An approximation method for evaluating flash flooding mitigation of sponge city strategies – a case study of central Geelong

Urban flooding is a severe problem and a growing development challenge for many cities around the world. Therefore, there is an urgent need to introduce mitigation measures through managing rainwater to reduce these risks. The sponge city system is regarded as an effective mechanism to manage rainwater and reduce flooding, as it is capable of promoting infiltration and retaining rainwater. However, the main challenge is how to efficiently quantify and visualise the flooding mitigation effects of a sponge city project with various technical measures. There is also a lack of cost analysis of sponge city strategies to reduce risks of a large-scale urban project. Thus, this paper aims to evaluate and visualise mitigation effects from sponge city strategies on urban floods. This paper establishes a rapid and simplified modelling framework for assessing the approximate effects of sponge city strategies on flooding mitigation through GIS simulation in a real-world environment. Various sponge city elements, such as green roofs and wetlands, are systematically integrated in a rainfall-runoff model. Central Geelong will be used as a case study. Scenarios are created in terms of project expenditure, green benefit and infiltration. With the GIS mapping technologies and MATLAB-based digital image processing, maps presenting results for the proposed scenarios are displayed to visualise the mitigation effects of the strategies on urban flooding inundation. The results reveal that the sponge city strategies are capable of generating varying degrees of mitigation of urban flash floods. The scenario with the largest infiltration volume has shown the most significant effect on the flooding mitigation, such as a reduction of 80% in the runoff volume during a one-in-50-year rainfall event, but it may cost twice as much as other options. Overall, this study could be applied to rapid assessment measures of sponge city strategies in the flooding control and decision making on urban green infrastructure planning. The approach proposed here could be adopted for disaster mitigation though sponge city technologies.