Vulnerability assessment of water supply systems for insufficient fire flows

Abstract

Water supply systems’ vulnerability towards physical, chemical, biological, and cyber threats was recognized and was under study long before September 11, 2001. But greater attention toward security measures for water supply systems was focused after the incidents of September 11, 2001. In response to those events, several acts have been passed by the United States Congress, and numerous vulnerability assessment tools and methodologies for water systems have been developed. Although water supply systems are vulnerable to many forms of terrorist acts, most of the vulnerability analysis studies on these systems have been for chemical and biological threats. Because of the interdependency of water supply infrastructure and emergency fire response, any substantial damage in a water system would be a significant threat towards the community. In this study, attention is focused toward physical threats on water supply systems during a fire flow condition, and a methodology is developed to determine the vulnerable components of a water supply system during a fire event. The methodology utilizes dynamic programming optimization procedure to determine maximized disruption of fire flows as a function of number of attacks and/or failures in the water distribution system. Disruption is quantified at specific fire hydrants in two schemes using normalized values of (1) available flow and (2) available pressure and distance to the nearest operational fire hydrant. It is found that the pressure-based quantity is inferior to the flow-based one. However, using the flow-based disruption metric, clear functions of disruption versus failure number can be determined that exhibit discernable properties of robustness and resiliency – and the sequential failures in each. This methodology is applied to the water supply system of Micropolis, a virtual city developed by Brumbelow et al. (2005), and vulnerability analysis is performed with fire at several possible locations. On the basis of the results, three mitigation strategies are proposed to harden specific sets of water mains and more simulations are performed on the hardened water supply system to assess its changed vulnerability. The results from the simulations of the mitigation strategies show that the recommendations on specific mitigation measures reduce the serious consequences from such threats.