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The availability of new technologies for 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) for 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 TIm 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, τ (e.g., equal to 2.89 for DEMs with cell size of 20 m). This allows the development of a simple procedure for the identification of flood-prone areas that requires only two parameters for the calibration: the threshold τ 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 11 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 of flooding areas in ungauged basins and in areas where expensive and time-consuming hydrological-hydraulic simulations are not affordable or economically convenient. [ABSTRACT FROM AUTHOR]

To prevent erosion of polluted land and discharge of contaminated groundwater and surface water into the Venice Lagoon, a 57 km long cutoff wall has been designed by the Venice Water Authority and is currently under construction along the canal banks of the Venice, Italy industrial harbor. To predict the impact on the hydrologic regime and mitigate the related inundation hazard in the nearby urban areas, a complex three-dimensional finite-element model has been developed and implemented over the multiaquifer system down to a depth significantly larger than the wall bottom. The model is initially calibrated against the regional piezometry and then refined locally to reproduce the groundwater volume drained along a 5-km long bank of a harbor canal already bounded. Major results from the simulations show that after the wall completion the subsurface discharge into the lagoon is successfully abated by as much as 85% relative to the preexisting rate with, however, an expected 1-m increase of the water table in the inland city of Mestre, Italy thus pointing to the need for implementing a drainage trench upstream the wall to properly reduce the raised groundwater level. The model would also indicate that the shallowest aquifers may receive underground water from a large number of old deeper boreholes abandoned in the past and not properly sealed. From a more general perspective, the present study constitutes an important example where the reliable and sustainable design of a complex engineered structure bound to impact significantly on the surrounding environment can be much helped and improved by the use of advanced numerical models capable to capture the essential features of the underlying geohydrological processes. [ABSTRACT FROM AUTHOR]

The genetic algorithm (GA) technique is applied to obtain optimal parameter values of the standard rating curve model (RCM) for predicting, in real time, event-based flow discharge hydrographs at sites receiving significant lateral inflows. The standard RCM uses the information of discharge and effective cross-sectional flow area at an upstream station and effective cross-sectional flow area wave travel time later at a downstream station to predict the flow rate at this last site. The GA technique obtains the optimal parameter values of the model, here defined as the GA-RCM model, by minimizing the mean absolute error objective function. The GA-RCM model was tested to predict hydrographs at three different stations, located on the Upper Tiber River in central Italy. The wave travel times characterizing the three selected river branches are, on the average, 4, 8, and 12 h. For each river reach, seven events were employed, four for the model parameters’ calibration and three for model testing. The GA approach, employing 100 chromosomes in the initial gene pool, 75% crossover rate, 5% mutation rate, and 10,000 iterations, made the GA-RCM model successfully simulate the hydrographs observed at each downstream section closely capturing the trend, time to peak, and peak rates with, on the average, less than 5% error. The model performance was also tested against the standard RCM model, which uses, on the contrary to the GA-RCM model, different values for the model parameters and wave travel time for each event, thus, making the application of the standard RCM for real time discharge monitoring inhibited. The comparative results revealed that the RCM model improved its performance by using the GA technique in estimating parameters. The sensitivity analysis results revealed that at most two events would be sufficient for the GA-RCM model to obtain the optimal values of the model parameters. A lower peak hydrograph can also be employed in the calibration to predict a higher peak hydrograph. Similarly, a shorter travel time hydrograph can be used in GA to obtain optimal model parameters that can be used to simulate floods characterized by longer travel time. For its characteristics, the GA-RCM model is suitable for the monitoring of discharge in real time, at river sites where only water levels are observed. [ABSTRACT FROM AUTHOR]

The paper describes the results of a study based on the integration of remote sensing and geographical information system techniques to evaluate a distributed unit hydrograph model linked to an excess rainfall model for estimating the streamflow response at the outlet of a watershed. Travel time computation, based on the definition of a distributed unit hydrograph, has been performed, implementing a procedure using (1) a cell-to-cell flow path through the landscape determined from a digital elevation model (DEM); and (2) roughness parameters obtained from remote sensing data. This procedure allows the taking into account of the differences, in terms of velocity, between the hillslopes and the stream system. The proposed procedure has been applied to two watersheds in Sicily, in order to establish the level of agreement between the estimated and recorded hydrographs, using as a tool to calculate the excess rainfall a simplified version of the probability distributed model. [ABSTRACT FROM AUTHOR]