‘What-if’ Water Conveyance Modeling

A new water source in Southern California is the Holy Grail of water sustainability. Once found and developed, the challenge becomes effectively integrating the supply into an established water conveyance system.

Often, the new source (e.g., desalinated groundwater or seawater, new wells, or imported water) is added at the smaller capacity fringe of the existing transmission system, requiring a plan to push water “in reverse” of how the system was originally designed.

One example is Irvine Ranch Water District’s (IRWD) new Baker Water Treatment Plant that will increase regional supply by adding local treatment capability for imported water and surface water.

The plant’s 43.5 cubic feet per second (cfs) output must be conveyed through the existing IRWD system with 33 cfs earmarked for delivery to the Santa Margarita Water District, Moulton Niguel Water District, El Toro Water District, and Trabuco Canyon Water District, which are sharing plant costs.

This creates a significant conveyance challenge because the IRWD system was not designed for that level of conveyance.

The large, new supply will significantly impact the IRWD distribution system’s performance, such as fluctuation in tank levels, system pressures and velocities, and available fire flow. Skilled hydraulic modeling has been done to evaluate these impacts. Dudek engineers helped IRWD’s staff run and analyze the hydraulic modeling to evaluate “what if” performance scenarios to determine the impacts of water facilities and operational modifications associated with the new Baker plant.

Modeling raised the difficult issue of balancing water levels in IRWD’s storage reservoirs (tanks) throughout the system, which are severely impacted by the new supply source. Water modeling had to be run and monitored over extended period simulations. Results were discussed with IRWD engineers and operators to devise alternative solutions. The alternatives were further analyzed to identify the optimal facilities and operational changes.

For example, to minimize cost, one solution included using flow from upper pressure zones as an insurance policy to maintain tanks at minimum water level, which was only anticipated under very extreme conditions. Other alternatives included an in-line booster station, changes to system configuration by closing and opening valves, and looping the system to reduce head loss.

The alternatives were compared in the water modeling, and the most energy-efficient, cost-effective solutions were finalized. Surprisingly, only minor new construction was required. IRWD relied mainly on modifying system operations and using existing IRWD facilities.

This finding was good news to the other four water agencies that are sharing costs. Without this “what if” process, supported by complex and detailed computer modeling analyses of various operational scenarios, it is likely that construction of additional new, costly facilities would have been needed to accommodate this new water source.