Question of the Month – Feb 24′ – Solar Arrays
This month we had a great question posed to the Post-Construction Committee about Solar Arrays
Question
I’m a stormwater quality engineer for El Paso County and we recently have had a few planned projects come in for an Early Assistance planning meeting with us for Solar facilities which include related buildings and solar arrays. In preparation for when these projects come in, I reviewed all of our exclusions in our MS4 permit and was thinking that possibly either the “Aboveground and Underground Utilties” exclusion or the “Sites with Land Disturbance to Undeveloped Land that will Remain Undeveloped” exclusion may apply to the solar arrays if the land under remains vegetated with limited changes to grade. But after discussing with our team we wanted to reach out to the CSC Post Construction team to see if anyone has experience with this and if exclusions have been applied to solar arrays previously.
Let me know if you have any experience with those types of projects and if applying those exclusions is a generally acceptable practice.
-Mikayla Hartford, El Paso County
Answers
They do not count because the solar panels themselves are impermeable. There has been some research about this.
https://www.nrel.gov/solar/market-research-analysis/pv-smart.html
Heather Otterstetter, City of Westminster
Following up on this email thread, here are a few items that may be useful in response to the question concerning solar fields.
- MHFD will be publishing an updated version of the USDCM Volume 1 Chapter 6 (Runoff) next month. One change to highlight is the updated recommended imperviousness values that will now include two tables related to imperviousness. The first table has percent imperviousness by land use (for planning purposes), and the second table has imperviousness by surface types (for site-level design). Both tables are supported by technical memorandums available via MHFD’s website under Resource Library.
- Related to solar fields, recommended imperviousness is included in both imperviousness tables in the updated chapter, which are based on WWE’s technical memorandum (link below).
- For planning purposes (master plans or when site layout is not available), recommended values for solar fields are conservatively provided for grass cover (~45%) and gravel cover (~60%).
- For site-level designs (when the layout is known), recommended values are further expanded to consider other design elements of the solar field, specifically (a) panel width, (b) panel spacing, and (c) panel orientation relative to the ground contours (parallel, diagonal, or perpendicular). The values in the current draft version of the chapter use imperviousness ranges for solar fields with grass cover (10%-45%) and gravel cover (50-75%), and refer back to WWE’s memorandum for more information about applying at a given solar field site.
- Refer to the memorandum for additional information on solar fields, specifically Table 8 & Table 9 (for grass cover with diagonal and perpendicular orientations) and Table 10 & Table 11 (for gravel cover with diagonal and perpendicular orientations).
- Link to WWE memo (re: solar fields) à https://mhfd.org/wp-content/uploads/2024/01/TECHNICAL-MEMORANDUM-DETERMINATION-OF-SOLAR-PANEL-FIELD-RUNOFF-COEFFICIENTS-2023.pdf
Brik Zivkovich, MHFD
Boulder has taken the utilities exclusion/land that would remain undeveloped in projects that were solely installation of solar panels. I think this is a bit of a gray area and requires an interpretation. In the cases in Boulder that were solely solar panel projects, they were open fields with no connecting storm infrastructure, and our municipal code did not otherwise require a drainage report or detention.
I think the MHFD guidance is really valuable but I think it still requires making the call on requiring a drainage evaluation and treatment up front and deciding on an exclusion. I don’t know if the Boulder interpretation was 100% correct but that is the way we went. If you do require treatment I would think the easiest method would be to calculate runoff reduction, it seems likely you would get 60% WQCV infiltrated in the field without needing other infrastructure for a solar panel alone project.
Kevin Koryto, City of Boulder
Thank you to everyone who takes the time to help one another! Each of our committees can be reached via the email addresses listed on our meeting agendas/minutes. Don’t hesitate to reach out and ask the appropriate committee when you have great questions like these.
