Between a Rock and the Mississippi River

Known collectively as the Quad Cities, the Illinois cities of Moline, Rock Island, Davenport, and Bettendorf are major Mississippi River commerce sites on the Illinois-Iowa border. The commercially advantageous location of one of them, however, also makes it extremely flood-prone. Bordered by the Mississippi on the north and the Rock River to the south, low-lying Moline suffers each rainy season, as drainage to the rivers does not occur quickly enough by gravity alone. Drainage to the Mississippi is obstructed by a levee system in some areas, and during particularly heavy downfalls, river water has backed up into the stormwater system.

Near the River WalkThe city has traditionally used a series of pumping stations to alleviate flooding during storms and the spring snowmelt. The pumps transfer stormwater runoff–including the twigs and other debris–from the storm sewer system and drainage ditches to the rivers. The older submersible pumps presented problems, however. Designed primarily for sewage treatment plant applications, they were not well suited for intermittent flood-control use. “Those pumps are good for what they’re made for, but when you don’t use them a lot and don’t maintain them a lot, they don’t work when you need them,” observes Moline City Engineer Frank Hodina. In 1997 the city contracted with Missman, Stanley & Associates Engineers of Rock Island to evaluate the problem and recommend solutions for the most flood-prone areas. Of special concern were an area of planned downtown development and a planned public River Walk area on the Mississippi riverfront along 16^th and 17^th Streets. Based on recommendations, the city installed five new 16-in. vertical electric stormwater pumps and associated controls at strategic locations along the two rivers.

This pump unit handles parking lot and street runoff. Discharge piping leads to the Mississippi River.The River Walk area received two new stormwater cutoff structures. At the critical 16^th and 17^th Street locations, the city installed a 100-hp, 10,000-gpm unit and a 50-hp, 7,300-gpm unit. A few blocks away a stormwater cutoff and pumping structure (100 hp, 9,650 gpm) was installed to handle parking lot and street runoff. Two pumps were installed in residential areas. One, a 100-hp, 10,000-gpm unit in a somewhat isolated area, handles flow from an open drainage ditch by discharging it through removable, temporary surface piping over a small levee and into another ditch that drains to the river. The other, a 100-hp, 11,200-gpm pump, is in a low-lying residential area with a small lake. Although the storm sewer handles lake overflow, the area had been prone to flooding. The pump at this location discharges into a marshy area that drains to the Rock River. As an indication of just how low and flood-prone the city is, the total dynamic head for these installations ranges from 7 to 14.5 ft. All stormwater collection areas have isolation flood gates to prevent backflow of river water during high-water conditions. Normally the gates remain open to allow gravity drainage to both rivers. As the stormwater level rises, float switches in the wells activate the pumps. At very high water levels, the gates must be closed manually.

An installation in an isolated residential areaFlooding last spring put the new systems, all by SRS Crisafulli of Glendive, MT, to a severe test. Hodina reports that they performed well. He describes the tradeoffs inherent in the stormwater pumps: To accommodate inevitable solid debris from the floods, the new pumps by design are of relatively low efficiency, but because they are used primarily in emergency conditions, they actually use only a small amount of energy annually. “They’re not very efficient, but you don’t run them that long–a few weeks every four years maybe,” says Hodina. An advantage is their correspondingly low maintenance. “You don’t have to exercise them,” he says. “When the Mississippi came up this spring, one of the pumps ran for three weeks straight with no problems. Now that the flood is over, we’ll pull them all out and have them cleaned up and drop them in and forget about them ’til the next time.”All five pumps discharge above grade on a sloped surface to the river, either through permanent or temporary discharge piping, buried piping, or directly onto riprap or grassy areas. “Sometimes the pumps worked all too well,” remarks Hodina. “We have to put in a little more erosion protection at the discharge pipes in a couple spots. You’ve got 10,000 gallons per minute coming out of there. With the flood, there was a lot more turbulence there than we thought, so in a couple of areas we have to go back, now that the river’s down, and address those.” Last June a nonfunctioning flood pumping station that previously contained a 20,000-gpm submersible pump was also upgraded with a 100-hp, 9,300-gpm Crisafulli vertical pump. The old pump drained the well too quickly, and the original plan called for replacing it with a smaller submersible unit. The city was concerned, however, that a smaller submersible pump might not be able to handle the debris that flowed into the well and instead opted for another vertical pump.