By Bas van Maren, EOS
Land has been reclaimed for many centuries, and with the present-day demand for land, this process will continue in the future. The impact of such land reclamations has, up to now, been evaluated on a case-by-case basis, while studies integrating a wider range of land reclamation impacts is missing. Insights into the way tides interact with basin topography and the complex feedback mechanisms associated with fine-grained sediments is crucial to understanding the long-term response of coastal systems to a land reclamation.
A new article in Reviews of Geophysics synthesizes earlier findings on the effect of land reclamations on the coastal environment. Here, the lead author gives an overview of land reclamations, their impacts on coastal environments, and challenges for future research efforts.
Why and where is land reclaimed?
Fertile and low‐lying coastal landscapes are often densely populated because of their food supply (agriculture, aquaculture, and fisheries) and easy navigability (shipping lanes). At present 10% of the global population lives in Low Elevation Coastal Zones (LECZ’s; less than 10 meters above Mean Sea Level) and the population growth in LECZ’s is larger than the global average, especially in river deltas.
As this population pressure drives a continuous need for land, much of the low-lying land that was regularly inundated by the sea has been converted to agricultural land or urban environments. Especially the deltas of muddy rivers are suitable for reclamation, because of their shallow coastal waters and high sedimentation rates.
In our study we investigate how tide-influenced, typically muddy areas with wide intertidal areas, respond to reclamation by analyzing long-term datasets on tidal and bed level changes.
What kind of land reclamation techniques are used in such tide-influenced coastal environments?
Humans have reshaped deltas and other coastal areas for thousands of years. Traditional land reclamation techniques include the construction of wooden structures to dampen waves, allowing fine sediments to deposit. Eventually the area becomes vegetated and reclaimed through the construction of dikes. Although centuries old, this practice continues today, for example, in the Dutch-German Wadden Sea.
Other techniques include the construction of levees to convert wetlands into aquaculture or salt ponds (see Figure 1), or concrete revetments on the intertidal areas which gradually advance seaward. Such latter techniques are especially employed along the megadeltas of Asia. The most recent technique is the closure of tidal (sub)basins using barriers. Such closed basins may be converted into dry land (through pumping, creating polders) or remain reservoirs. In both cases, they profoundly influence the tidal dynamics of the coastal environment seaward of the closure for many decades or centuries.
How do these land reclamations influence coastal environments?
Land reclamations influence both the hydrodynamics (water levels and flow velocity) and the morphological response (erosion and sedimentation) of coastal environments. Both the hydrodynamic and morphological response are controlled by the reclamation type, the hydrodynamic conditions and sediment availability, and the location of the reclamation within the coastal environment.
Based on an analysis of all studies describing the effects of land reclamations, we have developed a classification scheme to explain the impact of reclamations on the coastal environment. A first major distinction herein is whether the reclamation takes place along an open coast, within a bay, or within an estuary. Open coast reclamations may lead to both erosion or sedimentation, likely depending on sediment availability. Reclamations in bays reduce tidal flows and mixing rates, and therefore lead to a reduction in water quality. The largest variability in response is observed in estuaries, where tides may amplify or dampen, and channels may erode or fill in (Figure 2).
Why do estuaries display such a large difference in response?
The large variability in estuarine response is caused by tide-topography interactions. Intertidal areas flanking an estuary reduce tidal energy and amplitude as the tide propagates through the estuary. Reclaiming land along the length of an estuary removes these intertidal areas and therefore leads to tidal amplification, especially if the estuary becomes more funnel-shaped (bottom left in Figure 2).
In contrast, reclaiming the most upstream intertidal areas of an estuary especially leads to a reduced tidal discharge (less water flowing in and out of the estuary with each tide) and therefore lower flow velocities (top right in Figure 2). As most estuaries are rich in sediments, such a reduction in flow velocities usually leads to sediment deposition. Reclaiming land at the mouth of an estuary only limitedly influences the tidal discharge, but the resulting smaller channel width leads to higher flow velocities and deepening of the channel (bottom right in Figure 2).
Why is the impact of land reclamation sometimes very large and prolonged?
Land reclamations may lead to an increase in tidal range of several meters but also to complete infilling of tidal channels. These adaptations are typically slow and may take decades to centuries. The impact is so large and takes such a long time because fine-grained sediments introduce a number of positive feedback mechanisms strengthening the effect of the original disturbance. We have identified five such feedback mechanisms. One example is the infilling of channels because upstream land reclamation reduces the tidal discharge (Figure 3).
As channel infilling continues, the tidal discharge further declines, promoting more sediment deposition. This infilling process slowly progresses in the seaward direction, in time leading to complete abandonment of a tidal system. Such impacts are most pronounced in branching tidal channel networks, such as the Ganges-Brahmaputra delta in Bangladesh. In such networks, infilling in one tidal channel may lead to large-scale erosion in another because of channel rearrangement, with its devastating effects illustrated in Figure 4.
Why are the effects of land reclamations relatively unknown?
The physical impact of land reclamations (changing tides, erosion, or deposition) are surprisingly poorly known. The ecological effects of land reclamations have been extensively studied, and these studies in turn synthesized in several reviews. The impact on tides and bed levels has received much less attention and has so far only been investigated in individual case studies, which do not reveal the full extent of land reclamation impacts. We believe that the number of studies are limited because (1) large amounts of land were reclaimed before data was collected; (2) the response time is slow and variable (and therefore changes are insufficiently correlated with a reclamation); and (3) many contemporary reclamations are executed simultaneously with other interventions (deepening of channels for navigation; reduction of sediment supply by upstream reservoirs) obscuring the effect of the reclamation.
What are key challenges for future research?
We have identified three key challenges for follow-up research. Firstly, the impact of reclamations is so large and prolonged because of a number of positive feedback mechanisms. A better understanding of such mechanisms is needed to explain historic changes but even more to predict future impacts of present-day land reclamations (especially tidal amplification which may influence high water level increase in the coming decades much more than sea level rise).
Secondly, we infer that land reclamation leads to higher suspended sediment concentrations in estuarine environments, negatively impacting coastal ecosystems but also being a key factor driving the positive feedback loops. However, studies relating suspended sediment dynamics to reclamations are limited.
And finally, more attention should be given to the long adaptation timescales. Present-day reclamations will impact their coastal environment for the coming decades to centuries. Forecasting how coastal systems will respond to rising sea levels, for example, is only possible with sufficient understanding of their slow response to existing reclamations.
—Bas van Maren (Bas.vanMaren@deltares.nl; 
0000-0001-5820-3212), Delft University of Technology and Deltares, The Netherlands
Editor’s Note: It is the policy of AGU Publications to invite the authors of articles published in Reviews of Geophysics to write a summary for Eos Editors’ Vox.
Citation: van Maren, B. (2025), Echoes from the past: how land reclamation slowly modifies coastal environments, Eos, 106, https://doi.org/10.1029/2025EO255035. Published on 19 November 2025.
