The Civil Defense in Brazil needs to identify and differentiate floods and flash floods for the official registry. This study aims to quantitatively define and differentiate between floods and flash floods. Floods and flash floods are characterized by factors including the speed of water level rise and water flow; the hydrological response time to a rainfall event and the extension of the flood affected area. The hydrological definitions are ambiguous and make it difficult to distinguish

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... a flood and flash flood event. Even though some papers have mentioned that flash floods occur within 6 hours after an intense rainfall event, there is still certain subjectivity. This study proposes the use of the Operation Efficiency Index (OEI) as a quantitative means to distinguish between a flood and flash flood event. The OEI is defined as the rate of the time of flood concentration (Tc) to the operational response time (To) in the institution-community system. Tc and To are associated with environmental and human factors, respectively. When the OEI value is smaller than one, then flash floods occur. Otherwise the event can be defined as a flood.