“The La Canova Priority Site, along the Rio Grande, needed immediate action to protect local water resource irrigation infrastructure from further river and upland runoff erosion,” explains David Cline, P.E., a senior engineer with Tetra Tech Inc.

The Assessment
The site was experiencing active lateral erosion from the river, as well as surface erosion and rilling from overland flows. The combination of these upland and riverine erosive forces required a unique design solution for the site. In addition to protecting irrigation and roadway infrastructure, the Bureau of Reclamation is also responsible for considering the issues related to willow flycatcher habitats. The willow flycatcher bird species is listed as Threatened under the Endangered Species Act, and has known habitat areas within the Velarde reach of the Rio Grande near the project site.

Design Solutions
The design solution provided mitigation for protection of local irrigation and roadway infrastructure, as well as enhancing riparian vegetation and providing development of riparian flycatcher habitat over the long run.

“Integration of bioengineered bank stabilization features were appropriate in limiting surface erosion occurring on the upper portions of the bank. Use of a coir product provided a biodegradable growing media for planting and establishing native riparian vegetation,” said Cline. “Over time, the coir fabrics blocks biodegrade and the remaining plant community root structure provides the additional soil strength reinforcement and promotes bank stability.”

A composite bank structure was designed using rock toe protection up to the ordinary high water mark to prevent further river erosion, and the newly developed Rolanka BioD Block 400 series on upper portions of the bank to prevent further surface erosion. Soil treatments placed behind the prefabricated coir block forms included mixing of compost, topsoil and water saving polymers to condition the soils for landscape and planting treatments. The BioD Block terraces were then planted with a variety of native riparian plant species mimicking the natural species distribution found along the riparian areas of the Rio Grande including Rio Grande cottonwood, coyote willows, New Mexico olive and native bunch grasses.

Construction was completed in March 2005 with dormant planting of cottonwood poles and coyote willows on the lower riprap bench in the dryer and cooler winter months, and live planting of New Mexico olive and bunch grasses on the upper terraces (BioD Blocks) during the wetter monsoon season later in the summer. Follow on maintenance has required irrigation and watering of the bioengineered bank to ensure establishment of plant roots to strengthen the bank protection feature.

“The utilization of the BioD Blocks provided a simple method for constructing the terraced upper bank. The pre-formed coir blocks eliminate the need for soil forms during construction, which ultimately saves the client money. One year after construction and irrigation maintenance, the project is performing well and the native plant community is becoming well established,” concludes Cline.  SLDT