Jeff Decoteau, P.E., P.M.P., is vice president of water market operations for Stanley Consultants. Decoteau can be reached at [email protected].
In addition to the devastation and heartbreak that Hurricane Katrina brought to the Gulf Coast in 2005, lingering consequences hamper the quality of life and health in New Orleans today. Some of these consequences are related to the city’s water supply, which has suffered several boil-water advisories since 2012 to protect citizens from potentially tainted drinking water.
The Sewerage and Water Board of New Orleans (SWBNO) advisories are cautionary, and no events have resulted in harm. There are myriad environmental, maintenance and operational causes for water distribution pressure drops. Several short-term events, less than 24 hours, reduced pressures to less than 20 psi (boil order threshold).
There were a gamut of reasons for the pressure drop events, including power fluctuations at the onsite power plant; interruptions of the commercial power supply; thunderstorms that knocked out power; a major water main break in New Orleans East; commercial power surges; a mechanical failure of the onsite boilers; a fire at the power plant; a water main rupture in Uptown; and other impactful water main breaks. The consequences of these events were a hassle for citizens, a frustration for SWBNO maintenance and operations staff and a political issue for city officials.
After Hurricane Katrina, the Federal Emergency Management Agency (FEMA) launched a program to repair point leaks and full-line ruptures to the existing water distribution system. These system integrity improvements still limit the effectiveness of restoration work due to water hammer effects when power outages cause water distribution delivery pumps to shut down resulting in a rapid water pressure drop in the system.
Imagine a shift in water pressure resulting in powerful 5-ft waves slamming back and forth within the pipes, causing damage to pumps, angled pipe connections, valves and other system parts. Because of these conditions, the Water Hammer Hazard Mitigation Program (WHHMP) was initiated at SWBNO in conjunction with FEMA. The program’s basic goals were to:
• Develop a hydraulic model of three main pumping stations and the associated distribution system to understand
the problem;
• Determine the most cost-effective solution for eliminating water hammer effects; and
• Execute a design and construction program to bring the project to completion.
Sewerage & Water Board of New Orleans
The Sewerage and Water Board was created in 1899 to prevent disease and improve health conditions related to open sewer ditches in the city of New Orleans. The Carrollton Water Treatment Plant (WTP) was built between 1905 and 1909 to provide safe drinking water under controlled conditions. In its infancy, SWBNO also managed the city’s sewerage collection and storm water drainage control.
Not only was the original Carrollton WTP an engineering marvel for its time, the distinctive Mediterranean architecture of the original 10 buildings resembled a university campus rather than an industrial plant. This placed today’s facility on the historical landmark list for the Carrollton Historic District. Although it lost much of that campus feel as the plant was enlarged and modernized through the years, the architecture of this historic landmark area continues to play a role in shaping future plant improvements. Copper and red roofs, white stucco building facades and large arched windows seen today link the modern building to its past.
In 1973, the Louisiana Engineering Society awarded the SWBNO as one of the 10 most outstanding engineering achievements in the state of Louisiana. Today, the Carrollton WTP provides more than 170 million gal per day (mgd) of drinking and fire protection water, which serves the majority of New Orleans on the East Bank of the Mississippi River. The Algiers Plant supplies 53,000 people on the West Bank with 11 mgd of drinking and fire protection water.
Post-Katrina Damage
Even an award-winning water system will deteriorate with age when combined with the catastrophic forces of a hurricane. Katrina brought uprooted trees, saturated soils and an influx of contaminated floodwaters, which resulted in a complete loss of system pressure. Nearly a half-million residents were without water, sewage collection and stormwater pump protection. The Post-Katrina impact for non-revenue water (leakage) was a loss of nearly 90 mgd of treated water.
Federal money poured in to make repairs, and tens of thousands of point and line repairs were made, restoring operating pressure to nearly pre-storm levels. However, the improvements led to water hammer conditions that attacked the old parts of the system, causing further leaks.
FEMA-Proposed Solution
FEMA recommended the replacement of four 40 mgd pumps at the Claiborne station, two 45-mgd pumps at the Panola station, and two steam-driven pumps in the steam pump room. The new pumps, though of similar capacity, would bring better capabilities, such as variable frequency drives, slow opening and closing discharge valves with battery backup for each pump; inverter-duty motors with anti-rotation devices, and a vacuum priming system for each pump. Programmable logic controllers were proposed to minimize sudden pressure changes.
A key factor for successful water operations at the WTP is reliable power. Instead of a dedicated power source, 60-Hz power is supplied by the local utility, while antiquated 25-Hz power is generated onsite. A breakdown in any part of the power supply chain from either the public utility or the onsite plant would jeopardize pump service and result in a potential boil order advisory.
Surge Mitigation Analysis & Recommendations
The SWNBO authorized Stanley Consultants Inc. to initiate the WHHMP planning and design efforts. The WHHMP included improvements at the WTP to reduce the risk of transient pressures in the repaired water distribution system. The team confirmed the FEMA pump upgrade recommendations would mitigate high-pressure surges, but did not provide adequate protection for controlling low pressure fluctuations.
After reviewing various options, the resulting recommendation by Stanley Consultants included installing two, 2 million gal elevated composite concrete and steel storage tanks that would provide both high and low-pressure surge mitigation and keep the system pressurized for 40 minutes at 75 psi. This 40-minute design criteria allows enough start-up time for ramping backup pumps into operation to regain system pressure.
In conjunction with the elevated storage tanks, it was recommended to upgrade the Claiborne and Panola onsite pump stations to create a more robust, sustainable and redundant system. The improvements include new pumping equipment, variable frequency drives, electrical upgrades, discharge check valves, isolation valves, electric drives and miscellaneous piping appurtenances. To minimize pump-start times and reliable pump availability, each pumping station is designed with a central priming system that will keep each pump primed continuously. The WHHMP has a total estimated construction cost of more than $100 million.
Water Tower Design & Construction Challenges
The WTP is a 100-year old site where all utility lines are required to be buried, creating a logistical problem for creating a clear footprint for the placement of two 2-million-gal elevated storage tanks. A virtual rats nest of underground utilities was the main challenge: electrical duct banks, communications and data lines, water mains and branches, return lines, storm water drainage pipes, sanitary sewers, force mains, building piles and support systems.
In addition to typical engineering constraints, the elevated storage tanks needed to meet requirements maintaining the community’s historical identity. The stakeholders included members from the SWBNO, FEMA, Governor’s Office of Homeland Security and Emergency Preparedness, Louisiana State Historic Preservation Office, Carrollton and Riverbend Neighborhood Assns. and the Preservation Resource Center of New Orleans. As such, it was important to maintain the New Orleans heritage by including a roofed tank structure resembling the architecture of the French Quarter. Therefore, architectural gusset trusses were used in conjunction with the red roof to give the illusion of a roofed cistern.
After final design was completed by the Stanley Consultants team, the project was bid and awarded to B & K Construction Co. LLC of Mandeville, La., for $33.6 million. The contactor in partnership with Landmark Tank Structures completed the concrete tank pedestal bases using 14 vertical lifts and building a final tower dome tank bottom. The steel tank was fabricated onsite and lifted onto the base adorned with the classic SWBNO logo. The Claiborne tank is operational and was scheduled for final completion in July 2019. The Panola tank had a scheduled completion date of July 2019, as well.
The B & K foundation design for the tanks included a flexible design matrix of 12 different pile layout scenarios for accommodating the resulting pile load tests for both the Claiborne and Panola tanks. This design planning and innovation allowed for a quick foundation determination for each tank for maintaining the critical path schedule for construction. Ultimately, 128 piles in a four row configuration were used for the Claiborne storage tank and the Panola tank required 146 piles in a five row alignment.
More Water Hammer Hazard Mitigation Program Projects
The potable water system in New Orleans managed by SWBNO is an amazing and complex modern marvel that faces enormous challenges in the future because of water demands, local subsiding soils and exposure to the coastal ravages of hurricanes and sea level rise. The WHHMP represents a leap forward into modernizing the city’s water system and significantly reducing the adverse risks associated with boil order advisories.
The new Claiborne and Panola Elevated Storage Tanks and the Claiborne and Panola Pump Station upgrades will provide reliable safe water for years to come. SWBNO officials, engineers and operational and maintenance staff continue to seek ways to improve functionality, reliability and sustainability of the system to the citizens of New Orleans.
