Our Water Matters
The Center for Climate and Energy Solutions reports that “extreme precipitation events have increased in frequency and intensity in the U.S. and across many regions of the world since the 1950s … [and] scientists expect these trends to continue as the planet warms. For each degree Celsius of warming, the air’s capacity for water vapor goes up by about 7 percent. An atmosphere with more moisture can produce more intense precipitation events, which is exactly what has been observed.” The main threat posed by heavy precipitation is the potential for flooding, which can be particularly acute in urban centers with large areas of non-permeable pavement.
One such place is Dallas-Fort Worth, which is the fastest growing metropolitan area in the United States based on 2020 U.S. Census data. A study by the Nature Conservancy and Texas A&M Agrilife Extension titled “Green Stormwater Infrastructure for Urban Flood Resilience: Opportunity Analysis for Dallas, Texas” states that “with rapid and widespread conversion of natural land cover to impervious surfaces, stormwater management — for water quality and urban flooding — is an important challenge for municipalities in the region.” The study cites the U.S. Global Change Research Program’s 2018 Fourth National Climate Assessment, which states that “increases in the magnitude and frequency of heavy precipitation will place more stress on existing water resource infrastructure” in the southern Great Plains, including North Texas.
Stormwater management has traditionally involved concrete canals, culverts and other “gray” infrastructure to divert stormwater flows to rivers and creeks. But as rainwater events continue to intensify and impervious surfaces proliferate, this approach is proving increasingly inadequate. Green stormwater infrastructure (GSI), including rain gardens, bioretention areas, and rainwater harvesting cisterns, could provide a solution.
Rain gardens are vegetated, mulched depressions designed to capture surface runoff and infiltrate it into the ground before it leaves the property. The landscaping in these gardens is selected to filter the runoff from asphalt and concrete surfaces before it is absorbed into the soil. Bioretention areas are essentially sophisticated rain gardens and usually involve the replacement of the native soil with an engineered soil mix that increases infiltration. These systems often incorporate perforated pipes to facilitate drainage during consecutive storms and are commonly integrated into parking lots. Rainwater harvesting cisterns comprise collection tanks connected to downspouts on structures to collect runoff from rooftops. This water is then used to irrigate nearby native vegetation between rain events. These systems can be designed to slowly release up to 75% of a cistern’s total capacity, for example, so that the cistern is mostly empty by the time the next storm occurs.
The study relied on hydrologic modeling and spatial analysis to identify “hotspots,” or “specific locations where existing gray stormwater infrastructure as modeled is under capacity during simulated storm events,” as well as “opportunity zones” where the deployment of GSI “would have an immediate impact on reducing the flow to modeled system hotspots.” About 118,418 acres (or 53% of the city’s land cover) were included in the analysis, which translated this total available “space” into capacity in gallons and then estimated the maximum potential capacity for GSI to capture stormwater and reduce the overflow volumes generated by the storm scenarios considered.
Several different scenarios were modeled to project the expected increase in stormwater volumes, including 2-year, 10-year, and 100-year 24-hour storm events. These events represent the highest stormwater volumes over a 24-hour period within an interval of 2 years, 10 years, and 100 years, respectively. The study found a 58% increase in precipitation between the 10- and 100-year, 24-hour storm events and a 138% increase in precipitation between the 2- and 100-year, 24-hour storm events. These data are of particular interest because the City of Dallas currently requires that any upgrades to the stormwater drainage network must be designed to withstand 100-year 24-hour storm event conditions.
The study found “GSI to be 77% less costly than upgrading gray infrastructure alone, to meet modeled overflows.” The study also found that a combination of green and gray infrastructure would maximize cost-effective benefits. Bioretention areas had “the most potential beneficial impact, in terms of the site availability, volume captured, and associated costs,” because they “provided approximately 71% of the overall stormwater volume managed by GSI across all storm scenarios” with parking lots representing “the largest spatial opportunity for bioretention areas within the study area.” The authors also note that “parks, rain gardens, cisterns, and planting strips also offer substantial volume capture.”
The towns of Presidio and Marfa could greatly benefit from the application of GSI to address our own flooding hotspots, reduce road repair costs, prevent heat island impacts, and enhance community health.
Visit www.nature.org/content/dam/tnc/nature/en/documents/GSIanalysisREVFINAL.pdf for the full report.
Trey Gerfers is a San Antonio native and serves as general manager of the Presidio County Underground Water Conservation District. He has lived in Marfa since 2013.