Case Study of the Big Bay Dam Failure: Accuracy and Comparison of Breach Predictions

The Big Bay Dam embankment failure occurred on March 12, 2004, releasing 17,500,000?m3 (14,200?acre-ft) of water. In all, 104 structures were documented as being damaged or destroyed as a result of this failure. No human lives were lost. This paper documents data gathered and analyses performed on the hydraulics of the failure. High water levels from the failure were marked and measured. A HEC-RAS unsteady flow model was developed. Using observed breach geometry, HEC-RAS provided results that agreed with the measured high water marks from ?0.02?to??0.90?m and 0.01?to?0.62?m with associated modeled flow depths ranging from 9.3?to?5.7?m (from 30?to?19?ft). A peak breach flow of 4,160?m3/s (147,000?ft3/s) was predicted at the embankment. Breach peak flow prediction equations were found to substantially underpredict the peak flow indicated by HEC-RAS for this failure. HEC-RAS modeling utilizing predicted breach geometry and formation time also underpredicted the peak flow, but by a lesser amount. The National Resources Conservation Service models WinTR-20 and TR 66 were also assessed. WinTR-20 results compared reasonably well with the high water marks for this failure. TR-66 results did not compare well.     

Optimal Self-Scheduling of a Tidal Power Plant

This paper considers the optimal operation of a tidal power producer. Different generating cycles for the tidal power plant are considered. The paper provides a tool for self-scheduling of the tidal power producer so that the maximum profit from selling energy in an electricity market is achieved, while meeting all technical operating constraints of the tidal power plants. The formulation provided is also valid in a centralized operation framework; the only change needed is to define a different cost-related objective function. The resulting optimization problem, which is mixed-integer and linear, is solved using commercially available software. Results from realistic simulations are reported and analyzed.     

Seepage Modeling Assisted Optimal Design of a Homogeneous Earth Dam: Procedure Evolution

Presented herein is a procedure for arriving at an optimal design of a homogeneous earth dam laid on an impervious foundation and provided with a drain. The procedure, heavily dependent on variably saturated flow modeling, involves optimizing a multiobjective function comprising a weighted summation of four objective functions, viz., the dam section area, seepage discharge, wetted area of the dam section and the drain area. The design variables considered in the optimization are the upstream and downstream slopes and the drain dimensions. The optimization is carried out subject to the constraints ensuring safe upstream and downstream slopes and sufficient distance between the free surface and the downstream face. Two of the objective functions (viz., the seepage discharge and the wetted area) and the constraints are implicit functions of the design variables. Their values are obtained by employing a numerical model of two-dimensional (vertical plane) variably saturated flow in a homogeneous earth dam. Optimization, conducted by the sequential unconstrained minimization technique procedure is preceded by several runs of the model for various combinations of the design variables. The discrete values of the implicit functions so generated are invoked during optimization to compute the implicit objective functions and constraints. The results are presented in the form of nondimensional design tables/curves. The design procedure is illustrated with the help of few examples.     

Uncertainty of Predictions of Embankment Dam Breach Parameters

Risk assessment studies considering the failure of embankment dams often require the prediction of basic geometric and temporal parameters of a breach, or the estimation of peak breach outflows. Many of the relations most commonly used to make these predictions were developed from statistical analyses of data collected from historic dam failures. The prediction uncertainties of these methods are widely recognized to be very large, but have never been specifically quantified. This paper presents an analysis of the uncertainty of many of these breach parameter and peak flow prediction methods. Application of the methods and the uncertainty analysis are illustrated through a case study of a risk assessment recently performed by the Bureau of Reclamation for a large embankment dam in North Dakota.     

Model for Predicting Floods due to Earthen Dam Breaching. I: Formulation and Evaluation

This paper proposes a dam-breach model which predicts, in a simple but physically based manner, not only the peak discharge but the whole outflow hydrograph and breach development. The following aspects are taken into account: the geometry of the embankment, the shape of the reservoir, the hydraulic characteristics of the flow through the breach and its erosive capacity, and the shape of the breach. The model needs only one calibration parameter and can be easily applied to real cases. The application of the model with a single value of the calibration parameter produced excellent results in the simulation of 12 historic earthfill dam failures, with a discharge range covering three orders of magnitude.     

Model for Predicting Floods due to Earthen Dam Breaching. II: Comparison with Other Methods and Predictive Use

A sensitivity analysis of the model presented in the companion paper is made and, on the basis of the results a criterion is proposed for the choice of values to assign to the side slopes of the breach, in order to use the model for prediction. Moreover, the sensitivity analysis shows that the outflow hydrograph and its peak value depend not only on the dam height and the stored volume, but also on the vertical distribution of the water mass in the reservoir. The model is compared with some previously published methods and the disadvantages, limitations, and errors that can be made using parametric models and predictive equations are pointed out. Finally, easy to use equations interpolating the numerical results of the model are provided that predict not only the peak discharge but the whole outflow hydrograph for overtopping failures.     

Case Study: Dam Safety during Construction, Lessons of the Overtopping Diversion Works at Aguamilpa Dam

A study on risk analysis by overtopping of the diversion works of Aguamilpa Dam in Mexico was carried out to examine the overtopping event of January, 1992. Specifically, this study focuses on the upstream water surface elevation during the flood considering the tunnel discharge actual width and roughness. During this maximum flow event, the upstream cofferdam was overtopped and the discharge tunnels exceeded their maximum hydraulic capacity. The risk analysis was made considering the original hydrologic data (until 1992), the final constructed conditions, to establish a performance function that compares the risk of original deterministic analysis with a probabilistic analysis made in 1992. The study also focuses on practical improvements in the construction stage of tunnels; like lining the floor with hydraulic concrete and shotcrete in vaults and walls, that improves the safety of dams during construction and increases the real return period. The work here is presented as a case study because of the unique large-scale flow conditions that are registered in diversion works of dams.     

Analysis of Gravity Dam on a Complicated Rock Foundation Using an Adaptive Block Element Method

In this Technical Note, the adaptive block element method of rock masses is formulated, in which the elastoplastic characteristics of both rock blocks and discontinuities are taken into account. The concept of an overlay element is illustrated first; then the displacement fields of rock blocks are expressed as functions of so-called general degree of freedoms using the shape functions of the hierarchical finite element method; the governing equations of the rock block system are deduced on the basis of the virtual work principle; and the p-version adaptive algorithm based on the energy norm error estimation of each block element is proposed. The method is applied to the deformation and stability study of a gravity dam, and the parallel laboratory physical test is used to check the validity and ability of the method.     

Fully Coupled Analysis of Failure and Remediation of Lower San Fernando Dam

The extent of flow deformation in an embankment dam is determined by the driving forces and the residual strength of the soil, as well as by the kinematic constraints. The soil conditions of berm and buttress, as well as of foundation, are also critical factors affecting seismic performance of an embankment dam. A careful examination of these factors is necessary when proposing remedial measures to a seismically deficient dam. This paper presents a set of fully coupled finite element analyses of the response of the well-known lower San Fernando Dam during the 1971 earthquake. A critical state model incorporating the concept of state-dependent dilatancy was employed to describe soil behavior over the full range of loading conditions encountered. The results show clearly that a flow slide occurred on the upstream side, and indicate that a downstream flow slide would occur, too, if the downstream berm had not been constructed before the event. The analyses show also that the addition of an upstream berm could effectively prevent the upstream flow slide.     

Determination of Length of a Horizontal Drain in Homogeneous Earth Dams

An earth dam can be prevented from a seepage failure due to softening of the downstream slope by providing a rock toe or horizontal drainage blanket. Analytical solutions are not available for determining the length of the filtered drainage blanket and downstream slope cover, though graphical solutions are available for them. Explicit equations have been obtained in the present work for calculating the downstream slope cover and the length of the downstream horizontal drain in homogeneous isotropic and anisotropic earth dams. Similar equations have also been obtained for maximum downstream slope cover and minimum and maximum effective length of the filtered drainage. These equations are nonlinear and representative graphs have been plotted for them covering all the practical ranges of the dam geometry. The numerical example demonstrates that the proposed equations are simple to use, hence the designers may find these equations as an additional check to their design by the conventional flownet method.     

Earth Dam on Liquefiable Foundation and Remediation: Numerical Simulation of Centrifuge Experiments

A series of four dynamic centrifuge model tests was performed to investigate the effect of foundation densification on the seismic performance of a zoned earth dam with a saturated sand foundation. In these experiments, thickness of the densified foundation layer was systematically increased, resulting in a comprehensive set of dam-foundation response data. Herein, Class-A and Class-B numerical simulations of these experiments are conducted using a two-phase (solid and fluid) fully coupled finite element code. This code incorporates a plasticity-based soil stress–strain model with the modeling parameters partially calibrated based on earlier studies. The physical and numerical models both indicate reduced deformations and increased crest accelerations with the increase in densified layer thickness. Overall, the differences between the computed and recorded dam displacements are under 50%. At most locations, the computed excess pore pressure and acceleration match the recorded counterparts reasonably well. Based on this study, directions for further improvement of the numerical model are suggested.     

Endochronic-Based Approach to the Failure of the Lower San Fernando Dam in 1971

The Lower San Fernando dam failure, which took place in 1971, is one of the most reported cases of seismic liquefaction damage in the geotechnical literature. For this reason, it has been analyzed by almost all of the numerical models developed since that year. In this paper, a comparison between numerical simulations, using a new endochronic model, with measured response of this dam during the earthquake of 1971, as well as numerical results previously obtained by other researchers, is presented. The main particularity of this new constitutive law is a nonassociative flow rule, related to a parameter quantifying degradation with shaking duration, in terms of stiffness reduction. It is incorporated in the model to represent soil dilation. Furthermore, contractive, dilative, and collapse trends of soil behavior are jointly embodied into the new developed constitutive law, which has been implemented in a two-dimensional coupled finite-element model. By so doing, the failure mechanism and the critical locations of the dam are identified and compared with field observations, and the approximate time for the beginning of the upstream sand fill slide is determined.     

Lessons Learned from Field and Laboratory Testing of a DBR Project

Where faulting takes place due to the absence of dowel bars and inadequate subbase support in jointed concrete pavement (JCP), dowel bar retrofit (DBR) is used to improve load transfer efficiency (LTE) and to prevent further faulting of slabs at transverse joints. Even though DBR generally improves LTEs and overall performance of JCPs, not all DBR projects have been successful. Faulting reoccurred within 2?years after DBR treatment on US59 in Texas. An investigation from the cores taken in the project revealed excessive voids under a dowel bar due to poor consolidation of the grouting material. A laboratory investigation was performed to determine the most critical factors for adequate consolidation of grouting materials in DBR. Typical rapid-setting grout materials widely used in DBR were selected and full-scale specimens were made for evaluations. Four testing variables for consolidation performance were investigated: time of placement after mixing, vibration time, slot width, and maximum aggregate size. Maximum aggregate size and slot width were not critical factors for consolidation performance of grout. The most significant factor was vibration time. Twenty s of vibration is recommended. Placement time was also an important factor, with grout materials placed after initial set performing poorly. Delayed placement of grout materials without vibration led to the most voids under the dowel bars.     

Essentials of Culture Change: Lessons Learned the Hard Way.

This article presents an analysis of a failed culture change initiative and the derailment of a senior leader. Five lessons are drawn from this situation and are presented in a way that can be easily applied by consulting psychologists to their practice. These lessons include selecting leaders who have an insider's understanding of the organization, demonstrating the need for change, empowering subordinates to participate in the change process, balancing stakeholder issues, and developing leaders at senior levels. These lessons, although appearing obvious, do require thought and planning to implement. As process experts, consulting psychologists can help executives plan and execute a successful change initiative and avoid having to learn these lessons the hard way. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Case Study: Efficiency of Slit-Check Dams in the Mountain Region of Versilia Basin

Slit-check dams are widely employed in mountain river control. However an analysis of their performance in the field is still lacking. In the present work a field verification to evaluate the interaction between solid discharge regime and four slit-check dams built in two subcatchments of the Versilia River in Tuscany, Italy is presented. The analysis is based on a relatively detailed field knowledge consisting of hydrological, topographical, and sedimentological data, together with a recent model proposed by Armanini and Larcher. Slit-check dam efficiency is analyzed in terms of deposit formation during major floods and its influence on long-term sediment transport regime. Results suggest that the design efficiency is affected by the high sediment trapping capacity associated with the relatively minor floods. A comparison between the deposit geometry predicted by the theory and the field measurements gathered during a systematic monitoring activity shows good agreement.     

Meta-Analysis of 301 Slope Failure Calculations. I: Database Description

Since the early part of the twentieth century, two-dimensional limit equilibrium (2DLE) analysis has been the scientific community’s primary means of slope stability calculation. However, it is well established that the input parameters to 2DLE, namely, soil strength and anisotropy, slope geometry, pore water pressures, failure surface geometry, applicable correction factors, and loading conditions are all inherently uncertain. Effective modeling must account for these uncertainties statistically. Unfortunately, most of the key statistical parameters, such as the safety factor statistical distribution and standard deviation (sd), are unknown and must be estimated by the analyst. In response to this growing need for statistical information, a database was established from the literature of 157 different failed slopes and the corresponding published 301 safety factor (SF) calculations. The database, which covered more than five decades of slope stability research, also included a number of the slope stability factors, including analytical method used, stress approach (effective versus total), assumed slip surface geometry, slope type, applied correction factors, and soil Atterberg limits. A temporal analysis found no evidence that SF prediction or deviation had significantly changed. A log (base 10) normal distribution was found to adequately describe the SF data, with a (nontransformed) mean of 1.03 and a (transformed) sd of 0.087, but the pronounced curvature of the residuals indicated significant, unresolved slope factors, further investigated in the companion paper.     

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.     

一起锅炉仪控系统故障的深入思考

锅炉仪控故障影响锅炉的稳定运行,对莱钢130 t/h锅炉一起故障现象及原因进行了分析,深入思考问题原因,并阐述了相应的改进措施。     

铜钢变电所一次全停电事故原因分析及对策措施

介绍了武钢供电厂铜钢变电所一次全停电事故经过,分析指出ZW-7型户外真空断路器设计缺陷是导致事故发生的根本原因,针对断路器设计缺陷提出反事故措施。     

Factor and Cluster Analyses of Water Chemistry in and around a Large Rockfill Dam: Implications for Water Leakage

Based on water chemistry data, a rockfill dam in South Korea was investigated to identify major parts of substantial dam water leakage. Sinkholes in the dam crest and abnormal water leakage through the dam were observed since the first impounding of the dam. Previous investigations, including observational drillings, hydraulic tests, geophysical surveys, and tracer tests, revealed the development of a high permeability zone in the clay core and an abnormally high phreatic line zone in the left embankment. Sequential tracer tests provided further evidence of locations of preference and rapid water leakage pathways. In this study, we examined the usefulness of water chemistry data to confirm previous findings of water leakage. Factor analysis of the water chemistry data revealed major processes controlling the chemical composition, which include the wash-out and successive dissolution of fill materials in the dam interior, interaction between percolating water and dam core materials, and contamination of seeping water by grouting materials and previous tracer tests. Distinct spatial distribution of similar chemical groups was illustrated by cluster analysis and confirmed the location of major water leakage at the left abutment adjoining the spillway.