A record-breaking storm event wreaked havoc on a residential development and three watersheds in Arizona
What happens when three separate watersheds, a record-breaking storm event and a residential development all converge? The answer is flooding, and erosion, and a lot of it.
North Schultz Pass is located in Flagstaff, Arizona, in Coconino county. The area, like much of Arizona, is hot and dry with generally sparse vegetation growth. But after a fire destroyed a wooded area on the east side of the San Francisco peaks near Flagstaff, the eastside of the mountain saddle was left more vulnerable to erosion from heavy rains. Specifically, North Schultz Pass is an area in which the Thames, Peaceful Way and Copeland watersheds all flow converging on the edge of the city in a residentially developed area. In July of 2018, extreme flooding hit the area when a large storm event dumped nearly 6 inches of rainfall on the area within just two hours. The extent of this rainfall event was determined to be approximately a 1,000-year hydraulic event for the area, making it the largest rainfall event to occur in recorded history.
To make matters worse, this event was preceded by another large rainfall event that produced 2 to 4 inches of rain in the same area just four days earlier. When factoring in the loss of vegetation due to fire, not only was storm runoff impacting the area but also mountain debris, which funneled down the slope causing huge blowouts in the drainage channels on already overly saturated soils.
The results from the flooding event created massive amounts of soil erosion and instability along many of the existing mitigation areas, including ditches, emergency spillways and retention basins. Many of the areas had previously been designed using articulated concrete blocks around residential driveways and road bridges, some of which did not hold up. Other areas had been protected with a lesser erosion control system, but poor system anchoring resulted in huge soil and sand pillows forming beneath the existing erosion mat. Multiple residential homes and structures also were damaged from the flooding. In the aftermath, the Coconino County floodplain management team was tasked with evaluating the affected area to address the need for increased soil stabilization efforts to help mitigate future erosion issues in large floods.
To begin remediation, the county brought in design engineers to evaluate the existing flood control structures and to assess changes and adaptation needed to help expand the flood structure control against these larger events. Additionally, clean-up work began to remove flood-produced sediment from existing structures. Many existing structures needed a form of erosion control protection installed to help against future high-hydraulic conditions, one main need was a high-strength anchoring system that could help reinforce the channels, as these areas see slow vegetation establishment. The design engineers consulted with Western Green to design and select a high-performance erosion control system for the North Schultz area.
The Xtreme Armor system (XAS) was selected as the appropriate erosion control system for the job. The XAS is an adaptable Anchor Reinforced Vegetated System (ARVS) that includes an integrated design that combines high-performance turf reinforcement mats (HPTRM), percussion-driven anchors, pins and vegetation to create an erosion control system that can handle the high stress and strain from large hydraulic loading events. The configuration designed for North Schultz was specifically adapted for channel installations that are of greater-than-typical flows and hydraulic stress conditions. To build the site-specific XAS system, the PP5-Xtreme HPTRM was selected. The PP5-Xtreme is a woven HPTRM that creates a high-tensile strength, low-profile matting. These HPTRM attributes also make it suitable to handle wheel load and large animal traffic through the area, both of which were benefits to the use and maintenance of the project location.
The addition of the percussion-driven anchors and twist and straight pins were implemented to offer downward pressure on the HPTRM material, which helps to keep soils from shifting when under high strain. The Gripple TL100-TLA2 3-foot zinc-aluminum percussion-driven anchors were selected and ensured a high level of performance.
Installation began on the project site in July of 2019. Originally, the system was scheduled to be installed during the spring of 2019, but due to multiple delays, work didn’t begin until late summer. Due to delays, the crew was forced to hasten the installation pace in hopes of finishing the site protection before the monsoon season in August. Luckily, the system selected can be installed more quickly than some alternatives.
Due to the shape and size of the flood structures, the HPTRM was installed perpendicular to the channel flow. This direction of matting installation is a bit unconventional, as most installers choose to install parallel to water flow, but for this project, the direction was decided upon as it would result in zero material waste. The HPTRM could be cut to fit the channel dimensions specifically. The XAS system can be installed in either direction as long as proper shingle-style overlaps occur in the direction of water flow. Once the erosion control matting was placed, the percussion-driven anchors were installed along seams and every 4 feet across the matting. The PDAs created a working check slot to slow soil movement in ground saturated conditions, while resisting pull-out up to 1,000 pounds per foot. Additionally, 8-inch twist pins were installed as added fastening support. The twist pins are an inexpensive fastener that help add HPTRM -to-soil conformance using a screw-like install resulting in nearly 150 pounds of pull-out resistance in loam to clay soil types. Finally, 12-inch-long straight wire pins were installed between the rows of PDAs and twist pins as a final stop-check for system strength.
Beyond holding up to the hydraulic forces needed for the system, there were additional considerations for the selection of an erosion control system. Local soils and the lack of long-term vegetation were significant concerns. The Flagstaff area typically consists of nutrient-poor, alkaline soils that are heavy in clay. This soil type results in minimal vegetation growth, and what does grow tends to be slow growing natives. The optimized configuration of the system is ideal for the establishment and support of vegetation, an extra benefit with slow-establishing plants. Additionally, since the long-term vegetation will be sparse, the erosion control solution needed to hold up under added UV exposure. Luckily, the system is suitable for both of these issues. Together the PP5-Xtreme matting and the permanent percussion-driven anchors have been tested for long-term system longevity of 25 to 50 years.
Since the system's installation in the summer of 2019, the project site has held up through multiple seasons of wind and rain exposure. By fall, the channel installation looked very good, and naturalized vegetation was starting to grow through that XAS. The design engineer was pleased with the outcome of the design and felt confident that the extra fastening and added check slots would benefit the project from future events. Overall, the system has offered the Coconino County floodplain management team a cost-effective, low-impact solution for creating a high-performance system for floodwater management.