THE CONTRIBUTION OF GIS IN FLOOD MAPPING: TWO APPROACHES USING OPEN SOURCE GRASS GIS SOFTWARE

Abstract

The first step of a risk assessment analysis is the evaluation of flood-prone areas. Its importance is considered for both managing and planning emergency activities, such as hydraulic risk reduction management, and also town planning. Nowadays, using GIS technology for risk assessment analysis is very common. However, it is not widely used for defining inundated areas. LiDAR data, such as Digital Elevation Models (DEM), makes GIS numerical models attractive methods for obtaining a flooded area automatically. Using GIS tools, is beneficial for effective processing and accuracy assessment in comparison to the traditional methods which are based on topographic maps and field surveys. A first approach (Federici and Sguerso, 2007; Marzocchi et al. 2009) is the use of a GIS module in order to create perifluvial flood maps, having as prerequisites (i) the conformation of the river floodplain by a high resolution DEM and (ii) a water surface profile along the river axis calculated for a given water discharge through a generic one-dimensional (1D) hydraulic model (HEC-RAS, Basement, MIKE 11, etc). On the other hand, a second approach is the use of a 2D model GIS embedded in order to create flooded areas due to a dam break (Cannata & Marzocchi, 2012). This module solves the conservative form of the 2D Shallow Water Equations (SWE) using a Finite Volume Method (FVM). The intercell flux is computed by a one-side upwind conservative scheme extended to a 2D problem (Ying et al., 2004). The new developed GIS module gives as an output maximum intensity maps which can be directly used during the risk assessment process. Both models implemented in GRASS GIS software (GRASS, 2013) and two new commands (r.inund.fluv and r.damflood) have been created. They are all available on the official GRASS website and they are distributed under the terms of the GNU General Public License (GPL). In this work we present a comparison between the two models mentioned above. We analyse the possibility of integrating these two approaches. We intend to use the 1D model, GIS embedded if possible, to calculate the water surface profile along the river axis and the 2D numerical one to analyse inundation beside the river levees.