Tres Rios Environmental Restoration
A collaborative project in the Southwest desert has improved natural habitats and water quality while assisting a community with flood risk and wastewater management.
By Rob Infantino, Mike Stockham and Dusan Stanisic, P.E.
The Tres Rios Environmental Restoration, Phase II Flow Regulating and Overbanks Wetlands Project outside Phoenix was honored with the 2012 USACE Chief of Engineers Award of Excellence.
PHOTO BY TIM ROBERTS PHOTOGRAPHY
In southwestern Arizona, where three rivers meet near the City of Phoenix, an ambitious environmental engineering project is sustaining natural habitats, improving water quality, and helping a community with flood risk and wastewater management. This large-scale and comprehensive collaboration was conceived and developed to achieve many diverse objectives. And it is succeeding.
The multi-year design and construction effort administered by the U.S. Army Corps of Engineers (USACE) Los Angeles District sought to provide flood risk management for the local community and restore area ecosystems and natural habitat. Improving water quality, recreation and education played a role in enhancing a 7-mi-long tract of land where the Salt, Gila and Agua Fria Rivers converge.
The Tres Rios Environmental Restoration Project also involves site improvements to increase water quality by reducing nitrogen levels in wastewater from a nearby treatment plant. Landscaping to facilitate advanced nitrogen removal, as well as to help restore ecosystems, was a significant aspect of the project. The work has required an array of engineering disciplines to meet civil, geotechnical/foundation, and hydro design and construction challenges. A flood risk management levee, an effluent pump station, marshes and riparian corridors are just part of what is envisioned to be a long-term restoration endeavor.
The project has received various awards, including the prestigious USACE Chief of Engineers Award of Excellence in 2012. The honor recognizes design and construction accomplishment in large-scale projects; in this case, it recognizes the Phase II Flow Regulating and Overbank Wetlands portion of the Tres Rios restoration.
DEVISING A SOLUTION
The project’s origins date to the 1990s, when USACE and the City of Phoenix began to pursue a solution to flooding problems along the Salt River near a local wastewater treatment plant. A study conducted by the U.S. Bureau of Reclamation, the Arizona Game and Fish Department, and a consortium of municipalities called the Multi-City Sub-Regional Operating Group (SROG), examined ways to reclaim water and reuse wastewater effluent all while maintaining the natural habitat. One idea gaining traction was to treat effluent from the wastewater plant by employing natural methods instead of mechanical means. The City of Phoenix constructed several demonstration sites to validate the efficacy of this natural treatment method.
Over time, methods supported by wetlands “cells” that would “polish” treated wastewater were researched and modified until the process was refined. This “demonstration” effort, in which chlorine and metals were removed from effluent and its ammonia and nitrate levels reduced, ultimately led to the extensive Tres Rios project, with constructed wetlands commissioned into full operation in 2010. The approach not only showed it could improve effluent quality; it yielded the additional benefit of restoring wetlands in the Salt River basin.
A MONUMENTAL EFFORT
After the trial project, the full-scale work began in a two-phase design-bidbuild effort. Construction work, managed by Archer Western Contractors, included wetlands restoration and follow-on development of an overbank wetlands section. Treated effluent is used to help sustain an ecosystem that supports wildlife habitation, while the water “oasis” created in the desert and assisted by sophisticated pumping technology serves to fight flooding and expand area recreation options.
The magnitude of the project kept all parties engaged and committed to seeing the vision through. A monumental effort was put forth in partnering. In addition to the City of Phoenix, SROG, USACE and the construction and engineering consultants, the project involved representatives of the Flood Risk Management District of Maricopa County, a nearby Native American community, and various federal agencies, including the U.S. Environmental Protection Agency. The substantial site area (comprised of 580-acres containing engineered basins for effluent detention) was cleared of existing debris standing in the flood plain. Flood risk management was addressed through levee construction.
Water fills one of the newly constructed flow-regulating wetlands “cells” that was constructed at the project site.
PHOTO BY HUMBERTO CHAVARRIA
Downstream from the detention basins are three flow-regulating wetlands constructed as part of the second project phase. Roads, an earthen channel, flowcontrol structures, an analyzer/electrical building, and a complex system of piping also were constructed to sustain these wetlands.
Treated effluent is metered into the wetlands, where it undergoes biological treatment, occasionally augmented with chemical methods during peak flow periods. The automation-controlled wetlands are fed by secondary effluent from pump stations at the treatment plant via above- and below-grade pipes, leading to an inlet structure where the flow is bifurcated into two deep-water wetland basins. From there, water is carried by the channel to overbank wetlands before being released into the Salt River. The overbank and flow-regulating wetlands were constructed above the flood plain.
Construction of the regulating wetlands called for removal and disposal of debris. It required clearing and grubbing tree trimmings, corrugated metal pipe, reinforced concrete pipe, steel pipe, fence and gates, asphalt concrete roads, posts, headwalls and vegetation. Excavation and grading was needed to form the two deep-water zones and the three flowregulating wetland ponds. Demolition included removing and plugging abandoned utilities, terminating existing concrete irrigation ditches, and clearing buildings and other structures. Concrete flow-control structures utilizing sluice gates, weir gates and stop logs were built, along with discharge structures and spillways. The newly constructed analyzer/ electrical building houses all electrical components and the heating, cooling and analyzing equipment. Installed landscaping represents four categories: Riverine Terrace and Mesquite Bosque, Riparian and Transitional Zones, Emergent Marsh, and Aquatic.
Despite the heavy demolition and excavation work, the most challenging aspect of the project was developing a way to plant delicate wetlands vegetation, without, incidentally, benefit of the very water that would ultimately nourish it. During the time the pump station was being built, several issues arose that prevented water from being delivered during the growing seasons of the wetland plants. This prompted construction of an intricate temporary sprinkling system that spanned more than 40-acres and enabled the delivery of water to the sensitive plant species.
Though the project’s primary objective was restoring the Salt River’s bank and flood protection, one added benefit stands out. The wetlands, because of their flow-regulating ability, naturally remove residual chlorine from the wastewater treatment plant effluent prior to discharge into the river. This substantially reduces chemical use. And they are expected to significantly remove emerging contaminants such as pharmaceuticals.
While the Tres Rios Environmental Restoration has received many awards for its engineering and environmental work, perhaps more telling of its success is the wildlife that has been reported at the site in the two years since the wetlands basins were filled and native plants were reintroduced.
Beavers, bobcats and bald eagles are just a few of the creatures identified in the vicinity, in which more than 100 species of birds have been spotted. Where once it was barren, life again flourishes.