Meso-scale hazard zoning of potentially flood prone areas

•Probabilistic method for hazard zoning within the potentially flood prone areas, based on Topographic Wetness Index (TWI).•Methodology to build TWI-flood depth relationship for a given return period: the predictive model.•Development of visual and analytical checks of the proposed methodology.•Application of the procedure for hazard zoning within potential flood prone areas of the City of Addis Ababa, Ethiopia.

SummaryIdentification of flood-prone areas is a fundamental step for flood risk management. This work focuses on a fast procedure for hazard zoning within the flood-prone areas at a meso-scale in the absence of detailed hydrologic and hydraulic calculations for large areal extents. To this end, a probabilistic methodology for hazard zoning of the potentially flood-prone areas is presented. The main output consists of prompt GIS-compatible maps for the hazard zoning (by flood height) of the potentially flood-prone areas. A flood height-dependent extension of the Topographic Wetness Index (TWI) threshold is proposed as the lower-limit TWI for zones with flood height greater than a given value. This procedure, that starts from a consolidated geomorphic algorithm, relies on calibrating the flood height-dependent TWI threshold through maximum likelihood estimation and inundation maps carried out at micro-scale level for a given spatial window. This calibration is performed for different values of flood depth in order to investigate the correlation between the flood height-dependent TWI threshold and the flood depth conditioned on return period, through simple linear regression. The resulting regression model is used in order to up-scale the results from the micro-scale to the meso-scale level. The procedure is applied for the City of Addis Ababa, Ethiopia and the resulting map of flood hazard zoning is validated for a spatial window not used for calibration purposes.