Sedimentation enhancing strategies are crucial to keep deltas from drowning and their populations safe. Often strategies are implemented on a local scale, but it remains unclear whether they can be upscaled and applied in deltas more broadly. Water, Climate and Future Deltas hub researchers compiled a database exploring the efficiency and sustainability of nineteen different strategies in various deltas worldwide, including tidal river management in the Southwestern Ganges-Brahmaputra-Meghna delta.
Comparing different strategies
Developing a database
"Interestingly", Mandy Paauw, PhD researcher on socio-spatial inequalities in flood risk management, explains, “no framework exists to compare the efficiency and sustainability of sedimentation enhancing strategies”. Together with Eline Sieben, Master student in Marine Sciences, and other researchers at Utrecht University, they developed a comprehensive database of sedimentation enhancing strategies in deltas worldwide and a tool for decision makers to assess the applicability of different sedimentation strategies for certain deltas (Project: Sedimentation enhancing strategies for river deltas).
Functioning and effectiveness
Using the database, they assessed the functioning and effectiveness of nineteen sedimentation enhancing strategies in various deltas worldwide. It was found that 84% of strategies are capable of offsetting even the most extreme of sea-level rise scenarios. They also found that the success of the strategies heavily depends on socio-economic, legal and governance systems, including decision making processes, stakeholder engagement, legal restrictions, land ownership and environmental impact.
For sedimentation enhancing strategies to be sustainable in the long term, multiple strategies should be implemented at various locations
Due to the varying timescales, costs and land types created by the different strategies, it is increasingly likely that combinations of multiple, small-scale strategies will be required to offset land loss in deltas. Jana Cox therefore suggests: "For sedimentation enhancing strategies to be sustainable in the long term, multiple strategies should be implemented at various locations in deltas". Strategies also take time to be planned and implemented, and in most cases, it takes several years before they become effective, so “Deltas need to urgently consider these strategies for long term sustainable management”.
Tidal river management in the Ganges-Brahmaputra-Meghna delta
Reintroducing sediment in low-lying polders
The Ganges-Brahmaputra-Meghna delta in Bangladesh is threatened by future sea-level rise, land subsidence and a decrease in sediment delivery (Storyline: The future of the Ganges-Brahmaputra-Meghna delta). Dikes, protecting polders from flooding in the Southwestern part of the delta, have caused silting up of the river channels, whilst the polders suffer from water logging during the monsoon. Tidal river management is a strategy currently employed in the Southwestern part of the delta to reintroduce sediment in the lowest parts of embanked polders through controlled flooding. By applying this mechanism during several consecutive years, the polder surface rises to offset sea-level rise and land subsidence, and the sediment no longer clogs the river channels.
Factors that determine the potential success of tidal river management implementation are physical boundary conditions, trapping efficiency and operation scheme of inundation. PhD researcher Feroz Islam aims to identify optimum polder flood rotation schemes acceptable to all stakeholders by studying these factors. He explains, “Sediment deposition depends on the number of inlets and shows seasonal variability, with 20-30% more deposition during the pre-monsoon period. The potential of controlled flooding is highest in the tide-dominated coastal region, where sediment accumulation can be up to 28 times higher than in the more upstream river-dominated areas”.
Until now, proper implementation of tidal river management is hindered by conflicts between stakeholders and the top-down approach of government institutions
There are also socio-economic factors, such as livelihood security during tidal river management operation, that influence tidal river management implementation and therefore need to be considered. Frank van Laerhoven, associate professor of environmental governance, explains, “Until now, proper implementation of tidal river management is hindered by conflicts between stakeholders with different interests and by the top-down approach of government agencies, that often do not engage with local stakeholders, who might lose their land or livelihood for several years”. Our joint project with Bangladesh partners reveals that controlled flooding is for many locals not an acceptable option. This is due to the current lack of equity in terms of decision making and sharing cost and benefits among different stakeholders. For many local farmers, the long-term benefits are not clear, whereas they are directly affected by implementation. ‘Living with water’ is for many not a desired or targeted strategy but is only adopted when there are no other options.
The Living Polders project, led by Frank van Laerhoven, investigates the physical and societal requirements for successful application of tidal river management. In addition to the physical effects, it is important that the different stakeholder interests are made explicit in decision making. Therefore, a decision-support tool is being developed which will help stakeholders make informed decisions about tidal river management. The tool integrates governance guidelines that target the shaping of institutional conditions for effective governance with technical guidelines on the potential, design and operation scheme of tidal river management.