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By Scott Manning

The executive director of the Southern Sandoval County Arroyo Flood Control Authority (SSCAFCA) is confident that its facilities protecting Corrales can withstand increasingly severe storms caused by climate change —in the near term.

Chuck Thomas told Corrales Comment last month its flood control infrastructure can handle expected storm intensities since the authority’s dams, ponds and other facilities were designed to control floods in excess of the projected “once in a hundred years” storm events, and because timely maintenance is conducted. SSCAFCA uses models to understand storm water flow in southern Sandoval County. Using those data, the authority has designed infrastructure based on the probability of an intense downpour occurring within a 100-year period.

For example, if a storm water control facility would need to store 25 acre-feet of water in a major storm, SSCAFCA engineers design the facility to operate properly in more intense conditions by doubling the required storage capacity to 50 acre-feet. But, Thomas said, the authority lacks the data necessary to determine how climate change impacts weather events in this region. With a relatively small data set, it is difficult to identify weather trends, Thomas cautioned.

Yet SSCAFCA has commissioned research on the risks of climate change and how to communicate those risks. Presentations have been given to the authority’s board of directors on the implications of future storms of greater intensity and frequency. SSCAFCA participated in the Central New Mexico Climate Change Scenario Planning Project, led by the U.S. Department of Transportation’s Volpe Center, and by the Mid-Region Council of Governments (MRCOG) in Albuquerque.

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SSCAFCA first examined whether climate change corresponded to greater storm severity and increased flood risk in the southern Sandoval region. For this analysis, SSCAFCA studied a 56- square mile region just northwest of Corrales in the Rio Rancho. SSCAFCA evaluated general circulation models (GCM), that use atmospheric and oceanic circulation models to predict weather changes over time.

The project used GCM precipitation models from 1950 to 2099 to evaluate changing precipitation patterns.

Then, researchers considered three time periods in their analysis: 1950-1999, 2000-2049, and 2050-2099. Researchers focused on the predicted median precipitation in a 100-year storm to draw comparisons between the three time periods. Results indicated that differences were statistically insignificant between the median 100-year 24-hour precipitation value for the 1950-1999 period and the precipitation value for the 2000-2049 period.

However, the median precipitation greatly increased for the 2050-2099 time period. From this analysis researchers concluded that SSCAFCA should prepare for increasingly more severe precipitation events driven by climate change and changing weather patterns in the future.

Having established a link between climate change and increased storm severity in Sandoval County, SSCAFCA then considered the potential impact of increasingly severe weather events by considering the Calabacillas Watershed in Sandoval and Bernalillo Counties.

In 2014, SSCAFCA partnered with the Albuquerque Metropolitan Arroyo Flood Control Authority (AMAFCA) to develop a hydrologic model of the Calabacillas watershed.

First, the hydrologic model considered the impact of a current 100-year storm with an estimated 24-hour precipitation of 2.6 to 3.1 inches. Second, the model examined the impact of a hypothetical storm that produces 10 percent more rainfall than the current 100-year storm.

Third, the model examined the regional impact of a hypothetical storm that produced 25 percent more rainfall than the current estimates. Using this three-storm model, researchers evaluated the impact of severe storm events for this region. They determined that climate change could damage and overwhelm existing flood control infrastructure. More precipitation in a 24-hour period could generate higher peak flow rates.

For example, analysts identified Southern Boulevard in Rio Rancho as an at-risk road in the case of more severe storms. Already severe storms have threatened Southern Boulevard. But a 25 percent increase in the severity of a 100-year storm would increase water flow rates along Southern Boulevard by 75 percent. That example demonstrated that some flood control infrastructure simply lacks the capacity to handle more serious 100-year storms.

Second, investigators considered the impact of more severe storms on floodplains, the areas in the watershed that experience high levels of water flow in severe storm situations. Both the depth of a channel and the topography of the surrounding region affect flood patterns, so that researchers must study each channel separately to identify the risks to the floodplain.

In that study, they evaluated a housing development located directly east of an arroyo. Analysis demonstrated that a 10 percent increase in the severity of a 100-year storm would damage or destroy 30 homes in the housing development. That meant more work must be done to secure the floodplains.

Third, researchers concluded the study by examining how erosion in the watershed would change in cases of more severe storms. Erosion is a concern because the Calabacillas Watershed contains sandy loam soils that are highly susceptible to erosion. Erosion is damaging because the process changes the direction of water flow and deposits sediment downstream.

Studies compared the shape of the Calabacillas Arroyo from 1952 to 2012 to understand how erosion impacted the water feature. In this period, parts of the arroyo migrated laterally by over 300 feet. In highly populated areas, that kind of migration could harm private property and storm water control infrastructure. As the severity of storms increases, they expect the rate of erosion-driven lateral migration will accelerate.

The study concluded by acknowledging that climate change could lead to more severe 100-storm events that threaten infrastructure and private property in southern Sandoval County. SSCAFCA is currently conducting an evaluation of drainage on the western side of Corrales to identify storm water flow patterns and drainage concerns. This study is part of a continuing effort by SSCAFCA to serve as an available resource for Corrales.

Thomas is confident that SSCAFCA infrastructure will protect Corrales in future storm events because it has taken the necessary precautions in designing and in maintaining arroyo flood control infrastructure.

The authority maintains a robust maintenance program to ensure that all current infrastructure developments are up-to-date and operating properly. Thomas said he aims to conduct maintenance work twice a year on each facility. His crews perform the first round of maintenance before summer so that facilities are in good shape for severe storms during monsoon season. Then SSCAFCA performs the second round after the rainy season ends to repair any damage.

The agency operates uses a mix of property tax revenues and government grants. SSCAFCA receives a portion of property taxes from residents living in the region which pays for the authority’s operating budget and maintains a debt service fund.
In 2008, SSCAFCA issued general obligation bonds for $18 million which lasted through 2016. After 2016, the authority issued more bonds to raise $21 million which is expected to be sufficient for another decade.

To conserve its funding, SSCAFCA applies for supplemental federal grants. For example, a bank stabilization project currently under construction will cost $4.3 million. SSCAFCA was awarded a federal grant through the Federal Emergency Management Agency (FEMA) for $3.2 million for the project, so SSCAFCA only had to spend $1.1 million of its bond funds on that project.

Earlier this summer, two Columbia University researchers wrote an op-ed article for the New York Times warning that climate change is dramatically increasing the risk of catastrophic dam collapse. “There is no doubt that climate change is increasing the frequency of extreme rainfall and the risk of floodwaters over-topping dams, the main reason a dam fails,” according to Upmanu Lall, chairman of Columbia’s Earth and Environmental Engineering Department, and Paulina Concha Larrari, a researcher in the Columbia Water Center.

“But while climate change may not be so easily fixable, making sure dams can withstand flooding is, and it is much cheaper than the consequences.”

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