Cost-benefit analysis of NbS using flood resilient scenario modelling (FReSMo)

Coastal flood impact assessment on built infrastructure is critical for coastal resilience and sustainability, but the estimation process is rather complex. Especially for developing countries that have limited access to information regarding future flood extent, land use change and damage behaviour of elements at flood risk such as residential buildings. The rising impact of coastal floods on built infrastructure requires an economical and lucrative intervention that is impracticable without the estimation of the potential damage cost. Therefore, the proposed flood resilient scenario modelling (FReSMo) aims to estimate the probable cost of future damage that could be further used in determining the efficacy of IPCC defined nature-based solutions (NbS) in reducing flood impact on built infrastructure in Sagar Island, West Bengal. FReSMo is made up of a multivariate damage model that is developed to have a flexible application over spatial (local to regional) and temporal (flood character and intensity) dimensions. The future built-up area for 2050 is predicted using a patch-growing algorithm FUTURES model that generates realistic outcomes based on landscape metrics, temporal land use growth, demographic and socio-economic characteristics. The return period of high-water flooding is calculated using peak over threshold (PoT) analysis of long-term tidal data. The flood extent is mapped using a connected bathtub model derived from the Cartosat DEM. Residential building damage against a 100-yr flood event is calculated using FReSMo in pre- and post-mitigation scenarios. The cost-benefit analysis shows that an investment in mangroves will reduce the impact of floods by 70-75%, with a total benefit of about 261.3% for a 48-hr flood and 254.6% for a 24-hr flood.