Saved by sediment: a review of sedimentation enhancing strategies for river deltas

Deltas worldwide are at risk of elevation loss and drowning due to relative sea-level rise. Sediment is the primary building block of river deltas and can maintain elevation and area when used effectively. Management strategies which restore or enhance natural sedimentation processes on delta plains (sedimentation enhancing strategies, SES) are now being pursued in many deltas, but have not yet been evaluated in a global context. They also tend to be locally implemented events by engineering companies or government organisations with information limited to reports, often in the native language of the country, and absent from scientific discussion of sustainable deltas. We compared 19 existing and planned SES, including sediment diversions in the Mississippi delta and tidal river management in Bangladesh, reviewing their physical characteristics, funding, governance arrangements, stakeholder engagement, process of implementation, environmental impact, land use change, and potential for upscaling. We found that strategies can be effective at various scales, from ~0.05 km2 - 500 km2, which means there are demonstrated options for all river deltas. However, they range in costs from minimal (thousands of US dollars) to multi-million or billion dollar projects. Cheaper strategies are limited to short term impacts and small spatial scales, while more expensive strategies can have longer lifetimes. Most strategies create wetlands and flood storage. Some create opportunities for agriculture, aquaculture, housing, or recreational land use. 79% of strategies are capable of outpacing the highest projected rates of global sea-level rise. Combinations of SES will likely be more effective and sustainable for maintaining elevation in river deltas. Strategies can also take several years to be planned and implemented, and take further time to become effective, thus rapid implementation is required for the sustainable management of deltas.