Baldassare Bacchi

Identification and calibration of spatial correlation patterns of rainfall

Abstract The significance of spatial distribution of rainfall and its effects on spatial correlation functions are reviewed. A number of statistical terms and properties are explained in depth with reference to the theory of variograms originally adopted in geostatistics. This is followed by a detailed analysis of theoretical variogram models, effects of meteorological, orographic, and space-time aggregation scales and a discussion on network design, areal mean calculations and storm velocities. Empirical variogram and validation techniques are then examined. Some applications to two different parts of Lombardia (the mountainous Valtellina catchment and a large part of the flat area of the region) in northern Italy are given with a discussion of the results. In the Appendix, the technique of kriging is explained with reference to the estimation of the spatial distribution of the random rainfall field.

Bivariate exponential model applied to intensities and durations of extreme rainfall

Abstract The Poisson model for rainfall occurrences in which storm intensity and duration are represented by two independent random variables is extended to consider intensity and duration as bivariate random variables each with a marginal exponential distribution. A numerical optimization method using annual maxima is adopted for parameters estimation. Comparison is made with the results of a numerical procedure which uses the Gumbel distribution as an approximation to the probability distribution of the extremes of the bivariate exponential model. A case study is presented using data from 18 raingauge stations in northern Italy. For rainfall durations of practical interest the theoretically derived relationships between probabilities and intensities compare favourably with observed relationships.

On the derivation of the areal reduction factor of storms

Abstract A stochastic derivation of the areal reduction factor (ARF) of the storm intensity is presented: it is based on the analysis of the crossing properties of the rainfall process aggregated in space and time. As a working hypothesis, the number of crossings of high rainfall intensity levels is assumed to be Poisson-distributed and a hyperbolic tail of the probability of exceedances of rainfall intensity has been adopted. These hypotheses are supported by the analysis of radar maps during an intense storm event which occurred in Northern Italy. The reduction factor derived from this analysis shows a power-law decay with respect to the area of integration and the duration of the storm. The areal reduction results as a function of the storm duration and of its frequency. A weak, but significant decrease of the areal reduction factor with respect to the return period is shown by the functions derived, and this result is consistent with that of some recent studies on this topic. The results derived, although preliminary, may find useful applications for the definition of the design storm in urban catchments of a size greater than some square kilometres and with duration of some hours.

Proposal of a semi-probabilistic approach for storage facility design

Storage facilities are key devices in mitigating the urban drainage impact on receiving water bodies, but their design is still affected by high uncertainty. The analytical-probabilistic approach has recently raised interest, because the facility performances are directly related to probability. Starting from statistically independent storm events, distributions of the meteorological variables must be fitted. Rainfall series, recorded in three Italian raingauges, were examined for appraising two main concerns: the choice of proper probability distributions for rainfall volume and the sample sensitivity with respect to the analysis criterion. The analytical derivation of the model is then finally discussed.

Ten years of monitoring areal snowpack in the Southern Alps using NOAA-AVHRR imagery, ground measurements and hydrological data

Monitoring snow cover in alpine areas is important for the estimation of the water storage during the snowmelt season, especially in view of irrigation, hydropower production and water supply. Cost-efliciency and fine temporal resolution of images from the satellite-borne NOAA-AVHRR sensor indicate this source of information as a suitable candidate for monitoring snow cover extent. This information can also be used for validation of distributed snowmelt models. As a result of a long-term study, ten years of snow covered area depletion curves have been estimated using remote sensing in seven watersheds of size larger than 400 km 2 in the Southern Alps. Coupling of satellite imagery with detailed topographic data and some ground measurements of snowpack depth and density provides regional estimates of snow water equivalent in northern Italy, upstream of Lakes Maggiore, Como, Iseo, Idro, and Garda. The basin water equivalent estimates are compared with the values obtained from the hydrological water balance equation used in two of the selected watersheds and computed for different snowmelt seasons.

Flooding Hazard Mapping in Floodplain Areas Affected by Piping Breaches in the Po River, Italy

AbstractIn recent years, flood-related risk has been increasing worldwide, being inundations among the natural disasters which induce the maximum damage in terms of economic losses. In the research reported in this paper, a methodology to map the flooding residual hazard due to levee failure events induced by piping in embankments protecting flood-prone areas is proposed. Ensemble simulations are used to account for uncertainties in location, geometry, and time-evolution of the levee breaches. Probabilistic flooding-hazard maps are generated combining the results of 192 inundation scenarios, simulated by using one-dimensional (1D) and two-dimensional (2D) hydrodynamic models. The methodology is applied considering 96 different locations and sizes of breaches occurred along a 23-km reach protected by the right levee of the Po River, the right levee of the Taro River, and the left levee of the Parma River, which delimit a 100-km2 study area. The influence of obstacles to the flood propagation and consequent...

Distributed evaluation of the contribution of soil erosion to the sediment yield from a watershed

The correct determination of the sediment yield from a basin is of paramount importance in several hydraulic and environmental applications, such as the evaluation of the storage reduction of artificial reservoirs. However, due to the highly episodic nature of sediment supply and transport in many environments and to the extreme complexity of the processes involved, the evaluation of the sediment load in a river is still highly uncertain. When the time scale of interest i s sufficiently long, and when the primary sediment source comes from distributed erosion in the watershed, the problem can be tackled in an indirect fashion, by computing the contribution to the annual suspended sediment yield from soil erosion. In order to accomplish this task, we propose a distributed application of the widely used USLE formula. The formula is automatically applied along drainage networks derived from a digital elevation model and properly modified to take into account the presence of deposition zones in the watershed.  1997 John Wiley & Sons, Ltd. Earth surf. process. landforms, 22, 1239–1251 (1997) No. of figures: 5 No. of tables: 2 No. of refs: 24

Statistical characterization of spatial patterns of rainfall cells in extratropical cyclones

The assumption of a particular type of distribution of rainfall cells in space is needed for the formulation of several space-time rainfall models. In this study, weather radar-derived rain rate maps are employed to evaluate different types of spatial organization of rainfall cells in storms through the use of distance functions and second-moment measures. In particular the spatial point patterns of the local maxima of rainfall intensity are compared to a completely spatially random (CSR) point process by applying an objective distance measure. For all the analyzed radar maps the CSR assumption is rejected, indicating that at the resolution of the observation considered, rainfall cells are clustered. Therefore a theoretical framework for evaluating and fitting alternative models to the CSR is needed. This paper shows how the “reduced second-moment measure” of the point pattern can be employed to estimate the parameters of a Neyman-Scott model and to evaluate the degree of adequacy to the experimental data. Some limitations of this theoretical framework, and also its effectiveness, in comparison to the use of scaling functions, are discussed.

Simplified Method for the Characterization of the Hydrograph following a Sudden Partial Dam Break

This paper presents a simplified approach to the characterization of the hydrograph following the partial collapse of concrete gravity dams. The proposed approach uses a simplified representation of the reservoir geometry and is based on the numerical solution of shallow water equations to study the two-dimensional evolution of the water surface within the reservoir. The numerical results are made dimensionless and reorganized so as to compute the peak discharge, the duration and the recession limb of the dam break hydrograph. The proposed practical approach provides a quite satisfactory reproduction of the computed hydrograph for a wide set of realistic situations that have been simulated in detail.

Analysis of the relationships between flood peaks and flood volumes based on crossing properties of river flow processes

A stochastic approach is developed for the determination of the reduction ratio between the annual maximum flow averaged over a consecutive D hour period and the annual maximum of peak instantaneous flow with the same frequency of occurrence. Initially, crossing properties of the integrated flow process are related to those of the instantaneous flows, using a Gaussian hypothesis. The scale of fluctuation of the river flow process is the key parameter which governs the theoretical reduction function. Application is made to numerous historical series from Italy. To facilitate the applicability of the model, a method of calibration based on currently available hydrometric information is proposed. Quantiles of flood volumes are obtained from the theoretical reduction function. It is verified that the reduction ratio is independent of the return period and appears to be insensitive to the underlying distributions of the component processes. The non-Gaussian case and scaling properties are discussed.