The value of green infrastructure in urban flood risk management, case study: Gernika-Lumo, Spain

Different strategies for water security have been developed during the last decade. Water availability and quality have grabbed the attention of both public and private entities that are looking for strategies to conserve strategic ecosystems, and that provide a wide variety of ecosystem services, including water quantity, quality and sediments regulation among others. This research aims to propose a new methodology to valuate ecosystem services from a disaster risk reduction perspective, by the valuation of the regulation of water as an ecosystem service. To achieve these objectives, a case study in the city of Gernika-Lumo in Spain was selected. The city is located inside the Oka and Golako river basin, where heavy rainfalls have caused frequent urban floods, causing important damages to infrastructure, buildings, and properties. Green Infrastructure plays an important role in water regulation and has the potential to reduce the quantity of water that causes flooding inside urban areas. This research analyses the effect of different land-use cover scenarios, including the land use trends, focusing on the natural forest as green infrastructure inside the catchment. The land use cover changes were analysed using satellite images and GIS methods. Rainfall scenarios were calculated based on the historic rainfall data of the studied area, producing three storm scenarios for different return periods. Using all this information and the main physical characteristics of the Oka and Golako rivers, a hydrologic and hydraulic model were developed. The resultant flood depth maps were used to calculate the total damage per scenario, using the existing flood damage curves for the city of Gernika-Lumo. The results of this study reveal the big potential that green infrastructure has in the reduction of flood risk, by reducing the hazardous effect of floods. From 2015 to 2020, the flood modeling results shows that there was an increment in flood damage costs of € 7,016,285 for the worst hydrologic scenario. The conservation scenario shows an avoided damage cost of € 12,786,283 in comparison with the actual scenario for 2021. The conservation scenario consisted of the reforestation and afforestation of 1,595 hectares inside the Oka and Golako river basins. The result for the 500-years return period event shows that in every scenario there is a total avoided damage cost of € 8,000 per hectare of forest. There is an evident direct relationship between the reduction of economic losses and the area of green infrastructure cover that helps in the regulation of flow discharge inside the Oka and the Golako river basins.