Giulia Valerio

1923 Gleno Dam Break: Case Study and Numerical Modeling

On the morning of December 1, 1923, the Gleno Dam (located in the Central Italian Alps) suddenly collapsed a few days after the first complete reservoir filling. Nearly 4: 5×1 0 6 m 3 of water was released. The consequent inundation caused significant destruction along the downstream valley and a death toll of at least 356 lives. This failure is the only historical case of dam break caused by structural deficiencies that has occurred in Italy. As a result, it has deeply influenced the evolution of Italian regulations regarding dam design and hydraulic risk evaluation. However, in spite of its relevance, this event has never been characterized from a hydraulic standpoint. This paper reports the main information obtained from the analysis of a vast amount of historical documents regarding the Gleno Dam break to set up a case study useful for validating dam-break models in mountain settings. Moreover, it presents the main results of one-dimensional (1D) modeling of the dam break wave propagation accomplished with a first-order finite volume numerical scheme recently proposed in the literature for field applications. The overall effectiveness and reliability of the model are evaluated for this case characterized by very irregular topography. Finally, the practical relevance of several choices that the numerical reconstruction of this kind of event demands is tested. DOI: 10.1061/(ASCE)HY.1943-7900.0000327.

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.

A simple approach to the evaluation of the actual water renewal time of natural stratified lakes

In natural lakes, where thermal stratification hinders complete mixing, the theoretical value T0 of the water renewal time provides a low-order approximation to the time T37 when 37% of the original water is still present within the lake; this time could be operatively regarded as the actual value of the water renewal time. In this paper, we present a simple nonparametric model to estimate the age distribution of water within stratified natural lakes, taking into account fundamental aspects of its mass exchange and thermal evolution. This distribution provides a straightforward way to compute T37. The model is presented as a system of ordinary differential equations along with a MATLAB script for its numerical solution, so that it can be easily applied to lakes where a minimum of limnological data are available, without the need of extensive meteorological data set and modeling expertise that an hydrodynamic model would require to the same purpose. The case of a deep oligomictic Italian prealpine lake (Lake Iseo) is considered: after a positive comparison with the results obtained using a 1-D lake hydrodynamic model, the reiterated application to the available time series allows to approximate the water age probability distribution. This distribution is used to compute the actual value of the water renewal time, that resulted T37 = 1.6T0.

Dam-break modeling in alpine valleys

Dam-break analysis is of great importance in mountain environment, especially where reservoirs are located upstream of densely populated areas and hydraulic hazard should be assessed for land planning purposes. Accordingly, there is a need to identify suitable operative tools which may differ from the ones used in flat flood-prone areas. This paper shows the results provided by a 1D and a 2D model based on the Shallow Water Equations (SWE) for dam-break wave propagation in alpine regions. The 1D model takes advantage of a topographic toolkit that includes an algorithm for pre-processing the Digital Elevation Model (DEM) and of a novel criterion for the automatic cross-section space refinement. The 2D model is FLO-2D, a commercial software widely used for flood routing in mountain areas. In order to verify the predictive effectiveness of these numerical models, the test case of the Cancano dam-break has been recovered from the historical study of De Marchi (1945), which provides a unique laboratory data set concerning the consequences of the potential collapse of the former Cancano dam (Northern Italy). The measured discharge hydrograph at the dam also provides the data to test a simplified method recently proposed for the characterization of the hydrograph following a sudden dam-break.

Experimental investigation of reservoir geometry effect on dam-break flow by A. Feizi Khankandi, A. Tahershamsi and S. Soares-Frazão, J. Hydraulic Res. 50(4), 2012, 376-387

The Authors deal with an important topic that deserves continuous experimental, theoretical and numerical efforts to improve the methodologies of hazard reduction. Often sophisticated technologies cannot be used due to the lack of information resulting in simplifications to evaluate the hydrograph following a dam break. The Authors’ study on the effects of reservoir shape on a dam break wave is relevant. The Authors claim that “existing studies consider only the rectangular reservoir shape” and that “practice often requires quick and rough estimates of the peak discharge and maximum water levels”, underlining that the existing methodologies for evaluating this information are based on regression models derived from a limited database, so that the overall confidence on the quality of the results is moderate. However, the work of Pilotti et al. (2010) was overlooked, so that a formula for the peak discharge and a simple approximation to the entire hydrograph are presented. Only the hydrograph at the dam section is considered because its shape downstream of the breach is strongly conditioned by the local bathymetry (Pilotti et al. 2011). The observations are limited to the rectangular, wide reservoir; equations and tables of Pilotti et al. (2010) contain JHE (Journal of Hydraulic Engineering) added to the number. As to the measurement of peak discharge at the gate section, the Authors extrapolated the discharge using the data at location G4–G6, comparing it with that provided by empirical formulae. It is not surprising that there is a wide scatter between the results (up to an order of magnitude) in Table 6. The Authors’ results and these of Pilotti et al. (2010) may explain why empirical formulae may be so inaccurate. Pilotti et al. (2010) computed the hydrograph at the breach section for a partial dam break in a rectilinear, constant slope reservoir of cross-sectional area A = δh, in which h is the water depth and δ and l depend on the cross-sectional shape, ranging from rectangular (l = 1) to parabolic (l = 1.5) and triangular (l = 2). The explored breach ratio a/A0, in which a is the breach area and A0 the initial wetted area at the dam, ranges up to 1, so that the methodology applies also for the full dam break. The comparison is limited to the peak discharge because horizontal bathymetries were not considered. It is interesting to compare Eqs. (12 JHE) and (25 JHE) with the results of Table 6. For the long and 90◦ bend reservoir, Eq. (25 JHE) reduces to Ritter’s (subscript R) equation, providing for peak (subscript p) discharge Qp the value QR = 0.120 m3 s−1 versus the experimental values of 0.123 and 0.125 m3 s−1,

Measuring and Modelling the Nutrients Residual Load from the Combined Sewer of the Eastern Shore of Lake Iseo

The research is motivated by the need to understand the nutrient pollution dynamics in combined sewer overflows (CSOs) contributing to the eutrophication of Lake Iseo, the fifth largest Italian lake in terms of volume. To this end, the effectiveness of the combined sewer system along the lake’s eastern shore is assessed. The sewer’s efficiency is quantified with regard to the residual nutrients load from CSOs, which was originally reckoned at no more than 3% of the overall sewer load. A hydrodynamic model of the sewer system was developed by SWMM and calibrated by a long time series of measurements collected at two selected CSOs. This data allowed to investigate in detail the occurrence of first flush and to estimate the pollutant loads discharged during wet weather periods. The calibrated model then allowed to extrapolate the results of the year-long campaign to a 10-year simulation period providing, for the first time, quantitative information on the total residual loads to the lake. Such loads are at least 5 times larger than the design value. This research provides important insight into the potential impact of CSOs on the other deep lakes of the pre-alpine chain (e.g., lakes Como, Maggiore, and Garda in Italy), that are struggling with growing environmental stressors, opening the way to important technical and management considerations regarding remedial actions.

Steady-state distributed modeling of dissolved oxygen in data-poor, sewage dominated river systems using drainage networks

Abstract The determination of water quality along a river drainage network is of paramount importance for many environmental applications. Whereas several codes have been proposed to deal with this problem along a single reach of a river, no simple solution has been proposed for a complex river network. In this contribution, a methodology is presented for preliminary screening of river water quality at the watershed-scale and at steady state. The solution process exploits the information contained within the watersheds space-filling drainage network (SFDN), that is automatically computed by processing the raster map of the watersheds elevations. To this end, a visiting algorithm for non-binary trees is coupled to a Newton-Raphson method that locally solves the system of nonlinear water-quality mass balances. We believe that the resulting framework will be especially valuable for initial screening analyses in data-poor watersheds. A free code called Q2T that implements the described methods is provided.