Hondsbossche Dunes, the Netherlands

As a low-lying country, the Netherlands is more vulnerable to sea level rise associated with climate change. Dutch climate experts recently warned that the country could face more severe sea level rise than previously modelled. By the end of the century, sea levels could rise by more than 2 metres.  

The growing risks of inland and coastal flooding require constant upgrading of the Dutch flood protection infrastructure. In parallel with (or in place of) conventional reinforcements, infrastructure planners have increasingly chosen to implement nature-based infrastructure (NBI). For example, they create salt marshes, widen floodplains, and build soft coastal barriers out of sand.

IISD analyzed the value of the Hondsbossche Dunes as a form of nature-based coastal protection. Before its construction, the stretch of sea dike in the northern Netherlands no longer met Dutch flood safety standards. Instead of raising the dike, policy-makers decided to build an artificial beach and dune landscape on the seaside of the existing sea wall. It stretches along 7 kilometres of the Dutch North Sea coast between the villages of Petten and Camperduin. 

Since construction was completed in 2015, the Hondsbossche Dunes have protected the region from floods. They are designed to withstand severe storm surges that statistically happen once every 10.000 years. In case sea level rise requires further upgrading of the coastal defences, the NBI can easily be strengthened by adding more sand. In addition, the Hondsbossche Dunes offer valuable space for nature and recreation and support the tourism sector in local communities.  

 

An assessment with the Sustainable Asset Valuation (SAVi) methodology shows that the sand dunes outperform conventional flood protection infrastructure. In comparison to a grey infrastructure alternative of raising the sea dike, the NBI was cheaper to build and brings greater benefits for tourism. Based on our modelling, the sand dunes increase tourism revenue by almost EUR 203 million over 50 years, while the grey alternative would have increased it by only EUR 103 million.  

The SAVi assessment also demonstrates that the Hondsbossche Dunes offer cost-effective flood protection. We calculated the value of the NBI and the grey dike reinforcement under different scenarios. Assuming that they prevent one flood over the next 50 years, the sand dunes have a present value at construction of EUR 98.3 million. Under the same scenario, the grey alternative would have a negative value (-EUR 18.1 million). This is because raising the dike would cost more but provide fewer societal benefits. Under a scenario where they help avoid two floods, the dunes would have a present value of more than EUR 180 million, while the reinforced dike would be worth about EUR 65 million.  

To compare the performance of the NBI and a conventional dike reinforcement, we calculated diverse costs, benefits, and avoided costs from the investments, such as: 

• Costs for construction and maintenance, including embodied carbon 
• Avoided flood damages and avoided impacts on the agriculture sector 
• Avoided pollution with nitrogen and phosphorus 
• Increased tourism revenue 
• Carbon storage benefit.

A spatial analysis formed the basis for calculating some of these costs, benefits, and avoided costs. We used spatial models to find out which area would be flooded in case the coastal protection fails and what that would mean for the environment and economy. 

Map: Land use in the area protected by upgraded coastal infrastructure. Source: Author diagram

The Hondsbossche Dunes protect about 900 hectares of land, including agricultural land, villages, and natural areas with water and shrubs. In total, these ecosystems store about 45,000 tonnes of carbon that would be released into the atmosphere in case of a flood. Preventing such an event that contributes to climate change would avoid societal costs of nearly EUR 1.5 million. 

We also modelled that the study area retains 32.1 tons of nitrogen and 2.1 tons of phosphorus. If a flood would wash these nutrients into nearby water bodies, it could lead to lead to negative impacts like algae growth resulting in costs of about EUR 150,000. 

Map: The study area stores about 45,000 tonnes of carbon. Most carbon is stored in the agricultural land and natural areas with shrubs, as shown with the lighter colours in the map. Source: Author diagram.

The benefits calculated in the valuation are likely an underestimate because not all possible effects of the coastal protection investments were considered. In particular, the Hondsbossche Dunes include a wet dune valley, beach areas, and different types of dunes that are valuable habitats and support biodiversity. With its foot and cycling paths, the NBI can also improve people’s health by encouraging them to exercise more. 

Photo: The wet dune valley created in the Hondsbossche Dunes is a valuable habitat. Source: Carrie de Wilde.