Identification of Flood Prone Roads

Abstract : The availability of new technologies far the measurement of surface elevation has addressed the lack of high-resolution elevation data, which has led to an increase in the attraction of automated procedures based on digital elevation models (DEMs) far hydrological applications, including the delineation of floodplains. In particular, the exposure to flooding may be delineated quite well by adopting a modified topographic index (TIm) computed from a DEM. The comparison of TI", and flood inundation maps (obtained from hydraulic simulations) shows that the portion of a basin exposed to flood inundation is generally characterized by a TIm higher than a given threshold, T (e.g., equal to 2.89 far DEMs with cell size of 20 m). This allows the development of a simple procedure far the identification of flood-prone areas that requires only two parameters far the calibration: the threshold T and the exponent of TIm' Because the modified topographic index is sensitive to the spatial resolution of the DEM, the optimal scale of representation for the performance of the method is investigated. The procedure is tested on the Arno River Basin by using the existing documentation of flood inundations produced by the Arno River Basin Authority for calibration and validation. This approach is applied on Il subcatchments with areas ranging from 489-6,929 km2 utilizing DEMs of different resolutions with cell sizes ranging from 20-720 m. Results show that the proposed procedure may help in the delineation of flood-prone areas, especially in basins with marked topography. The method is sensitive to the DEM resolution, but a cell size of -100 m is sufficient for good performance for the catchments investigated here. The procedure is also tested by adopting DEMs from different sources, such as the shuttle radar topography mission (SRTM) DEM, ASTER global DEM (GDEM), and national elevation data. This experiment highlights the reliability of the SRTM DEM for the delineation of flood-prone areas. A useful relationship between model parameters and the reference scale of the DEM was also obtained, providing a strategy for the application of this method in different contexts. The use of the modified topographic index should not be considered as an alternative to standard hydrological-hydraulic simulations for flood mapping, but it may represent a useful and rapid tool for a preliminary delineation offlooding areas in ungauged basins and in areas where expensive and time-consuming hydrological-hydraulic simulations are not affardable or economically convenient.
 ? As long-term inundation by floodwater is not a very common phenomenon in these countries, research in this regard is almost non-existent. ? The importance of developing new infrastructure in a climate-resilient way and adapting existing infrastructure to changing environmental conditions is thus evident. ? The probability of flooding has been assessed, maintenance of culverts (e.g., regular unclogging of flood-prone intersections) and the adaptation of existing infrastructure to the predicted changes in climate conditions can be prioritized accordingly.
 ? The delineation of these areas raises complex problems regarding the definition of flood events and the parameterization of models far flood wave propagation in river beds and in surrounding areas. ? The problem is selecting the right threshold and identifying the correct exponent that optirnizes this method. ? The effect of depressions and obstac1es were treated using the AT search algorithm, which is a least-cost path algorithm designed to minimize the impact of areas of low slope and DEM data errors.
 • This provides justification for the use of SMCE in order to identify flood-prone areas that are needed for mapping purposes. • The approach used in this study was for screening purposes, other studies should be conducted based on a quantitative approach at other scales in order to obtain the direct and indirect losses for elements at risk (assets). • The prioritization based on the exposure analysis was conducted by intersecting the flood-prone areas with roads that were ranked similar to the method proposed in France for analyzing potential flood damage to transportation networks at the meso-scale.
 ? The performance of the method tended to increase for basins with a higher slope ratio. ? The preliminary results of the flood prone area determination, as a basis to perform more comprehensive synthesis such as flood risk and flood hazard mitigation. ? The visualization of the suggested models by means of the flood thermometer allows for the comparison of their performance to that of the Blue Spot method.

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