Part I of II
In New Mexico’s arid and semi-arid climates there are a unique set of human and environmental factors that make green stormwater infrastructure an important, but often misunderstood, practice.
Green Stormwater Infrastructure in Arid Climates
By Tess Houle, Landscape Designer, MRWM Landscape Architects
Green vs. Grey: The Basics
Most precipitation in Central New Mexico comes as rain during brief, high-intensity summer storms. In order to benefit from this critical resource, soils and plants must be able to absorb as much rain as fast as possible. Areas where rainfall can collect and soak in (infiltrate) support the healthiest soils and plants, which return many benefits to people.
Rain that falls on undeveloped, permeable land in the Middle Rio Grande Valley in New Mexico has the best opportunity to infiltrate. What doesn’t soak in runs to sand-bottomed arroyos, where infiltration rates increase. From there the water heads to the Rio Grande River.
Land where rainfall can soak in.
When the land is made impermeable by roads, parking lots, and buildings, rain is unable to infiltrate into the soil, which causes soils to dry up and essentially die. In addition, when rainfall flows over paved surfaces, it collects sediment and a variety of pollutants, including E.coli bacteria, polychlorinated biphenyls (a carcinogenic chemical), heavy metals, grease, fertilizers, and trash.
Grey stormwater infrastructure uses concrete pipes and channels to direct this polluted stormwater from roads, parking lots, and buildings directly to arroyos, creeks, and rivers, where it poisons soils, water, plants, and animals, prevents safe recreation in the Rio Grande, and disrupts the fragile desert ecology. Grey stormwater infrastructure also increases potable water use, as it prevents rainfall from reaching landscape plants. Instead of stormwater being directed to trees, water is piped hundreds of miles and then treated so that it can be used for irrigation, while runoff from hard surfaces is sent straight to the storm drain! There is a common misconception that all stormwater must go directly to Texas in order to meet obligations under the Rio Grande Compact. This is simply not true! Read this helpful document for clarification.
We can improve this situation through the redesign of stormwater systems.
Grey infrastructure is made primarily from concrete and metal.
Green stormwater infrastructure (GSI) practices reconnect water to the land and use natural systems to filter pollutants and decrease potable water use through reduced irrigation needs. Though the concept of green stormwater infrastructure is ancient, it was not used during the construction of most U.S. cities. However, the overwhelming number of benefits has led many cities, such as Philadelphia, Seattle, Denver, and Tucson, to incorporate GSI into standard stormwater management practices.
Green stormwater infrastructure practices include bioswales, bioinfiltration basins, stormwater tree trenches, and permeable pavement. Most GSI practices use some combination of soil, rocks, plants, and sometimes a little concrete, to slow water down, clean it, and allow it to soak into the ground. Here are a few benefits of this method of stormwater management:
- Capture and treatment of pollutants
- A healthy watershed
- Reduced potable water use
- Healthy flora and fauna
- Reduced erosion
- Increased shade/ reduced temperatures
- Groundwater recharge
- Reduced flooding
- Carbon sequestration
- Beautiful and comfortable outdoor spaces
- Potential for decreased cost of grey infrastructure
An infiltration basin at CNM’s Smith Brasher Hall (designed by MRWM, 2015) captures runoff form the roof and
parking lots to provide a multitude of benefits for the community.
Successful and cost-effective GSI projects require the collaboration of many disciplines to meet flood control requirements while also maximizing the benefits listed above. Landscape architects are uniquely qualified to coordinate teams of engineers, architects, planners, arborists, water quality regulators, biologists, and contractors, to ensure that GSI structures make a lasting impact at site and system scales. In fact, the founder of landscape architecture in the United States, Frederick Law Olmsted, designed large GSI systems on the east coast in the late 1800’s that still work today. Despite all of these benefits, there is a lack of understanding about the important role of GSI in arid climates.
Why GSI is Especially Important in New Mexico
Although New Mexico does not have the problem of stormwater and sewage overflows that has motivated the use of GSI practices in Seattle and Philadelphia, we do have another water quality issue: infrequent rainfall means that pollutants accumulate on surfaces (such as roads and parking lots) for months before being washed off during a storm. This means that the pollutant concentration in the first rinse of stormwater in arid climates is much higher than in wetter climates like the Northwest and Northeast. Greet stormwater infrastructure is imperative to the capture and treatment of these concentrated pollutants.
In the Middle Rio Grande Valley, the U.S Environmental Protection Agency issues the Municipal Separate Storm Sewer System Permit (required to discharge stormwater to the Tijeras Arroyo and the Rio Grande), which requires on-site stormwater treatment in all new- and re- developments in order to decrease pollutants entering the Rio Grande. The EPA recognizes the important role that GSI has to play in decreasing pollutants entering the waterways, and encourages it to be used whenever possible.
At the UNM College of Education, roof runoff irrigates healthy plants, providing a beautiful landscape with shade and habitat (designed by MRWM 2008).
In addition to unique pollutant concerns, soils in arid and semi-arid climates tend to be highly erodible due to low organic content, and runoff volumes tend to be higher due to low vegetative cover. Erodible soils and high runoff volumes mean that erosion is a more significant issue in arid climates than it is in wetter climates. Concrete and rocks are often used to address erosion issues, but they often just create a bigger erosion problem downstream, or sometimes fail altogether. GSI offers long-term stability to erosion-prone areas by building healthy soils, root systems, and plants- all while creating a cooler and more beautiful place to live.
Another important difference about development in semi-arid and arid cities in the U.S. is that they tend to sprawl – think Phoenix, Los Angeles, and Albuquerque. A lot more land is covered in parking lots, roads, and one-story buildings than in cities of comparable population on the east coast. More asphalt means more surface area to collect pollutants. It also means more surface area to cause the ‘urban heat island effect’, which is especially severe in Southwest cities and has many significant negative impacts on human health and energy consumption. GSI captures runoff from sprawling surfaces, cleans it, and can use it to irrigate trees, which shade parking lots, buildings, and roads, reducing the urban heat island.
One of many expansive and unused parking lots in Albuquerque.
GSI promotes water conservation by directing stormwater runoff to planted areas, which significantly reduces potable water use for irrigation. In some locations, GSI can even contribute to groundwater recharge. Water management practices play a unique and significant role in the histories and cultures of New Mexico. Ceremony, celebration, recreation, and agricultural practices show respect for this limited resource upon which we all depend. Contaminating, wasting, and otherwise disrespecting water goes against deep-rooted cultural practices. GSI offers a respectful alternative to destructive methods of water management.
Look for our next post on what aspects of design are most important for high-performing GSI in an arid climate.