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Two countries. One troubled watershed. No solution—yet.

By Lisa Owens Viani

Named for the walnut trees that used to line its banks, the Arroyo de los Nogales, a tributary of the Santa Cruz River, flows from south to north, descending from the high Sonoran desert in Mexico into Arizona. The main arroyo and its many smaller tributaries form a watershed, shaped roughly like a human heart, that is broken in two by the U.S.–Mexico border wall. Facing each other across the wall, in the river’s floodplain, are two cities, each named Nogales, that share social and environmental problems—including repeated flooding caused by rapid urbanization, ineffective flood control efforts, and the border wall itself.

Gabriel Díaz Montemayor, ASLA, an assistant professor of landscape architecture at the Fay Jones School of Architecture and Design at the University of Arkansas, and Francisco Lara-Valencia, an associate professor at the School of Transborder Studies at Arizona State University, have a greener vision for these border cities (together called Ambos Nogales), whose streets and arroyos often run brown with sediment and sewage in heavy storms. Díaz Montemayor and Lara-Valencia want to increase permeability throughout the watershed, slow peak flows in heavy storms, and develop more ecological connectivity between the two cities, despite the dividing presence of the wall.

They hope their ideas for an extensive network of green infrastructure can transform the way the cities develop, not only to improve water quality and flood management but also to provide more green space for residents. As the cities have grown, impervious surfaces have too, destroying natural areas. Both cities lack green space: There is just 1.1 square meter per person in Nogales, Mexico, and only 2.2 square meters per person on the U.S. side, Lara-Valencia says.

“We are not saying development shouldn’t happen,” Díaz Montemayor says. “We’re saying, ‘Let’s provide a structure for that development to happen [that] is based on natural systems.’”

With multiple collaborators, including the Instituto Municipal de Investigación y Planeación, the Arizona Department of Environmental Quality, the U.S. Geological Survey, the universities of Arizona and Arkansas, and two nonprofits, Díaz Montemayor and Lara-Valencia have applied for a U.S. Environmental Protection Agency Border 2020 grant to fund further development of their ideas: In 2010, they completed a conceptual green infrastructure plan for Nogales, Mexico, called City of Green Creeks. At the urging of Claudia Gil, a former planning director in Nogales, Mexico, who now works for the Arizona Department of Environmental Quality, they are expanding their plans to include Nogales, Arizona.

“Over time we have learned that the only way to address the issue of flooding is to do it binationally,” says Lara-Valencia. “The two cities are really connected historically, economically, and socially. It’s a limitation of the imagination to focus on just one side.”

Although the cities share a watershed, most of the new development is taking place in Nogales, Mexico, which has a much larger population than Nogales, Arizona, of close to a quarter-million people, and is a hub for U.S.–Mexico trade-related industry. New immigrants are constantly arriving from other parts of Mexico and Central America looking for work in downtown Nogales. A new highway and border crossing planned at the city’s eastern edge will likely increase urbanization as well, says Díaz Montemayor. He and Lara-Valencia recognize that they cannot stop development, but their plan will recommend that new structures not be built in the floodplain or on steep slopes as has been common in the past. “Otherwise business as usual will continue to happen on the creeks,” Díaz Montemayor says.

While the two researchers are promoting nonstructural stormwater solutions, including bioswales, rain gardens, permeable pavers, green roofs, created wetlands, terraced gabion structures, restored creeks, and other capture and filtration systems, they realize that some hard structures will ultimately be needed to solve water quality problems, especially when it comes to sewage. In some parts of the city, there is no connected sewer service, so during large storms, raw sewage flows from random, disconnected pipes directly onto the ground—and directly into the arroyos along with the stormwater.

As the Nogales Wash enters downtown Nogales, Mexico, it has been confined in a concrete channel as an attempt to control flooding. It then flows into a concrete tunnel that carries stormwater beneath the border. The channel and tunnel frequently become overwhelmed by debris and sediment and back up during heavy storms, flooding both cities. The border wall itself sometimes acts as a dam, too, says Díaz Montemayor. In one instance, U.S. border patrol agents installed a fence inside the tunnel, causing an enormous flood. Adding to the flooding issues, a large sewer pipe was built directly beneath the concrete channel and tunnel in the 1930s, to carry sewage from the two cities to the Nogales International Wastewater Treatment Plant in Arizona. In heavy storms, the sewer pipe sometimes ruptures when the concrete flood conveyances on top of it fail; sewage then mixes with stormwater.

These overflows degrade water quality in the wash and its tributaries, the cities, and ultimately the Santa Cruz River, says Ben Lomeli, a board member with Friends of the Santa Cruz River. He strongly supports planning for green infrastructure, but says the sewer pipe also must be removed from beneath the concrete channel and tunnel and relocated to a less risky location.

Díaz Montemayor and Lara-Valencia are trying to tackle these flooding and water quality problems in their plan by reducing peak flows. Because the city has no traditional underground stormwater infrastructure, the streets and arroyos are the de facto stormwater management system, as in many developing countries, Díaz Montemayor explains. “The general approach has been to channel creeks into concrete and move water away as quickly as possible. Now we know that’s nonsense; eventually the water is going to reclaim its space,” he says.

Because the arroyos often double as streets in the urbanized center, and in some cases are the only access points for neighborhoods, Díaz Montemayor and Lara-Valencia have created designs to green the streets using bioswales alongside them and to increase infiltration through permeable pavers, rather than by trying to restore riparian habitat as they recommend for arroyos in the upper watershed and less urbanized areas.

Laura Norman, a supervisory research scientist with the U.S. Geological Survey, says that nearly every year, the state of Arizona declares an emergency at the Nogales, Arizona, border because of flooding. “Almost every August or September in monsoon season, we’ll see something in the news, so it’s a regular occurrence.”

Norman, who will be conducting some of the stormwater modeling for the new plan if funded, says the other factor that contributes to flooding in the two cities is the sediment that washes from the steep slopes in Nogales, Mexico, into the arroyos in heavy storms. “The green infrastructure being proposed would not only help control the flood events but also detain a lot of that debris that clogs the arroyos,” she says. Restoring vegetation along the arroyos would help slow flows by creating more roughness and encouraging sediment to drop out, she explains. “As things get greener, it slows down the flow and traps that sediment. It’s possible to slow down the water upstream in the tribs so it doesn’t get to the downtown area in such velocities.”

The plan, called City of Green Creeks, presents 10 green infrastructure site-level design strategies that can become part of a large network of green nodes and corridors consisting of greenways and restored arroyos. The nodes and corridors, which will help reestablish some ecological connectivity in a fractured landscape, vary in scale and design, depending on whether their location is urbanized or less developed. The goal is to introduce small-scale and cost-effective interventions to slow flows and increase permeability at the source, say Díaz Montemayor and Lara-Valencia.

For Nogales, Mexico, they’ve identified four upper watershed nodes, totaling 5,436 acres; these nodes would receive the major stormwater retention infrastructure elements, including restored arroyos, and connect green space with rural outskirts. Fifteen city nodes in the urbanized areas, totaling 704 acres, would consist of parks and green space and infiltration basins. Twenty nodes totaling 445 acres would be created at the neighborhood scale and consist of green corridors with green infrastructure and restored tributaries wherever spatially possible. “The general idea is that we look at the specific location of the potential sites for water management infrastructure, and that location has different capacities depending on where it is in the city,” Díaz Montemayor explains.

Initial concepts in the City of Green Creeks plan include nodes such as the larger scale El Nogal Regional Park, which would benefit low-income families by creating park space with wetlands that double as flood-retention facilities and by providing pedestrian access. Another project, the Arroyo Cuesta Blanca Green Development, preserves the site’s arroyos but also adds retention dams to slow flows and increase infiltration in strategic areas. “It’s a very simple idea—develop it and continue to provide housing, but don’t ruin the creeks,” says Díaz Montemayor. “Use them as green corridors.”

The El Ranchito de los Alisos project, located near a middle school, cement plant, and power generation station, creates a park for locals as well as a streamside buffer between the arroyo and human activities. The buffer will shade the stream and absorb floods, and families in the area have expressed their desire to operate small food enterprises and horseback riding and camping services for visitors.

An informal vacant lot called the Parque Nuevo Nogales would be repurposed to infiltrate stormwater using retention and infiltration dams, which will also act as bridges, allowing people to cross the Arroyo el Rastro. Slopes would be re-engineered with a series of retention walls—possibly even using discarded tires (a huge problem in Nogales)—that provide space for pedestrian paths and terraces for plantings. The terraces would be built in a trapezoidal design so that water flowing down the slope would irrigate the planted areas while filtering and storing runoff from the slope and the public park on the terrace’s shoulder. Over time, the planted terraces on the slope will be able to support communities of trees and other plants running alongside the arroyo, say Díaz Montemayor and Lara-Valencia. This prototype could be a model for other vacant lots and older unused industrial parks.

On the U.S. side, Díaz Montemayor and Lara-Valencia would like to add green nodes to the city’s arroyos, which have not been as urbanized as on the Mexico side, and tie new green spaces and parks to the Paseo de Los Nogales central corridor, giving it formal designation with amenities such as paths, bike lanes, benches, and other places to linger.

The first step in the next phase of the plan, if funded, will be to identify more spots for multibenefit flood mitigation based on the results of rainfall and runoff modeling. Norman will also model the effect of green infrastructure interventions on the hydrograph. “With the modeling we will then design a more detailed final project,” says Díaz Montemayor. He also plans to work with his students to design a set of pilot projects at two middle schools, one in each Nogales, while Lara-Valencia will partner with local nonprofits on community outreach. One of those is Seeds/Semillas, a binational grassroots group of 13- to 18-year-olds that implements green infrastructure to improve livability, sustainability, and equity in Ambos Nogales (the two cities). Lara-Valencia will also research laws and regulations that may be acting as inadvertent barriers to green infrastructure, with the hope of developing and incorporating better guidelines into the cities’ municipal codes.

“We want to convince people that this is for real,” says Lara-Valencia. “When you go and talk to some people who are always thinking about dams and pipes, and you tell them about another approach that is probably more cost-effective, they normally are skeptical. We have to show them that it works.”

When extensive work was done to bolster the border wall in the 1990s, says Díaz Montemayor, it disrupted the strong symbiotic relationship between the two Nogaleses, affecting families and friends who lived and worked in both cities. He believes the plans to solve flooding and other issues that span the border could unite the two cities not only ecologically but also by contributing to a more positive perception of the border region in both countries. “This is like laying the groundwork for something deeper, socially and environmentally, to happen in the near or midterm future,” he says. “A stronger sense of place will make a stronger community, and the perception of the impenetrable artificial border might be weakened so that more initiatives to foster and reunite the community in various and diverse ways come into play. History tells us that political barriers are not forever.”

Lisa Owens Viani is a Bay Area-based freelance writer and LAM contributing editor who specializes in ecological and water-related topics.