As the United States continues to shift toward alternative sources of energy, the spread of solar development has uncovered as many questions as it has answers, particularly with uncertainty around stormwater analysis of these projects. Most stormwater management ordinances adopted by Authorities Having Jurisdiction (AHJ) were written largely to address more traditional development with the understanding that the soils beneath these improvements are unable to infiltrate the rainwater shed from such impervious surfaces.
Ground-mounted solar arrays typically allow for the infiltration of rainwater below the raised panels. This raises a series of questions from solar developers and stormwater reviewers alike, focusing mainly on whether the solar panels should be treated as impervious surfaces or if the resulting hydrology of an array is like that of an open field.
Developers have argued that no stormwater management requirements are warranted, especially if the access roadways to the arrays are permeable. Plan reviewers pushed back, arguing the installation process alone would influence the hydrologic characteristics of a site.
In January 2013, the Maryland Department of the Environment (MDE) issued Stormwater Design Guidance for Solar Panel Installation. The guidance emphasized that nonstructural techniques were a low-cost alternative to treating runoff from narrower impervious areas (including solar panels) by promoting overland filtering and infiltration.
The following conditions needed to be met to take credit for the impervious disconnects:
- The vegetated area receiving the runoff must be equal to or greater than the disconnected impervious surface.
- Runoff must sheet flow onto and across vegetated areas to maintain the disconnection.
- Disconnections should be located on gradual slopes (< 5%) to maintain sheet flow. Slopes between 5% and 10% can be addressed via level spreaders, terraces or berms but slopes greater than 10% will require an engineered plan to ensure a non-erosive conveyance.
- Disconnections work best in undisturbed soils.
- Groundcover vegetation must be maintained in good condition in those areas receiving the runoff.
The Maryland Guidance was quickly brought to the attention of other AHJ’s outside of Maryland. As expected, some localities accepted the guidance while others stiffly rejected it.
In April of 2017, North Carolina Department of Environmental Quality published a section in their Stormwater Design Manual focused specifically on Solar Farms. The recommendation differences from Maryland’s Guidance are noted below:
- Avoid Compaction of Subsoil; Difference: Discussed the tilling of soil if construction traffic was unavoidable.
- Disconnect Runoff from Solar Panel Arrays; Difference: Referenced slopes less than or greater than 8%.
- Minimize Use of Herbicides and Fertilizers; Difference: Suggested the use of mowing for vegetation control rather than herbicides and fertilizers, which may degrade water quality.
- Plant Mix of Warm- and Cool-Season Grasses. Difference: Maryland’s Guidance did not specify the types of grasses to be planted, only that they be maintained in good condition.
- Limit Vertical Clearance to < 10 Feet. Difference: Maryland Guidance did not address a vertical limit.
In January 2019, Pennsylvania Department of Environmental Protection (PA-DEP) issued a FAQ sheet on the Permitting of Solar Panel Farms for Erosion and Sediment Control and Stormwater Management. Five major differences from prior jurisdictions include:
- The post construction condition should have a minimum uniform 90% perennial vegetative cover with a density capable of resisting accelerated erosion and sedimentation. This differs from the standard PA requirement of 70% perennial vegetative cover since the vegetation functions as the primary PCSM BMP (Post Construction Stormwater Management Best Management Practice) for the solar farm.
- The disconnection length utilized in the PCSM calculations extends from the panel drip edge to the next panel drip edge (at a minimum this is 2 times the length allowable in prior jurisdictions).
- Projects that need grading within array areas with slopes between 10-15% may be acceptable upon site evaluation and certification by a qualified professional engineer and the addition of slope protection, if necessary.
- The project should minimize the vertical clearance of the solar array from the ground, utilizing additional controls to address accelerated erosion where the lowest vertical clearance exceeds 10 feet.
- If the solar panel support structure/foundations result in more than 5% of the project site area, the applicant will be required to conduct a stormwater analysis in accordance with standard NPDES requirements.
With the passage of the Virginia Clean Economy Act of 2020, and the implementation of the Permit-By-Rule (PBR) process, Virginia ranked within the top 4 states for solar development in 2021. The Commonwealth’s Stormwater Management Program (VSMP) also appeared to be more attractive to solar developers as ground-mounted solar installations were not required to include the imperviousness of the solar panels within the stormwater calculations, instead only the foundations were considered.
Virginia’s Department of Environmental Quality (DEQ) later determined that this method underestimated the post-development runoff from solar panels and negatively impacted downstream property owners.
On March 29, 2022, the DEQ issued a memo that the following stormwater directives should be integrated into ground-mount solar installations:
- Quantity: Solar panels are to be considered unconnected impervious areas when performing post-development water quantity calculations.
- Quality: Solar panels are to be considered impervious areas when performing post-development water quality calculations using the VRRM (Virginia Runoff Reduction Method). The panel area may be considered “simple disconnection” for these calculations.
- Alternative Methods of meeting stormwater quantity and quality requirements are not prohibited.
The memo was made effective immediately to all ground-mount solar projects, “regardless of the stage of design.” On April 14, 2022, DEQ issued a clarification memo that the stormwater directives were only subject to “any project(s) that does not obtain an interconnection approval by a regional transmission organization or electric utility by December 31, 2024.”
In April of 2022, Ohio EPA Division of Surface Water published the “Guidance on Post-Construction Stormwater Management for Solar Panel Fields”. Noteworthy guidelines included a requirement of short-term vegetation establishment period (first two years) to ensure that the vegetation would effectively control stormwater both immediately after construction and long-term; and an actual limit specified for construction vehicle axle loading allowed to work within disconnection areas prior to the need of soil tilling (axle load limit = 6 tons).
Patterns are beginning to develop among the stormwater management requirements with respect to solar development, including:
- Keep the row spacing equal to or greater than the table width.
- Keep the grasses native and healthy.
- No heavy equipment on the disconnect areas.
- Don’t put the panels too high.
Thankfully, due to the collaboration of multiple states and agencies throughout the east coast, the stormwater requirements for solar panel installation throughout the remainder of the U.S. will be relatively easy to predict.
- Cook, L. and R. McCuen, 2013. Hydrologic Response of Solar Farms, J. Hydrologic Engineering. 18(5):536-54. American Society of Civil Engineers. Reston, VA.
- Maryland Department of the Environment. 2013. Stormwater Design Guidance – Solar Panel Installation.
- North Carolina Department of Environmental Quality. 2018. Stormwater Design Manual, E-6 Solar Farms.
- National Renewable Energy Laboratory (NREL), Photovoltaic Stormwater Management Research and Testing (PV-SMaRT) Project. https://www.nrel.gov/solar/market-research-analysis/pv-smart.html
- Ohio Environmental Protection Agency, Division of Surface Water. 2022. Guidance on Post Construction Stormwater Management for Solar Panel Fields
- Pennsylvania Department of Environmental Protection, Bureau of Clean Water. 2019. Chapter 102 Permitting for Solar Panel Farms, Frequently Asked Questions. January 2, 2019. Rev. April 30, 2021.
- Solar Energy Industries Association. 2022. State Solar Spotlight, Virginia Solar Factsheet 2022-Q2.
Jared Pantella, is a civil engineer with 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. With a focus in design-build projects, Jared specializes in renewable energy, power generation, heavy industrial, and commercial projects. His experience in survey, site-development, stormwater management and erosion, and sediment control is key for civil design and permitting. Based in Frederick, MD, his geographical footprint extends from New York to Virginia.
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