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[HCG23-05] Simulation for residential relocation to adapt to sea level rise in Japanese coasts
Keywords:climate change, sea level rise, adaptation, retreat, relocation, economic valuation
Introduction
It is almost certain that sea levels will continue to rise throughout this century due to climate change. The associations of coastal structures with high maintenance costs, environmental damage and hazards during development have led to growing interest in residential relocation. However, conventional residential relocation cost estimation methods are inadequate. Therefore, we developed a simulation that relocates residents based on their distance from inundated sites to estimate the cost of relocating residents of Japanese coastal areas, including costs associated with building new residential areas. We also analysed land-use changes and transfer distances in residential relocation.
Methods
In this study, future climate and socioeconomic scenarios were based on Shared Socioeconomic Pathways (SSPs)/Representative Concentration Pathways (RCPs) SSP1-2.6 (i.e. SSP1 and RCP2.6) and SSP5-8.5 (i.e. SSP5-RCP8.5).
Elevation, administrative-boundary, tide, sea-level-rise, land-use and population data were used to estimate inundated areas and the number of households affected by flooding per tertiary mesh every decade during 2020–2100.
Tertiary meshes that met the following conditions: elevation and administrative code data available, no inundation for 30 years, maximum slope of < 30 degrees, and total area of agricultural land, forest and wasteland of > 3,300 m2 were designated as candidate meshes for relocation.
In the simulation, we calculated distances between an inundation mesh and all candidate meshes, and then allocated affected households within the inundation mesh to candidate meshes in ascending order of distance until the capacity of each candidate mesh was reached. This process was performed for all inundation meshes.
To evaluate the effects of differences in processing order on the simulation results we processed the inundation meshes in mesh-code-ascending (MCA), population-ascending (PA) and population-descending (PD) order, and processed the candidate meshes in MCA and cost-ascending (CA) order.
Results
The distribution of relocated households around Tokyo Bay under SSP 5-8.5 using MCA order for both inundation and candidate meshes is shown in Fig. 1.
Fig. 1. Distribution of relocated households around Tokyo Bay under SSP5-8.5, with both inundation and candidate meshes processed using mesh-code-ascending order.
Total relocation costs and costs of land acquisition and development are shown in Figs. 2 and 3. Total relocation costs are close to the lower end of the range estimated by Imamura et al. (2022). Land-acquisition and development costs varied depending on the inundation-mesh processing order.
Fig. 2. Relocation costs under SSP5-8.5 Left, total costs. Right, land-acquisition and development costs.
Fig. 3. Relocation costs under SSP1-2.6. Left, total costs. Right, land-acquisition and development costs.
Our land-use change analysis showed that use of wasteland increased over time, whereas use of agricultural land and forest decreased. Transfer distance decreased over time as population decline led to increased relocation to available land closer to inundated areas. The inundation-mesh processing order had non-negligible effects on both land use and transfer distance.
Conclusion
Our relocation costs are lower than damage costs in previous studies, but higher than the protection costs in those studies. The significant impact of the inundation-mesh processing order implies the necessity of coordination among relocation areas. However, in densely populated areas, protection tends to be less costly than relocation; therefore, coordination may be necessary only under certain circumstances.
Acknowledgements
This study was funded by the S-18 project (JPMEERF20S11811).
It is almost certain that sea levels will continue to rise throughout this century due to climate change. The associations of coastal structures with high maintenance costs, environmental damage and hazards during development have led to growing interest in residential relocation. However, conventional residential relocation cost estimation methods are inadequate. Therefore, we developed a simulation that relocates residents based on their distance from inundated sites to estimate the cost of relocating residents of Japanese coastal areas, including costs associated with building new residential areas. We also analysed land-use changes and transfer distances in residential relocation.
Methods
In this study, future climate and socioeconomic scenarios were based on Shared Socioeconomic Pathways (SSPs)/Representative Concentration Pathways (RCPs) SSP1-2.6 (i.e. SSP1 and RCP2.6) and SSP5-8.5 (i.e. SSP5-RCP8.5).
Elevation, administrative-boundary, tide, sea-level-rise, land-use and population data were used to estimate inundated areas and the number of households affected by flooding per tertiary mesh every decade during 2020–2100.
Tertiary meshes that met the following conditions: elevation and administrative code data available, no inundation for 30 years, maximum slope of < 30 degrees, and total area of agricultural land, forest and wasteland of > 3,300 m2 were designated as candidate meshes for relocation.
In the simulation, we calculated distances between an inundation mesh and all candidate meshes, and then allocated affected households within the inundation mesh to candidate meshes in ascending order of distance until the capacity of each candidate mesh was reached. This process was performed for all inundation meshes.
To evaluate the effects of differences in processing order on the simulation results we processed the inundation meshes in mesh-code-ascending (MCA), population-ascending (PA) and population-descending (PD) order, and processed the candidate meshes in MCA and cost-ascending (CA) order.
Results
The distribution of relocated households around Tokyo Bay under SSP 5-8.5 using MCA order for both inundation and candidate meshes is shown in Fig. 1.
Fig. 1. Distribution of relocated households around Tokyo Bay under SSP5-8.5, with both inundation and candidate meshes processed using mesh-code-ascending order.
Total relocation costs and costs of land acquisition and development are shown in Figs. 2 and 3. Total relocation costs are close to the lower end of the range estimated by Imamura et al. (2022). Land-acquisition and development costs varied depending on the inundation-mesh processing order.
Fig. 2. Relocation costs under SSP5-8.5 Left, total costs. Right, land-acquisition and development costs.
Fig. 3. Relocation costs under SSP1-2.6. Left, total costs. Right, land-acquisition and development costs.
Our land-use change analysis showed that use of wasteland increased over time, whereas use of agricultural land and forest decreased. Transfer distance decreased over time as population decline led to increased relocation to available land closer to inundated areas. The inundation-mesh processing order had non-negligible effects on both land use and transfer distance.
Conclusion
Our relocation costs are lower than damage costs in previous studies, but higher than the protection costs in those studies. The significant impact of the inundation-mesh processing order implies the necessity of coordination among relocation areas. However, in densely populated areas, protection tends to be less costly than relocation; therefore, coordination may be necessary only under certain circumstances.
Acknowledgements
This study was funded by the S-18 project (JPMEERF20S11811).