Flood Hazard Mapping in Alluvial Fans with Computational Modeling

Abstract

Floods occurring in alluvial fans are very peculiar and hazardous, due to the mobility of the channel in the regions. Such mobility increases the uncertainty of the course to be occupied by the water or the debris flow, which makes hazard mapping more complex. Since the flow in alluvial fans is three-dimensional, the flood mapping in these regions without hydrological data can be unfeasible. Then, at first the present study applied to the Upper Mampituba river basin characterized with three main alluvial fans (Molha Coco and Malacara Fans (MCMFs) and Boi Fan), southern Brazil, three models: Hydrological Engineering Center – River Analysis System 2D (HEC-RAS), FAN Model and Height Above The Nearest Drainage (HAND). The FAN and HAND models are considered geomorphological models and comparatively simpler. Then the combination between these two models was applied to generate flood hazard maps. The proposed combination model presented results more coherent with alluvial fan feature, representing satisfactorily the flood extent of the most recent event occurred in the study area (showing good performance metrics: statistical F (%) 72.65, General Accuracy (%) 79.82, FIT(%) 38.44 and False Alarm Rate (%) 20.50).