is an assemblage of plants and the ground cover they provide. It is a general term, without specific reference to particular Taxon, taxa, life forms, structure, Spatial ecology, spatial extent, or any other specific Botany, botanic ...

serves important functions: it attenuates incoming waves and encourages sediment deposition. The resulting rise in land elevation allows some wetlands to keep up with sea-level rise. Many wetlands have been converted to other land uses by constructing dikes, seawalls and embankments to prevent water encroachment. As a result, wetlands are disconnected from hydrological input and no longer receive sediment, which inhibits land raising and can result in land elevation loss. One strategy to restore wetlands is depolderisation, which entails breaching dikes and reconnecting wetlands to rivers,

estuaries An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environm ...

or the

, restore the natural

hydrology Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and drainage basin sustainability. A practitioner of hydrology is called a hydro ...

and land-building capacities of wetlands.

Biesbosch, the Netherlands

Depolderisation has occurred in a polder in the

Biesbosch De Biesbosch National Park is one of the largest national parks in the Netherlands and one of the last extensive areas of freshwater tidal wetlands in Northwestern Europe. The Biesbosch ('forest of sedges' or 'rushwoods') consists of a large ...

under the Dutch Room for the River program. The Biesbosch is a 9000-ha freshwater tidal wetland in the southwestern part of the Netherlands. Water and sediments were reintroduced to the Noordwaard, an agricultural polder in the Biesbosch, in 2008. The embankments were lowered by 2 meters to reconnect the Biesbosch wetlands with the Merwede river, a distributary of the lower

Rhine The Rhine ( ) is one of the List of rivers of Europe, major rivers in Europe. The river begins in the Swiss canton of Graubünden in the southeastern Swiss Alps. It forms part of the Swiss-Liechtenstein border, then part of the Austria–Swit ...

. This project aimed to allow flooding during peak discharges of the Rhine and

Meuse The Meuse or Maas is a major European river, rising in France and flowing through Belgium and the Netherlands before draining into the North Sea from the Rhine–Meuse–Scheldt delta. It has a total length of . History From 1301, the upper ...

rivers, with the restored tidal and flood dynamics encouraging

ecosystem restoration Ecological restoration, or ecosystem restoration, is the process of assisting the recovery of an ecosystem that has been degraded, damaged, destroyed or transformed. It is distinct from conservation in that it attempts to retroactively repair ...

. The results of this restoration effort were that the Biesbosch area trapped approximately 46% of the incoming sediment, and the average

aggradation Aggradation (or alluviation) is the term used in geology for the increase in land elevation, typically in a river system, due to the deposition of sediment. Aggradation occurs in areas in which the supply of sediment is greater than the amount o ...

rate was 5.1 mm per year. In February 2020, the Noordwaard polder flooded for the first time due to high water levels in the rivers caused by a

and

spring tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables ...

Sacramento-San Joaquin delta, California, USA

Wetlands in the Sacramento-San Joaquin delta are rapidly losing elevation. Under natural conditions, wetlands in the delta were frequently flooded. The soil was waterlogged and

anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: *Adhesive#Anaerobic, Anaerobic ad ...

, and under these conditions

organic carbon Total organic carbon (TOC) is an analytical parameter representing the concentration of organic carbon in a sample. TOC determinations are made in a variety of application areas. For example, TOC may be used as a non-specific indicator of wa ...

accumulates faster than it decomposes, resulting in soil accumulation. However, wetlands in the Sacramento-San Joaquin delta have been drained for agricultural purposes, so the soil is now situated at or above the

water table The water table is the upper surface of the phreatic zone or zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with groundwater, which may be fresh, saline, or brackish, depending on the loc ...

where it can

oxidize Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

and decompose quickly resulting in a loss of elevation. Many former wetlands in the area are now more than 6 meters below mean sea-level and

rates of up to 5 cm per year have been found. Shallow flooding of land is a strategy used to reduce subsidence and restore wetlands in the delta. Adding a layer of water to the soil restores anaerobic conditions, which results in the accretion of new

peat Peat is an accumulation of partially Decomposition, decayed vegetation or organic matter. It is unique to natural areas called peatlands, bogs, mires, Moorland, moors, or muskegs. ''Sphagnum'' moss, also called peat moss, is one of the most ...

and increases surface elevation. Mean rates of land surface elevation gain in the study wetlands were 4 cm per year.

Mangrove restoration

Mangrove A mangrove is a shrub or tree that grows mainly in coastal saline water, saline or brackish water. Mangroves grow in an equatorial climate, typically along coastlines and tidal rivers. They have particular adaptations to take in extra oxygen a ...

s provide a wide range of

s, such as

habitat In ecology, habitat refers to the array of resources, biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species' habitat can be seen as the physical manifestation of its ...

for aquatic species,

carbon sequestration Carbon sequestration is the process of storing carbon in a carbon pool. It plays a crucial role in Climate change mitigation, limiting climate change by reducing the amount of Carbon dioxide in Earth's atmosphere, carbon dioxide in the atmosphe ...

, and their

root systems In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the surfa ...

reduce the impact of incoming waves and capture sediment resulting in land elevation gain. Mangroves also play a role in mitigating the effects of

and

extreme weather Extreme weather includes unexpected, unusual, severe weather, severe, or unseasonal weather; weather at the extremes of the historical distribution—the range that has been seen in the past. Extreme events are based on a location's recorded weat ...

events. For all these reasons, mangrove forests are one of the most powerful

nature-based solutions Nature-based solutions (or nature-based systems, and abbreviated as NBS or NbS) describe the development and sustainable use, use of nature (biodiversity) and natural processes to address diverse social issue, socio-environmental issues. These is ...

to climate change. However, almost 70 percent of mangroves are currently lost or degraded, and they are still rapidly deteriorating. Mangrove forests can be restored in several ways, for example through providing space for expansion or by replanting. If relieved from human pressures, mangrove species can quickly recolonise degraded areas, depending on the availability of

s and the ability of seeds to access degraded areas. In areas where seeds cannot easily migrate, replanting is the best option. Mangrove restoration efforts have taken place in the Mahakam delta,

. From the 1990s onwards, the mangrove forests in the delta have been under intense pressure from

: 60-75% of mangrove forests in the Mahakam delta have been converted into shrimp ponds. Since 2000, private oil and gas companies have funded various mangrove replanting efforts. From 2001 to 2005, Total E&P Indonesia planted over 3.5 million trees in the delta, covering an area of 646 ha. Total E&P invest in mangrove rehabilitation for various reasons, for instance to reduce

erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...

and ecosystem degradation which is seen as a threat to gas operations, and because pipelines installed to transport oil and gas caused mangrove clearing. Additionally, between 2002 and 2007 the Department of Forestry of the Indonesian government also planted 819 ha of mangrove forests. Restoration programs funded by the government and the oil and gas industry focus on replanting mangroves in abandoned shrimp ponds and encouraging combined mangrove-shrimp aquaculture, also called silvofishery. Mangroves can recover rapidly in the area if the physical environment of the delta is not destroyed: every year, hundreds of hectares of cleared areas in the Mahakam delta are naturally recolonised by mangrove vegetation, causing accretion. There is also evidence of sedimentation in restored mangroves in Vietnam. Danube_delta_satelite

Construction of channel networks

The construction of dams reduces the sediment load in rivers downstream. Levees and embankments also inhibit the deposition of sediment on the delta plain, resulting in the loss of land elevation. Research has shown that cutting and dredging of shallow, narrow

channels Channel, channels, channeling, etc., may refer to: Geography * Channel (geography), a landform consisting of the outline (banks) of the path of a narrow body of water. Australia * Channel Country, region of outback Australia in Queensland and pa ...

on the delta plain can be an effective strategy to increase the input of freshwater and sediments to

floodplain A floodplain or flood plain or bottomlands is an area of land adjacent to a river. Floodplains stretch from the banks of a river channel to the base of the enclosing valley, and experience flooding during periods of high Discharge (hydrolog ...

s and

lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'') an ...

s in deltas. Shallow, narrow channels have been dug in the

Danube delta The Danube Delta (, ; , ) is the second largest river delta in Europe, after the Volga Delta, and is the best preserved on the continent. Occurring where the Danube, Danube River empties into the Black Sea, most of the Danube Delta lies in Romania ...

(

Romania Romania is a country located at the crossroads of Central Europe, Central, Eastern Europe, Eastern and Southeast Europe. It borders Ukraine to the north and east, Hungary to the west, Serbia to the southwest, Bulgaria to the south, Moldova to ...

). The main reason for digging the channels was that

in the Danube delta were negatively impacted by the limited freshwater delivery to the deltaic

s and

s. The construction of the channel network in the Danube delta almost tripled the water influx toward the delta plain. However, at the same time sediment delivery in the lower Danube river reduced due to the construction of dams upstream. Interestingly, sediment deposition on the delta plain did not decrease after

constructions. It has been estimated that the average sediment flux in the Danube delta increased from 0.07 g/cm² under natural conditions to 0.09-0.12 g/cm² after the construction of shallow, narrow channels, which could mean a

rate of 0.5-0.8 mm per year. This suggests that the artificial channels function as sediment traps that can assist in preventing delta drowning due to sea-level rise. However,

along the Danube coast has increased since the construction of channels. Similar results have been found in the Ebro delta: channels dug there for

rice cultivation The history of rice cultivation is an interdisciplinary subject that studies archaeological and documentary evidence to explain how rice was first domesticated and cultivated by humans, the spread of cultivation to different regions of the planet ...

deliver sediments to the delta plain resulting in land accretion rates that may be fast enough to keep up with

Breaching levees

Flood A flood is an overflow of water (list of non-water floods, or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are of significant con ...

ing is a vital source of

fresh water Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salt (chemistry), salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include ...

and sediment supply to

s, important for land elevation maintenance, soil fertilization, and the support of healthy wetland ecosystems. Levees prevent

s, creating

s that no longer receive water or sediment and therefore lose elevation. Additionally, due to the construction of polders in upstream parts of deltas, floodwater can no longer be stored on upstream floodplains, causing larger floods downstream. A strategy to restore the input of freshwater and sediment to floodplains is intentionally breaching or significantly lowering levees to allow flooding to occur during peak discharges. Tractor_Mekong_Delta_Vietnam

Plans are being made to lower and breach levees in the upper

Mekong delta The Mekong Delta ( or simply ), also known as the Western Region () or South-western region (), is the list of regions of Vietnam, region in southwestern Vietnam where the Mekong, Mekong River River delta, approaches and empties into the sea th ...

Vietnam Vietnam, officially the Socialist Republic of Vietnam (SRV), is a country at the eastern edge of mainland Southeast Asia, with an area of about and a population of over 100 million, making it the world's List of countries and depende ...

near the border with

Cambodia Cambodia, officially the Kingdom of Cambodia, is a country in Southeast Asia on the Mainland Southeast Asia, Indochinese Peninsula. It is bordered by Thailand to the northwest, Laos to the north, and Vietnam to the east, and has a coastline ...

, an area that would normally flood during peak water discharge season from July to December. However, in many areas high levees have been constructed to protect against flooding year-round. With this full flood protection, farmers in the Mekong delta can produce more rice crops per year compared to a system with lower or no levees. However, preventing floodwater and sediment from entering the Vietnamese floodplains resulted in increased peak river discharges and flood risks downstream, reduced flood retention capacity of floodplains, accumulation of

agrochemical An agrochemical or agrichemical, a contraction of ''agricultural chemical'', is a chemical product used in industrial agriculture. Agrichemical typically refers to biocides (pesticides including insecticides, herbicides, fungicides and nematicide ...

s in the soil, and reduced or eliminated sediment deposition contributing to accelerated land elevation loss. To ameliorate these negative impacts, steps are being taken in the upper Mekong delta to lower levees. This would allow flood water to enter the plains only during peak season. During the rest of the year, the lower embankments provide sufficient protection for farmers to cultivate their lands.

Tidal replicate method

A novel eco-engineering solution to preserve existing

intertidal The intertidal zone or foreshore is the area above water level at low tide and underwater at high tide; in other words, it is the part of the littoral zone within the tidal range. This area can include several types of habitats with various sp ...

wetlands from

has been implemented in a coastal

Kooragang Island Kooragang () is the largest suburb of the city of Newcastle, in the Hunter Region of New South Wales, Australia. Dominated by Kooragang Island, the eastern part of the suburb is primarily industrial, while the western part of the suburb consists ...

in Hunter Wetlands National Park,

Newcastle Newcastle usually refers to: *Newcastle upon Tyne, a city and metropolitan borough in Tyne and Wear, England, United Kingdom *Newcastle-under-Lyme, a town in Staffordshire, England, United Kingdom *Newcastle, New South Wales, a metropolitan area ...

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

. Due to the construction of

s and internal

drainage Drainage is the natural or artificial removal of a surface's water and sub-surface water from an area with excess water. The internal drainage of most agricultural soils can prevent severe waterlogging (anaerobic conditions that harm root gro ...

in the area during the 20th century, tidal water was prevented from entering the wetlands. Although tidal flows were already reintroduced in the early 2000s, the site's

and

topography Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the landforms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sci ...

favoured the expansion of

s. This created a situation in which mangroves expanded rapidly at the expense of other

saltmarsh A salt marsh, saltmarsh or salting, also known as a coastal salt marsh or a tidal marsh, is a coastal ecosystem in the upper coastal intertidal zone between land and open saltwater or brackish water that is regularly flooded by the tides. It ...

, resulting in a deeper tidal inundation similar to that experienced with sea-level rise. To recreate desired natural tidal conditions, a strategy called the tidal replicate method was applied. The tidal replicate method creates an artificial tidal regime through an automated tidal control system which the authors call SmartGates. The gates manipulate the tidal flow reaching the

area and mimic the tidal conditions necessary to recruit and establish wetland vegetation. The site, which would have been inundated under natural conditions, has effectively re-established saltmarsh vegetation following the implementation of the novel method. Although the primary aim of this strategy is restoring saltmarsh vegetation, vegetation captures sediment and can therefore enhance natural

processes.

References

{{reflist Sedimentary basins Environmental management schemes

Benefits of sedimentation enhancing strategies

Limitations of sedimentation enhancing strategies

Types of sedimentation enhancing strategies

River diversions

Mississippi River delta, Louisiana, USA

Canal del Dique, Colombia

Tidal flooding of previously enclosed areas

Ganges-Brahmaputra-Meghna delta, Bangladesh

Western Scheldt, the Netherlands

Creation of low energy aquatic conditions

Ems-Dollard estuary, the Netherlands and Germany

Wulan delta, Indonesia

Wetland restoration

Biesbosch, the Netherlands

Sacramento-San Joaquin delta, California, USA

Mangrove restoration

Construction of channel networks

Breaching levees

Tidal replicate method

See also

References