Pumping stations are a last line of defense in flood-prone developments.

By Lynn Merrill

A 100-year storm is predicted in your area, and already rain splatters the streets and streaks the windshield of your pickup. Your crews are on standby in case there's a microburst mixed somewhere in that hovering mass of darkening clouds, ready to dump more stormwater into the channel than it can handle. As the evening wears on, the rain falls more heavily, flowing deeper and deeper in the river. You drive down the gravel road to one of the pump stations that were built for nights such as this. Already the water has risen high enough to set off the automatic start feature on the pump, and the diesel engine that provides power is reverberating through the thickening night. Just in case, you check your personal digital assistant and place a call to one of the local pump vendors to make sure that it can provide you with portable pumps if the situation worsens. Now it's just a matter of waiting to see what the river will do.

Whether it's the banks of the Ohio River or a retention basin in the Everglades, when the prediction is for heavy and continuous rains, stormwater managers' thoughts immediately turn to their pumping systems as the backbone defense against rising waters. To be effective, a pumping system may include both permanent and temporary pumps sized and designed to move water quickly from the collection point to the discharge point.

Permanent pumps must be highly reliable, able to sit unused for weeks, then run continuously without failure until the flood problem has abated. Because floodwaters contain high levels of sand, silt, and debris, the system must be able to withstand the abrasive materials and resist clogging. Power sources for the pumps must also be considered because interruption of electrical power to a pump site is probable during a flood event. Even the siting of diesel power needs to be taken into consideration, because a rapidly rising stream can quickly inundate a poorly sited power source, rendering the system ineffective when needed most.

A Primer for Designing a Complete Pumping System

The design of a pumping system begins with an understanding of the physical, environmental, and geological parameters of the area, as well as the hydrological conditions that can be expected. Understanding the potential capacities and flow rates and the location of the watercourses most likely to be affected during various storm events is critical. In urban regions, the areas where potential loss of property is most likely to occur through flooding should also be determined. Almost all municipal stormwater managers have reasonable knowledge of where problems might occur and can perform risk assessments to help design appropriate pumping systems.

Designing, whether it's a single pumping station or an entire system, can be performed either in-house by hydrologic engineers, through partnerships with other agencies such as the US Army Corps of Engineers (USACE), or by private consulting firms. USACE provides a variety of technical manuals that offer information pertinent to the design of pump stations; these manuals are available on the corps' Web site at www.usace.army.mil.

According to the USACE manual, Engineering and Design—General Principles of Pumping Station Design and Layout (EM-1110-2-3102), several factors influence the design and layout of civil works pumping stations used for interior drainage removal from areas protected by local flood protection works:

·      Dependability."Pumping stations are one of the more vulnerable features of a flood protection project. The failure of a pumping station during a flood could result in considerable damage within the protected area. Consequently, station dependability must be the primary consideration, during the design and pump selection process."

·      Economics."While the cost of the pumping station is generally but a small percentage of the cost of the entire project, this does not mean that the designer is to proceed without any consideration of cost. Because of the infrequent operation of the majority of local flood protection pumping stations, efficiency can be sacrificed to a certain extent in favor of equipment with a lower first cost. However, higher pump efficiencies can actually lower the installed horsepower requirements and reduce operating costs for large stations that have frequent usage. The extra costs to provide higher efficiency equipment should be studied on a life cycle cost basis over the project life."

·      Refinements."The equipment selected must be rugged, reliable and well suited for the required type of service. Refinements which make no realistic contribution to usability or dependability should be strictly avoided."

Another USACE manual, Engineering and Design—Mechanical and Electrical Design of Pumping Stations (EM 1110-2-3105 [Change 2]), provides addition details about types of pumps and their applicability. According to the manual, there are three centrifugal classes of pumps used for flood-protection pumping stations:

·      Axial-Flow Impeller Type. "The impellers of these pumps have blades shaped like a propeller. This type of pump develops most of its head by lifting action of the blades on the liquid. The pumped fluid travels in a direction parallel to the shaft axis. Axial flow pumps are primarily used to pump large quantities of water against low heads and are typically used in open sump pumping stations in a vertical configuration." Axial-flow impeller-type pumps can include fixed-blade vertical type, adjustable-blade vertical type, and fixed-blade horizontal type.

·      Mixed-Flow Impeller Type. "The impeller of these pumps develops head or discharge pressure by a combination of both a lifting action and a centrifugal force. The path of flow through the impeller is at an angle (less than 90 degrees) with respect to the pump shaft. The pump can be constructed similar to an axial flow pump with water flowing axially from the pumping element, or the impeller can be placed in a volute (spiral casing), where the water flows from the pump radially. The volute design would be used either for large pumps where a volute would allow the pump to operate at lower heads or for small pumps where it is desirable to have a dry pit installation with the discharge pipe connected near the pumping element."

·      Centrifugal Volute or Radial-Flow Type. "The impeller of these pumps develops head only by centrifugal force on the water. The path of flow through the impeller would be at a 90-degree angle with respect to the pump shaft. A special design of this pump has a non-clog impeller that makes it useful for pumping sewage. This type of pump is used for pumping small flows and in applications where a dry pit sump is desirable."

Factors that have to be considered in the design and installation of a permanent pumping system include the arrangement of the pump in the facility. Here, the designer would need to be concerned about whether the pump should be vertical or submersible, with vertical pumps the most common type in flood control pumping stations. Selection of pump types should be based on the life cycle cost of the pumping station, the capacity of the pump, the head requirements, the net positive suction head, and the efficiency of the pump. All of these parameters would be determined from the hydrology requirements of the station. Other considerations include incoming electrical service, foundation conditions that might make it prohibitively expensive to lower the sump to the level required by certain pumps, the available space for the proposed station, and whether the pumps might be subject to clogging because of low discharge capacity.

Although this information provides basic guidance as established by USACE, the actual design of the pumping station and selection of pumps must be determined for each site through appropriate hydrological modeling and detailed engineering.

Pumping Action: Case Studies

The results of a well-designed pumping system are most evident when the rain begins to fall and the water rises. This is literally when the floodwater meets the intake. The success may be measured in terms of the number of lives protected and the value of property loss avoided.

Urbandale, IL

Urbandale-area flooding.

Urbandale is a small community near Cairo, IL, and the confluence of the Mississippi and Ohio Rivers. In May 2002, an overnight storm dumped at least 8 in. of rain, inundating the community and threatening its sole industry, a lumberyard with a wood-drying kiln. The major concern was that floodwaters could inundate the kiln, possibly resulting in an explosion.

"We got the call from our area commander who was monitoring that area," says Danny Max, acting chief of the Readiness Branch for the Memphis, TN, District of the Corps of Engineers. "The local sponsors had two different pumping stations there to lift the interior drainage over the levee during high stages, and the gravity gates have to be shut to keep the Mississippi and Ohio Rivers from flowing backward through the levee. The pumping stations could not keep up with all that water."

The corps began mobilization to provide additional pumping equipment to the local community. "We provided several tractor-driven-type pumps, and we pumped around the clock for eight or 10 days," recalls Max. "We partnered with the State of Illinois and Alexander County. We were providing all of the support for those pumps, while the state and the county manned other pumps that we had available."

During potential flood situations, the corps monitors the water levels in the river, and it has phased plans for various stages. In the Urbandale incident, a Phase II flood plan operation was in effect. "In a Phase I, we put people out in the field to monitor the conditions of the levee systems that we have constructed and turned over to the local sponsors for their operation and maintenance," Max explains. "Phase I keys off the Cairo gauge at 49 feet. Phase II is 52 feet at Cairo, and that's when we actually mobilize field offices." This mobilization includes provision of sand and sandbags, portable pumps, and technical assistance to the local agencies.

One of the major concerns during a flood event is the generation of sand boils. "Sand boils are areas that, as the river rises, put pressure underneath the levee," describes Max. "This water pressure finds places to go underneath the levee, and it comes up on the land side of the levee as a sand boil, similar to an artesian well. If it starts piping material from underneath the levee, then it could cause a levee failure. What we do is equalize the head pressure on the water by ringing a sand boil with sandbags and bringing it up to an elevation where it negates the pressure so the pressure evens out."

Maryanna, AR

At the confluence of the St. Francis and Mississippi Rivers stands the W.G. Huxtable Pumping Plant. Using 10 4,000-hp diesel engines turning pumps with 10-ft.-diameter impellers, this colossus handles 12,000 ft.³/sec. or 5 million gal./min. of water, protecting a 2,000-mi.² watershed from flooding. "During normal flows on the St. Francis River, the river flows through gravity bays at the center of the pumping station," explains Max. "We can gravity-flow several times more than we can pump. The St. Francis used to back up during flood times. There was nothing to prevent the Mississippi and all the localized rainfall from flooding thousands of acres of land. As long as we can get positive drainage through the station, we do not turn the pumps on. But on the occasions where the Mississippi gets higher than the St. Francis, we have to shut the gravity gates and begin pumping."

Originally constructed in 1977, the Huxtable Pumping Plant was recently refitted. "The contract [to refit] was actually awarded on what was considered best buy, based on wanting a better-design pumping station," says Max. The original design blades were made of softer metal, and it was desirable to go to a stainless steel for improved wear. During the 25-year life of the original impellers, the tolerances on the blades began to increase, reducing the efficiency of the pumps. The new impellers are designed to last for an estimated 50 years. "We wanted to do this once and we wanted to do this right," relates Max. "We wanted to spend the minimum of the taxpayers dollars but come out with something that had a 50-year life and not a 25-year life."

Palm Beach, FL

The South Florida Water Management District covers a 16-county region of 17,930 mi.² that extends from Orlando to the Florida Keys and serves 6 million residents. The district operates and maintains 1,800 mi. of canals and levees, 25 major pumping stations, and 200 larger and 2,000 smaller water control structures.

"Pumps have been a central component of operations, particularly in agricultural areas, but we do have a couple of pump stations that deal with urban areas," points out Tommy Stroud, director of the district's Operations, Control and Technical Support Department. "The wet season generally starts around June and extends through November. During the wet seasons, the pump stations operate pretty frequently. When we have rain events that are moderate but short of flood threatening, we may still operate hose pumps 24 hours a day to remove that volume of water so that we avoid a flood."

The district's largest urban pumping station is in the drainage basin of western Fort Lauderdale. "That area back in the '50s was part of the Everglades," Stroud points out. "The pumps were basically part of a system [that includes] levees to hold water back from the adjacent Everglades. The pumps were the mechanism by which any rainfall that was introduced into these basins - that had now been drained and levied—was pulled out, so those areas could become economically viable for either agriculture or urban growth."

Many of the design parameters used when the pumping stations were first built assumed an agricultural land use. "The demand for home sites and urban development overtook those agricultural land uses," says Stroud. "In Broward County, it is almost all residential urban development that these pump stations now protect. It becomes an interesting water resource management issue because the capacity of the pump station assumed there would be agricultural operations in the basin, which are a little more [tolerant of] flooding. An urban landscape can't tolerate the same level of flooding, so we really have a management challenge in how these developments were constructed in the 40 years between the design and the implementation of the pump station." He adds that many residents are relatively new to the region. "They don't recognize that the area they currently live in used to be a fairly significant swamp. They certainly don't want to tolerate water in their yards for long periods of time, so there is a lot pressure to remove the water as quickly as we can."

The district has developed two approaches to deal with urbanization. The first is to require new developments to include pump stations that are owned and operated by the development. The second involves the installation of large lakes in which to temporarily store the water during major storm events. "A combination of those two things allows us to not have to enlarge the current facility," says Stroud. "However, some of the older communities don't have these pumps or the large lake areas in which to store stormwater. What we are wrestling with now is that some of these older developments are now impacted by the improvements in some of the new developments that displaced the farmland. There are a lot of options on the table. They could go to a pump type of system to disconnect them from the effects of the conditions downstream in our canal. We've looked at reconfiguring or adding pumps. We've talked about operational protocols where we are looking at conditions during a storm event in the entire basin. If one community is in sufficiently good shape, they can back off their pumping to allow capacity in the canals to be dedicated to an area that's not [in] as good of shape. Cost has become an issue, so we are trying to find the least cost but the most effective solution to the problem."

When pumps are the alternative of choice, the pump selection criteria are determined through hydrological modeling and other project parameters, explains Zan Kugler, director of the district's Engineering and Project Management Division. "Our end of the business is more the detailed design and preparation of the specifications for the pump stations. The operation and maintenance of the pump are extremely important. We want to design a very reliable pump because this is for flood control, and reliability is the most important criterion. Service life is extremely important, so we look to specifying performance that is based around maybe a slower-speed pump that wears less than a faster-speed pump."

Deltona, FL

Three straight months of rain in Deltona in 2002 led to increased flooding.

Deltona is a retirement community of 72,000 located along the eastern side of Florida. Developed in the 1970s, the community included retention basins as part of the original flood control strategy. But development pressures and growth resulted in conversion of the retention basins into housing tracts, with a resulting increase in flooding. Starting in June 2002, the community experienced three straight months of rain, greatly increasing the number of flooding situations.

"We have nine stationary pumps throughout the city," says Wesley Keeney, the city's stormwater supervisor. "With our daily thunderstorms, it was hard to get caught back up. We currently have 27 pumps rented, and we're just trying to keep the water levels down out of people's houses."

Keeney turned to Thompson Pumps in Port Orange, FL, to provide him with the temporary pumps. Most are 6-in. centrifugal force pumps, but two 10-in. pumps and one 8-in. pump are being used in the hardest-hit area. "We are doing a lot of chain-pumping [relaying water from one location to another] because we have to pump for such a long way, and we're also using high-pressure head pumps to force that water."

Although the city had a stormwater master plan developed by a consultant, the unexpected 50 in. of water required many adjustments in the field. "We had to adjust everything on the fly," recalls Keeney. "The Storm Water Master Plan helped us out some because we knew where we could shift the water to. A lesson that we learned is to plan ahead and look way into the future. Get a lot of pumps and find your resources ahead of time."

Many pump manufacturing companies provide emergency assistance to communities such as Deltona, reports Majid Tavakoli, vice president of products at Thompson Pumps. "We have an emergency response team, and based on the severity of the emergency, we are set up to respond at different levels." Tavakoli's team collects data from the municipality, including rainfall levels, volume of water, and surface areas, in order to design a solution. "We calculate the amount of rain that is going to come, the ground infiltration, and the area that is flooded in order to make the recommendations."

The company will take information from the municipality and even make recommendations relating to size of the piping, street crossings, and other concerns. Response time is critical, with a goal of responding as quickly as two hours from the time of the call. "It depends on the location," says Tavakoli. "Sometimes we have the equipment available behind our trucks. We know about it and we just take it into the area, start setting up, and pump." In Deltona, Keeney reports, the response was immediate. "They started rounding up pumps. I called them on a Monday evening, and by Tuesday morning they had pumps rolling from everywhere. They were even letting us rent pumps that just came off the manufacturing line."

As a result of the floods, Keeney says, the city will be reevaluating its storm drain system, working with a contractor. "We prioritized a list; of course now that we had all this rain, we are going to be changing that list somewhat," he states. "It mainly goes from where water is entering structures all the way down to where it is standing in swells. Where we have water entering structures, that would be the first place the engineers would be checking out. We have such a low geography; in some areas we are only 12 feet above sea level. These areas are going to require pumps to move the water."

Guest author Lynn Merrill is director of public services for the City of San Bernardino, CA.

SW - January/February 2003

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