We are using these Best Available Control Measures (BACM) to mitigate dust emissions on Owens Lake: shallow flooding, managed vegetation, tillage and gravel.
LADWP has installed one of the world’s largest shallow flooding systems to reduce dust on Owens Lake and reach the LADWP’s ultimate goal; to improve air quality around the dry lake. The shallow flooding system consists of a network of computer-controlled sprinklers placed across more than 14 square miles (so far) of the ancient lakebed. The overall irrigation system will have over 60 miles of pipelines and over 2,000 miles of buried drip irrigation lines. Shallow flooding is the most widely used BACM at Owens Lake. Water is spread across a graded surface with a minimum of 72-75 percent (depending on the dust control area) of the surface covered with standing water or surface-saturated conditions during the peak dust season between mid-October and mid-May. A variety of different water delivery systems are used for this BACM, including water supply through lateral pipes and distributed sprinklers. The presence of standing water eliminates dust generation from the wetted surface and also traps blowing sand that enters the ponded area. Approximately 80,000-acre feet of water are used on an annual basis to control dust. In addition, water-based dust-control measures help provide a habitat for wildlife, providing a key stopover location for migratory birds.
Precision surface wetting as demonstrated in the Shallow Flooding Wetness Curve Refinement Field Test (SFWCRFT; LADWP, 2019b) represents a modification to the existing shallow flooding BACM. Precision surface wetting utilizes reciprocating sprinklers or perforated whip lines to wet circular areas of the lakebed to target a specific wetted percentage. Testing has been conducted in the SFWCRFT to examine approaches to using precision surface wetting to reduce water use while controlling dust emissions.
Brine with Shallow Flooding BACM Backup
The brine BACM consists of three dust-mitigating surfaces: brine, evaporite salt deposit, and capillary brine salt crust. The liquid brine serves in the same manner as the shallow flooding BACM, eliminating any sand or dust sources as well as capturing saltating particles. The evaporite crust that forms subaqueously from evaporation of standing brine, serves as the armoring of the surface to reduce dust emissions. This crust is primarily evaporite minerals (solid phase salts as well as the potential for interstitial brines) and is not easily eroded by wind. Capillary brine crust, termed from its formation during the capillary rise of shallow brine in the sediments, forms from evaporation of shallow groundwater, precipitating salts both within and on top of the lake sediments (GBUAPCD, 2016b).
Dynamic Water Management
An operational modification of the shallow flooding BACM that allows for later start dates and/or earlier end dates to reduce water use in areas with historically low PM10 emissions.4 Areas under dynamic water management are carefully monitored, and reflooding is required when specific performance criteria are exceeded (e.g., sand flux greater than 5 g/cm2 day, visible dust observations, or visible dust emissions when induced particle emission testing5 is performed at the reference test height). Dynamic water management was approved in 2014 during an extended drought to provide LADWP with flexibility to reduce water use on 13.15 square miles.
The amount of open space in the Owens Valley has not changed much in 100 years. What has changed is that almost one third of the water once sent south for a growing metropolitan population is now being used directly in the Eastern Sierra for enhancing wildlife and plant habitats, restoring the Owens River, and creating even better habitat and recreational opportunities. Management of water resources has adjusted to the changing perspectives of modern times where environmental needs are considered in water supply decisions. As part of the many restoration and revegetation projects that the LADWP undertakes, staff specialists are involved in vegetation mapping, rare plant studies, growing native plant stock for revegetation projects, and ongoing monitoring of plant community health. More than 700 plant species have been identified in the Owens Valley, living in alkali shrub, desert scrub, alkali grassland, riparian, urban, and irrigated agriculture plant communities. While some of these species can be found in parts of the Sierra Nevada, Great Basin, and Mojave Desert regions, the Owens Valley provides an environment where they all can thrive. LADWP cares for these treasures with botanists and range management specialists who map the resources, study rare plant distributions, eliminate noxious species that interfere with native vegetation, revegetate where needed, and constantly monitor the area’s vegetation.
Native Plant Seed Farm
Native plants produce seeds intermittently during naturally wet years. However, native plant seeds are needed on an ongoing basis in many of the LADWP’s revegetation efforts in the valley. To give nature a hand, the LADWP is developing seed farms near Bishop that will produce a reliable supply of seeds every year. These seeds will be used in the coming years for restoring native vegetation where needed in the region. LADWP is growing salt grass for dust mitigation as shown in the photo to the right. The Owens Valley is home to an astoundingly wide array of plant, animal, and bird species. Wildlife inventories show that 299 species of birds, 73 species of mammals, 14 species of fish, 32 species of reptiles, six species of amphibians, and hundreds of species of invertebrates inhabit Owens Valley. LADWP and its team of contractors are installing one of the world’s largest shallow flooding systems to reduce dust on Owens Lake and reach the LADWP’s ultimate goal; to improve air quality around the dry lake. The shallow flooding system consists of a network of computer-controlled sprinklers placed across more than 14 square miles (so far) of the ancient lakebed. The overall irrigation system will have over 60 miles of pipelines and over 2,000 miles of buried drip irrigation lines.
In addition, 2,400 acres of saltgrass have been planted on the lake playa, and a massive irrigation system was installed to water the plants. About 30 million saltgrass seedling plugs were planted on Owens Lake over two months in 2002. Saltgrass anchors the dust and holds water in the top layer of soil. The saltgrass has stabilized and is already working effectively to reduce dust emissions from the ancient lakebed.
By increasing the surface roughness, tillage also reduces the wind speed at the surface by shear stress partitioning and the creation of turbulent eddies. This effect on the wind field is most effective when the direction of tillage and the ridges created are perpendicular to the dominant wind flow direction. For this reason, tillage patterns that deviate from linear are more effective at reducing surface wind speed for winds of all directions. Finally, the surface ridges and clods provide shelter angle protection that prevents wind-carried sand particles from striking a flat horizontal surface and ejecting more particles (Potter et al., 1990).
Tillage is a proven method for reducing surface erodibility (Fryrear, 1984; Potter et al., 1990). Studies at Owens Lake showed that when the performance criteria were maintained, tillage generally resulted in de minimis levels of sand flux and PM10,8 which was considered equivalent to a control efficiency of 99 percent or greater sand flux (Air Sciences, Inc., 2015). Exceedances were attributed to the tilling events, construction activities, and off-site sources. The field tests at T12 in heavy clay soils were tilled to achieve a ridge spacing of 12-14 feet and ridge heights of 1.6-2 feet (total distance between furrow bottom and ridge top of 3.2-4 feet), resulting in starting roughness values between 6 and 8.75,9 although the furrow depths and ridge heights did decrease somewhat over time. Different tilling spacing was not tested. There was no contemporaneous untreated control area during the evaluation of tillage performance, but several years of pre-tillage horizontal mass flux measurements were made at dust control area T12. In addition, the tillage test at T12 is one of the few DCMs to have performance evaluated using direct measures of PM10 at upwind and downwind locations. Tillage can also benefit adjacent dust control areas because the aerodynamic roughness it creates can slow near-surface wind speeds immediately in the lee of the tilled area.
Tillage with Shallow Flooding
Tillage with the shallow flooding BACM backup was approved as a modification to the shallow flooding BACM in 2014.6 Tillage controls soil erosion by wind and fugitive dust emissions in several ways. Tillage, as practiced on the Owens Lake bed, creates oriented beds and large surface aggregates (termed oriented and random surface roughness, respectively; see Figure 4-5). Surface roughness has long been known to reduce surface erodibility and was one of the five factors in the first predictive equation for wind erosion (Woodruff and Siddoway, 1965). In general, soil particles and aggregates greater than 0.84 mm in diameter are considered non-erodible (Chepil, 1962; Fryrear, 1984; Zobeck et al., 2003) because the aggregates are too large to be entrained in all but the most intense windstorms. By increasing the surface roughness, tillage also reduces the wind speed at the surface by shear stress partitioning and the creation of turbulent eddies. This effect on the wind field is most effective when the direction of tillage and the ridges created are perpendicular to the dominant wind flow direction. For this reason, tillage patterns that deviate from linear are more effective at reducing surface wind speed for winds of all directions. Finally, the surface ridges and clods provide shelter angle protection that prevents wind-carried sand particles from striking a flat horizontal surface and ejecting more particles (Potter et al., 1990).
Gravel cover is a zero-water-use DCM that involves distributing a layer of gravel on an emissive lakebed to protect it from wind (see Figure 4-9). Gravel protects the bare ground underneath it against wind erosion by substantially reducing the capillary rise of saline groundwater and salt and crust formation. Some areas are covered by 4 inches of gravel (GBAPCD, 2003), while others are covered by 2 inches, underlain with a permanent permeable geotextile fabric to prevent settling of the gravel (GBAPCD, 2013b). The gravel, which is mined and transported to the site, is required to be of similar color to that of the lakebed soils and be at least 0.5 inches in diameter. The geotextile fabric is a 2.3-mm thick (90 mils) artificial fabric that is permeable to draining and resistant against acids and alkali elements of the soils. To protect the gravel-covered area from flooding, channels and drains are incorporated in the area surrounding the control area (GBUAPCD, 2008, 2013b).