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
wetlands. Diversion structures can range from simple gates to more complex siphon or pump systems.
Mississippi River Delta, Louisiana, USA Over the 20th century the
Mississippi Delta lost approximately 25% of its land. Currently, land is disappearing at a rate of almost 11,000
acres per year. 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.
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. The construction of this channel increased the flow of water and sediment into the Bay of Cartagena. 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. Tidal water can bring in large concentrations of sediment from the
sea 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. 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. 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.
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. 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. 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.
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
energy 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,
twigs and brushwood.
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. 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. Silt concentration in the Ems-Dollard estuary increased from 40 mg/L in 1954 to 80–100 mg/L currently, 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.
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. 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
mangrove forests to
aquaculture,
land reclamation for coastal infrastructure, and
groundwater extraction causing land
subsidence. 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.
Wetland restoration Coastal wetlands are
ecosystems temporarily or permanently flooded by water. Wetland
vegetation 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 or the
sea, restore the natural
hydrology and land-building capacities of wetlands. 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. 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. 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.
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. 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. 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. For all these reasons, mangrove forests are one of the most powerful
nature-based solutions to climate change. However, almost 70 percent of mangroves are currently lost or degraded, and they are still rapidly deteriorating. Mangrove restoration efforts have taken place in the
Mahakam delta,
Indonesia. From the 1990s onwards, the mangrove forests in the delta have been under intense pressure from
aquaculture: 60-75% of mangrove forests in the Mahakam delta have been converted into
shrimp ponds. Total E&P invest in mangrove rehabilitation for various reasons, for instance to reduce
erosion and ecosystem degradation causing accretion. There is also evidence of sedimentation in restored mangroves in Vietnam.
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 on the delta plain can be an effective strategy to increase the input of freshwater and sediments to
floodplains,
lakes and
lagoons in deltas. 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.
Breaching levees Flooding is a vital source of
fresh water and sediment supply to
floodplains, important for land elevation maintenance,
soil fertilization, and the support of healthy wetland ecosystems. Levees prevent
floods, creating
polders 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. 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. Due to the construction of
levees and internal
drainage 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
hydrology and
topography favoured the expansion of
mangroves. This created a situation in which mangroves expanded rapidly at the expense of other
saltmarsh vegetation, resulting in a deeper tidal inundation similar to that experienced with sea-level rise. 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
wetland 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
sedimentation processes. == See also ==