By Brad Stebleton, CFM and Andres Sanchez, PE

One of the most confounding aspects of the arid west for transplants from the eastern U.S. are the many watercourses in our region that are completely bereft of any water for the vast majority of the year. The newcomers often hail from places where streams and creeks have water in them all year around and assume (to their detriment) that these channels never see water. As the locals know, these watercourses can go from bone dry to raging torrent in a matter of minutes, particularly during summer downpours. These flash flooding events can destroy property and endanger lives, making them a very dangerous reality of our arid environment.

The responsibility of addressing these threats often falls on local flood control agencies. In the south-central portion of Sandoval County, New Mexico, that agency is the Southern Sandoval County Arroyo Flood Control Authority (SSCAFCA). For the uninitiated, “arroyo” is a commonly used term in our area to describe an intermittent or ephemeral watercourse. In other parts of our region these are described as a “wash,” “draw,” or “gully.” Formed in 1990, SSCAFCA’s jurisdiction covers more than 128,000 acres, including the communities of Rio Rancho and Corrales, as well as a portion of Bernalillo, the County seat. This area also contains a majority of the County’s population and is a region that has seen rapid growth over the past several decades, a period in which Sandoval has been the fastest growing County in New Mexico.

Obviously, this is a daunting task for any agency, but SSCAFCA has proven itself to be more than capable over the 30+ years of its existence. Known as a careful steward of the taxpayer dollar, the agency has seen most of its bond issues pass by wide margins. As a result, SSCAFCA has been free of the controversies and bad press that have plagued other governmental entities in New Mexico. Before green infrastructure became trendy, SSCAFCA was very well known for its naturalistic flood control facilities, which stand in stark contrast with the large concrete edifices that typify flood control in many parts of our region. In fact, only 7% of the drainage courses in the SSCAFCA are hard-lined. Not only is this approach better for the natural environment, it can be more economical to construct and easier to maintain these facilities.

IMAGE 1: Eastern reach of the Coronado Arroyo, photo courtesy of the Southern Sandoval County Arroyo Flood Control Authority.

SSCAFCA brought this approach to its flood control project addressing the problem of the Coronado Arroyo. Named after the first Spanish explorer to enter (in 1540) what would become the American Southwest, this channel conveys storm water from portions of Rio Rancho and northwestern Bernalillo, ultimately discharging to the Rio Grande. This watercourse is located in an area of Bernalillo that has seen rapid growth in recent years. The Town’s population grew by 35% from 2000 to 2020, and the bulk of that growth occurred in this area. Many residences were constructed in close proximity to the arroyo, which had been become badly silted and often overran its banks, threatening those homes. During summer rain events, stormwater ran very rapidly in the channel, further exacerbating the problem. To make things worse, the eastern outlet of the arroyo to the Rio Grande had become badly choked off by the exotic and invasive vegetation that people in our region know well, including Tamarisk, Trees of Heaven, and Siberian elms.

IMAGE 2: Western reach of the Coronado Arroyo, courtesy of the Southern Sandoval County Arroyo Flood Control Authority.

Utilizing an internal engineering design team, SSCAFCA saved taxpayer money on the project by not contracting that task out to a private firm. Besides addressing the drainage issues in this channel, SSCAFCA also focused on the water quality challenges present in the Coronado Arroyo. Water quality is often degraded by rapidly flowing channels that drain areas of human habitation. Fast moving runoff can contain hazardous chemicals and waste products such as motor oil that are then carried into rivers and streams like the Rio Grande. With this in mind, the design concept for this project was to divert runoff from smaller, more frequent storms into a meandering water quality (WQ) side channel where the flow would be passively treated before returning to the main channel.. As stormwater flows enter the meandering WQ portion of this project, it has the opportunity to infiltrate into the earth, taking advantage of the area’s sandy soils and being cleaned in a variety of ways. Highlights of the project include:

The WQ channel includes gabion basket side walls. The gabion baskets were filled with basalt rock, a porous igneous rock, which promotes a “wicking” effect on the passing storm water and allows it to soak into the soil and provides moisture to the native vegetation.
The WQ channel uses a natural soil bottom rather than concrete. The sandy soils in this area have excellent infiltration rates. This sand acts as a natural filter for oils and other suspended contaminants commonly found in urban/rural storm water discharges.
The longitudinal slope of the WQ channel is very flat in comparison to the main channel. This flat slope works to reduce flow velocities and allows the storm water to infiltrate.
The WQ channel outlet pipe, which conveys flow from the WQ channel back into the main arroyo, is set above the channel grade, creating a ponding effect which further promotes infiltration of storm water.
The project provides a more efficient, trapezoidal design of the arroyo to increase flow-carrying capacity and protect adjacent property and citizens from runoff resulting from the 1% Chance Annual Storm, commonly known as the 100-year Storm.
The channel banks are strategically lined with riprap armoring to reduce project cost and impact to the environment.
Grade control structures were installed in order to mitigate increased flows from future upstream development and to preserve the channel’s “equilibrium slope.”
The project provides maintenance access for the arroyo (where there was previously none) in order to monitor the condition of this system and conduct annual maintenance as required.
A weather station and flow gauge were installed in order to improve SSCAFCA’s in-house storm data collection and provide the public with realistic “on-the-ground” information on rainfall events.
The exotic and invasive trees that constricted the channel were removed, helping to restore its natural flow to the Rio Grande. This also saves water in an arid environment, as some exotics, particularly Tamarisk, consume a great deal of this precious resource.

IMAGE 3: Eastern reach of the Coronado Arroyo with the Rio Grande Valley and Sandia Mountains in the background. Note the horseshoe shaped Water Quality Channel. Courtesy of the Southern Sandoval County Arroyo Flood Control Authority.

The project was completed in two phases, the first in 2017 and the second in 2020. Total cost was just over $1 million, with a portion of that paid for by state capital outlay and grant funds. So far, the project is functioning very well. Besides the obvious flood control benefits, the WQ channel portion of the project has provided a major positive impact on stormwater quality in the area. A large pipe that conveys stormwater from a nearby commercial area, including a Wal-Mart Super Center, now discharges to the WQ Channel, where this water is cleaned before reaching the Rio Grande. Prior to the project those pollutants were rushed to the river untreated. Since the Rio Grande serves as a key area for recreation, a corridor for wildlife in the region, and a source of drinking water for human consumption, this project will provide a multitude of benefits for many years to come. In addition, the weather station and flow gauge will provide vital information on precipitation and storm water events in an era of changing climate.

IMAGE 4: Western reach of the Coronado Arroyo looking West. Courtesy of the Southern Sandoval County Arroyo Flood Control Authority.

Every project is a learning opportunity, and Bosque de Bernalillo was no exception. Including water quality elements into flood control projects has in years past been an afterthought, if it was thought of at all. By making the WQ Channel an integral element of this project, SSCAFCA was able to secure grant funding from the New Mexico Environment Department to help defray its cost. More importantly, that Channel helps the Coronado Arroyo to function better and will help to safeguard the quality of water in the nearby Rio Grande by removing pollutants from the stormwater. In an arid environment where water is a very precious resource, this is a very important benefit indeed.

ABOUT THE AUTHORS

Brad Stebleton is a senior planner for Sandoval County, NM. He is a Certified Floodplain Manager. He currently serves as secretary on the board of Western Planning Resources and is the chair of The Western Planner Editorial Board. Andres Sanchez, PE, Design Services Director of the Southern Sandoval County Arroyo Flood Control Authority (SSCAFCA).