Sedimentation enhancing strategy
Sedimentation enhancing strategies are environmental management projects aiming to restore and facilitate land-building processes in deltas. Sediment 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. Sedimentation enhancing strategies aim to increase sedimentation on the delta plain primarily by restoring the exchange of water and sediments between rivers and low-lying delta plains. Sedimentation enhancing strategies can be applied to encourage land elevation 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 of deltas.
Benefits of sedimentation enhancing strategies
When compared to conventional flood protection infrastructure such as embankments and seawalls, sedimentation enhancing strategies provide various benefits. Firstly, flood protection structures can exacerbate environmental problems in deltas: land reclamation and levee construction result in a loss of water storage area during peak river discharges, which may cause an increased risk of flooding 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 flood risks while simultaneously restoring ecosystems, enhancing production and cultural ecosystem services.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. Particularly considering uncertain future scenarios due to climate change, 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
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 regions in the world. Land-use change 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, 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 wetlands. 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 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.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 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 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 to build land.Canal del Dique, Colombia
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 flooding of previously enclosed areas
of polders entails breaching dikes and allowing tidal water to flow into an embanked area during high tide. 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. We distinguish between tidal river management, implemented in the Ganges-Brahmaputra-Meghna delta, Bangladesh, and exchange polders, implemented in the Rhine-Meuse delta, the Netherlands.Ganges-Brahmaputra-Meghna delta, Bangladesh
Polders, known as beels in Bengal, have been constructed in the Ganges delta since the 1960s. 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. Tidal river management 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. 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 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 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.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. 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.In the Western Scheldt, 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 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. 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. 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 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.