Cascading Failures in Complex Networks

Cascading failure is a potentially devastating process that spreads on real-world complex networks and can impact the integrity of wide-ranging infrastructures, natural systems, and societal cohesiveness. One of the essential features that create complex network vulnerability to failure propagation is the dependency among their components, exposing entire systems to significant risks from destabilizing hazards such as human attacks, natural disasters or internal breakdowns. Developing realistic models for cascading failures as well as strategies to halt and mitigate the failure propagation can point to new approaches to restoring and strengthening real-world networks. In this review, we summarize recent progress on models developed based on physics and complex network science to understand the mechanisms, dynamics and overall impact of cascading failures. We present models for cascading failures in single networks and interdependent networks and explain how different dynamic propagation mechanisms can lead to an abrupt collapse and a rich dynamic behavior. Finally, we close the review with novel emerging strategies for containing cascades of failures and discuss open questions that remain to be addressed.Comment: This review has been accepted for publication in the Journal of Complex Networks Published by Oxford University Pres

Competitive percolation strategies for network recovery

Restoring operation of critical infrastructure systems after catastrophic events is an important issue, inspiring work in multiple fields, including network science, civil engineering, and operations research. We consider the problem of finding the optimal order of repairing elements in power grids and similar infrastructure. Most existing methods either only consider system network structure, potentially ignoring important features, or incorporate component level details leading to complex optimization problems with limited scalability. We aim to narrow the gap between the two approaches. Analyzing realistic recovery strategies, we identify over- and undersupply penalties of commodities as primary contributions to reconstruction cost, and we demonstrate traditional network science methods, which maximize the largest connected component, are cost inefficient. We propose a novel competitive percolation recovery model accounting for node demand and supply, and network structure. Our model well approximates realistic recovery strategies, suppressing growth of the largest connected component through a process analogous to explosive percolation. Using synthetic power grids, we investigate the effect of network characteristics on recovery process efficiency. We learn that high structural redundancy enables reduced total cost and faster recovery, however, requires more information at each recovery step. We also confirm that decentralized supply in networks generally benefits recovery efforts.Comment: 14 pages, 6 figure

Network resilience

Many systems on our planet are known to shift abruptly and irreversibly from one state to another when they are forced across a "tipping point," such as mass extinctions in ecological networks, cascading failures in infrastructure systems, and social convention changes in human and animal networks. Such a regime shift demonstrates a system's resilience that characterizes the ability of a system to adjust its activity to retain its basic functionality in the face of internal disturbances or external environmental changes. In the past 50 years, attention was almost exclusively given to low dimensional systems and calibration of their resilience functions and indicators of early warning signals without considerations for the interactions between the components. Only in recent years, taking advantages of the network theory and lavish real data sets, network scientists have directed their interest to the real-world complex networked multidimensional systems and their resilience function and early warning indicators. This report is devoted to a comprehensive review of resilience function and regime shift of complex systems in different domains, such as ecology, biology, social systems and infrastructure. We cover the related research about empirical observations, experimental studies, mathematical modeling, and theoretical analysis. We also discuss some ambiguous definitions, such as robustness, resilience, and stability.Comment: Review chapter

Governing Cascade Failures in Complex Social-Ecological-Technological Systems: Framing Context, Strategies, and Challenges

Cascade failures are events in networked systems with interconnected components in which failure of one or a few parts triggers the failure of other parts, which triggers the failure of more parts, and so on. Cascade failures occur in a wide variety of familiar systems, such as electric power distribution grids, transportation systems, financial systems, and ecosystems. Cascade failures have plagued society for centuries. However, modern social-ecological-technological systems (SETS) have become vast, fast moving, and highly interconnected, exposing these systems to cascade failures of potentially global proportions, spreading at breathtaking speed, and imposing catastrophic harms. The increasing potential for cascade failures of the magnitude of the 2008 financial system collapse, which had a truly global reach and affected systems well beyond finance, screams out for clear thinking about governing vulnerability to cascade failures in SETS. Yet, legal scholarship on the theme is essentially nil, and a more comprehensive, generalizable governance theory leveraging knowledge from scientific research on cascade failures has not emerged. Research initiatives are needed to forge ground on three fronts: (1) system modeling and monitoring; (2) event prediction; and (3) event prevention, response, and recovery. This Article is a first step in that direction. Part I outlines the cascade failures problem. Part II summarizes the scientific research on cascade failures. Part III identifies strategies for controlling cascade failures. Part IV explores the governance challenges of deploying those various strategies in large-scale SETS. Part Vextends the analysis to the special case of cascade failures within ecological systems and the difficulties of managing them through the strategies coming out of cascade failures science. Lastly, Part VI suggests directions of future research on governance of cascade failures

The Visibility of (In)security: The Aesthetics of Planning Urban Defences Against Terrorism

Urban defences against terrorism have traditionally been based on territorial interventions that sought to seal off and surveil certain public and private spaces considered targets. Lately, though, a much wider range of crowded and public spaces have been viewed as potential targets and thus have been identified as requiring additional security. This has immense implications for the experience of the ‘everyday’ urban landscape. Drawing on contemporary notions that incorporate the study of aesthetics and emotions within critical security and terrorism studies, this article discusses the visual impact of counter-terrorism security measures. It analyses the ‘transmission’ of symbolic messages, as well as the variety of ways in which security might be ‘received’ by various stakeholders. The analysis takes place against the backdrop of concern that obtrusive security measures have the capacity to radically alter public experiences of space and in some cases lead to (intended and unintended) exclusionary practices or a range of negative emotional responses. The article concludes by outlining a ‘spectrum of visible security’ ranging between traditional obtrusive fortified approaches and approaches that embed security features seamlessly or even ‘invisibly’ into the urban fabric

Network Interdependency Modeling for Risk Assessment on Built Infrastructure Systems

As modern infrastructures become more interconnected, the decision-making process becomes more difficult because of the increased complexity resulting from infrastructure interdependencies. Simulation and network modeling provide a way to understand system behavior as a result of interdependencies. One area within the asset management literature that is not well covered is infrastructure system decay and risks associated with that decay. This research presents an enhanced version of Haimes\u27 input-output inoperability model (IIM) in the analysis of built infrastructure systems. Previous applications of the IIM characterized infrastructure at the national level utilizing large economic databases. This study develops a three-phased approach that takes component level data stored within geographic information systems (GIS) to provide a metric for network interdependency across a municipal level infrastructure. A multi-layered approach is proposed which leverages the layered data structure of GIS. Furthermore, Monte Carlo simulation using stochastic decay estimates shows how infrastructure risk as a result of interdependency effects changes over time. Such an analysis provides insight to infrastructure asset managers on the impact of policy and strategy decision-making regarding the maintenance and management of their infrastructure systems

Conviviality in Bellville: an ethnography of space, place, mobility and being

This study provides insight into the experiences of mobility and migration in contemporary South Africa, contributing to a field of literature about multiculturalism and urban public space in globalizing cities. It is a study of how the mystique of conviviality configures amongst a diverse migrant and mobile population that frequents Bellville's central business district surrounding the train station - an area located approximately 25 kilometres from Cape Town, and a prominent destination for informal trading, shop keeping, and other ad hoc livelihoods. Understanding the emergence of conviviality and the forms it takes in this particular locality lies at the heart of this thesis. I argue that conviviality emerges out of shared understandings of Bellville as a zone of mobility, of safety and of livelihood opportunities; and of negotiated meanderings within particular spaces of the Bellville central business district. Bellville's migrant networks become convivial when individuals innovatively sidestep away from tensions broiled in rhetoric of the "outsider" and instead negotiate space - both physical and social - to derive relations that often result in mutual benefits. This study also takes into consideration the greater international political and local socio- economic factors that drive migration, relationships and conviviality, and how they are intertwined in the everyday narrative of "insiders" and "outsiders" in Bellville. The Bellville central business district demonstrates the realities of interconnected local and global hierarchies of citizenship and belonging and how they emerge in a world of accelerated mobility. Ethnographic research in Bellville further demonstrates how the emergence of conviviality in everyday public life represents a critical field for contemplating contemporary notions of human rights, citizenship and belonging