Laying the groundwork
This spring, Phoenix finished long-term repairs to a stormwater channel in the city’s northeast business district that frequently needed restoration to its banks following rainstorms. The new channel is designed to handle stormwater flows in excess of 10 feet per second and complement the landscaping of nearby businesses.
The previous channel was a 25-foot-wide, 10-foot-deep natural wash with no reinforcement on the banks or channel bottom that ran alongside 56th Street for approximately one mile. Rapid development in the surrounding areas, particularly upstream, increased the amount of impervious surfaces, causing more runoff to flow through the channel during storms. “The channel down 56th Street carries over 2,000 cubic feet per second, and [the runoff] would wash away the channel every storm,” says Laura Fritschi, project manager for the city. The resulting erosion threatened the adjacent roadways, 56th Street and Mayo Boulevard. It also posed a hazard to parking structures and access to a health care facility located nearby.
Because of the stormwater velocity and shear stresses, city engineers initially considered installing traditional hard armor systems, such as rock riprap, concrete lining and soil cement, to stabilize the side slopes and protect the channel bed from scouring. Rigid linings generally withstand high discharges, flow velocities and shear stresses. However, they can fail if a portion of the lining is damaged, and they can increase runoff velocities and discharges. Tenants of the health care facility also objected to the appearance of the rigid linings, saying they contrasted with the site’s unique landscaping.
City engineers next turned to permanent turf reinforcement mats, which promote vegetation growth and reduce erosion by shielding the soil surface from falling rain. The mats disperse and decrease the velocity of the surface flow, allowing sediments to drop out or deposit, and restrain soil movement by holding its structure and porosity with plant roots.
Consulting engineer Mike Delmarter, of the Phoenix-based engineering firm Kimley-Horn and Associates, and city Engineering Supervisor Dan Matthews chose to install Pyramat manufactured by Chattanooga, Tenn.-based Propex. The product consists of an ultraviolet-stabilized, three-dimensional woven synthetic fiber that is available in a tan color to match Phoenix’s sandy soils.
In March 2006, city crews began installing approximately 15,000 square yards of mat over the channel, and they seeded over the mat with a plant mix native to the Phoenix valley. The slopes of the channel were then landscaped with trees and shrubbery, matching the well-manicured design of the surrounding property. Crews completed the $2 million project in May.
Within a few months of project completion, a major storm that produced a large amount of runoff tested the channel. Flows were estimated to be two to three feet deep and moving at eight to 10 feet per second. The channel functioned well without any erosion. As a result, Phoenix plans to continue using the material for future channel and slope erosion control projects.