In thriving urban areas, new development often results in a higher volume of stormwater run-off into local streams, leading to channel instability and stream bankstreambank erosion downstream. These are among the “growing pains” associated with increased development. This was the case in St. Charles, MissouriMO, where the city’s Department of Public Works recently completed 13 channel stabilization projects using gabions to prevent further channel migration and arrest stream bankstreambank erosion in residential neighborhoods. “St. Charles has grown exponentially in the last 20 years,” saysid notes Tim Baker, acting director of Public public Worksworks. “Unfortunately our creeks couldn’t keep up with it, and some of the older homes in the older parts of town felt the brunt of the expansion by the erosion in their creeks.” Site Selection and Priorities
In 1996, the St. Charles Department of Public Works identified and prioritized locations for stream bankstreambank stabilization. “We looked at areas that had a need for it based on our knowledge of the locations of downstream erosion and a lot of input from homeowners,” Baker saysrecallsid. For the most part, these creeks are tributaries of the Mississippi River, which borders the city on to the North. In 1997, the city passed a $30 million bond issue, of which $6 million was allocated to stream bank stabilization based on Public Works’ general cost estimates for the projects. Assessing AlternativesThe project team performed a preliminary assessment of each site, analyzing the feasibility and estimated costs of various methods of channel stabilization, including gabions. Other alternatives included a crib wall with a combination of geosynthetic and soil material to create a reinforced embankment, a crib wall with vegetation, a geosynthetic-wrapped riprap with vegetated geo-grid, and vertical concrete retaining walls.Use of gabion baskets to stabilize channel banks offered several advantages. It is a cost-effective technology, which can be constructed with relative ease and in a nearly vertical pattern, minimizing the intrusion into residents’ property. “Gabions are also time tested in the St. Louis area,” Baker sayspoints outid. Based on the feasibility, cost, and aesthetics, the city specified use of gabions in all but one case, where a vertical concrete retaining wall was specified because ofdue to space limitations. . “People preferred the gabions in their back yards to a concrete retaining wall,” Baker saysobservesid.Public Involvement
The Public Works Department developed a program to inform the citizens of St. Charles – homeowners in particular – , affected homeowners, of the plans and methodology for creek stabilization. The city conducted public hearings followed by one-on-one meetings between Public public Works works engineers and specific homeowners affected by the projects. After development of construction drawings and detailed engineering, Woolpert LLP also made a presentation to the St. Charles City Council. Baker believessaid public involvement was essential to the success of the projects. “You can perform the best-engineered project, but if you don’t have public involvement, it is not going to succeed,” he saysaid. “. The citizens of St. Charles are very sensitive, as they should be, because these are their houses. If they are not comfortable with the project, it doesn’t matter how good the plans are. We recognize that, and that’s why we try to involve the public early and often and let them have input.” Homeowners were most concerned about tree removal. “In some cases, tree removal was necessary,” says Baker said. “To offset that, we implemented a tree replanting program through a local nursery.” Laying GroundworkThe project team performed a geotechnical evaluation of the soils at each site. Each site also was professionally surveyed to assess its topography, identify property boundaries, and delineate easements. Design was completed ahead of schedule, and permitting was obtained through the US Army Corps of Engineers and Missouri Department of Natural Resources (MDNR). ConstructionContractors began the construction process with preparation of channel sites, including earthwork and grading. Gabions were constructed starting at the bottom of the channel, stacking ; a series of gabion baskets were stacked upwards to the specified height, which varied from three 3 to 15 feetft. The baskets were then filled with rock using construction equipment and/or by hand. Once full, the wire lids were placed on top and secured with metal ties. Fences were constructed as necessary to meet code based on where the height from gabion top to channel exceeded four fee4 ft. Properties were replanted with grass and trees. Public Works supervised and inspected construction. The average length of the projects was approximately 250 feetft., with the longest some 400 feetft., and construction time ranged from two to four months. Challenges The contractors successfully met the challenges associated with gaining construction access with the least disturbance to homeowners’ properties and, on some sites, construction in tight spaces. In fact, the biggest challenge in projects of this nature is not the engineering and design; rather, it is earning and maintaining the public’s confidence and ensuring their its satisfaction with the city’s solution to the erosion problem. Except where projects were extended at homeowners’ request, the projects were completed on schedule last fall. The community of St. Charles considers the project a success, a measure of that success being the growing list of homeowners who want the city to stabilize creeks bordering their properties. “I think once we were finished, the citizens and homeowners were very happy,” Baker saysnotesid. “We got a lot of compliments. In fact, many more homeowners are requesting this solution.” A New Rating SystemThe city has identified additional sites using a system to rate the seriousness of the erosion threat to homeowners’ property. This involves looking at the broad picture, from starting to stopping points of the current projects. The city is also rating sites based on, for example, distance of structures from the creek, and is sending this list to the homeowners who have called about the status of their creek. For future projects, the city anticipates specifying vegetated gabions, which use a mixture of rock, soil, and plants to achieve a more natural stream bank appearance. “Right now we’ve switched our focus to strictly use the vegetated gabions,” Baker explainsed. “MDNR wants something less intrusive to the natural habitat.”Meanwhile, many of the growing pains associated with upstream development have been eased for homeowners and citizens of St. Charles.
