It’s inevitable. No matter how hard departments of transportation try to avoid it, construction projects have an impact on their surrounding environment.
The impact becomes even more pronounced when a project is in an area renowned for its lakes, rivers and streams.
Such was the case when the Missouri Department of Transportation (MoDOT) was preparing for construction of a four-lane expressway in the Lake of the Ozarks area of Camden County in 2007. The 54,000-acre lake and surrounding state park is a popular tourism attraction because of its abundance of aquatic activities, from boating and water skiing to fishing and swimming. Construction of the new expressway had become necessary to improve safety and relieve congestion through the lake area as travel volumes on the existing Rte. 54 were ranging from 43,000 in off-peak times to over 50,000 during peak periods, such as holidays and summer weekends. With many access points and traffic signals along the existing route, the high volume of motorists often made it difficult for traffic to flow efficiently through the area.
The scope of the expressway project required MoDOT to mitigate, or compensate, for the environmental impacts it would have on the area. Mitigation can take place in a variety of forms. However, since mitigation banks are the preferred option of the U.S. Army Corps of Engineers, MoDOT selected multiple low-water crossings located in the same stream reach and collectively proposed them as a “bank.”
Forming its first stream mitigation bank would be just as challenging for MoDOT as it would be beneficial. It would require multiagency approval, but it also would optimize the ecological benefits of removing multiple barriers from a large stream reach and generate a greater amount of credits that could be banked for immediate and future stream mitigation needs.
As a result, the department was able to couple the use of its greatest asset, engineering, with natural design concepts to produce an excellent mitigation project and save taxpayer dollars in the process.
Opening a bank
Prior to these projects, MoDOT had found it difficult to develop a stream mitigation bank because of the amount of land acquisition required to generate bank credits.
Fortunately for MoDOT, the Missouri Department of Conservation had voiced a need to address the low-water crossings within the habitat of the federally threatened Niangua Darter, because they were believed to be one of the reasons the fish population was declining. Because this species is considered threatened, improving its habitat to provide long-lasting stream benefits was a priority. All four of the low-water crossings selected for modification were along the Little Niangua River in Camden County, one of the streams in which the Niangua Darter resides.
Although the location of these projects proved ideal for a stream mitigation bank, there was still another hurdle to cross. Up to this point, there was no accepted methodology to calculate the amount of stream mitigation credit received by modifying or replacing low-water crossings.
Before any of these low-water modification projects could begin, MoDOT had to collaborate with all five Missouri Corps districts, Region 7 Environmental Protection Agency, the Missouri departments of Conservation and Natural Resources and U.S. Fish and Wildlife Service-Region 3 to gain approval for the survey methodology that now determines the amount of compensation that can be received for low-water crossing improvements.
In the end, MoDOT’s first stream mitigation bank was approved by the regulatory and resource agencies. With the methodology now in place, the process required for future mitigation using low-water crossings will be much simpler and will result in more timely reviews and approvals by the U.S. Corps of Engineers when using this type of mitigation. By consolidating efforts for stream mitigation into one bank proposal, future mitigation needs will be met until all bank credits have been used. Another benefit of the stream mitigation bank is that it eliminates the need to commit other MoDOT resources (such as survey, right-of-way, design, etc.) as would be required for individual mitigation projects.
Crossing county lines
With the process approved, MoDOT was ready to go to work on the mitigation projects. However, the crossings were located on county roads, so MoDOT had to secure an agreement with Camden County to work on each crossing. Prior to every project, MoDOT sought input from county commissioners and engineers regarding alignment, width of the bridge and safety improvements. Once an agreement was signed, it allowed the department to temporarily take that crossing into the state’s system; design, bid and construct the project; and then transfer it back to the county once construction was complete.
“These low-water crossings were on county roads, but the economic and environmental benefit made them ideal for MoDOT to help meet our federal requirements for our construction projects in Camden County,” said Nicole Hood, MoDOT project manager. “Our standard mitigation efforts for general construction projects usually include planting trees or stabilizing nearby stream banks. The amount of stream mitigation needed for multiple construction projects forced us to look at a large-scale mitigation project. This project has allowed us to make several improvements to the transportation system and provide great benefits to the area’s natural habitat and wildlife.”
To date, the department has completed modifications to four low-water crossings, all on county roads over the Little Niangua River in Camden County. Those include Griswald’s Slab on County Road N-145, Green’s Slab on County Road N-166R, Howard’s Ford on County Road NN-166 and Bannister Ford Crossing on County Road N-165.
All of the low-water crossings were modified using precast concrete beam spans to enable the natural stream bottom to adjust with flow events and allow more natural sediment to flow through the structure. Each of the new crossings was made 3 ft wider than the existing ones to a width of 17 ft, with the exception of the Bannister Ford Crossing. At the request of Camden County, it was widened from 14 ft to 15 ft 4 in., with 10-in. curbs on each side. Practical design was applied, allowing MoDOT to keep the existing roadway alignment on each project.
Construction of Griswald’s Slab and Howard’s Ford included the removal of the 60-ft opening at each of the existing crossings where low water was flowing most of the time. The old concrete was replaced with two new 30-ft concrete slabs. Griswald’s Slab was raised 1 ft, but Howard’s Ford remained at the same elevation. MoDOT recycled the concrete pavement it removed from each crossing, broke it up and reused it as stream-bank protection. Both Bannister Ford crossing and Green’s Slab were completely replaced, leaving none of the previous structure. Green’s Slab involved the replacement of the 60-ft crossing, with two new 30-ft slabs 21?2 ft higher than the original structure. Bannister Ford required two 45-ft slabs to replace the 90-ft crossing. The new structure was elevated a ½ ft higher than the one it replaced.
Credit is due
The last low-water crossing modification was completed in September 2009. One year later, a 5-mile section of the new Rte. 54 Expressway was opened to traffic. The next 4-mile stretch of the $48.7 million project is scheduled to open in late 2011.
While the expressway, once fully completed, will improve safety and traffic flow for residents and tourists, the low-water crossings are producing their own multiple benefits now and for years to come. From an environmental standpoint, the new crossings provide a great benefit to area wildlife. The new, higher crossings have improved water flow and increased fish migration, especially for the Niangua Darter.
“Low-water crossings on Missouri county roads can limit aquatic organism movement because they create barriers for fish and other aquatic organisms moving upstream and downstream,” said Melissa Scheperle, MoDOT senior environmental specialist. “By modifying these structures, populations are reconnecting, which increases opportunity for genetic diversity and boosts population growth.”
But the benefits do not stop there. Because low-water crossing improvements provide greater benefits to stream habitat than other smaller projects, they generate a greater amount of stream compensation, allowing MoDOT to have credits available for future projects. So far, this process has saved Missouri taxpayers nearly $3 million.
The process also will help deliver some future projects faster, because the department no longer has to take the time to locate, secure and design project-specific stream mitigation for future projects, because the credits are already available for use.
For motorists, these improvements have helped increase safety. The design of the new structures has decreased the frequency and duration of flooding, making it easier and safer for local residents, mail carriers and school buses to use them.
Finally, the new structures MoDOT built have resulted in reduced main-tenance needs and expenses for Camden County.
Overall, the low-water crossing projects have been a resounding success, and not just in MoDOT’s opinion. The effort has been rewarded at both the state and national level, receiving the 2009 Practical Design Awards for Excellence from the Transportation Engineers Association of Missouri and the prestigious 2009 Environmental Excellence Award from the Federal Highway Administration.
Looking ahead, MoDOT will carefully weigh the economic and environmental benefits that additional low-water crossings may provide as mitigation on future projects.
For more information about Missouri’s low-water crossings improvements, go to www.modot.org/ehp/StreamMitigation.htm.
