Retaining sediment

Strategies that retain sediment, also called sedimentation enhancing strategies, aim to restore and facilitate land-building processes in deltas. They encourage land-elevation gain primarily by restoring the exchange of water and sediment between rivers, estuaries or beaches and (low-lying) delta plains. The Water, Climate and Future Deltas hub assesses the impact of these strategies on sediment dynamics and budget of the Rhine-Meuse delta. 

Room for the River

Room for the River was a nationwide project adopted by the Dutch government from 2005-2015 as a reaction to severe floods in 1993 and 1995. Various measures were taken along the Rhine, Meuse and their tributary rivers to prevent high water levels, improve landscape quality of river areas and restore ecological value. The hub carried out research in floodplains and the Biesbosch to evaluate the Room for the River project.

Figure of typical landscaping measures implemented in the Room for the River project
Selection of measures taken in the Room for the River project. Source: Straatsma et al. (2019), reprinted with permission

Increasing sediment flux to floodplains

Locally increased sediment trapping

Measures taken in the Room for the River project, such as the lowering or removal of embankments, floodplain lowering and constructions of side channels, increased the supply of water and suspended sediments to the floodplains. These measures, together with more frequent floods due to climate change, locally increased the trapping efficiency of floodplains. However, higher flow velocities in the floodplains due to the Room for the River measures also reduced sediment settling.

These effects combined result in a very small, almost negligible net increase of sedimentation

Unaltered floodplains are inundated less often because the measures cause an overall reduction in water levels, thereby decreasing sediment deposition in these unaltered floodplains. Marcel van der Perk, associate professor engaged in research on floodplains, further elucidates, "Recent modelling work shows that these effects combined result in a very small, almost negligible net increase of sedimentation in flood plains after the Room for the River project".

Depoldering in the Biesbosch

Fresh-water delta

The Biesbosch is a small fresh-water inland delta, formed by the Waal and Meuse rivers after 15th century storm surges created a large ingression from the sea into this area. During the Room for the River project it was partly re-opened for river water from the Waal to reduce flood water levels. The new de-poldered area – the Kleine Noordwaard – is an excellent 700-ha test area to study re-sedimentation in a tidal freshwater wetland. The Kleine Noordwaard currently traps ~37.3 thousand m-3 per year, which corresponds to an area-averaged deposition rate of 6.4 mm per year, with a trapping efficiency of 12%. Most of the sediment ends up in older channels within the former polder.

Map of impact of climate change on sedimentation/erosion patterns in the de-poldered area de Kleine Noordwaard
Impact of climate change on sedimentation/erosion patterns in the de-poldered area de Kleine Noordwaard. Source: adapted from Verschelling et al. (2018), basemap:

Sediment trapping

Eveline van der Deijl and Eelco Verschelling, who studied the Biesbosch during their PhD projects, demonstrated how sediment trapping in the area is determined by the interplay of river discharge, tide and wind, driving sediment and water flow through the channel-wetland. They explain, "The inlet size strongly controls sedimentation, as it determines the amount of incoming water and sediment, their residence time within the area and water flow velocities occurring within the polder. If the water flow velocity is too high or if the wind causes waves to develop, sediment will not settle and just move out of the polder during ebb tide".

Counteracting sea-level rise?

The low sediment concentration in the feeding Waal river and resuspension of material result in low deposition rates of about 6 mm per year on the intertidal flats. Whether this accumulation is sufficient to counteract sea-level rise in the future strongly depends on the future sediment load in the feeding river, and on the tidal range which determines the trapping efficiency of the polder. 

More storylines about Room for the River

Exchange polders

Using natural sedimentation processes

Exchange polders, also called “transitional polders” or “wisselpolders”, use natural sedimentation processes to raise land in between two dikes (Storyline: Double dike). A temporary opening is made in the most seaward dike to allow tidal water to flow into the polder, while the second dike prevents the water from flowing further inland. By reconnecting polders to the tidal influx of water and sediment, they gradually silt up. In this way, exchange polders create an area of elevated land along the estuary, providing extra protection for the hinterland while at the same time creating new land for agriculture and nature.

Exchange polders are a no-regret investment

Tjeerd Bouma, professor of bio-geomorphological ecology of estuaries, deltas and coasts, explains, “Due to the high return on investment and the addition of new economic and natural functions to the landscape, exchange polders are a no-regret investment”.

Conflict with traditional flood protection

However, flood protection and land reclamation are deeply rooted in Dutch tradition. Giving land back to nature goes against this tradition. Moreover, local farmers argue that transitional polders may increase soil salinization, which would negatively impact agricultural productivity. 

Figure of a conceptual comparison between a traditional dike and an ecosystem-based solution with a transitional polder between two dikes
Conceptual comparison between a traditional dike and an ecosystem-based solution with an exchange polder between two dikes. Source: adapted from Van Belzen et al. (2021)

Blow-outs in foredunes

Dynamic, climate-resilient foredunes

Management practices, such as planting grasses and building sand fences, have stabilised many foredunes. Consequently, the more landward back dunes can no longer naturally grow with sea-level rise and their habitat diversity is degrading. To make dune coasts more dynamic and climate resilient, openings comparable to natural blowouts are increasingly constructed in the foredunes to facilitate wind-driven (aeolian) transport of beach sand onto the back dunes.

Blowouts function as effective corridors for aeolian transport into the back dunes

Long-term evolution of blowouts

Gerben Ruessink, professor of wave-dominated coastal morphodynamics, together with colleagues at Utrecht University has examined the development of these human-made blowouts to increase our understanding of the evolution of blowouts, especially on time scale of years and longer (Project: Ecological restoration of coastal dunes). He explains, "Blowouts can strongly increase dune dynamics and function as highly effective corridors for aeolian transport into the back dunes". Before the start of the largest experiment in the Netherlands, all wind-blown beach sand was deposited on the seaward side of the foredune. After the construction of the openings, 75% of the sand blown from the beach deposited landward of the foredune, while prior to the construction the back dunes gained no sand at all. Satellite imagery is additionally used to examine how the interaction between the deposited sand and the vegetation shapes the long-term evolution of human-made and natural blowout systems.

Photo of a blowout in a foredune on the barrier island Terschelling
Blowout in a foredune on the barrier island Terschelling. Credits: Gerben Ruessink

Gaps in dunes require a paradigm shift

Although openings in the foredunes are sufficiently high to prevent the sea from entering the hinterland during a severe storm, creating openings in dunes requires a paradigm shift. There can be resistance from local people who are not necessarily convinced that gaps in dunes, which are expected to provide flood protection, are desirable.