Flooding can be caused by heavy rain falling for an unusually long time. It can be caused by an unusually large snowpack followed by a sudden thaw. It can be caused by unusually high tides, tsunamis, dam failures, deforestation, poor drainage, or a high proportion of impervious land, or by a combination of any of these factors.
Because the most vulnerable areas are low-lying, the first method of flood control is often to build berms, levees, and floodwalls. Another is to dig down and construct ponds and stormwater structures to detain floodwater, and storm sewers, canals, and ditches to convey it into adjacent water bodies outside the flood protection zone.
These methods all depend on gravity to move the water out of the flood zone, but when the water outside is higher than inside, flood control engineers need a different strategy.
This is where pumps come in. Whether they’re on high ground or underground, in concrete pump stations, bolted in place, or portable, they lift the floodwater up and into the adjacent water body. The pumps a district chooses depend mainly on the frequency of the flooding and the availability of funding.
Coralville, IA
The winter of 2007–2008 was especially severe in eastern Iowa, and soil moisture conditions in the Iowa and Cedar River watersheds were near capacity.
Then the snow pack melted in late May, and heavy spring rains began in early June, according to the flood mitigation case study by Dan Holderness, P.E., Coralville city engineer, and Teresa Stadelmann, P.E., of the design consultant firm HR Green, and commissioned by the University of Iowa after the flood. The floodgates of the Coralville Reservoir above the city of Coralville, a suburb of Iowa City on the Iowa River, were opened to balance increasing flows into the reservoir, and on June 5 the flooding began.
The Coralville area is flat and low, with heavy clay soils. The Iowa River meanders by on its way south from the Coralville Reservoir, which was built by the US Army Corps of Engineers in 1958, to the Mississippi River. At Coralville, west of the river, Biscuit Creek joins Clear Creek, a tributary of the Iowa River. The CRANDIC (Cedar Rapids and Iowa City Railroad) embankment runs alongside a western section of the river, between the river and the 1st Avenue corridor in the city. Both the railroad and 1st Avenue have bridges that crosses Clear Creek near the river.
By the second week of June, the river had surpassed the flood stage, and by June 10, water had filled the reservoir, according to the study. Outflow began overflowing its emergency spillway, and the flow downstream was dramatically increased.
On June 13, the river surpassed its record 1993 elevation. With the river still rising, the railway embankment failed. Land in Coralville flooded, and existing stormwater pump stations were inundated. On June 15, 10 days after the flooding began, the river crested, but it wasn’t until July 7 that it fell below flood stage. Ultimately, nearly 273 acres of land were flooded. Commercial damages totaled $21 million and residential damages were at $4 million. Damage to the city’s infrastructure was estimated to be at $7 million.
“The city had been protected through the 1990s for a 100-year flood,” says Andrew Marsh, an engineer with HR Green. “The 2008 flood was much closer to an estimated 500-year flood event.”
Within one month, the city commissioned HR Green to complete a flood control and storm sewer improvement study, including the projected capital costs for the improvements. The study was completed by November.
The study resulted in a series of flood protection projects that required substantial funding to implement. They included building additional–and higher–berms and floodwalls and installing backflow prevention valves in the storm sewer systems draining to the Iowa River, Clear Creek, and Biscuit Creek. Recommendations also included repairing the existing stormwater pump stations, adding new ones, and raising their level of protection to 1 foot above the 2008 flood, the new flood of record.
“Securing funding for the projects was the biggest challenge,” says Marsh. “The city has done a fantastic job securing funding.”
I-JOBS, a state investment program, awarded the city $27.1 million for berms, walls, pump stations, the reconstruction of 1st Avenue, and the construction of the new 1st Avenue Bridge, as well as $3.6 million for flood protection along Clear and Biscuit creeks. The Economic Development Administration gave $7.1 million and the CDBG Community Development Block Grant (CDBG) gave $3.1 million for flood protection along the CRANDIC embankment. The CDBG also gave the city $600,000 for storm sewer backflow prevention and $10 million for pump stations for the new 4th Avenue storm sewer system.
Because of the cost and the scope of the work, the area was divided into flood protection regions, with the lowest and most flood-prone areas protected first.
The four existing pump stations and the six new ones the city bought after the flood are all from Flygt (now Xylem Water Solutions) in Charlotte, NC, says Marsh, now project manager for all the projects that involve the pump stations.
“It’s at the city’s request,” he says. “They’re happy with that equipment and they want to keep the same operation and maintenance throughout the city. Also, the city has used the dealer, Electric Pump in Des Moines, in the past, and they’re happy with their service and products.”
The city now has a total of 10 small concrete pump stations. Of the four that were installed before the flood, three are along Clear Creek and one is on the railroad embankment. After the flood, these four were upgraded and six new pump stations were built, two on Clear Creek and four on 1st Avenue. They all will have emergency power capabilities with installed diesel generators.
“There was a lot of construction sequence coordination,” says Marsh. “Coralville is a growing community, and accommodating active businesses without disrupting services like electrical traffic, water, and sanitary sewers takes a lot of coordination.”
All the new pumps are submersible, propeller, axial flow pumps in Flygt’s PL-7000 series. Axial flow pumps are made especially for transporting large volume of water, according to the company. Because they’re submersible, the motor and hydraulics are integrated into one compact unit, so they’re smaller than other pumps. They’re also less complex to build and have virtually no noise or cooling problems, and they aren’t affected if the pump station floods, although the electrical controls and switchgear are above water.
In Coralville, stormwater runoff passes through a coarse screen that traps trash, enters stormwater pipes under the roads, and flows to Clear Creek or the Iowa River. When levels of the creek and the river rise enough to prevent normal drainage, the runoff flows to pump station wet wells, which accommodate the pumps and enough volume for the runoff to drain to for subsequent pumping. Inside each wet well are two identical pumps that operate in sequence to manage the flow.
A wall whose height was set at the 2008 flood level plus 1 foot separates the pumps from the creek and the river, and sluice gates on the stormwater pipes are closed to prevent river backup to the inland areas.
“Runoff can no longer use gravity to flow into the flooded river,” Marsh says. “The pumps drain the runoff by pumping it over the wall to the river channel.”
Working with Electric Pump, the city has developed a maintenance plan for the Flygt pumps. It includes periodic pump operations to drain the wet well, the operation of the generators on a monthly basis, and periodic inspection and maintenance by Electric Pump.
“There’s an extensive network of berms and walls to protect the city from the 2008 flood level plus one foot freeboard–a safety factor,” says Marsh. “There are more than before, and they’re higher.”
The city built flood control berms and walls in addition to the new pump stations to protect the 1st Avenue corridor. 1st Avenue was reconstructed from Clear Creek to south of 6th Street and has an improved storm sewer system. The reconstructed 1st Avenue Bridge over Clear Creek is higher and longer and allows floodwater to pass without backing up.
Overall project work continues into 2012. Flood protection along the CRANDIC railroad embankment includes earthen berms and concrete and removable floodwalls. The railroad bridge over Clear Creek will be replaced.
“Both the railroad bridge and 1st Avenue bridge replacements will reduce the risk of flooding upstream,” says Marsh.
The flood control and stormwater improvement study identified 42 storm sewer outfalls to the Iowa River, Clear Creek, and Biscuit Creek. These storm sewers all will have backflow protection to prevent stormwater from backing up into the streets.
The city also is focusing on increasing its amount of open space. “It’s for the aesthetics and the environmental benefits,” says Marsh. “A lot of our green initiatives area based on providing a more natural system for infiltration.”
This includes green space along Clear Creek between Biscuit Creek and 1st Avenue, and The Iowa River Landing Wetland Park on the inside curve of the river. The park itself is larger than 5 acres and includes 4.76 acres of wetlands, which provide water treatment, flood storage, erosion control, wildlife habitat, and groundwater recharge. In addition to flood protection walls and berms, walkways, trails, interpretive signage, and a comfort station, it will be planted with native vegetation.
Most of the work on these projects will be completed in 2012.
The Red River Valley
The Red River Valley is not a valley at all; it’s the floor of what was once Lake Agassiz, a glacial lake that once may have covered as much as 170,000 square miles. Because it’s flat and low, floodwaters are usually widespread and shallow, but along the border between Moorhead, MN, and Fargo, ND, it narrows to just a few hundred feet wide.
The Red River forms the boundary between Minnesota and North Dakota from its source near the southern border of Minnesota. It flows north and drains into Lake Winnipeg in Manitoba, Canada. Water from Lake Winnipeg flows into the Nelson River and empties into Hudson Bay. The soils in the basin are fertile, but mostly clay from sediments deposited during the last ice age.
According to the US Geological Survey, there have been significant floods here since the first anecdotal accounts in the mid-1700s. Some have been caused by heavy rain alone and by heavy rain on frozen ground, but the majority have been caused by the melting of above-average snow packs in the spring. Since the flood of 1997, though, when the water rose to 39.57 feet, flooding seems to be increasing in intensity as well as in frequency.
“In 2009, they had a record event,” says Jim Schulz, senior engineer with the city of Moorhead. “It made the 1997 flood seems like small potatoes.” The 2009 flood, the new flood of record, rose 40.84 feet. The city used a combination of sandbagging and clay levees as temporary contingency measures, Schulz says, but many of the storm sewers didn’t have gates and required temporary measures such as plugs and sandbags. These temporary measures worked, but they were time consuming and not as reliable as gated structures.
The city has been working on a number of flood protection projects since then, both permanent and temporary. “There’s generally a consensus with the city of Fargo on what needs to be done,” says Schulz.
One measure Moorhead has taken is to develop an action plan based on the depth of the flooding. For example, at 15 feet, city workers raise the bike bridges. By 17 feet, they begin to close roads. By 28 feet, they begin to close bridges, and by 35 feet, they begin to build temporary levees.
The city is also using computer modeling to create interactive maps, a depth grid, and a graphic that depicts flow conditions and the estimated arrival time of the flood peak. For example, Schulz says, if the National Weather Service reports that a flood is moving at 32,000 cubic feet per second, the model will show what it will look like.
Moorhead is building protection to the flood of 2009, 40.84 feet, plus 3 feet of freeboard, he says. The US Army Corps of Engineers estimates that a 100-year flood would be close to 42.5 feet without a diversion around the city. With a diversion in place, it estimates that a 100-year flood would be 30.8 feet and a 500-year flood, 40 feet.
The temporary measures are adequate in conjunction with the permanent ones, first because the main reason for flooding in the spring is snowmelt, which flows slowly, so to date there has been time to prepare; and second, because many permanent measures have been constructed since the 2009 flood. These reduce the number of temporary measures required and therefore the time required to construct them.
Workers are lining a ditch with a concrete liner so floodwater can move more efficiently through the city. Additional levees are also being built. The city is upgrading the storm and sanitary sewers and installing sanitary sewer isolation valves on homes in the floodplain. The cities are waiting for approval and funding from Congress for a huge diversion project that would reroute floodwater around both Moorhead and Fargo. Moorhead is also adding new pumps to its existing ones.
“We are generally going with Crisafulli [SRS Crisafulli in Glendive, MT],” says Schulz. With just one source, the parts are interchangeable; the pumps are cost effective for pumps that are used only in the event of major floods, and the power takeoff (PTO) pumping stations don’t need electricity to operate. In addition, Schulz says, the dealer, Northwestern Power Equipment Company in Rosewood, MN, “has been good to work with. I’ve worked with them with sluice gates, too.”
All the pumps are at storm sewer outlets that outfall to the river. The city has five permanent pumping stations with electric Crisafulli pumps, which operate automatically with level switches that turn the pumps on and off. These are in 10-foot-square pumping stations approximately 15 to 20 feet below ground.
Six permanent PTO pumping stations are powered by tractors brought in during large flood events and operated manually as needed. These pumps are typically Crisafulli L-Series Vertical PTO Gearbox Driven Pumps and are bolted in place.
“The maintenance is pretty simple,” says Schulz. “They maintain them annually by turning them and greasing the bearings.”
Most of the pumps are temporary. “We don’t anticipate flooding every spring to the level that we require all permanent pumps,” he says. “The flooding doesn’t warrant electric motors, switches, and automated switching stations.” The city set up approximately 42 small gas motor pumps of various models, which it leases during flood events, in low-lying areas to handle smaller capacity storm sewers.
During a flood event, floodwater in the city flows into storm sewers. Because most of the storm sewers go under the levees and then drain to outfalls on the river, they become a potential conduit for floodwater to back up into the streets. At points on the storm sewers, typically under the levees, gates have been installed to prevent floodwater from backing up. When the river gets too high and starts to back up into the storm sewers, the gates close. This also prevents precipitation or snowmelt in the storm sewers from reaching the river, however. The pumping stations and temporary pumps are used to pump this floodwater over the gates and levees and into the river.
The city also is building permanent and temporary levees on land adjacent to the river, most of which is residential. On properties where there is room to build a permanent levee between the home and the river and the homeowners have agreed to an easement, the city builds permanent earthen levees, constructed primarily of clay. Homeowners who didn’t agree to an easement were offered a buyout of the home. If they didn’t agree to an easement or a buyout, they were allowed to stay. During major flood events, the Army Corps of Engineers builds temporary clay levees on these properties and connects them to the permanent ones.
“The city has done a lot of buyouts with a large grant from state,” says Schulz. “Most of the homes on the lower elevations have sold, but there are a lot of holdouts at the higher elevations. There is a risk even for homes on the dry side of the levees because the permanent levee isn’t continuous, but we’re confident that we’d be able to do okay with permanent and temporary levees.”
The diversion project is another reason for the temporary flood control measures.
“A diversion is many years out in the best case, and with three major floods in three years, doing nothing and waiting for it to be completed was not a good option,” he says. “If a diversion is constructed, the flood mitigation measures being completed by the city will work to further reduce the risk from flooding.”
The project would be able to handle a 100-year flood, the equivalent of the record 2009 flood. Computer modeling is being used to quantify its impact both downstream and upstream.
The project also affects North Dakota, Schulz says, and in fact it would probably be built on the North Dakota side. There would be some extra cost to the state as well as some extra benefit. The project would create a 10-square-mile retention pond upstream of the two cities, which would fill during a 100-year flood event. Floodwater from the pond would enter a 36-mile ditch that would route it back to the river. It would cost $1.8 billion and flood three towns as well as nearby farms.
The Minnesota Department of Natural Resources (DNR) has awarded the city $45.5 million in grants for flood mitigation since 2009. As a contingency of receiving this grant money, the city had to commit an amount equal to a percentage of the median household income times the number of households, which came to approximately $12 million. If more grant money became available from the DNR, the city wouldn’t be required to come up with additional matching funds.
However, there are projects the city wanted to complete for which grant funds were not available. “The city has moved forward at its own expense and has committed an additional $16 million,” says Schulz. “We have the measures to protect the city.”
