A new nonlinear programming (NLP) formulation of the conventional flexibility index model (Swaney and Grossmann, 1985) has been developed in this work for flexibility analysis of single-contaminant water networks. Since this improved model is constructed on the basis of a single critical point instead of the entire region of uncertain parameters, the iterative optimization process converges at a much faster rate. A systematic flexibility assessment procedure has also been devised to analyze and modify a given network so as to achieve the desired level of operational resiliency. Specific design steps are followed sequentially to check the feasibility of a nominal design and to improve its flexibility index by (1) relaxing the upper limit of freshwater capacity and/or (2) adding new pipelines and/or removing existing ones. The effectiveness of the proposed approach is demonstrated with two examples in this paper.