Pierfranco Costabile

Enhancing river model set-up for 2-D dynamic flood modelling

Flood hazard mapping is a topic of increasing interest involving several aspects in which a series of progress steps have occurred in recent years. Among these, a valuable advance has been performed in solving 2-D shallow water equations in complex topographies and in the use of high resolution topographic data. However, reliable predictions of flood-prone areas are not simply related to these two important aspects. A key element is the accurate set up of the river model. This is primarily related to the representation of the topography but also requires particular attention to the insertion of man-made structures and hydrological data within the computational domain. There is the need to use procedures able to 1) obtain a reliable computational domain, characterized by a total number of elements feasible for a common computing machine, starting from the huge amount of data provided by a LIDAR survey, 2) deal with river reach that receives significant lateral inflows, 3) insert bridges, buildings, weirs and all the structures that can interact with the flow dynamics. All these issues have large effects on the modelled water levels and flow velocities but there are very few papers in the literature on these topics in the framework of the 2-D modelling. So, in this work, attention is focused on the techniques to deal with the above-mentioned issues, showing their importance in flood mapping using two actual case studies in Southern Italy. In particular, the simulations showed in this paper highlight the presence of backwater effects, sudden and numerous changes in the flow regime, induced by the detailed river model, that underline the importance of using 2-D fully dynamic unsteady flow equations for flood mapping. Correct representation of the flood-prone areas topography.Getting from LIDAR data a computational grid feasible for a common computing power.Interaction between hydrologic and hydraulic models.Insertion of structures that can interact with the flow dynamics.

Flood mapping using LIDAR DEM. Limitations of the 1-D modeling highlighted by the 2-D approach

Nowadays, the use of 2-D fully dynamic models represents the most reliable approach for flood inundation and flood hazard studies, especially in complex applications. However, 1-D modeling is still a widely used approach due to the reduced computational time and cost. The introduction of LIDAR technique has stimulated a more detailed topographic description of river reaches. As a result, this huge amount of topographic data can lead to significant improvements in the 1-D computations. Therefore, the main purpose of this paper is to realize how the improvements in the topographic description can reduce the difference between 1-D and 2-D models, highlighting at the same time the critical aspects and the limitations of 1-D approach in the hydraulic simulation as well as in the spatial representation of the results. The analysis presented in the paper refers to two actual case studies for which terrestrial and airborne LIDAR DEMs were collected on purpose. The results of those applications show that the use of 1-D models requires a greater hydraulic skilfulness than the use of 2-D model.

Analysis of One-Dimensional Modelling for Flood Routing in Compound Channels

In this paper a number of hydraulic aspects concerning the flood routing in compound channels are discussed. In particular, attention is firstly focused on the management of boundary conditions for compound channel flow when 1D modelling is used. In this context, the characteristics theory was used to obtain the complete expression of the eigenvalues of the system and the consequences of the simplification associated to the commonly-used approximation were highlighted. The analysis has shown that the number of physical conditions to be imposed at the boundaries is influenced not only by the Boussinesq coefficient but also by its derivative over the water level. The second part of the paper is devoted to the analysis of unsteady flow simulations. Attention was focused on the role played by the lateral momentum transfer, between main channel and floodplains, within the 1D flood propagation model. In particular, the simulations showed that significant differences may occur between the traditional approach and the methods able to take into account the momentum transfer mechanisms.

Visual Risk Communication of Urban Flooding in 3D Environments Based on Terrestrial Laser Scanning

In this paper, attention is focused on the extension of the urban flood-mapping concept from an eminently cartographic context to a three-dimensional one, by applying techniques for the representations of urban flood inundation in a virtual environment for hazard communication purposes. In particular, visual risk communication of urban flooding in 3D environments, based on terrestrial laser scanning, has been analyzed here. Moreover, a web-based rendering of point clouds for the visualization of urban flooding has been explored in order to highlight its benefits in urban flood risk communication.

The Power Function for Representing the Reservoir Rating Curve: Morphological Meaning and Suitability for Dam Breach Modelling

In this paper the authors demonstrate that the simple power function gives a good description of the reservoir morphology. This function proved to be suitable for application on 97 reservoirs taken from three different geographical regions in the world. The power function is suitable to obtain an accurate fitting of the reservoir rating curve using a very limited number of surveyed elevations and volumes or areas. In particular it is shown that two points are enough for a good fitting of the curve, or even only one if volume and surface for an elevation close to normal or maximum pool are both available. The results obtained for dam breach calculations using this equation, have the same quality of those achieved using the elevation-volume table. The paper stresses that the exponent of power equation can be expressed by a formula which has a precise morphological meaning, as it represents the ratio between the volume which the reservoir would have if it were a cylinder with its base area and height equal to the respective maximum values of the actual reservoir, and the real volume of the reservoir.

Final Document on the Third Year, Second Activity: 'General Methodologies and Hydrological-Hydraulic Parameters Supporting the Definition of a Climate Index for Changing Flood Risk Assessment'

This report is the final document related to the third year, second activity whose title is: General methodologies and hydrological-hydraulic parameters supporting the definition of a climate index for changing flood risk assessment. The purpose of the collaboration between LAMPIT (department of Soil Defence, University of Calabria) and CMCC is to develop an hydrometeorological chain in order to obtain a reliable tool in the context of flood evolution prediction able to provide quantitative information of practical importance within the civil protection activities. The LAMPIT contribution to the project concerns the mathematical description of both generation and propagation of flood events at basin scale. The work here presented has been carried out in close cooperation with dr. Pasquale Schiano and dr. Paola Mercogliano. In order to embrace the problem as a whole, this report starts from a general overview of the characteristics of climate change according to IPCC evaluations, highlighting the main causes that may induce a variation of flood features (chapter 1). The term flood is often generically associated to a number of natural events that significantly differ in terms of phenomenological generation. So it seems necessary to recall in mind a possible main classification of flood phenomena in order to underline the flood type discussed herein and his peculiarities in relation to climate change (chapter 2). In a catchment-wide perspective there is the need to take into account non-climatic drivers, such as land-use change, in the evaluation of changing flood risk: land cover change within a watershed is recognized as an important factor affecting runoff and it is possible that the transformation of land across the globe could have a greater influence on runoff than climate change; that question is explained in the chapter 3. Chapter 4 is devoted to the general methodologies developed to evaluate the effects of climate change on runoff. They are mainly based on the interconnection between climate and hydrologic models; some features of each model are also analysed to highlight those aspects useful for the evaluation of climate impact on river flooding that link climate models to hydrological model. In order to better characterize the general methodologies, a number of typical results, presented in the literature, are illustrated in the chapter 5. The analysis related to the impact of climate change on flood risk is affected by a significant degree of uncertainty; some consideration on the uncertainty source is presented in the chapter 6. Finally in the chapter 7, the concept of climate elasticity of streamflow, considered to be an important indicator identifying the sensitivity of streamflow to climate change, is presented.

Final Document on the Third Year, Third Activity: 'Coupling Meteorological and Hydraulic Modelling to Develop a Hydro-Meteorological Chain: Simulation of a Test Case'

The purpose of the collaboration between LAMPIT (Department of Soil Defence, University of Calabria) and CMCC is to develop an hydro-meteorological chain in order to abtain a reliable tool in the context of flood evolution prediction able to provide quantitative information of practical importance within the civil protection activities. The LAMPIT contribution to the project concerns the mathematical description of both the generation and propagation of flood events at basin scale. The work here presented has been carried out in close cooperation with CIRA researchers (dr. Pasquale Schiano and dr. Paola Mercogliano) and DISTART (Dipartimento di Ingegneria delle Strutture, dei Trasporti, delle Acque, del Rilevamento, del Territorio - University of Bologna) researchers (Prof. Armando Brath, Ing. Elena Toth and Ing. Alessio Domeneghetti). The mathematical representation of the flow processes is based on the fully dynamic shallow water equations. The solution of these equations, excluding some simplified cases, can be obtained by numerical integration only. Many schemes based on fully dynamic and simplified shallow water equations have been proposed in literature. Some of these schemes have been implemented in the LAMPIT laboratory and applied to simulate simple cases of overland flow as already presented. Afterwards the inplemented codes have been applied to simulate overland flow over real topography. In this contest some numerical anomalies appeared due to the presence of small water depth over high slope and irregular topography. According to that a careful analysis of these problems has been made and some numerical techniques have been implemented in order to prevent them. Finally the developed model has been applied to simulate a rainfall event in a Reno sub basin. A Digital Elevation Model (DEM) with 20 m cell size has been generated from topographical maps of the area in 1:10000 scale, using a GIS software and the Corine map has been used to obtain the land use/land cover for the watershed. Varying in time and in space effective rainfalls, computed by DISTART researchers, have been fed in the model as input data. The model has been calibrated and validated using the rainfall event occured in Reno basin on 7-9th November 2003. The numerical results have been compared with the observed data and found to be satisfactory. A sensitivity study of the roughness parameter has been also carried out. Finally the hydro-meteorological chain has been implemented to simulate the same rainfall event on Reno basin at Pracchia Station starting from rainfall data simulated by the CIRA researchers using a meteorological model. The developed model seems to be an useful tool for the simulation of overland flow events.