Comparison of one-dimensional and two-dimensional GRASS-GIS models for flood mapping

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 Geographic Information System (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 traditional methods which are based on topographic maps and field surveys. A first approach (Federici & Sguerso, Bollettino SIFET 4:25–42, 2007; Marzocchi, 2009) uses a GIS module in order to simulate 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 uses a GIS-embedded two-dimensional (2D) model in order to simulate flooded areas due to a dam break (Cannata & Marzocchi, Nat Hazards 61(3):1143–1159, 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., J hydraul eng-ASCE 10:977–98, 2004). The newly developed GIS module gives as output maximum intensity maps which can be directly used in a risk assessment process. Both models have been implemented in geographic resources analysis support system (GRASS)-GIS software (Neteler et al., Environ Model Softw 31:124–130, 2012; GRASS, 2013) and two new commands (r.inund.fluv and r.damflood) have been created. Both are available on the official GRASS addons website and 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.