Launch Slideshow

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Bringing drinking water to an island community

Bringing drinking water to an island community

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    The first settlers came to Roanoke Island in 1585. Like the Dare County Water Department's other 12 service areas, the 18-square-mile island between North Carolina's coast and the Atlantic Ocean experienced chronic saltwater intrusion. Map: Mark Darwin, CDM Smith; Photos: CDM Smith

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    Horizontal directional drilling has been used to install 1,010 linear feet of 16-inch HDPE pipe over three locations; jack-and-bore for 36 road crossings in carrier sizes from 6 to 16 inches. This is a pipe-jacking heading east from the northwest corner of an intersection on Roanoke Island.

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    In addition to 56 miles of 6- to 16-inch pipeline, the Dare County Water Department's construction manager at risk (CMAR) contract with CDM Smith includes improvements that will increase pumping capacity from 800 - 900 gpm to a maximum 1,950 gpm under typical operating conditions. New, 100-hp pumps have replaced two 30-hp high-service pumps; transmission pumps have been rehabilitated.

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    When the project's completed this year, North Carolina's Dare County Water Department will have more than doubled Roanoke Island's storage capacity. This new, 300,000-gallon tank is located at an abandoned county facility.

OVERVIEW:

Owner: Dare County Water Department
Where: Kill Devil Hills, N.C.
Project: Roanoke Island Water Treatment System Expansion
Procurement method: construction manager at risk (CMAR)
Guaranteed maximum price (GMP): $20,387,815
Expected completion date: June 2012

Often called the Lost Colony, Roanoke Island was the first English colony settled in the New World in what was then Virginia. The tiny island — just 11 miles long and 2 miles wide — is now part of North Carolina's Dare County. Nestled between the mainland and barrier islands near Nags Head, the island is a popular tourist attraction for both its history and natural beauty. The Outer Banks beaches are a coveted destination in the summer, when the island's population swells from 10,000 year-round residents.

Saltwater intrusion is an ongoing challenge in the Outer Banks, but Roanoke Island presents a particular problem. The Dare County Water Department's 6-mgd Skyco ion-exchange treatment plant serves the town of Manteo and limited unincorporated areas in the central part of the island, but residents in northern Roanoke Island, the southern area of Wanchese, and other unincorporated areas rely on private wells supplied by two aquifers. Tourist attractions including the Lost Colony, Fort Raleigh, and an historic fishing village in Wanchese also are in these unserved areas.

For many years, the private wells supplying these areas have experienced low water levels and saltwater intrusion, a common problem where groundwater is pumped from coastal wells. Because the private wells provided no reliable fire protection, public safety was another concern. Local fire departments had asked the county board of commissioners to expand public water service to ensure adequate supplies for fire suppression.

The county replaced many of the shallow wells in Wanchese with deeper wells in the 1980s, but saltwater again became an issue in the late 1990s. A study concluded that well water quality was deteriorating because the replacement wells were poorly constructed. But extending municipal water service to the rest of the island, which would be the county's largest water services project since the early 1950s, wasn't without issues, either. There was the potential impact of widespread construction on tourism, the island's primary industry, as well as wildlife and wetlands, to consider. Like many communities, Dare County faced revenue shortages and budget concerns. Finally, retrofitting homes — some 50 years old — with new pipes would pose logistic and communications challenges.

Modeling strategic improvements

For more than 20 years, the county has collaborated with CDM Smith on water supply, treatment, and distribution projects. In 2005, the Water Department enlisted the consulting engineering firm to conduct a system planning study to determine what improvements were necessary to expand water distribution on Roanoke Island. The firm developed existing and future demand projections, created a hydraulic model, and recommended infrastructure improvements based on projected water demands through 2025.

To assess the county's needs, a computer model of the distribution system was developed using billing data, population projections, parcel data, and land use data. Projected conditions — including average day, maximum day, maximum day plus fire flow, and peak hour demands — were simulated and observed. Because the Skyco plant's existing storage tank and mains wouldn't meet the projected daily demand of 2.83 million gallons, the firm's 2006 recommendations included:

  • Improving the facility's high-service pumps and piping
  • Building a 300,000-gallon elevated storage tank to supplement an existing 200,000-gallon tank, which will remain in service
  • Building a 3-million-gallon ground storage tank
  • Installing more than 50 miles of distribution piping.
  • Minimum fire flow capacities were determined based on standards by the Insurance Services Office (ISO), a private rating service that insurance companies use to establish fire flow requirements for communities. Flows are based on maintaining a residual pressure of 20 psi to overcome frictional losses through the hydrant and hoses and maintain positive pressure on the suction side of a fire department pumper truck.

    For residential areas, ISO recommends fire flow capacity of no less than 750 gallons per minute (gpm) for one- and two-family dwellings not exceeding two stories in height and spaced 31 to 100 feet apart. This describes the majority of structures in the service area. Requirements for commercial and industrial areas are generally higher, so CDM Smith developed specific flow requirements for commercial and industrial developments as well as public facilities such as schools, hospitals, and nursing homes.