Paolo Villani

Best linear unbiased design hyetograph

Under the conditions of system linearity and stationarity of the precipitation process we derive the hyetograph associated with any given flood discharge Q, using best linear unbiased estimation (BLUE) theory. The BLUE hyetograph depends explicitly on the correlation characteristics of the rainfall process and the instantaneous unit hydrograph (IUH) of the basin. When the correlation time of the rainfall process is short relative to the dispersion of the IUH, the shape of the BLUE hyetograph is the mirror image of the IUH itself. In the opposite case the design hyetograph is the mirror image of the rainfall correlation function. The procedure is extended from lumped to semidistributed and distributed basins. In the latter case we obtain simultaneous design hyetographs for each subbasin or each site in the basin. The analytical findings and the possibility of regionalization are verified using synthetic and recorded rainfall time series, with excellent agreement in both cases. Because of its simplicity, robustness, and theoretical basis the proposed procedure should be appealing for practical use, particularly when not only the peak but the entire flood hydrograph is required.

Regional flood estimation methods

Regional methods for flood estimation are examined and discussed, with reference to a three level regionalization scheme, to take into account the relative role played by spatial and sampling variability of the parameter estimate. At the first level, the flood distribution is identified and its shape parameters are estimated. Spatial variability is much smaller than sampling variability and the homogeneity hypothesis can be made. At the second level, the estimation of scale parameter requires an exploitation of the physical meaning of the flood distribution parameters to transfer information from rainfall to flood. The homogeneity hypothesis still holds for the rainfall distribution scale parameter, even if a spatial interstation correlation must be taken into account. At this level, the estimation of the flood distribution scale parameter could require some geomorphologic variates. The purpose of the third level of regionalization is to obtain regional estimation of the mean annual flood (MAF). Its spatial variation is generally much higher than its sampling variation, and hence, the MAF is the least suitable parameter for regional estimating. This leads to the need for causative relationships between MAF and geomorpho-climatic parameters, which can be of a merely empirical regressive type or of a more physically based conceptual type.

A project for regional analysis of floods in Italy

Regional methods for flood estimation are examined and discussed in a companion paper (Rossi and Villani, this volume). The results of the application of a three-level regionalization scheme based on the TCEV distribution to a case study, Italy, are here referred as work in progress. These results are part of the special VAPI project, a large-scale research program being conducted by the National Group for the Prevention of Hydrogeological Disasters (CNR-GNDCI). The whole of Italy is subdivided into three homogeneous regions at the first regionalization level to identify the shape distribution parameters. Several homogeneous subregions in the Apennine Region are identified at the second level through behavior analysis of the skewness and variation coefficients of the maximum daily rainfalls. This is achieved by means of clustering techniques and decision criteria for the optimal value of a number of subregions. Sampling interstation correlation and theoretical spatial variability are taken into account by considering synthetic generation mechanisms with incorporated sampling interstation autocorrelation structure and by means of geo-statistical analysis. Empirical relationships between rainfall and flood scale parameters are proposed. At the third level of analysis, the mean annual flood is derived using multiregressive relationships that are either purely empirical or have a structure identified by a rational approach or by a geomorphoclimatic model.

Experimental Investigation on the Effects of the Fixed Boundaries in Channelized Dry Granular Flows

The dynamics of granular mixtures, involved in several geophysical phenomena like rock avalanches and debris flows, is far from being completely understood. Several features of their motion, such as non-local and boundary effects, still represent open problems. An extensive experimental study on free-surface channelized granular flows is here presented, where the effects of the fixed boundaries are systematically investigated. The entire experimental data set is obtained by using a homogenous acetal-polymeric granular material and three different basal surfaces, allowing different kinematic boundary conditions. Velocity profiles at both the sidewall and the free surface are obtained by using high-speed cameras and the open-source particle image velocimetry code, PIVlab. Significantly, different sidewall velocity profiles are observed by varying the basal roughness and the flow depth. Owing to sidewall friction and non-local effects, such profiles exhibit a clear rheological stratification for high enough flow depths and they can be well described by recurring to composite functions, variously formed of linear, Bagnold and exponential scalings. Moreover, it has been discovered that transitions from one velocity profile to another are also possible on the same basal surface by merely varying the flow depth. This shape transition is due partly to the sidewall resistances, the basal boundary condition and, in particular, the occurrence/inhibition of basal grain rolling. In most of the experiments, the normal-to-bed velocity profiles and the velocity measurements at the free surface strongly suggest the occurrence of a secondary circulating flow, made possible by a chiefly collisional regime beneath the free surface.

At which time step do we need to monitor the stream hydro-chemistry properties for low flow characterization?

The low flow regime features are frequently described in terms of global indices and, among these, the most relevant are the baseflow index and the flow duration curve. Low flow indices have to be computed from observed streamflow data, but monitoring campaigns are time and cost consuming activities. The aim of the present study is then to identify the impact of the monitoring time step resolution, with particular reference to the low flow regime. To the scope, the hydro-chemographical dataset recorded, during a two year monitoring campaign, at the T. Ciciriello experimental catchment, a 3km2 watershed located in the Cilento, European and Global Geopark (Southern Italy), have been analyzed. Water depth (D), discharge (Q), electrical conductivity (EC) and rainfall time series are available. Assuming the daily time scale as the baseline, a fictitious monitoring experiment has been performed, sampling the observed daily time series at different multiple-day time steps. Main global statistic are computed for the different fictitious series and compared to the daily series. The baseflow patterns, filtered with the use of a mass balance method, appear statistically similar and the differences in terms of low flow indices fall within a 10% range.

Hydrological Modelling For River BasinManagement In A Highly Hydro-geologicalConditioned Environment

Water resources management represents a present key issue in hydrology, and hydrological models generating streamflow time series are useful tools in this field. It is possible to refer, in the extreme, to lumped or fully distributed approaches, but when river basins with particular features have to be modeled it is possible to take advantage of a semi-distributed formulation. In this study we propose a semi-distributed conceptually based modeling approach, supported by field measurements collected within several seasonal campaigns, that has been set up for the Bussento river basin, located in southern Italy, characterized by soils and rocks with highly different hydraulic permeability and above all a highly hydro-geological conditioning. The proposed approach, which joins together all hydraulic, hydrological and geological data, is able to reproduce the river discharge mean characteristic.

Water Resources Assessment for Karst Aquifer Conditioned River Basins: Conceptual Balance Model Results and Comparison with Experimental Environmental Tracers Evidences

Water resources management, more and more limited and poor in quality, represents a present key issue in hydrology. The development of a community is highly related to the management of the water resources available for the community itself and there is a need, for this reason, to rationalize the existing resources, to plan water resources use, to preserve water quality and, on the other hand, to prevent flood risk. The importance of decision support systems tools, such as hydrological models, generating streamflow time series which are statistically equivalent to the observed streamflow time series, is even more important considering the combination of multiple and complex issues concurring in the definition and optimization of water resources management practices. When river basins with particular features have to be modelled, both traditionally conceptually based models and more recent sophisticated distributed models appear to give not very reliable results. In those cases it is possible to take advantage of a semi-distributed formulation, where every sub-catchment is modelled to account for its features and informations coming from all the sub-catchments are related to each other in order to improve the system description. In this study, starting from the application of a catchment scale modelling tool, we propose a semi-distributed conceptually based framework, able to describe the sub-catchment scale systems hydrological response. The modelling approach is supported by field measurements collected within several seasonal campaigns, that has been set up for the Bussento river basin, located in Southern Italy, well known to hydrogeology and geomorphology scientists for its karst features, characterized by soils and rocks with highly different hydraulic permeability and above all an highly hydrogeological conditioning. The groundwater circulation is very complex, as it will be later discussed, and groundwater inflows from the outside of the hydrological watershed and groundwater outflows toward surrounding drainage systems frequently occur. With the aim to enhance the knowledge of the interaction between the groundwater and surface water and acknowledged the substantial help given by natural isotope tracers experiments to solve hydrological complex systems circulations problems, radon-in water concentrations have also been collected, in a limited number of cross sections, along the upper Bussento river reach.