St. Croix Crossing designed and built to protect environment
From the very beginning, the Minnesota Department of Transportation and the Wisconsin Department of Transportation have worked to consider the environmental impact of the St. Croix Crossing. We’ve done all we can to avoid environmental impact, and have planned and executed the most extensive mitigation package in our history to offset those impacts.
Last month, we (Terry Zoller, St. Croix Crossing construction engineer, and Tim Mason, Wisconsin project supervisor) explained what construction activities will take place during the final construction season at the project. Now, we want to describe how MnDOT and WisDOT managed the environmental impact of the bridge throughout every aspect of the project.
Our designers, engineers and crews faced challenges at every stage of the project because the bridge is located in a national scenic riverway, next to a historic area, and in an area where threatened and endangered species live. Every challenge gave us the opportunity to meet or exceed established rules for protecting sensitive ecosystems.
Architectural design and engineering
During the bridge’s planning and design phase, stakeholders chose an extradosed bridge structure to reduce the visual and environmental impact of the bridge. This type of bridge, which combines the benefits from both the boxed-girder bridge and the cable support design, reduced the total number of piers required in the waterway. It also allowed for a lower tower height, and provided a more elegant look with elements such as the stay cables attached to the bridge deck. With the extradosed design, we were able to make the bridge structure slim and reed-like, enhancing the natural beauty of the landscape instead of becoming the focal point of it.
We surveyed ravines during the planning phase and aligned the bridge with an existing ravine on the Wisconsin side. This reduced the need cut into the bluff and created a smooth transition from the highway to the bridge. Tan was chosen as the bridge paint color so it would mirror the hue of the river bluffs. And the St. Croix Crossing’s lighting system will use directional instruments that ensure a safe driving surface while minimizing nighttime light spillover into the river valley.
The bridge’s drainage system was carefully designed to meet or exceed stormwater quality requirements—in fact, the water coming out of the new bridge’s holding ponds will be cleaner than the water currently running off the Lift Bridge into the St. Croix River. The system features 16 ponds which filter stormwater—10 in Minnesota and 6 in Wisconsin. The bridge’s stormwater system carries rainwater and snowmelt directly to the ponds to be treated before being released back into the river. By filtering out materials such as sand, sediment, gravel and nutrients, the holding ponds will benefit the water quality and reduce phosphorus by approximately 23% in the stormwater.
Federally protected and endangered species also presented unique challenges. For example, the interchange at Highway 36 and Highway 95 was designed to avoid impact to a bald eagle nesting tree. In addition, Higgins eye pearlymussels living in the bridge construction area were relocated before construction, and the U.S. Fish and Wildlife Service was compensated to raise new mussels at an off-site facility. These native mussels will eventually be placed in the St. Croix River and other tributaries. In Wisconsin, seeds of endangered Dotted Blazing Star flowers were gathered from existing flowers and stored for future planting before crews relocated the flowers.
Additionally, historic properties were assessed and mitigated appropriately. For example, we preserved and relocated the Shoddy Mill and Warehouse north along Highway 95 in Stillwater.
During construction, we diligently worked to protect water quality. Frequent, routine water measurements during key phases of construction determined pH levels and required water to be at a safe, neutral level before being released back into the St. Croix River. We collaborated with private citizens who have monitored water quality for decades and shared data to gain the most accurate measurements. We also built a water treatment plant and brought water containing construction sediment there to be treated. Turbidity curtains temporarily controlled soils within work zones from migrating downstream or to other areas, and concrete barriers and silt fencing guarded wetland edges from construction traffic.
When working near the Wisconsin bluffs, construction crews used less invasive construction techniques that included constructing the drainage structure by hand, operating small machines and equipment, and using a temporary trestle system that reduced impact to the bluffs below Pier 13 at the east abutment.
At the barge unloader facility near the bridge, crews avoided disturbing an active bald eagle nest by keeping a strict 300-foot perimeter. The policy must have worked well because, this spring, we had three eaglets in the nest instead of the usual one hatchling. In addition, managers and crews took steps to prevent the introduction of invasive species into the river ecosystem.
We recently removed the King Plant mooring cells from the St. Croix River, spreading the aggregate from inside the cells along the riverbed to provide aquatic habitat for fish spawning and mussels. Construction activities also met or exceeded requirements regarding noise, vibration, and air quality.
Long after the St. Croix Crossing opens to traffic in late summer, the bridge’s environmental measures will continue to preserve and improve the pristine quality of the surrounding natural environment.
In 2019, when the Stillwater Lift Bridge is reopened to the public after extensive renovation, the five-mile Loop Trail will connect the St. Croix Crossing with the Lift Bridge, enabling hikers and bikers to safely travel from one bridge to the other and back again, on both sides of the river. Currently open, the scenic overlook on the bluff in Oak Parks Heights offers an outstanding vantage point to observe the new bridge. As part of the project, the scenic overlook was restored to the way it looked in 1929.
The St. Croix Crossing bridge is a great example of how stakeholders and departments of transportation can respect the environment and still meet transportation needs for generations to come.
Aug. 2 has been announced as the day of the dedication ceremony for the St. Croix Crossing—about a month away. The bridge will open to traffic not long after the ceremony. Until then, there is still much work left to finish on the bridge, including chip sealing, painting, electrical work, lane striping and more. In the meantime, you can look at our web cameras or visit the scenic overlook to view the bridge from a safe distance.
In our next blog post, we will discuss the practical benefits of the bridge, including impacts to traffic patterns.