From Dikes to Mangroves, or Both?

Nature-Based Solutions


Traditional interventions are expected to become difficult to sustain under high-end climate scenarios. Natural (BwN) solutions are better able to cope with varying wave conditions and rising sea-levels due to lower energy needs and costs associated to their implementation.

This reality has given rise to efforts to make greater use of ecosystem-based approaches to reduce risks from coastal storms. These approaches draw from the capacity of wetlands, beaches and dunes, biogenic reefs, and other natural features to reduce the impacts of storm surge and waves. In addition to providing engineering functions related to reducing risks from coastal storms, Natural and Nature-Based Features (NNBF) can provide a range of additional ecosystem services, including those supporting coastal ecosystems and communities.

A true systems approach to coastal risk reduction and resilience requires consideration of the full range of functions, services, and benefits produced by coastal projects and NNBF.

Wetland outside a city

Muddy Systems


Muddy coasts are common around the world. Most are partly vegetated, as these coasts are often sheltered from extreme conditions. Historically, solutions for stabilizing or extending temperate muddy salt marsh coasts have used brushwood groyne systems along tens of kilometres of coastline. Examples are found in the Netherlands along the muddy shores of the Wadden Sea.

Within tropical regions, mangrove belts are very effective stabilizers of muddy coasts. Both systems reduce erosion, can adapt to sea level rise and enable inclusive economic growth, so that coastal communities are safe and can prosper.

Building with Nature for Stable Coastlines in Java


There seemed to be no means of stopping the erosion and the floods. Major investments were made in seawalls and wave barriers, but all these efforts failed. Mangrove replanting was not successful as the conditions were not right, such as the input of sufficient sediment. The floods kept creeping further into the village and taking the land by 100 meters every year. People became desperate and made plans to move away, having witnessed neighbouring villages already being swallowed by the sea.

But then a new solution was introduced in the village, called “Building with Nature”. This solution entails the placement of permeable dams (see picture) and was made possible by Wetlands International, Deltares and the Indonesian Ministry of Marine Affairs and Fisheries (MMAF). The permeable dams break the waves and trap sediment, thus reclaiming land. Once the land is back, mangroves can recolonize the area and help protect the coastline against erosion.

Villagers had been waiting impatiently to see evidence of success of this new ‘permeable structure’ approach. This day had come. The waves are much lower inside the grid of permeable dams than outside. In some cases, pioneering mangrove trees are already testing the ground.

A major breakthrough came when the village signed a decree, demarcating 100 hectares of the lost land as protected area. This means that once the mangrove belt is restored, it will not suffer the same fate. The potential and hope is created for a new landscape in which mangroves and aquaculture can be combined sustainably.

Nature based solutions
Nature-based solutions using local resources
Aerial view of wetlands

Created in partnership with: EcoShape

Sand Engine

Researcher in Deltares

The "sand engine" is a new coastal maintenance strategy designed to use the power of winds, waves and currents to protect part of the Dutch coast while encouraging the development of new dunes and the valuable flora and fauna associated with them.

In this approach, a mega nourishment of 21.5 million m3 of sand was introduced, rising up to 5 meters above mean sea level. The sand is gradually redistributed along the shoreface, beach and dunes by wind, waves and currents. By using natural processes to redistribute the sand, this innovative approach aims to limit the disturbance of local ecosystems, while also providing new areas for nature and more types of recreation.

The Sand Engine is a coastal mega-nourishment pilot project which was constructed in 2011, with the means to improve the efficiency and effectivity of the Dutch coastal nourishment strategy.

Sand Engine located in the Netherlands
Impressions of the Sand Engine
Observed bathymetry August 2012 vs computed bathymetry August 2020

Dynamic Adaptive Policies

Researcher in Deltares

The speed and the magnitude of sea level rise and socio-economic developments are uncertain. Most adaptation measures require large investments, which are not always feasible. This calls for an approach to support decision-making under uncertainty that produces a dynamic, flexible plan that can be adapted as conditions change and funds become available. This approach is called Dynamic Adaptive Policy Pathways.

Dynamic Policy Pathways

Coastal Archetypes and Adaptation Pathways

Researcher in Deltares

Generic adaptation pathways for the 6 coastal archetypes; These generic maps will serve as inspiration to develop tailor-made adaptation pathways for any specific coastal area pertaining to the archetype(s); Linked to local policy objectives, adaptation measures and their tipping points. Pathway Generator: to explore policy pathways in an interactive way.

Adaptation is possible, effective and economically robust. In many cases, hard protection may not be practical or desirable (e.g. disruption of sediment supply, viability, access) and innovative adaptation options need to be implemented. Costs (for EU28) can reach up to or even exceed 1.5% of the annual GDP for some nations, but are considerably more affordable for others.

Avoided damage: By 2050, for RCP4.5 and RCP8.5 less than 0.1% of GDP; for the high-end 0.5% of GDP by 2050 and up to 14% by 2100. No severe financial or technological limits to reduce flood risk in the 21st century. Coastal flood risk can be maintained at or below today's levels, provided that large upfront investments can be mobilised where needed. On the financial side, adaptation often requires large upfront investments which are difficult to mobilize, even if the investment is cost efficient on the long run. From an institutional point of view, there is lack of appropriate mechanisms to deal with different stakes at hand.

Adaptation Pathway Generator
Costs and benefits of pathways