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Loudoun Water Raw Water Supply System Phase I Preliminary Engineering Services

In-depth water planning identifies $135 million in infrastructure needs to serve fastest-growing county.
  • Geologists collect digital photogrammetry data  Gannett Fleming

    Gannett Fleming geologists collect digital photogrammetry data for Loudoun Water.

  • Water model demand allocation  Gannett Fleming

    A water model illustrates 2040 demand from the Potomac River and two quarries.

  • Safe yield for various water system configurations  Gannett Fleming

    A graph shows the safe yield for various water system configurations.

  • Potomac Raw Water Pump Station rendering  Gannett Fleming

    A rendering of the Potomac Raw Water Pump Station.

Loudoun Water

Loudoun County, Virginia

Our Role
Analysis, Report Preparation, Consulting.

40 mgd projected demand
1 year 5 months
  • Identified and evaluated alternatives for satisfying projected water supply deficit

  • Calculated safe yield model to set design criteria and size of water facilities

  • Created master plan report to provide clear path forward through final design, construction, and commissioning.

Faced with a projected water supply deficit of 40 mgd in the next three decades, Loudoun Water began assessing its options to expand its water supply. As engineering consultant, Gannett Fleming developed a comprehensive preliminary engineering report for the Loudoun Water Raw Water Supply System (RWSS), including safe yield modeling. The safe yield reflects the average of the varying daily demands on the system for a calendar year at which it would be able to serve through the worst drought of record with no unsatisfied demands.

Our master plan summarized the system’s anticipated water demands through 2040, the availability of water supply to meet those demands, the facilities needed, and their estimated construction costs. The final report provides the utility with clear guidelines on how to meet the water demands of a rapidly growing community.

What We Did

Gannett Fleming analyzed the proposed Loudoun Water RWSS, a complex arrangement of multiple water supply sources and up to four quarries for water storage. To determine the available withdrawal rate that would be in accordance with Virginia law, we created a custom RWSS model, evaluating the safe yield of the system’s supply in terms of average day demand. 

In addition to the typical model run, we created routines to determine the safe yield of each operating scenario from 2016 through 2040. After calculating the safe yield during the drought of record, we performed analyses on each historical hydrologic year to determine its safe yield. By plotting, studying, and analyzing the model outputs, Gannett Fleming obtained answers to Loudoun Water’s longstanding questions about its system’s operation and reliability. The report featured the safe yield for future years, as well as system operating statistics, anticipated compliance with permit conditions, and assistance with master planning. We identified and developed improvements to approximately 30 percent complete design. Our team presented our findings and recommendations through workshops with Loudoun Water personnel and vetted them with Luck Stone Corporation, the quarries owner, as well as appropriate regulatory agencies. Armed with this information, Loudoun Water was able to confidently move ahead with its expansion plans to meet the needs of the next 30 years.

Key Features

  • Detailed daily water resources model to estimate safe yield of raw water supply system during drought of record

  • Analysis of each historical hydrologic year to verify safe yield

  • Modeling of projected system capacities to illustrate changes over time

  • Development of alternatives for raw water intake, Potomac Raw Water pumping station, and Quarry Raw Water pumping station

  • Evaluation of land use, zoning requirements, and easements

  • Collaborative workshops to identify and evaluate improvement alternatives

  • $135 million in improvements programmed to satisfy projected water demands and presented in a master plan report.

Sustainability Features & Outcomes

  • Determined safe yield of quarries, Potomac River, and city of Fairfax, Virginia, to avoid damaging or depleting groundwater supply

  • Plan avoided environmental impacts to the extent possible and minimized impacts where unavoidable.

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