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As the United States continues to shift to alternative energy sources, the spread of solar development has revealed as many questions as answers, especially with the uncertainty surrounding stormwater analysis from these projects. Most stormwater management ordinances passed by the Authorities Having Jurisdiction (AHJ) have been written largely to cater for more traditional development, with the understanding that the soils beneath these improvements are unable to infiltrate into the watershed. rainwater from these impermeable surfaces.

Ground mounted solar panels generally allow rainwater infiltration under the raised panels. This raises a series of questions from solar developers and stormwater reviewers, focusing primarily on whether solar arrays should be treated as impermeable surfaces or whether the resulting hydrology of an array resembles that of an open field.

The developers argued that no stormwater management requirements are warranted, particularly if the access roads to the networks are permeable. Reviewers of the plan pushed back, arguing that the installation process alone would influence the hydrological characteristics of a site.

Maryland

In January 2013, the Maryland Department of Environment (MDE) released stormwater design guidelines for installing solar panels. The guidance highlighted that non-structural techniques were a low-cost alternative to treating runoff from tighter impermeable areas (including solar panels) by promoting filtering and infiltration over land.

The following conditions had to be met to take credit for sealed disconnects:

  1. The vegetated surface receiving runoff must be equal to or greater than the disconnected impermeable surface.
  2. Runoff must flow over and through vegetated areas to maintain disconnection.
  3. Disconnects should be located on gradual slopes (
  4. Disconnects work best in undisturbed soils.
  5. Ground cover vegetation should be maintained in good condition in areas receiving runoff.

The Maryland Guidance was quickly brought to the attention of other AHJs outside of Maryland. As expected, some localities accepted the guidelines while others strongly rejected them.

North Carolina

In April 2017, the North Carolina Department of Environmental Quality released a section of its stormwater design manual focused specifically on solar farms. The differences between the Maryland recommendations and guidelines are shown below:

  1. Avoid subsoil compaction; Difference: Discussed tilling the ground if construction traffic was unavoidable.
  2. Disconnect runoff from solar panels; Difference: Referenced slopes lower or higher than 8%.
  3. Minimize the use of herbicides and fertilizers; Difference: Suggested use of mowing for vegetation control rather than herbicides and fertilizers, which can degrade water quality.
  4. Mixture of warm and cool season grass plants. Difference: The Maryland guidelines did not specify the types of grasses to be planted, only that they should be maintained in good condition.
  5. Limit vertical clearance to Difference: Maryland Guidance did not address a vertical limit.

Pennsylvania

In January 2019, the Pennsylvania Department of Environmental Protection (PA-DEP) released an FAQ sheet on permitting solar panel farms for erosion and sediment control and stormwater management.. Five major differences from previous jurisdictions include:

  1. The condition after construction should have a minimum of 90% uniform perennial vegetation cover with a density capable of resisting accelerated erosion and sedimentation. This differs from the standard PA requirement of 70% perennial vegetation cover, as the vegetation functions as the primary PCSM BMP (Post Construction Stormwater Management Best Management Practice) for the solar farm.
  2. The disconnect length used in PCSM calculations is from the drip edge of the panel to the drip edge of the next panel (at a minimum, this is 2 times the length allowed in previous jurisdictions).
  3. Projects requiring grading in network areas with slopes between 10 and 15% may be acceptable after site assessment and certification by a qualified professional engineer and the addition of slope protection, if necessary.
  4. The project should minimize the vertical clearance of the solar array from the ground, using additional controls to deal with accelerated erosion when the lowest vertical clearance exceeds 10 feet.
  5. If the solar array support structure/foundation is more than 5% of the project site area, the applicant will be required to perform a stormwater analysis in accordance with standard NPDES requirements.

Virginia

With the passage of the Virginia Clean Economy Act of 2020 and the implementation of the Permit-By-Rule (PBR) process, Virginia ranked among the top 4 states for solar development in 2021. Commonwealth Stormwater (VSMP) also appeared to be more attractive to solar developers, as ground-mounted solar installations were not required to include solar panel waterproofing in stormwater calculations, only the foundations were taken into account.

Virginia’s Department of Environmental Quality (DEQ) later determined that this method underestimated post-development runoff from solar arrays and negatively impacted downstream homeowners.

On March 29, 2022, the DEQ issued a memo stating that the following stormwater guidelines should be incorporated into ground-mounted solar installations:

  1. Quantity: Solar panels should be considered unconnected impermeable areas when calculating water quantity after development.
  2. Quality: Solar panels should be considered impervious areas when performing post-development water quality calculations using the Virginia Runoff Reduction Method (VRRM). The panel area can be considered a “simple disconnect” for these calculations.
  3. Alternative methods of meeting stormwater quantity and quality requirements are not prohibited.

The memo went into effect immediately for all ground-mounted solar projects, “regardless of design stage.” On April 14, 2022, the DEQ issued a clarification note stating that the stormwater guidelines were only subject to “any project(s) that do not obtain approval for interconnection by an agency regional transmission or electric utility by December 31, 2024.

Ohio

In April 2022, the Ohio EPA Surface Water Division released the “Guidance on Post-Construction Stormwater Management for Solar Panel Fields.” Noteworthy guidelines included a requirement for a short-term vegetation establishment period (first two years) to ensure vegetation would effectively control stormwater both immediately after construction and over the long term. ; and a specified actual limit for the axle load of construction vehicles allowed to work in the disconnect zones before the need to till the ground (axle load limit = 6 tons).

Patterns are beginning to develop among stormwater management requirements for solar development, including:

  1. Keep line spacing equal to or greater than the width of the table.
  2. Keep grasses native and healthy.
  3. No heavy equipment on disconnect areas.
  4. Do not place the panels too high.

Fortunately, with the collaboration of several East Coast states and agencies, stormwater requirements for solar panel installation in the rest of the United States will be relatively easy to predict.


References:

  • Cook, L. and R. McCuen, 2013. Hydrological response of solar farms, J. Hydrological Engineering. 18(5):536-54. American Society of Civil Engineers. Reston, Virginia.
  • Maryland Department of Environment. 2013. Stormwater Design Guide – Installing Solar Panels.
  • North Carolina Department of Environmental Quality. 2018. Stormwater Design Handbook, Solar Farms E-6.
  • National Renewable Energy Laboratory (NREL), Photovoltaic Stormwater Management Research and Testing Project (PV-SMaRT). https://www.nrel.gov/solar/market-research-analysis/pv-smart.html
  • Ohio Environmental Protection Agency, Surface Water Division. 2022. Advice on Post-Construction Stormwater Management for Solar Panel Fields
  • Pennsylvania Department of Environmental Protection, Bureau of Clean Water. 2019. Chapter 102 Permits for Solar Panel Farms, Frequently Asked Questions. January 2, 2019. Rev. April 30, 2021.
  • Association of Solar Energy Industries. 2022. State Solar Spotlight, Virginia Solar Fact Sheet 2022-Q2.

Jared Pantella, is a civil engineer at LaBella Associates. Pantella has 15 years of experience in civil engineering and surveying. His roles have included Survey Field Crew Leader, Civil Designer, Civil Engineer, Project Manager and Senior Project Manager. Specializing in design-build projects, Jared specializes in renewable energy, power generation, heavy industrial and commercial projects. His experience in surveying, site development, storm water and erosion management and sediment control is essential for civil design and permitting. Based in Frederick, MD, its geographic footprint extends from New York to Virginia.

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