References
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“Conceptual Hydrologic and Hydraulic Report for Earth Mount Solar Farm.” Taney Engineering Report. January 2022. Taney Engineering Report.
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Greene, Todd, Murawski, Nichole, and DaPonte, Ryan. “Solar and Stormwater.” September 9, 2020. Stormwater Magazine. https://www.stormh2o.com/home/article/21148549/solar-and-stormwater
Summary
The findings discussed in this paper demonstrate that Earth Mount Solar systems perform exceptionally well during extreme rain events. When comparing Earth Mount Solar sites to natural, undeveloped sites, there is very little difference in the measured flow depths and velocities. In other words, the Earth Mount Solar system, which sits flush on the ground with a 90% ground coverage ratio and an imperviousness rating of 98%, functions hydrologically like natural earth when properly sized retention ponds are installed. This is in stark contrast to fixed tilt and tracker systems, which can scar the earth with rows of channelized erosion and create unnatural runoff behavior.
Given the near-natural hydrological performance of Earth Mount Solar systems, drainage control on sites is simple. Smaller installations may require no drainage control at all, while larger installations, such as the 50MW system modeled in the Taney report, may require only 1.5% grading and a retention basin that has been sized to accommodate a 100-year rain event specific to that location. Additional drainage control measures not discussed in the report, including exact channeling and pathing specifications, are highly dependent on the site’s specific contour map and depend heavily on the conditions and contours of the location where an Earth Mount Solar system will be installed. Additionally, water treatment regulations and path access codes, among other factors, can vary widely across locations. With this in mind, we strongly recommend conducting site-specific studies to determine your exact drainage control needs and ensure your Earth Mount Solar systemperforms as expected.
Figure 6 — Size of retention pond needed at Earth Mount Solar sites per 50MWac package
Minimum Retention Pond Size
With the maximum flow depths and velocities calculated for each site, Taney then identified the size of the retention pond that a 50MWac Earth Mount Solar system installed at each parcel would require in order to successfully manage the hydrologic impact of a 100-year event:
The exception was Parcel 4, which modeled flow depths and velocities for each site assuming a slope of 0.2%. Under this condition — which represents almost perfectly level terrain — a site with Earth Mount Solar installed experiences higher flow velocities (and therefore lower flow depths) than an “existing” site. To prevent this outcome and ensure proper drainage control, we recommend Earth Mount Solar sites to be civil engineered to have a slope gradient of at least 1.5%, although site-specific conditions and the size of the installed system should be closely considered before making this determination.
Figure 5 — Average max flow and velocity differences for Parcels 1, 2, 3, and 5 (FPS = feet per second)
The results in Figure 4 represent plausible worst-case scenarios for each of the modeled locations and enable reasonable assessment of how an Earth Mount Solar project site can be expected to perform during extreme — but rare — hydrologic conditions. It is clear from the modeled data that Earth Mount Solar sites, both with and without drainage improvements, perform nearly identically to “existing” or natural sites during extreme precipitation events. For example, consider Parcels 1, 2, 3, and 5. The average maximum flow depth and velocity differences between the “existing” and “proposed” conditions for these parcels across all five project sites are as follows:
Figure 4 — Depth flows, flow velocities (FPS = feet per second), and rainfall intensities for 100-year events, organized by slope, location, and level of site development
Results
Modeling each of the five project sites with each of the five parcel assumptions led to 25 distinct flow depth and flow velocity datasets, as indicated in Figure 4.
It’s worth noting that flow velocities and depths are affected by the roughness and topography of the surface terrain. To account for this effect, a composite overland flow roughness value (also known as a Manning’s N-value) of 0.055 was assigned to each parcel.
Figure 3 — Size of watershed area and average slope percentage for each parcel
The drainage patterns and flow rates at these sites were simulated under three different conditions: “existing,” which assumed natural land with no Earth Mount Solar installation and no drainage improvements; "developed," which assumed a 50MW Earth Mount Solar installation with no drainage improvements; and “proposed,” which assumed a 50MW Earth Mount Solar installation with drainage improvements.
100-year precipitation events were then modeled for each of the three conditions at each of the five sites. Each event case was assumed to have a duration of six hours and occur contemporaneously over the entire domain. The six-hour precipitation depths for each site were obtained from the NOAA’s Precipitation Frequency Data Server.
These simulations — 100-year rain events on “existing,” “developed,” and “proposed” sites across five locations — were performed for five different slope percentage values at each site. The different slopes are represented as “parcels” in the parlance of the study (Figure 3).
Figure 2 — 100-year rain depths for modeled project sites
Study Methodology
The purpose of the Taney study was to model the hydrologic impacts on a range of project site types where Earth Mount Solar systems could potentially be installed, and to identify how best to protect and maintain each project site type in the face of extreme precipitation events. Five sites in five different states were selected, representing different site typographies and 100-year-event precipitation depths (Figure 2).
With a ground coverage ratio of 90%, an Earth Mount Solar system is nearly impervious, meaning most of the water from a major rain event flows over the land-following PV array, largely eliminating channelized erosion and simplifying drainage control. Runoff in an Earth Mount Solar site, even during a precipitation event of significant magnitude, can be simply and efficiently directed into on-site retention ponds, thus mitigating erosive impact to the land underneath the Earth Mount Solar system.
An independent hydrology study performed by Taney Engineering found the most appropriate runoff curve number — i.e., the imperviousness percentage — for the Earth Mount Solar system to be 98. In other words, the study estimated that only 2% of the water flowing over and against the array made it to the soil underneath the modules, nearly eliminating erosion beneath the array; the rest was runoff [1].
The runoff curve number of 98 is meaningful because it demonstrates that the land underneath an Earth Mount Solar array is largely protected from infiltration, even during 100-year storm events, assuming proper site selection, flow routing, and runoff mitigation — all of which are standard in the development of an Earth Mount Solar installation.
As noted on page 12 of the Taney report, this results in 100-year peak runoff flows at developed Earth Mount Solar sites that are nearly identical to the 100-year peak runoff flows at natural, undeveloped sites. Whereas a mounted tracker installation will produce “a concentrated discharge of stormwater runoff at the solar panel drip line, which can act like un-guttered roofs that channelize and accelerate water flow,” [2] the water flow on an Earth Mount Solar site is more uniformly distributed across the site and highly similar to the water flow seen on undeveloped land.
Figure 1 — Picture of Earth Mount Solar installation near Chowchilla, California showing minimal gaps between modules
Earth Mount Solar™ Imperviousness Rating
Earth Mount Solar arrays are installed directly on bare earth that has been compacted to 90% or more. Because the high-density, dual-glass modules in an Earth Mount Solar system are placed against one another with a minimal average gap of less than half an inch, as shown in Figure 1, the system achieves a ground coverage ratio of approximately 90%. A concrete perimeter encloses each 0.5MW island, four of which make up a standard 2MW block. This combination of near-flush ground coverage, dual-glass construction, and soil compaction reduces hydrological impact to the site, even during extreme rain events.
Abstract
A hydrologic study of the Earth Mount Solar™ system was conducted by Taney Engineering, an independent, professional civil engineering and
land surveying firm. From this study, the imperviousness rating of Earth Mount Solar systems was determined, as were the expected flow depths
and velocities encountered by these systems during 100-year rain events.
A variety of conditions and assumptions were modeled — including soil type, slope, precipitation amount, and whether or not drainage control was employed on the site — with the results comparing favorably, and in fact almost identically, to those achieved by natural, undeveloped land when recommended site-specific drainage control improvements are implemented.