David M Schaaf, Jeffrey A Schaefer, Rick W Schultz, Jason T Needham
Abstract: As one of the main federal agencies with responsibility to build, operate, and maintain large dams in the United States, the US Army Corps of Engineers (USACE) is developing a risk based framework to better manage their portfolio of 600+ dams in terms of risk management and prioritization of funding. A key element to this effort is the development of risk-based analytical tools to evaluate primary features for applicable failure modes. These are used in conjunction with loading and consequence modules to assess the overall risk associated with the dam in terms of lives and economic damages. The focus of this paper is on the engineering analysis modules used to generate fragility curves for dam features.
The analysis modules are broken into three main categories by engineering discipline: geotechnical, structural, and mechanical/electrical. The risk based assessment tools associated with geotechnical failure modes include Seepage & Piping, Embankment Stability, Seismic Performance, and Erosion of Unlined Spillways. The structural assessment tools include Concrete Monolith Stability, Spillway Gates, Scour of Concrete Lined Spillways, Spillway Training Wall Stability, Performance of Pipes through Dams, Hydropower Superstructures and Intake Towers. The mechanical and electrical are primarily focused on the performance of machinery used to operate dam gates.
This paper gives a broad overview of the main characteristics and methods used for each of these analysis tools. Some of the modules use historical performance to establish failure rates, while others are more analytically based. The context of each within the framework of the overall risk assessment effort of USACE dams is covered.
Keywords: risk based analytical tools, fragility curves, risk assessment, US. Army Corps of Engineers, portfolio, dams.
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Karen Riddette, David Ho, Mike Phillips
Abstract: Stepped spillways have seen a resurgence in popularity in recent years as new research has better described flow conditions and quantified the energy dissipation capacity of this type of spillway.
While carrying out hydraulic modelling for the Hinze Dam Stage 3 upgrade project using Computational Fluid Dynamics (CFD) analysis, extensive numerical testing and validation of the CFD model for the proposed 0.8H:1V stepped spillway design was undertaken. It was found that for the expected Probable Maximum Flood (PMF) discharge, the high flow depth would result in a non-aerated, non-uniform flow, typically seen in the developing flow region just beyond the spillway crest, occurring over the entire length of the stepped spillway. To date, only limited laboratory studies have been published concerning energy losses in this particular flow regime.
This paper examines the ability of CFD modelling to compute energy dissipation and air entrainment effects in stepped spillways under extreme flow conditions. Where possible, the computed results were validated against limited published data. Some new data obtained by the CFD model is presented for energy loss in the developing flow region that has not been established before.
Keywords: stepped spillway, CFD, energy loss, developing flow.
Andrew Shields, Dr Mark Bailey, Graeme Hannan
Abstract: In recent years, water resources have declined markedly under the effects of long-term drought and climate change. Resource planning has turned from a process of routine allocation to the specialised management of a limited and highly valued commodity. Probabilistic outlooks of water availability based on historical inflows have proven an important component of the drought communication strategies used by Goulburn-Murray Water. In turn, Goulburn-Murray Water has examined alternative means of securing additional water for its customers, including revised operating criteria and pumping from the dead storage of its dams.
With the focus on meeting essential domestic water needs and boosting irrigation supplies, Goulburn-Murray Water has used the opportunity afforded by water levels to review the capacity of its storages. The regular, but unwelcome, outcome of reduced storage capacities from these studies, and the predictions of continuing drought, present Goulburn-Murray Water and its customers with an ongoing challenge.
Keywords: water resources, allocation outlook, capacity table, dead storage, resource position
Eric Lesleighter, Erik Bollaert
Abstract: Many of the dams in Australia, and other countries, are potentially unprotected from the occurrence of extreme floods which would discharge either over the dam and/or the abutments due to inadequate spillway capacity.
The paper commences with the presentation of the evaluation procedure that has been applied by the first author for a number of dams in Queensland, initially with a detailed description of dams, the original hydraulics studies, the current hydrology, and the geological information. The procedure then comprises analysis of the extreme flood hydraulics and an erodibility assessment which leads to an estimate of the possible erosion of the rock for a range of flood magnitudes. A description of the components of the final evaluation and the typical conclusions is presented with reference to the Julius Dam in Queensland.
The paper includes a description of an alternative or complementary procedure that comes from the work of the second author. Comparison is made with the Comprehensive Scour Model (CSM) developed by Bollaert (2002, 2004). This model is physically based and allows estimating scour formation in rock or concrete as a function of time duration of discharge. The model compares the resistance against fracturing of concrete or rock layers with the hydrodynamic pressure fluctuations exerted by a turbulent aerated jet impacting in the plunge area. Once the fracture network is formed, dynamic uplift of single rock or concrete blocks in computed. Based on fracture mechanics and air-water hydrodynamics, a detailed time-evolution of scour formation is obtained. The model is generally used for projects involving plunge pool floors, ski-jump spillways, fractured rock and so on, where sufficient data are available on both rock quality and duration and intensity of spill from the dam. In the present paper, the CSM has been used to model scour of non-overflow abutment sections when there is overtopping flows.
Keywords: dams, spillways, extreme floods, rock scour, erosion, dam safety
Randy J James, Yuyi Zhang, Gabriela Lyvers, David Schaaf
Abstract: Following the flooding disaster in New Orleans, Louisiana, due to Hurricane Katrina in August 2005, the U. S. Army Corps of Engineers (COE) initiated a comprehensive program to survey, evaluate, and rank all dams and levees in the COE’s portfolio for risk of structural failure and associated economic consequences. One objective of this program is to improve safety and risk through efficient allocation of resources for rehabilitation efforts when needed. One area of great concern is internal stresses in aging concrete monoliths causing cracking. While some cracking in concrete monoliths is a common condition having little effect on the structural performance, extended cracking can lead to instability in parts of the monolith. Mass concrete monoliths generally are not reinforced, and cold joints at lift interfaces are a potential source of weak planes. Failure of concrete monoliths due to sliding instability along internal cracked planes can have serious consequences for loss of pool. This failure mode can occur quite suddenly, and detection of such cracking or the extent of such cracking is very difficult to establish from visual inspections or even core sampling. To help in this portfolio risk assessment, analysis methodology has been developed for establishing the structural risk due to cracking in mass concrete monoliths. Finite element modelling with automated mesh generation and employing advanced concrete constitutive relations for crack initiation, propagation, and arrest, are used to establish internal cracking. Monte Carlo based probabilistic analysis methods, directly coupled to the finite element analyses, are used to evaluate uncertainties and establish the probability of failure for increasing pool elevations and seismic hazards. The objective is to provide a probability of failure for possible pool elevations under current site conditions given that there is always some possibility of a range of seismic events that could occur at any given time.
Keywords: concrete dams, cracking, structural reliability, risk, and safety
G. Shams Ghahfarokhi, PHAJM van Gelder, JK Vrijling
Abstract: Risk and reliability analysis is presently being performed in almost all fields of engineering depending upon the specific field and its particular area. Probabilistic risk analysis (PRA), also called quantitative risk analysis (QRA) is a central feature of hydraulic engineering structural design.
Actually, probabilistic methods, which consider resistance and load parameters as random variables, are more suitable than conventional deterministic methods to determine the safety level of a hydraulic structure. In fact, hydraulic variables involved in plunge pools, such as discharge, flow depth, and velocity, are stochastic in nature, which may be represented by relevant probability distributions. Therefore, the optimal design of a plunge pool needs to be modelled by probabilistic methods.
The main topic of this paper is concerned with the reliability-based assessment of the geometry of the plunge pool downstream of a ski jump bucket. Experimental data obtained from a model of a flip bucket spillway has been used to develop a number of equations for the prediction of scour geometry downstream from a flip bucket spillway of a large dam structure. The accuracy of the developed equations was examined both through statistical and experimental procedures with satisfactory results. In addition, reliability computations have been carried out using the Monte Carlo technique.
The main conclusions are that structural reliability analysis can be used as a tool in the dam safety risk management process and that the most important factors for further analysis are erosion, friction coefficient, uplift and self-weight.
Keywords: risk analysis, reliability, plunge pool, Monte Carlo simulation, flip bucket, large dams