Solar Energy and Stormwater: Erosion, Sediment and Their Controls

As solar power generation expands, so does the potential for stormwater runoff. The rapid growth of solar energy has transformed former farm fields, landfills and industrial sites into utility-scale solar facilities that now power surrounding communities. Throughout this solar boom, TRC has been at the forefront of monitoring solar farms during construction, helping address erosion and sediment control challenges to protect surrounding land and waterways.

When designing stormwater management for solar projects, it’s common to focus only on the long-term design elements. Traditional industrial sites often rely on catch basins, oil water separators and flow meters. But solar farms are different. Typically built in agricultural or meadowed fields and supported by racking posts driven into the ground, solar sites rely on post-construction stormwater management practices that are generally limited to detention basins and swales. In our experience, the real potential for stormwater to wreak havoc occurs during the construction phase before vegetation is established.

Overland Flow: How Bigger is Not Always Better

Surface runoff comes in two main forms; sheet flow and concentrated flow. Sheet flow is a wide, shallow « wave” of runoff that flows across a wide area. Concentrated flow is just the opposite; a volume of water collected into one or more localized channels. As a rule of thumb sheet flow can only be sustained for 100 feet before it accumulates into concentrated flow.

Think of it like the relationship between force and area. If you step on a single nail, you are certainly in for a safety incident report. Yet, if you lay across a bed of nails, your downward force spreads across multiple points and injury can be avoided. The same applies for stormwater control – shallow, dispersed flow contains less concentrated force.

The Solar Farm Challenge—Disperse!

Solar farms present an interesting challenge for stormwater control designers. They span relatively large areas, at times exceeding one-thousand acres. Large portions of those sites may need to be graded or disturbed during construction. On that scale, even light rain can result in hundreds of thousands of cubic feet of water across the site. How do designers even attempt to control this amount of water? The answer is to disperse it as much as possible!

Key Construction Phase Stormwater Controls

During construction, stormwater practices must be simple, effective and maintainable. Common controls include:

• Silt Fence: A permeable woven mesh fence which allows water to trickle through slowly and retain a fraction of suspended sediment; often placed around site perimeters as a final step to direct and mete out stormwater.
• Filter Rolls or “Socks:” Long permeable tubes filled with mulch which retain sediments while stopping sheet flow from forming into concentrated flow.
• Check Dams: Piles of stone which when laid in natural swales or drainage courses help slow down concentrated flow and relieve strain on perimeter silt fence.
• Sediment Basins: Temporary basins excavated only for the construction phase of the project. These help detain sediment-laden water before decanting water off the top towards perimeter controls.

The goals of these controls are to slow down the stormwater. Perimeter silt fences help filter particulates to some degree, but more importantly act as a final way to guide water to check dams or stormwater outlets reinforced with stone and to prevent torrents of runoff from leaving the site directly. Thousands of feet of filter sock may be laid perpendicular to flow paths across these sites to “reset” the sheet flow and keep it dispersed. On a smaller site, these controls are easy to install and effective. On a site that may have dozens of acres stripped of vegetation at any one time and may consist of thousands of acres for that water to concentrate over, it can be a constant fight to keep the water from organizing.

What Can Go Wrong and How to Fix It

When construction phase erosion and sedimentation controls (ESC) fail, it can result in environmental harm, regulatory action and even lawsuits. Sediment entering wetlands and waterbodies can harm the flora and fauna. Breached perimeter controls can damage neighboring properties, eroding off-project land and depositing sediment from the project site elsewhere.

Even the best laid controls are subject to failure. Silt fence can become laden with sediment or undermined, filter socks can be quickly channeled under by stormwater and check dams can wash away during torrential rains. The key is to perform regular maintenance when damages are small, before they compound into serious problems.

Temporary structural controls, however, will never provide optimal erosion control. The most effective ESC is vegetation. It is vital that as soon as is practical, barren earth areas be seeded and monitored to ensure hearty species of plants like clover and fescue can take root.

On solar sites, permanent vegetation offers more than just erosion control. It creates a renewable, community-friendly landscape. Rather than having hundreds of acres of concrete and pavement, solar farms become functional meadows. Pollinator seed mixes can enhance erosion control vegetation into habitats for bees, butterflies and other sensitive pollinator species.