Fernando Nardi

Hydrogeomorphic properties of simulated drainage patterns using digital elevation models: the flat area issue

Abstract Flat areas are a critical issue for the characterization of drainage patterns using digital elevation models (DEM). In this work, flat area removal and flow direction algorithms are implemented, and also a physically-based DEM correction model is introduced, for investigating their influence on the topological properties of the channel network, the Hortonian parameters and the hillslope width function. Differences of results, as compared to the standard procedures implemented in widely-used GIS-based hydrological packages, show the importance for hydrogeomorphic modellers to consider the use of more detailed approaches.

A parsimonious geomorphological unit hydrograph for rainfall–runoff modelling in small ungauged basins

Abstract In this study, a parsimonious hydrological modelling algorithm is proposed based on the automated DEM-based geomorphic characterization of runoff dynamics in scarcely monitored river basins. The proposed approach implements the instantaneous unit hydrograph (IUH) concept, estimated using the width function (WF), for characterizing the travel time distribution using just one parameter, the river network flow velocity. Hillslope velocities are defined using spatially-distributed empirical formulas based on slope and soil-use information extrapolated from digital topographic data. Case studies are presented for testing model performance and comparing simulated and observed hydrographs of 25 selected flood events, as well as investigating the differences with the geomorphological instantaneous unit hydrograph (GIUH) model results. The calibration of the WFIUH channel flow velocity parameter using the concentration time is investigated providing interesting insights for the use of such a method for hydrological prediction in ungauged basins. Editor D. Koutsoyiannis Citation Grimaldi, S., Petroselli, A. and Nardi, F., 2012. A parsimonious geomorphological unit hydrograph for rainfall–runoff modelling in small ungauged basins. Hydrological Sciences Journal, 57 (1), 73–83.

Detailed Simulation of Complex Hydraulic Problems with Macroscopic and Mesoscopic Mathematical Methods

The numerical simulation of fast-moving fronts originating from dam or levee breaches is a challenging task for small scale engineering projects. In this work, the use of fully three-dimensional Navier-Stokes (NS) equations and lattice Boltzmann method (LBM) is proposed for testing the validity of, respectively, macroscopic and mesoscopic mathematical models. Macroscopic simulations are performed employing an open-source computational fluid dynamics (CFD) code that solves the NS combined with the volume of fluid (VOF) multiphase method to represent free-surface flows. The mesoscopic model is a front-tracking experimental variant of the LBM. In the proposed LBM the air-gas interface is represented as a surface with zero thickness that handles the passage of the density field from the light to the dense phase and vice versa. A single set of LBM equations represents the liquid phase, while the free surface is characterized by an additional variable, the liquid volume fraction. Case studies show advantages and disadvantages of the proposed LBM and NS with specific regard to the computational efficiency and accuracy in dealing with the simulation of flows through complex geometries. In particular, the validation of the model application is developed by simulating the flow propagating through a synthetic urban setting and comparing results with analytical and experimental laboratory measurements.

On the impact of urbanization on flood hydrology of small ungauged basins: the case study of the Tiber river tributary network within the city of Rome

The small ungauged basins of the highly urbanized area of the city of Rome are often the subject of critical flood conditions for the significant human-made transformations. In this work the EBA4SUB framework, implementing the hydrogeomorphic width function instantaneous unit hydrograph rainfall run-off model, and using digital elevation model, land use and synthetic precipitation as main input information, is applied for evaluating extreme hydrologic forcing conditions at the basic scale. The goal is to understand the rationale behind the observed increasing frequency of local urban inundations that are also observed in the uplands. Results present the impact of urbanization expressed by both the run-off coefficient, the artificial drainage, impacted by paved surfaces and a dramatic number of river–road intersections (i.e. culverts), and the upstream to downstream non-natural scaling behaviour of hydrologic parameters and in particular the peak discharge per unit drainage area.

Approach to Digital Elevation Model Correction by Improving Channel Conveyance

AbstractDigital elevation models (DEM) are important inputs for topography in modeling floods for remote and inaccessible regions. DEMs often lack in accuracy near water bodies and rivers. The objective of this research is to present a DEM correction technique to improve the accuracy of flood simulation and inundation mapping. The key feature of this method is the variability in thalweg (deepest point along a cross section) locations and depth based on the river meandering, width, and side slope. The DEM correction technique is demonstrated by adjusting a national elevation dataset (NED) DEM along the Cumberland River near Nashville in Tennessee. The original (base DEM) and modified DEMs are used as main input of the 1D Hydrologic Engineering Center River Analysis System (HEC-RAS) model and corresponding performances were analyzed. The model using surveyed topography was calibrated for a high flood event (May 2010) and later validated for an intermediate flood event (2003), a high flood event (May 2010), ...

Landslides in Bududa, Eastern Uganda: Preliminary Assessment and Proposed Solutions

Severe rains at the beginning of March in eastern Uganda caused fatal landslide in village Nametsi, district Bududa. On March 1st 300 people were missing or dead as the result of this event. More than 8,000 people from nearby villages were evacuated in UN funded temporary camp in Bulucheke. Results of preliminary assessment of the landslide situation showed development of new scarp within 300 m from the head of the Nametsi landslide. Absence of drainage systems, steep slopes and changes of the land cover exacerbate the potential for new landslides and will lead to new disasters in the future. Bududa area is known for Arabica coffee plantations that provide cash and jobs to local residents. Establishment of a simple monitoring system and education of local population regarding mitigation measures will reduce the risk of future disasters and provide better and safe environment for the coffee production. This will also lead to the reduction of poverty in the area and creating a potential for the future economic growth.

Epitomes of Bottom-Up Hydro-Geo-Climatological Analysis to Face Sea Level Rise in Complex Coastal Ecosystems

Climate affects ecosystems in multiple ways. An increased intensity of flooding events would occur if there is continuous sea level rise (SLR). SLR is a major threat to coastal ecosystems and the biota they support. In this chapter we provide a brief overview of flooding phenomena and their effects on cities, geomorphic elements, and species. In particular, we focus our attention on the relationship between climatological, hydrological, and ecological processes through data analysis and modeling. We consider two case studies as complex socioecological systems: Venice (Italy) and Florida. The persistence of the baroclinic pressure gradient over the Adriatic Sea and the SLR can potentially contribute to an increased flooding frequency of the city of Venice, causing an enormous economic and social impact. However, a trend assessment based on tidal observations found a reduction in extreme tidal levels. This is supported by recent results that suggest that the frequency of extreme tides in Venice might largely remain unaltered under the projected twenty-first century climate simulations because of the reduced frequency of storm surge events. For Florida, the observed increased frequency of the strongest tropical cyclones and the reduced frequency of the low-category cyclones will reduce the overwash that naturally shapes the beach habitat. Many shoreline-dependent species, such as some threatened and endangered shorebirds, will dramatically be affected by the inversion of the positive cyclone–species habitat feedback. At the same time, the negative effect of cyclones on infrastructure and inhabitants is forecasted to be very high. Thus, ‘contextual vulnerability’ approaches start by the definition of risks that are assessed by the integration of historical data analysis, models, and uncertainty evaluation of the predicted scenarios. This is fundamentally important to plan management policies under climate impacts, which will hopefully decrease the loss of cities, coastal landscapes, and species.

Convolution of linear system using geomorphological watershed information

Rainfall-runoff modelling is a relevant topic in water resources management in particular for small and ungauged basin; nowadays starting from Digital Elevation Models (DEMs) and assuming that the watersheds runoff response is linear and time-invariant, it is possible to determine the hydrograph corresponding to any given rainfall excess optimizing the geomorphological information provided by the basin topography: in the present work a case study is discussed within the framework of the Width Function Instantaneous Unit Hydrograph (WFIUH), i.e. a basin characteristic that can describe, using just one parameter, the spatial distribution of residence times.

Assessing hydrological extreme events with geospatial data and models

Prediction of river basin hydrological response to extreme meteorological events is a primary concern in areas with frequent flooding, landslides, and debris flows. Natural hydrogeological disasters in many regions lead to extensive property damage, impact on societal activities, and loss of life. Hydrologists have a long history of assessing and predicting hydrologic hazards through the combined use of field observations, monitoring networks, remote sensing, and numerical modeling. Nevertheless, the integration of field data and computer models has yet to result in prediction systems that capture space-time interactions among meteorological forcing, land surface characteristics, and the internal hydrological response in river basins.