coastal protection

Part of the larger The impacts of climate change on Bonaire (2022-present) report available here - https://www.dcbd.nl/document/impacts-climate-change-bonaire-2022-present

Summary

Bonaire is as a small and low-lying island state vulnerable to the impacts of climate change. In the future decades, sea level rise and tropical cyclones are expected to increase coastal flood risk on the island. Yet, it is still unknown to what extent coastal flooding will increase and which areas on Bonaire are expected to flood. Therefore, this study aims to assess the impacts of climate change on coastal inundation on Bonaire. First, a static bathtub model is used to estimate the future coastlines of the island under multiple sea level rise projections. Second, the SFINCS model is applied to incorporate the dynamic storm components of storm tide and waves in addition to sea level rise. The results of the inundation models indicate that coastal inundation becomes critical for large parts of Kralendijk in 2150 under scenarios SSP5-8.5 and SSP5-8.5 LC. Under the more optimistic scenarios SSP1-2.6 and SSP2-4.5 coastal inundation remains limited to the nature reserves of Klein Bonaire, Lac Bay and the saliñas. Therefore, the results of this study indicate the importance for Bonaire of globally limiting climate change to a lower-end future climate scenario.

Small islands are particularly vulnerable to climate change because of their fragile ecosystems, small economies, and often extensive, low-lying coastal areas.

Therefore, small islands, such as present in the Caribbean Netherlands, are expected to suffer excessively from rising temperatures, changes in precipitation, sea level rise, coral bleaching, cyclones, droughts and floods. Yet, scientific evidence of these effects in the Caribbean Netherlands is scarce.

In this study, an analysis is conducted assessing the impacts of climate change for the island of Bonaire. A mix of methods is used to estimate the impacts of climate change, including climate and flood models, ecological-economic models, as well as social-science methods such as social media analysis and participatory mapping. Four sub-studies can be distinguished: the estimation of the biophysical impacts, the modelling of economic effects, the identification of socio-cultural effects, and the exploration for potential adaptation options.

The main findings of the study include the following:

• Already by 2050, sea level rise will cause permanent inundation of parts of the low-lying nature reserves of the saliñas, Lac Bay and Klein Bonaire. The flooded surface area will increase further by 2150, threatening Bonaire's high-risk built-up areas such as Belnem and other areas in Kralendijk. The loss of coral reefs as a natural buffer will amplify these effects.
• The economic impact of climate change is mainly felt through damage costs caused by floods as well as negative effects on tourism caused by the loss of corals. Storms and floods are expected to cause an estimated damage costs of US$317 million by 2050. The degradation of coral reefs leads to the degradation or loss of the majority of dive sites on Bonaire, which may cause a reduction in tourist arrivals of more than 100 thousand visitors.
• Climate change is also expected to severely cause loss of cultural heritage and health impacts on Bonaire. Among others, this includes the permanent flooding of key locations with cultural significance for Bonaireans, such as the slave huts and the house at Boca Slagbaai. Additionally, climate change is expected to affect Bonaireans’ health, such as changes in vector-borne disease incidence and heat-related stress.
• Potential adaptation strategies against climate change on Bonaire include nature-based solutions such as the conservation of coral reefs and the restoration of coastal vegetation, which contribute to the prevention of flooding. We conclude that, although the impacts of climate change necessitate immediate action, decision-makers should also focus on the longer term, such as 2150 and beyond, as the effects of climate change will worsen significantly over time.

For more information about the study or sub-studies, download the following reports:

• Synthesis report
• Impacts of climate change on coastal inundation on Bonaire
• Coral reef degradation and the impacts on tourism and economic development
• Impacts of climate change on the built environment
• Impacts of climate change on cultural heritage
• Impacts of climate change on public health
• Adaptation strategies against coastal flooding

Abstract

Human development, extreme weather events and rising sea levels are driving the loss anddegradation of many of the world’s mangrove ecosystems. Mangrove forests are natural coastalbarriers that provide protection from erosion and storms while also supplying material andbiodiversity services tohuman andnon-humancommunities worldwide. In recent years innovativeand novel products of ecological engineering have emerged to safeguard and restore coastalecosystems. Biodegradable Ecosystem EngineeringElements(BESE) are one such innovation thathave been shown to stabilize sediment, attenuate waves, and mimic dense root mats to facilitatethe establishment of coastal vegetation. This studyinvestigatesthe effect of a built BESE structureon abiotic factors andRhizophora mangletransplant responsesalong a high-wave energy coastlineon the island of Bonaire, Dutch Caribbean. Results reveal that BESEelementsdo not significantlyalter wave-energy and sediment mobility, or increase transplant survival, shoot growth rate andbiomass allocation. Survival rate of transplants was significantly higher when secured by rockscompared to BESE structures and bare sediment, highlighting simple, local, and low-cost methodsfor restoration. Despite not being significant, there were indications of the potential for BESEstructures toattenuate a minimal amount of waves and stabilize propagules for greater investmentin above-ground biomass during establishment. Yet, longer monitoring is needed to validate this.Limitations ofthe BESE structuressuggest thattheymay be more optimally used below-ground onsediment substrate, in larger clustered structures, or within interconnected foreshore ecosystemsfor landscape-level coastal protection and resilience.

Abstract

Seagrasses are increasingly being recognized for their potential in protecting the coastlines from flooding and erosion. As ecosystem engineers, seagrasses can attenuate waves as well as trap and stabilise sediment, causing the seabed to become more stable, which can contribute to coastal protection. Where many tropical countries lack the means to undertake traditional coastal protection measures, natural ecosystems, such as seagrass meadows, can provide cost-effective alternatives to protect coastal communities from natural hazards and improve their well-being, as ecosystem services are continuously provided. The Caribbean regions holds a large cover in seagrass relative to the coastlines, yet few studies have focused specifically on seagrass and sediment stability in this region. Over the last decades, native seagrass meadows have been degrading and the non-native Halophila stipulacea, originating from the Red Sea, has been spreading rapidly through the region relatively recently. A knowledge gap exists in whether this opportunistic species could provide essential ecosystem services, such as coastal protection, where native species have been lost. The extent to which seagrass species contribute to coastal protection services depends on the growth properties of these species as well as the environmental conditions. Therefore, a need remains to study sediment dynamics in seagrass meadows in different scenarios. The present study aims to increase the understanding of the role of H. stipulacea in sediment dynamics, specifying on the coast St. Eustatius, Dutch Caribbean.

First, current available knowledge on sediment stabilising properties of seagrass species in the wider Caribbean region was reviewed. Second, an experiment was set up at a study site with a depth of 18 m, in a monospecific H. stipulacea meadow, and in a sand patch within the meadow. In addition to measuring the growth properties of H. stipulacea, sediment stability was investigated by measuring changes in seabed level over time, as well as using sediment traps to calculate the sediment deposition rate. Data on meteorological conditions was retrieved to investigate correlations between the weather and sediment change.

Results showed that H. stipulacea at a depth of 18 m at the coast of St. Eustatius, grew a new shoot within 3.3 days, over twice as fast as measured for its native range. Average leaf length was 4 cm while eight shoots were observed per strand. Similar values were observed in its native range. Although the leaf length was smaller than most native seagrasses in the Caribbean, H. stipulacea showed a higher density, which likely contributed to the findings of this study. The sediment deposition rate was much lower in the H. stipulacea meadow than in the sand patch, which is probably the result of limited resuspension within the seagrass meadow. After 14 weeks, the sediment in the H. stipulacea meadow had eroded. This is believed to be the result of erosion of the surface sediment layer, a less stabilised layer of sediment that was potentially resuspended and carried away when the weather became more turbulent after months of calm conditions. The subsurface layer, however, is believed to be more stabilised by the root and rhizome system of H. stipulacea, which explains why smaller and relatively constant changes in sediment level were found after weather conditions had increased. The sediment level in the sand patch showed a larger variance over time, also varying much more spatially between erosion and accumulation.

This study showed that H. stipulacea seems to stabilise the subsurface sediment layer via its root and rhizome system, which is assumed to contribute to a more stable seabed. However, this result only became apparent after the erosion of the surface layer and might even disappear with more extreme weather conditions. This study urges for future research to investigate the sediment stabilising effect of H. stipulacea at shallower depths as well as over longer terms, as different results might be found for different seasons.

Presentation from the Mangrove Restoration Workshop.  

The coastal protection value (CPV) of coral reefs is one of the ecosystem services that contribute to the economic value of coral reefs. The basic principle of coastal protection by coral reefs is the observation that reefs dissipate wave energy either by wave breaking or friction by reef structures. In this study, the coastal protection value (CPV) is estimated on 30 * 30 m grid cell level, which gives a more spatially explicit estimation of the CPV of coral reefs. The annual coastal protection values of the coral reefs of Bonaire for short-term (i.e. within 10 years) and long-term processes (i.e. beyond 10 years) are estimated at $33,000 and $70,000, respectively.