Giuseppe T. Aronica

Changing climate shifts timing of European floods

Flooding along the river Will a warming climate affect river floods? The prevailing sentiment is yes, but a consistent signal in flood magnitudes has not been found. Blöschl et al. analyzed the timing of river floods in Europe over the past 50 years and found clear patterns of changes in flood timing that can be ascribed to climate effects (see the Perspective by Slater and Wilby). These variations include earlier spring snowmelt floods in northeastern Europe, later winter floods around the North Sea and parts of the Mediterranean coast owing to delayed winter storms, and earlier winter floods in western Europe caused by earlier soil moisture maxima. Science, this issue p. 588 see also p. 552 Climate change is affecting the timing of river flooding across Europe. A warming climate is expected to have an impact on the magnitude and timing of river floods; however, no consistent large-scale climate change signal in observed flood magnitudes has been identified so far. We analyzed the timing of river floods in Europe over the past five decades, using a pan-European database from 4262 observational hydrometric stations, and found clear patterns of change in flood timing. Warmer temperatures have led to earlier spring snowmelt floods throughout northeastern Europe; delayed winter storms associated with polar warming have led to later winter floods around the North Sea and some sectors of the Mediterranean coast; and earlier soil moisture maxima have led to earlier winter floods in western Europe. Our results highlight the existence of a clear climate signal in flood observations at the continental scale.

Effects of forest fires on flood frequency curves in a Mediterranean catchment

Abstract The effect of land-use change on the flood frequency curve (FFC) in a natural catchment is analysed. To achieve this, a simple methodology for the derivation of FFCs in land-use change scenarios is proposed. The adopted methodology, using a stochastic model in Monte Carlo simulation of FFCs, was found to provide a useful framework for detecting changes in flood magnitudes in both pre- and post-fire conditions. In particular, the importance of the antecedent soil moisture condition in the determination of the flood frequency distribution was analysed. The analysis of FFCs for pre- and post-fire conditions shows an increase in the average value of Curve Number and a decrease in the catchment time lag. The derivation of FFCs shows a clear increase in flood quantiles. For the post-fire conditions, the FFC exhibits higher quantiles of the peak discharges showing a reduction in frequency of occurrence. This variation is more significant for low-return period quantiles than for high-return period quantiles. The results of the catchment studies reported here support the hypothesis that the hydrological response of the watershed changes as a result of fire, especially during the first years following a fire event.

Adaptation to flood risk - results of international paired flood event studies

As flood impacts are increasing in large parts of the world, understanding the primary drivers of changes in risk is essential for effective adaptation. To gain more knowledge on the basis of empirical case studies, we analyze eight paired floods, that is, consecutive flood events that occurred in the same region, with the second flood causing significantly lower damage. These success stories of risk reduction were selected across different socioeconomic and hydro-climatic contexts. The potential of societies to adapt is uncovered by describing triggered societal changes, as well as formal measures and spontaneous processes that reduced flood risk. This novel approach has the potential to build the basis for an international data collection and analysis effort to better understand and attribute changes in risk due to hydrological extremes in the framework of the IAHSs Panta Rhei initiative. Across all case studies, we find that lower damage caused by the second event was mainly due to significant reductions in vulnerability, for example, via raised risk awareness, preparedness, and improvements of organizational emergency management. Thus, vulnerability reduction plays an essential role for successful adaptation. Our work shows that there is a high potential to adapt, but there remains the challenge to stimulate measures that reduce vulnerability and risk in periods in which extreme events do not occur.

Pluvial flooding in urban areas: the role of surface drainage efficiency

Pluvial flooding in urban areas may derive from the limited or temporarily reduced efficiency of surface drainage, even when the underlying storm sewers are properly designed. This study focuses on the impact of uncertainties in the operational condition of the surface drainage system on pluvial flood hazard. The flood propagation model FLURB-2D is implemented on a selected study area in the town of Genoa (Italy). Synthetic hyetographs based on the Chicago and bivariate copula methods with suitable return periods are used as input. While simulating the design rainfall, inlet operational conditions are varied stochastically using a Monte Carlo approach. Results confirm that microtopography has the potential to impact the efficiency of surface drainage and consequently to produce local flooding, with significant water depth in zones of flow concentration. Furthermore, the derived inundation maps allow the highlighting of areas with insufficient design of the surface drainage system (inlet size and positioning).

Estimating Temporal Changes in Extreme Rainfall in Sicily Region (Italy)

An intensification of extreme rainfall events have characterized several areas of peninsular and insular Italy since the early 2000s, suggesting an upward ongoing trend likely driven by climate change. In the present study temporal changes in 1-, 3-, 6-, 12- and 24-h annual maxima rainfall series from more than 200 sites in Sicily region (Italy) are examined. A regional study is performed in order to reduce the uncertainty in change detection related to the limited length of the available records of extreme rainfall series. More specifically, annual maxima series are treated according to a regional flood index - type approach to frequency analysis, by assuming stationarity on a decadal time scale. First a cluster analysis using at-site characteristics is used to determine homogeneous rainfall regions. Then, potential changes in regional L-moment ratios are analyzed using a 10-year moving window. Furthermore, the shapes of regional growth curves, derived by splitting the records into separate decades, are compared. In addition, a jackknife procedure is used to assess uncertainty in the fitted growth curves and to identify significant trends in quantile estimates. Results reveal that L-moment ratios show a general decreasing trend and that growth curves for the last decade (2000–2009) usually do not stand above the others, with the only exception of the ones related to the outer western part of Sicily. On the other hand, rainfall quantile estimates for the same period are the highest values almost all over the region. An explanation can be found in the increase of subregional average medians, largely caused by recent severe local storms.

The Impact of Scale on Probabilistic Flood Inundation Maps Using a 2D Hydraulic Model with Uncertain Boundary Conditions

Recent flood events internationally have demonstrated the large damages and potential loss of life that flooding can cause. In order to accurately quantify flood risk and how it may change in the future we need to improve our understanding of the uncertainties associated with making such predictions. Improvements in remote sensing have facilitated the use of high resolution models; however without a consideration of the uncertainties these models can produce spuriously precise predictions. This paper has explored this by assessing how changing the scale of flood inundation models impacts on probabilistic flood inundation and flood hazard maps for the Imera basin, Sicily. We have demonstrated that although coarse scale models can perform just as well as fine scale models, the results are dependent on both grid scale and likelihood measure. Furthermore these results have highlighted the uncertainty associated with flood inundation predictions and the need to consider these uncertainties when performing flood risk analysis.

Debris Flow Damage Incurred to Buildings: An In‐Situ Back Analysis

In an attempt to conduct a back analysis of the damage caused to a 19th century masonry structure due to the October 2009 flash flood/debris flow event in Scaletta Zanclea, a flood discharge hydrograph is reconstructed in the ungauged conditions. The hydrograph for the solid discharge is then estimated by scaling up the liquid volume to the estimated debris volume. The debris flow diffusion is simulated by solving the differential equations for a single-phase two-dimensional (2D) flow that employs triangular mesh elements, also taking into account the channelling of the flow through the buildings. The damage to the building is modelled using 2D finite shell elements and is based on the maximum hydraulic action caused by the debris flow. The boundary conditions provided by the openings, floor slabs, orthogonal wall panels and the foundations are also modelled. The reconstruction of the event and the damage to the case-study building confirm the location of the damage induced by the event.

Finding Simplicity in Storm Erosivity Modelling

Precipitation variability and extremes have always been part of the Earth’s climate system, though they can manifest in many ways, both spatially and temporally. The chapter explores quantitative concepts of rainfall (storm) erosivity useful for soil erosion monitoring as well as for hydrological extreme events assessment. In this way, a review of the storms erosivity models was done in order to account indicators of climatic changes on both spatial and temporal domains. Most models here summarized were run to estimate erosivity for specific time aggregation levels (from event to multidecadal). For erosion modelling, it would be also preferable to be able to calculate an estimated erosivity value for a particular site. However, different parameterisation options were given in the chapter for accounting of the geographical location effects. The purpose of this chapter was to synthesize and stimulate new research on important issues in climatology, geomorphology, and agricultural engineering, and to provide an intensive and comprehensive review of current modelling and practice.

GIS Analysis of Debris Volume Mobilized by Heavy Rainstorm in North-Eastern Sicily

On October 1st 2009, after a very rainy season, a violent rainstorm poured down Giampilieri, in North-Eastern Sicily with damages to property, buildings, roads, bridges and moreover, loss of human lives. The main focus of this work concerns the analysis of debris volume that has been carried out during this event through the definition of methodologies and determination of techniques that can be applied to evaluate assessment, define crisis scenarios and predict situations of possible debris-mud flows. Here, a series of open-source GIS-based algorithms using spatial analysis has been used in order to perform the calculation of debris-mud flows volume moved during rainstorm. By comparing the volume and run out of past events Rickenmann(Nat Hazards 19(1):47–77, 1999), it is possible to identify empirical site-specific formulas to estimate run out distance of potential events. The analysis issue helps in assessing risk for establishing policies of land planning and civil protection activities.