Flowing through the center of the downtown business district of Iowa’s vibrant capital city is the Des Moines River, a 525-mile-long tributary of the great Mississippi, and the longest river in the state.
Named for Iowa’s largest city, the river has long beckoned tourists to the downtown area for a variety of festivals, celebrations and entertainment venues. Now, a recent infrastructure improvement project promises to further enhance the downtown experience; affording visitors the opportunity to explore both banks of the river whether walking, jogging, biking, blading, or dining at one of several planned riverfront restaurants.
The Principal Riverwalk is a multi-year, downtown improvement initiative jointly funded by the City of Des Moines, the State of Iowa, Army Corps of Engineers, Federal Highway Administration and the Principal Financial Group, one of the city’s largest employers. When complete, the Principal Riverwalk will feature a 1.2-mile recreational trail connecting the east and west sides of downtown via two pedestrian bridges, and a 12-foot-wide multi-use trail along the balustrade at river level for joggers and bikers. Night-time lighting and security will turn the river loop into a 24/7 attraction as downtown Des Moines prepares to take on a new and exciting recreational role.
Planning officials agree that while city expansion projects — especially those designed to attract tourists — are exciting and beneficial for municipalities, new development can also place additional strains on existing infrastructures, especially storm-water and sanitary sewer services. The Des Moines Riverwalk project is no exception. Yet advance planning by city engineers and cooperative efforts among all parties afforded officials the opportunity to incorporate a much-needed upgrade to aging sewers, while addressing an ongoing threat posed by the river nearly every spring — flooding.
After the historic flood of 1993, earthen berms and levees — many of which failed to protect the downtown area — were raised and reinforced. While the levee enhancements proved effective in containing the rising water within the river’s banks during a subsequent flood event in 2008, the aged, inadequate storm-water sewer system was unable to prevent flood waters from backing up into the downtown area again, surrounding many businesses. City officials focused on developing a solution. David Kamp, an engineer with the city and instrumental in the design of the project, explains the plan.
“In the downtown business district, there are storm sewer systems, sanitary sewer systems and combined sewer systems,” Kamp says. “During large rain events the combined sewers become very full, very quickly and usually become pressurized. These can cause backups to properties which have their sanitary sewers connected to the storm sewers. Also, during these deluges the combined sewers cannot handle all the flow and the only alternative is for the combined sewer to overflow directly into the river. Therefore, the plan specifies separating the storm flow from the sanitary sewage flow; an approach with a number of benefits.”
According to Kamp, by reducing the demand on the combined system, there are fewer backups into properties, which allow more capacity for sewage in the systems. Having separate storm sewers provides the storm water runoff a direct outlet to the river. The greatest challenge, however, is when these large rain events occur and the river level is already high. The solution — provided by the new sewer system design — includes a gatewell which is a large structure with a cast-iron gate that can close and not allow river water to backflow into the storm sewer.
Coined the Court Avenue and Water Street Sewer Separation Project, city officials unanimously approved nearly $10 million in funding to address the inadequacies of the existing sewer system. The project includes the construction of storm sewer along Court Avenue from Water Street west to 2nd Avenue; then north three blocks to Grand Avenue; and west again to 3rd Street — a distance stretching approximately six blocks.
The second component includes the construction of a storm-water pump station positioned directly across the earthen levee from the river at the corner of Water Street and Court Avenue. The pump station incorporates an existing gatewell structure that was built in response to the 2008 floods. The gatewell was built in advance of the pump station and was constructed in the winter during low river levels. Since the bottom of the pump station is situated well below the river, construction would not be delayed if river elevations would have risen the following spring.
The larger storm sewer was specified as RCPP; a reinforced concrete pressure pipe with a gasketed joint. The deeper sewers are below the river level and will always have infiltration into the pipe. Also, during high river levels, the pipe will become pressurized. Therefore, the gasketed pipe will reduce the infiltration and material being conveyed into the pipe through the joints.
H&W Contracting LLC, based in Sioux Falls, SD, was awarded the sewer separation contract with construction beginning in early spring 2011. The first phase from Water Street along Court Avenue and north up to Grand Avenue was completed on schedule last fall. The open-cut method identified for the majority of the project was influenced mostly by cost. Yet there were also locations along the sewer alignment in which open-cutting the pipe was not feasible; hence, five trenchless installations were completed along the project.
Rognes Excavating worked with RTL Equipment, also based in Ankeny, IA, to secure a Doosan crawler excavator to complete the trenching and installation of the 66- and 54-inch reinforced concrete pressure pipe (RCPP) sewer. The RCPP along Court Avenue is all 66-inch-diameter, while the pipe diameter reduces at 2nd Avenue to 54 inches for the remainder of the upstream system.
“There were confined space and lane width considerations given that this is a high-traffic, often-congested downtown area,” says Doug Rhodes, sales representative with RTL Equipment, Ankeny, IA. “We recommended the Doosan DX235LCR excavator for the job mainly because of its zero clearance capabilities. This excavator has just three inches of overhang and has the ability to maneuver within inches of construction barriers. The DX235LCR also has extended vertical reach that requires repositioning of the excavator less often, and is capable of loading spoil into high-frame truck boxes.”
Subsurface exploration was extremely critical. As a result, there were no delays due to utility conflicts. Communication was also an important component to the project’s success. Contractors, businesses and media were kept informed continuously, not just about specifics of the project — the what, where and when — but also of progress and delays. Aesthetically, the walls surrounding the station complement the design of a river-view kiosk situated directly across the street adjacent to the Court Avenue bridge. Kamp explains this is one of the unique components of the project overall.
“The need and funding for the pump station came after the design of the Riverwalk project,” Kamp explains. “Once the location was determined, there was a coordinated effort with the architect that designed the kiosk building and the engineer of the pump station to best fit with the completed design elements. Normally a pump station is less about aesthetics and more about function. But in this case, we were able to get project engineers and architects together to find a way to get both. There was compromise from all sides. Given the high profile location, limited space and established Riverwalk aesthetics, there are many elements of the project that would not typically be in a pump station project. Aesthetics had to be adjusted to ensure function of the station was not compromised.”
The project was funded by a combination of state, federal and local funds. There was a $4 million grant from Community Development Block Grant funds, $3.5 million of I-Jobs funding, and the balance funded by Storm Water Utility revenue bonds. As a result, businesses and the thousands of downtown loft and apartment dwellers are now better protected; the result of an improved infrastructure that helps ensure the downtown area remains economically viable while supporting recreational use — all in an aesthetically-pleasing manner.
“Aside from some unavoidable traffic backups and congestion during peak times and a few minor, unsightly inconveniences to the thousands of Downtown Farmer’s Market goers, the project has gone really well,” Kamp says. “Contractors, workers and the city tried to minimize inconvenience and mitigate the occasional frustration of drivers and downtown pedestrian traffic, but some of the challenges were simply unavoidable. The vast majority of Des Moines residents are unaware of just how significant this project really is, or what has taken place underground, making the downtown area safer from flooding. It really is an out-of-sight, out-of-mind sort of thing.”
Mary O’Keefe, senior vice president and chief marketing officer at Principal Financial Group, and chair of the Riverfront Development Authority, accompanied the Riverwalk team to other cities with similar river attractions to get a sense of the operations.
“We learned from other cities what to do and what not to do,” O’Keefe says. “Working together with the city engineers, we were able to plan ahead and coordinate efforts. As a result of the project, flood control is much better for the businesses and residents of downtown Des Moines; and it’s incorporated into the Riverwalk project, often disguised as part of the attraction. These flood control improvements, all part of the overall Riverwalk design, also allowed for removal of grass mounds that largely blocked the view of the river from street level. The project is ongoing; we view it as something that will always be evolving.”
FOR MORE INFORMATION:
Doosan Equipment, (877) 613-7970, www.DoosanEquipment.com
H&W Contracting LLC, (605) 339-8834, http://hwcontracting.org
Rognes Bros. Excavating Inc., (641) 592-8932, http://www.rognesbrosexcavating.com