Knowledge note 1: Assessing and communicating urban flood risk

Understanding the causes of flood disasters, and ways to mitigate them, is an essential first step toward the integrated management of flood risk in cities. This can help decision makers guide investments out of hazardous zones, save lives and property, as well as ensure that investments take into account both expected and unprecedented disaster events. Careful disaster planning is critical in complex urban environments that are home to large populations as well as multiple infrastructure and service networks. Though risk assessments are of various types, depending on the needs and objectives of the stakeholders involved, all share the objective of enhancing the knowledge and management of risk. To this end, it is also important to consider how assessment results are communicated to stakeholders, as well as to the wider public. In Japan, risk assessments are generally used to inform flood risk management plans and investments, as well as to communicate flood risk information to enhance preparedness and evacuation plans. This Knowledge Note examines the different types of urban flood risk, and the roles of specific sectors and stakeholders in conducting flood risk assessments in Japan. It provides an overview of the various objectives and methodologies used. Examples include risk assessments conducted to inform the design of structural flood management measures and the planning of nonstructural flood evacuation, as well as to raise community awareness and integrate climate change considerations in risk management efforts. These examples showcase how and why various risk assessments are conducted, and how they are used to enhance decisions and actions. The note also considers the effectiveness of risk communication measures in Japan, and analyzes the prevalent methods and technologies used to strengthen flood risk mitigation, preparedness, and evacuation plans in Japanese cities.