Firefighting capacity evaluation of water distribution system subjected to multi-ignitions of post-earthquake fires
Introduction
Post-earthquake fire (PEF) is one of the most hazardous secondary disasters caused by earthquakes, and the multi-ignition of PEF led to heavy casualties and severe property damage according to historical records. For example, the 1906 San Francisco earthquake led to multiple fire ignitions in downtowns where wooden structures were densely distributed [1]. Around 500 blocks were burnt out and life loss induced by PEF was estimated to be more than 3000. In recent occasions, about 110 fire outbreaks were reported to be attributed directly to the effects of the 1994 Northridge earthquake [2]. Fire following the 1995 Hanshin earthquake in Japan destroyed more than 7000 houses and killed more than 500 individuals [3]. In view of the tremendous damage PEF might bring about, cities should maintain adequate firefighting capabilities subjected to the attack of catastrophic earthquakes.
In order to effectively control and put out fires, adequate water for firefighting should be provided by means of fire cisterns, fire trucks and hydrants connected to water distribution systems (WDS). Yet the fire cisterns are available only around specific places such as hospitals and some high-rise buildings. Seismic damage to tanks and pipes connected may reduce the water supply capability. Water from fire trucks are also unstable because of the limited amount water trucks can carry. As a result, the WDS is still the major and most stable fire water source in most occasions. However, the firefighting capacity of urban WDS is likely to be impaired during earthquake excitations. On the one hand, urban WDS is composed of widely distributed buried pipelines, which are likely to suffer from heavy damages in seismic events, as is shown in previous catastrophic earthquakes such as the 1994 Northridge earthquake [2], the 1995 Hanshin earthquake in Japan [4] and the 2008 Wenchuan earthquake in China [5]. On the other hand, compared with typical customer’s basic demands, water demands for firefighting are much larger, which would bring down the global operating pressure in the pipeline network, in turn impair the water delivery capability of the entire WDS. Without sufficient water supply, fires tend to develop beyond control, and lead to serious losses of lives and property.
Although a number of researches have been conducted concerning the performance of urban WDS in seismic events, only a handful of work focuses on the capacity of WDS to provide water for firefighting after an earthquake. Bristow and Brumbelow [6] presented a methodology to link WDS model with PEF spread & suppression model; and then they used the number of people displaced after the disaster as the damage index, and discussed the effectiveness of different mitigation measures based on simulation results. Kanta [7] introduced a methodology integrating vulnerability, risk analysis and risk optimization to evaluate and enhance the firefighting capacity of urban WDS based on dynamic programming. In their work, multiple failure modes were considered including seismic induced failures, accidental pipe failures due to soil-pipe interaction and pipe failures from malevolent action, and the damage function was described by the ratio of fire flow shortage to the required fire flow. Li et al. [8] developed a multi-scenario simulation method to evaluate the water supply capability in seismic events considering both original demands and random firefighting demands. The method was applied to a Chinese city and it was found that the concentrated flow for firefighting has significant impact on the serviceability of the WDS, especially in the first few hours after the earthquake. In most existing literatures, the indexes for firefighting capacity of WDS are derived from the comparison between the available amount of water for firefighting and the required water flow to put down the fire. Although these indexes show the fire water shortage quantitatively, they fail to show how great the city is exposed to the risk of uncontrolled PEFs as a consequence of the damaged WDS functionality by earthquake. Besides, it should be noted that unlike daily fire events which usually happen once at a time, multiple fires following earthquake tend to take place simultaneously or subsequently in a short time. Water demands for firefighting could create heavy burden to the water network and reduce the water supply capability notably. Therefore, not only the damage degree of the WDS, but also the excessive water demands for firefighting should be taken into consideration while evaluating the firefighting capacity of urban WDS. However, none of the aforementioned existing methods considers the multiple simultaneous demands of water in hydraulic modelling analysis of WDS, which is very likely to overestimate its water supply capability.
In this work, an integrated procedure to evaluate the firefighting capacity of urban WDS after seismic events is proposed considering multiple simultaneous PEF ignitions. The probability of supplying sufficient water to suppress PEFs is proposed as the firefighting capacity index of WDS, and the evaluation is accomplished by Monte-Carlo simulation considering the uncertainties associated with both pipeline damage and fire ignitions. An actual WDS of a city in southwest China is used for illustration of the proposed method, and discussions are made on the influences of multiple simultaneous water demands on firefighting capacity evaluation of WDS and the possible applications of the proposed method.
Section snippets
Seismic fragility model of pipelines
Typical water distribution system is composed of water works, pumps, pipelines, tanks, etc. Their capabilities to keep functional following seismic event are different. Among these basic components, nodal elements are usually able to preserve certain level of functionality after seismic events. For example, the shutdown probability of medium-sized water plants with unanchored components is less than 3% under earthquake excitation with a peak ground acceleration (PGA) of 0.5 g according to
Influence of water demands on firefighting capacity of undamaged WDS
The basic function of a municipal WDS is to provide sufficient water to end users for different purposes, such as domestic use, industrial use, urban sanitation, and firefighting. As all water is transported in the same distribution network, the evaluation of water supply capability should consider all water demands in the network. In this section, through hydraulic analyses of a typical municipal WDS, we would discuss how different sorts of water demands influence the water supply capability
Evaluating seismic firefighting capacity of WDS considering stochastic pipe failures and simultaneous ignitions
Sufficient water supply for firefighting is essential for confronting post-earthquake fires. If fire ignited in a place where water supply for firefighting is in shortage, the fire is likely to grow out of control and leads to severe economic loss and casualties. In existing work concerning firefighting capacity of WDS after seismic events, the index is based on comparison between required fire flow and available water flow for firefighting under certain conditions [7]; however, the available
Conclusions
In this study, a simulation-based method is proposed to evaluate the seismic firefighting capacity of urban WDS accounting for the effects of multi-ignitions of PEFs. With uncertainties associated with pipe failures and PEF ignitions accounted for, the probability of uncontrolled fire occurrence is proposed as the firefighting capacity index of WDS. A Chinese city and its WDS are applied for illustration. Discussions on the influences of multiple simultaneous water demands and base demands on
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The research described in this paper was supported by the China National Nature Science Foundation (51778337 and 51890901). These supports are gratefully acknowledged. However, the views in this paper represent those of the authors, and do not represent the views of the sponsoring organization.
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