Even When It Rains You Need to Drought Plan: Dynamic Modeling Application to Water Supply Planning in Virginia

CH2M HILL working with the City of Portsmouth conducted a Water Supply Master Plan Study that includes evaluation of the safe yield of the City's reservoirs, evaluation of water quality and treatment issues, and development of system-operating rules. The purpose is to develop an integrated water supply plan that both supports new state requirements as well as assists the City in making decisions involving the future direction of water supply and water sales development. The new state regulations call for "Water Plans" to be developed. Requirements for the Water Plan include an assessment of needs and current supplies, development of a Drought Response and Contingency Plan, description of demand management measures in place and articulation of a Statement of Need. This paper will describe CH2M HILL's efforts to assist the City of Portsmouth in developing its Water Plan, including the application of the dynamic simulation model VOYAGE. One major part of the work involved the development of the drought response plan and reservoir operating rules. The foundation for both the plan and the rules was the testing of different reservoir operating modes. The operating modes consisted of different combinations of withdrawal rates, reservoir routing, and groundwater well pumping. The two limiting factors considered were available useable storage in the reservoirs and fluoride levels in the finished water. Each operating mode was modeled and optimized using CH2M HILL's water system simulation model VOYAGE. This tool is an object oriented model created in the Extend simulation platform. VOYAGE was designed to quickly evaluate numerous potential solutions and identify the optimal combination of water supply sources to meet demands based on given constraints and the operation that will optimize system performance. Input to the model included system configuration data, plant upgrade scenarios, water usage, meteorological data, and the results of an unaccounted-for water study. Twelve mode of operation scenarios were tested using a weekly time step over a four year period that included the 2001 – 2002 drought. The scenarios were divided into three groups. The first group assessed the combination of existing surface supplies and production wells. The second group included only wells as the raw water source during drought. The third group added an emergency pipeline from a neighboring reservoir owned by the City of Norfolk. VOYAGE allowed fast evaluation of changes in operating rules, for example, comparing the advantages of releasing water from upstream reservoirs based on total useable volume of the reservoir system with releases based on only the downstream reservoir's useable volume. Other modes of operation tested the optimal trigger points for well activation during a drought as well discharge location within the system. Efficacy was based both on available water for withdrawal and water quality at the intake.