Efficient and effective? The 100-year flood in the communication and perception of flood risk

This paper presents a synopsis of several terms used to describe US policy's benchmark flood and a preliminary study of how such terms are interpreted. Questionnaire surveys were conducted in a flood prone community with residents living in and out of official flood plains. Comparable questions regarding uncertainty, perceived need for protection, and concern were asked in connection with four descriptive methods: a 100-year flood; a flood with a 1 percent chance of occurring in any year; a flood with a 26 percent chance of occurring in 30 years; and a flood risk map. Statistical analysis and qualitative observation showed a disjuncture between understanding and persuasion, potential problems with the 26 percent chance method, and a preference for concrete references in describing risk.