- Prior research has identified a suite of mitigation methodologies to improve water systems' resilience against accidents, natural disasters, and terrorist attacks which simultaneously disable the water distribution system and start an urban fire. The results of vulnerability analyses and mitigation scenario evaluation recommended enlargement of critical water main sections in order to improve fire suppression effectiveness by increasing water flow to fire hydrants. This recommendation for optimal emergency response can potentially conflict with guidance on optimal water main sizing for daily water use. This work investigates a variety of alternative water main sizing strategies to determine the optimal combination of design for emergency response and day-to-day water quality requirements. These strategies are evaluated by testing them on the Micropolis virtual city model under both normal and emergency conditions in order to gauge their effect on a realistic water system. An analysis of the costs and benefits of each design strategy informs the study's final recommendations on optimal water main sizing and water system design to meet water system demands under differing use conditions. 2008 ASCE.