Post-Construction Storm Water Management/Ground-Mounted Solar: Wisconsin Department of Natural Resources Guidance Memorandum

The Wisconsin Department of Natural Resources (“WDNR”) issued a document titled:

Post-Construction Storm Water Management for Ground-Mounted Solar (“Guidance”)

See EGAD Number: 3800-2022-OX.

The site design of a solar project can impact whether it will improve or negatively affect water quality. Existing federal and state stormwater requirements have generally not been developed taking into consideration of ground-mounted solar energy facilities.

Note that the Virginia Department of Environmental Quality issued a policy document in March 2022 addressing this issue.

WDNR’s stated rationale for issuing the Guidance is the increasingly frequent construction of these facilities and its belief there is a need for:

. . . an approach to post-construction stormwater management that meets the post-construction performance standards in ss. NR 151.122 to 151.124, Wis Adm. Code that reflects the unique hydrological conditions created by large areas of ground-mounted solar and is simple to implement.

The stated objective of the Guidance is to identify the conditions under which the vegetation under and around ground-mounted solar arrays may be considered a stormwater management practice sufficient to satisfy post-construction performance standards.

In addressing background issues/definitions, the Guidance states:

• Areas under solar arrays are generally vegetated, creating a landscape that is a combination of meadow/impervious surface
• Former land uses associated with newly sited solar array installations are often row crops
• Emerging research on hydrologic response of solar installations confirms a reduction in runoff from predevelopment to post-development conditions (noting limitations of research)
• Tools used to estimate runoff and pollution control better assess pervious areas and different types of vegetation

The Guidance specifically addresses certain performance standards which cover:

• Total Suspended Solids
• Peak Flow
• Infiltration performance standards
• Conditions in the Guidance are based on:
  • WinSLAMM modeling of a series of pitched roof source areas in series with vegetated buffer strips and TR-55 hydrologic modeling methodology
• Description of a test case
• Analysis of differences between turf and prairie for purposes of defining scenarios with or without pollutant trading (with the assumption prairie will be used in all cases)

The Guidance describes in detail what is required to satisfy post-construction performance standards for the vegetation under, between, and around ground-mounted solar arrays within the cited Wisconsin Administrative Code.

A copy of the Guidance can be downloaded here.