Mississippi River delta, Louisiana, USA

Over the 20th century the Mississippi delta lost approximately 25% of its land.^[10][12] Currently, land is disappearing at a rate of almost 11,000 acres per year.^[13] To combat these rapid rates of land loss, 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.^[14][10] 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 of approximately 130 km² within three months.^[10] The currently planned Mid- and 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 to build land.^[15]

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.^[16] The construction of this channel increased the flow of water and sediment into the Bay of Cartagena.^[17] Sediment deposition in the canal, connected lakes and swamps, 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 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.^[17][16]

Ganges-Brahmaputra-Meghna delta, Bangladesh

Polders, known as beels in Bengal, have been constructed in Bangladesh since the 1960s.^[18] The embankments provide flood protection and initially increased agricultural production. However, together with a decrease in water supply due to upstream dam construction, the embankments caused an increase in riverbed sedimentation and congestion, hampering water drainage and navigation. Another issue in Bangladesh is waterlogging, which negatively impacts the agricultural productivity of the region.^[18] 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 measure due to its potential to raise land through sedimentation and allow residents to cope with changing environmental conditions. TRM involves temporarily breaching levees around low-lying polders to allow river water to flow in.^[18][19] When the water flows into embanked areas during high tide, water flow velocities reduce and sediments deposit.^[21][22] During low tide, water flow velocity increases again as the water is pulled back through the channels toward the sea, causing deposited riverbed sediment to erode. This increases the drainage capacity and navigability of the channels.^[18][23] TRM has been implemented in five beels in the south of the Ganges-Brahmaputra-Meghna delta. The implementation of TRM by local people (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.^[18] 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.^[24]

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 storms and extreme weather conditions, amongst which the flood disaster of 1953 which led to the construction of the Delta Works.^[20] The construction of dams, 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.^[20]

In the Western Scheldt, a strategy similar to TRM has been proposed to naturally raise the land.^[25] During high tide, the Western Scheldt delivers sediment to the areas outside of the embankments. As a result, these areas naturally rise with water levels.^[26] 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.^[25] Following this example, exchange polders, in Dutch called wisselpolders, are proposed. Exchange polders make use of natural sedimentation processes to create a buffer of elevated land along the estuary, protecting the land behind the dikes against flooding.^[20] 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.^[26] The area between the embankments would be reconnected to the Western Scheldt and should therefore gradually silt up as the tidal water slows down.^[25] 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.^[27]

Ems-Dollard estuary, the Netherlands and Germany

The Ems-Dollard estuary is located on the border between the Netherlands and Germany and has a high silt concentration.^[29] However, the silt cannot settle on the delta plains due to flood control levees that disconnect the land from the water. Additionally, channels in the area have been widened and deepened over time for navigation, increasing the strength of the tidal inland flood current and weakening the ebb current back to the sea, resulting in a surplus of silt being transported from the sea into the estuary.^[29][30]

Silt concentration in the Ems-Dollard estuary increased from 40 mg/L in 1954 to 80–100 mg/L currently,^[29] significantly reducing the water quality. The more silt water contains the more turbid the water is, which reduces the amount of light that can penetrate the water and inhibits algae growth. Algae are primary producers: they use CO₂, water and light to produce oxygen and food for other aquatic animals. Reduced algae growth therefore impacts oxygen and food availability for the entire food chain.^[29][30] Climate change climate change-induced sea-level rise may negatively impact primary production and the food chain, 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 groynes, wooden posts connected with branches, in the ground along the kwelder, slowing down the water and encouraging sedimentation, which may eventually create new land.^[31]

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, where silt can settle more easily due to low flow or stagnant water conditions. The settled silt can be used to make clay which is used to strengthen and raise dikes in the area.^[32]

Wulan delta, Indonesia

The Wulan delta is located in the Demak district, northern Java, Indonesia. Northern Java's deltaic shorelines suffer from severe coastal erosion.^[33] More than 3 kilometres of the Demak shoreline has already been lost to the sea.^[34] The main causes of coastal erosion are the conversion of mangrove forests to aquaculture, land reclamation for coastal infrastructure, and groundwater extraction causing land subsidence.^[34][35] 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 exposure, submersion time and sediment conditions were no longer optimal.^[34] Instead, a strategy similar to the willow groynes in the Ems-Dollard estuary was implemented. Semi-permeable barriers were built along the Demak coast using local materials such as bamboo, twigs and other brushwood.^[33] 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 seeds will colonise the area naturally when the shore bed level accretes high enough.^[34]

Initially, the permeable structures captured considerable amounts of sediment raising bed levels behind the structures. Some locations were naturally recolonised by mangroves, 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.^[36]

Biesbosch, the Netherlands

Depolderisation has occurred in a polder in the Biesbosch 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.^[39] The embankments were lowered by 2 meters to reconnect the Biesbosch wetlands with the Merwede river, a distributary of the lower Rhine. This project aimed to allow flooding during peak discharges of the Rhine and Meuse rivers, with the restored tidal and flood dynamics encouraging ecosystem restoration.^[40][41] The results of this restoration effort were that the Biesbosch area trapped approximately 46% of the incoming sediment, and the average aggradation rate was 5.1 mm per year.^[42][43] In February 2020, the Noordwaard polder flooded for the first time due to high water levels in the rivers caused by a storm and spring tide.^[44]

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, and under these conditions organic carbon 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 where it can oxidize and decompose quickly resulting in a loss of elevation.^[45] Many former wetlands in the area are now more than 6 meters below mean sea-level and subsidence rates of up to 5 cm per year have been found.^[46][47] 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 and increases surface elevation. Mean rates of land surface elevation gain in the study wetlands were 4 cm per year.^[47]