Aerial view of the filled impoundment in rolling, wooded landscape.

Bel Air Impoundment

The largest Maryland dam project in the past 30 years blazes a new trail for the design of water storage reservoirs in the U.S. The 90-million-gallon Bel Air Impoundment, featuring a 62-foot-high earthen embankment with a 20-foot-wide crest, secures a reliable backup water supply for 20,000 residents of Harford County. Gannett Fleming served as lead designer for the project, which draws and stores water from a nearby stream. The facility allows Maryland American Water to maintain enough raw water to provide service to its Bel Air-area customers in the event of drought or other water emergency.

The design team selected a bituminous geomembrane (BGM) liner to create a watertight barrier inside the dam embankment and impoundment floor. The project marks only the second time that a BGM liner has been used to line a water supply reservoir in the U.S., and the first time on the East Coast. Sustainable water-quality improvement systems maintain water quality in the impoundment and inhibit the growth of algae, which can affect water’s taste. Natural wetlands and chemical-free algae control systems help mitigate environmental concerns by reducing chemical usage and related costs by an estimated 20%.

What We Did

Because it does not require a soil cover, the BGM liner reduced construction costs by approximately $450,000. The project showcases the benefits of using a BGM liner in an exposed liner system: The liner has a friction angle of 34 degrees, making it suitable for most embankment slopes. Its dimensional stability provides permanent contact with substrate, making it ideal for differential soil settlement, which is common during embankment construction.

The team designed the facility to maximize sustainability. It features man-made floating wetlands to reduce algae-promoting nutrients in stored water, as well as ultrasonic algae control and water mixing systems that eliminate up to 90% of existing algae. The impoundment was positioned so that gravity conveys raw water to a nearby treatment plant, eliminating the need for an electric-powered pressure system. Extensive materials testing allowed the team to set acceptable parameters for using on-site soil that accounted for extreme variations in the soil’s composition, eliminating the need for costly soil disposal or hauling.

Key Features

  • BGM liner provides watertight barrier inside 2,025-foot embankment and impoundment bottom.
  • Highly durable, resistant to punctures and ultraviolet radiation, and textured to provide traction, the liner demonstrates an optimal solution for an exposed liner system.
  • Ultrasonic algae control and water mixing systems reduce existing algae by up to 90%, maintaining water’s clean taste.

Awards & Recognition

  • American Water EPA Aquarius Excellence in Problem Solving, 2022, Maryland American Water.
  • Best Water/Environment Project, 2019, Engineering News-RecordMid-Atlantic, Regional Best Projects Awards.
  • Top Project, 2019, Water & Wastes Digest, Top Projects Awards.
  • Outstanding Project Award, 2019, American Council of Engineering Companies of Maryland, Engineering Excellence Awards.
  • National Recognition Award, 2019, American Council of Engineering Companies, Engineering Excellence Awards.


  • Reliable backup water supply for 20,000 residents of Harford County.
  • Enough raw water maintained to last as long as the 100-day drought of record.
  • Exposed liner system saved an estimated $450,000 in construction costs.
  • Substantial completion seven weeks early exceeded client expectations.


Maryland American Water


Bel Air, Md.


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