By Robert Gay Sept. 1, 2020
Arizona Department of Transportation (ADOT) has completed the physical portions of two anti-scour projects on bridges on SR82 that cross Sonoita Creek in Patagonia, and over the creek from Big Casa Blanca Canyon, a tributary a few miles north. The purpose of the project is to strengthen the stream bed with an anti-scour slab of reinforced concrete, now buried 3 to 4 feet below the restored original bed level.
Scouring is defined as “any removal of silt and sand in the bed of a stream by a strong current.” Monsoon flooding in the area has been known to change stream bed levels by a foot or more, either by removing or depositing material. When removal happens near bridge abutments, the bridge itself can be at risk from structural damage. The deposit of material poses a different risk to bridges by decreasing the passage under the roadway for tree trunks and other flood debris – but it’s the scouring effect as a stream lowers its bed level which is the concern of ADOT’s current project.
At both bridge projects the original bank shapes were recreated, and at the east side of the bridge near the Patagonia Post Office, “walkability” from the roadway to the Railroad Trail has been restored, although with a crook in the trail where it drops to a lower level.
At both projects, all surfaces above the streambed were sprayed by a wet hydroseed mix applied from a truck to revegetate the disturbed area. The hydroseed applicator explained that there are 30 types of local seeds in the mix. It is applied to ground which has deliberately been “scarified” to provide a receptive grip for the seeds. Besides seeds and fertilizer, the hydroseed mix includes psyllium seed, known as “tackifier,” to glue the mix to the roughened earth so the seeds and soil are not washed away with rains, and get a chance to germinate. ADOT also notes with some pride that their crew has been careful to remove invasive plants, like tumbleweed, from project areas.
The revegetation effort was done under strict ADOT protocol and includes occasional monitoring of new plant establishment over this growing season. An inspector on the project’s crew concluded “We put the mix in place, and then Mother Nature does her thing.” Most of the hydroseeded areas seem enthusiastic in sending up a variety of plants, including grasses, thistles, datura, and poppies.