Stormwater standards in the United States were first implemented in 1972 through the Clean Water Act, long before the commercialization of solar energy systems. The standards are intended to protect surface waters and groundwaters from the effects of land development and they focus greatly on determining whether or not surfaces are pervious and able to allow water to percolate to the area underneath (e.g., vegetation), or impervious (e.g, pavement). However, since these standards were developed prior to the widespread use of solar energy systems, permitting standards and processes can be highly variable and unpredictable for solar development. As a result, solar development sites can often face high development costs - from soft costs to infrastructure costs to mitigate potential stormwater runoff.
Fortunately, the Photovoltaic Stormwater Management Research and Testing (PV-SMaRT) project has performed extensive real-world research and modeling to provide guidance for the solar industry on how to overcome solar project permitting challenges related to stormwater management. Below we cover the key guidance solar practitioners can follow to reduce project costs and prevent any unnecessary permitting delays in light of stormwater management requirements.
1. Compaction — Managing soil compaction and bulk density across the site
The PV-SMaRT modeling results demonstrated that compaction is the single most important consideration when it comes to determining stormwater runoff. Compacted soils limit stormwater infiltration, and highly compacted soils act as impervious surfaces by creating significant runoff. According to the research, having compacted soil between the arrays increased runoff by almost 100 percent, on average. Modeling results indicated that target bulk densities should be within 1.1 to 1.5 grams per cubic centimeter, depending on the soil texture or classification, in order to minimize runoff. Additionally, looser soils allow improved and more rapid vegetative establishment. When looser soils are combined with deep-rooted ground cover, this helps to maintain lower bulk densities.
Paying close attention to compaction is particularly important for solar energy sites, in which soil compaction frequently occurs from heavy equipment driving over the site, and sometimes from intentional compaction to stabilize pilings or foundations. Soil compaction may also take place post-construction due to ongoing operations and maintenance. Sites that are heavily graded, or sites in which topsoil is removed have a high chance of compaction, which then requires mitigation to achieve requisite bulk densities.
2. Soil depth — Including soil depth in stormwater modeling and design
Also known as the rooting depth, soil depth defines the ability of the site to have water infiltrate. Soil depth measures from the soil surface to the first impervious layer past which water, and thereby roots, cannot infiltrate. According to the PV-SMaRT research, soil depth is the second most important factor of the site when it comes to stormwater management. In fact, there was an estimated 78 percent increase in runoff as soil depth decreased from 1.5 to 0.5 meters. As larger soil depths enable a much greater capacity to absorb stormwater, recognition of this capacity by authorities having jurisdiction (AHJs) could significantly alter the need for engineered stormwater infrastructure.
Although soil depth cannot be easily improved on a site, since the impervious layer would have to be broken through, plowed, or have additional soil added to the site, soil depth can be altered in certain development scenarios, such as those that involve extensive grading. For sites with shallow soil depth, understanding the need for additional mitigation will prevent post-construction issues as well as costly retrofits.
3. Ground cover — Installing, establishing, and maintaining appropriate vegetated ground cover between and under the arrays to facilitate infiltration
The factor that has the third largest effect on stormwater management is ground cover, such as bare earth, gravel, native grassland, or pollinator habitats. The PV-SMaRT research found that there was a significant increase in stormwater runoff, up to 38 percent, for vegetation with shallow roots or intermittent density (e.g., poorly managed row crop or mowed turf grass) compared to deep-rooted perennials like prairie. Additionally, synergistic benefits are provided to host communities from deep-rooted pollinators, such as the creation or restoration of habitat, pollinator services to adjacent agriculture, and aesthetic site improvements.
Today, most solar development projects rely on fast-establishing fescues or other turf grasses as the final vegetative ground cover. Unfortunately, these ground covers typically have far more shallow root systems that do not provide the same infiltrative benefits as deep-rooted natives or pollinator mixes. On the other hand, native or pollinator seed mixes are more expensive. If appropriately addressed in stormwater modeling though, pollinators and similar ground covers can avoid capital costs of additional or larger capacity stormwater infrastructure.
The primary challenge with using deep-rooted vegetation though is that most stormwater permits require 70 percent coverage one to two years after development. This is often impossible with deep-rooted, native vegetation plant species. Some states and local jurisdictions have overcome this issue by allowing for alternative forms of stabilization, such as cover crops that allow permanent vegetation to be established.
4. Disconnection — Ensuring appropriate distance between arrays for infiltration
The areas underneath and between solar arrays can be designed and maintained as infiltration spaces to disconnect impervious surfaces from receiving bodies of water. Disconnecting impervious surfaces from surface waters and drainage systems is what differentiates solar development from other kinds of development. For disconnection to reduce stormwater management risks, the disconnected areas must:
infiltrate water quickly and reliably (e.g., deep-rooted ground cover and compaction/bulk density)
have volumetric capacity (e.g., soil depth); and
have sufficient vegetation cover to both slow and improve water flow (e.g., ground cover)
The PV-SMaRT research discovered that the distance between solar arrays is a key factor when it comes to managing stormwater. For example, runoff increased by 14 percent for an array spacing of 15 feet (piling to piling) as opposed to 35 feet. Solar arrays examined in the study had a spacing of 25 feet, which provided sufficient disconnection areas to allow proper water infiltration. For sites with highly compacted soils or steep slopes, increasing the disconnection between arrays can help mitigate these conditions.
If you have any civil engineering questions for your utility-scale solar projects, get in touch with an expert at Castillo Engineering.