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The Challenge – Erosion Control and Native Revegetation

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The Central Arizona Project (CAP) is a 336-mile long aqueduct and protective embankment system extending from Lake Havasu to Tucson, Arizona. This system consists of cut, fill, or partial fill sections of aqueduct, and has a number of pumping plants that are used to elevate the water to the next aqueduct. The entire system, cutting a swath through central and southern Arizona, has varying levels of limited precipitation and a multitude of soil types. The system is subject to all types of water erosion including the effects of splash, sheet rill and gully, stream/channel, and internal erosion.

The Quattro team of experts was brought in to provide environmental design and implementation management services for re-establishing native plant growth for long-term erosion control at the Hassayampa and San Xavier pumping plants.

The Sonoran Desert area is one of the wettest deserts in North America, averaging up to 10” of rainfall a year delivered during two rainy seasons (summer and winter) causing unusually severe erosion. It is a land of extremes with temperatures ranging from 32°F to 134°F. The two dominant vegetation communities on the site are creosote-bursage desert scrub and saltbush desert scrub. However, this desert contains more plant species than any other in North America.

The challenge for the Quattro team was to provide immediate slope and soil stabilization, while simultaneously implementing the Quattro Living Soil Membrane (LSM) in an environment of high sun and wind exposure, an arid desert climate subject to periodic heavy precipitation events, and with soil types typically consisting of well-degraded sands, silts and granular material.

The Quattro Process

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In December 2011, revegetation was initiated on 18 acres of excavation cuts at the Hassayampa pumping plant and 4 acres at the San Xavier pumping plant. No topsoil had been saved during construction of these pumping plants. Sampling of the seedbed soil was undertaken to develop a comprehensive understanding of the growing soil challenges inherent in re-establishing a functioning soil community, the critical key to sustainable growth of a diverse palette of native grasses, forbs and shrub species. Interpretation of the soil analyses resulted in a plan to buffer high salts/high pH values and balance minerals to counter macro- and micro-nutrient deficiencies.

Plant seed species representative of the diversity of the Sonoran Desert were selected and supplied from collections in the area. Two native seed mixes were used: Saltbush Scrub mix at Hassayampa and Saltbush Creosote Bursage Community mix at San Xavier. The highest percentage of seed within each mix was early seral stage, rapidly growing pioneer species.

Prior to seeding, the benched slopes were ripped/furrowed on contour to both contain sediment runoff that might result from heavy rainfall, and to intercept and contain rainfall runoff to enhance seed germination and facilitate long-term plant establishment. The custom-blended Quattro Living Soil Membrane was applied via a hydro-seeder.

Products

To stabilize the slopes against both wind and storm water erosion until such time as revegetation was accomplished, BIND l ATLAS SUPERDUTY (formerly known as Atlas SoilLok) the world’s #1 heavy-duty soil binder, BIND l ARMOR (formerly known as Tackifibers), BIND l STRONGHOLD (formerly known as Stronghold Fibers) and hydro-straw mulch were applied.

To begin growing the soil for long-term sustainable vegetation with Quattro’s LSM technology, GROW l NUTRI BOOST (formerly known as Kiwi Power), GROW l NUTRI BASE (formerly known as Fertil-Fibers protein-based nutrient), GROW l ENRICH (formerly known as Humic Shale), and a custom-blended minerals package containing sulfur, zinc, manganese, magnesium, iron and calcium, was applied.

Sustained Performance – No Soil Erosion and Strong Native Vegetation Growth

In May 2012, five months after seeding, the site showed signs of germination. As expected, the growth was a mixture of seeded native plant species and common weeds (tumbleweed was minimal). Of the nine plant species used, all were growing except Globemallow and Creosote (both species grow better in the winter). Desert marigold was also present. The extent of plant density and diversity was significant, especially when compared to the areas that received no soil treatment. Surrounding areas with no treatment were mostly barren and even devoid of weeds.

One hundred percent of the Holding Soil combination of Quattro products and wood fiber mulch was still intact and holding the slopes and soil, notwithstanding the effects of a torrential rain event in the spring. It was also apparent that the furrowing on the contour had significantly increased moisture retention; areas that were tilled displayed more plant growth owing to the increased water capturing.

By November 7, 2012, nearly every plant species applied in the mix germinated. These plants were extremely healthy and were even producing new seed. Over 50 percent of the weed growth present in spring was gone. Both plant density and plant diversity was strongly apparent. The untreated areas showed only moderate, anemic weed growth.

The integrity of Quattro’s Holding Soil erosion control BMP was still very much intact. This lattice-like living soil membrane is expected to remain in place throughout the 2012-2013 winter rains.

For what was essentially a soil stabilization challenge that has frustrated us for a number of years, the Quattro team recommended a two-pronged plan of attack: to work as a collaborative team to design and implement a state-of-the-art revegetation plan re-establishing site-specific native plants, in concert with re-establishing a functioning soil community in the sterile drastically disturbed and re-contoured desert soils. Project implementation was completed on time and within budget. Furthermore, we achieved our first benchmark of success after a year with minimal soil loss from the steep slopes surrounding our pump stations, while plant growth has demonstrated both encouraging density of coverage as well as plant species diversity.

Robert Hitchcock, Civil Engineer
Maintenance Engineering
Central Arizona Project

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