Lessons learned: Solar projects present unique stormwater management challenges

By Jason Sharp, Adam O’Connor and Mark Priddle

With the passing of the Green Energy Act in Ontario in 2009, the design and construction of utility-scale solar projects in Ontario blossomed. Between 2010 and the end of 2016, it is estimated that more than 100 ten (10) Mega Watt (MW) solar farms were constructed in Ontario. Approvals for these sites were issued by the Ontario Ministry of the Environment and Climate Change (MOECC) under Environmental Compliance Approvals (ECA) or more commonly Renewable Energy Approvals (REA).

A typical 10 MW photovoltaic (PV) installation requires about 40 ha of land for solar panels. Sites selected for such solar projects in Ontario range from flat former agricultural fields with clay soils to rolling diamicton hills and areas of very shallow bedrock. Former land uses (prior to solar development) range from airport properties to scrub forests. Locations for such projects are found all across Ontario but all essentially with a humid continental climatic zone with, on average, about 900 mm of precipitation per year and a water surplus of approximately 200 mm per year.

During the course of construction of a number of these solar projects, unique challenges associated with stormwater management (SWM) arose. Issues relating to turbid runoff occurred, with subsequent impacts to nearby watercourses, neighbouring properties and other downstream locations.

This discussion paper presents a review of the issues with stormwater management, impacts that were caused and remedial measures. The focus is on the experience gained at solar projects located in southeastern Ontario in rolling terrain with adjacent farmlands and watercourses. Overall, the objective is to provide a series of SWM “lessons learned” in the context of utility-scale solar farm, design, construction, operation and maintenance in Ontario.

Site Selection

The selection of a site for solar development is typically based on a number of factors. These include:

• Land availability;
• Land cost;
• Topography;
• Existing site conditions (vegetated field (grass/hay), farmed (row crop), forested, etc.)
• Constraints (water courses, Provincially Significant Wetlands (PSW), incompatible uses, prime agricultural zoning);
• Community acceptance; and
• Proximity to power grid for connection.

In hindsight, it has become apparent that the selection of sites must place great significance on topography, existing site conditions and constraints such as nearby watercourses and soil types. All of these factors readily influence the volume and flow rate of runoff that, if not properly managed, can result in negative impacts to downstream and neighbouring properties.

In general, undeveloped sites either did not possess known existing stormwater concerns or were in locations where seasonal flooding occurred. In either case, fields would remain untouched until they were stable enough to be worked. However, in a situation where a solar farm is constructed on an existing row crop, the land is drastically transformed from a site that would see minimal usage/disturbance until planting to a site that requires complete year-round accessibility by machines and workers during construction and early stages of operation.

Regulatory involvement, review processes and contractual issues

Under the REA process, public consultation and regulatory review are to be undertaken. Typically, comments are received from the public, all municipal levels of government and Conservation Authorities (if they exist for the area). While the MOECC is the overall approval authority, the process relies on the proponent and their experts to design the project such that potential impacts to the natural environment are mitigated both during construction and when built-out and in commercial operation.

As it relates to the stormwater management component of the review process, these utility scale projects are approved based on a conceptual stormwater management report. These reports typically provided high-level information such as:

• Delineation of the site watersheds;
• Identification of internal drainage areas;
• Completion of high level calculations (typical reports rely on the rational method for calculating peak flows);
• Confirmation that stormwater management is or is not required for the site; and
• Estimation of the scale and locations to accommodate the required stormwater storage.

In some cases, preliminary grading plans are not provided, which may pose the question of whether the proposed drainage areas are constructible. While this information is very useful to identify potential concerns with regard to proximity to watercourses and requirements for stormwater management at a high level, once approved, the vast majority of projects enter into Design Build (DB) contracts with an Engineering, Procurement and Construction (EPC) Contractor. The EPC Contractor is then responsible for taking the preliminary information and bringing the project to completion.

EPC contracts must place appropriate emphasis on completing grading, stormwater management and erosion and sediment controls prior to installing panels on racking, cabling installation, making transformers operational, etc. Otherwise, civil work after the fact, which could include grading around piles and panels, would need to be completed. This can be especially challenging as some solar projects have more than 10,000 piles and more than 40,000 live panels.

Design (Stormwater Management)

The design of large (10 MW or more) solar projects in Ontario has experienced a learning curve with respect to minimizing stormwater management issues. A 40 ha solar farm represents a hard surface with concentrated flow developing during a precipitation event. This being said, the hard surface may be discontinuous, with solar panels arranged in rows, but with vegetated surfaces (post-construction) in the dripline of each row. This differs from the design of a typical hard surface such as a roof or parking lot. The design of stormwater management and grading for a solar project is markedly different from how such issues are addressed in urban development, municipal road projects or the construction of provincial highways.

The key issues are: 1) the amount of hard surface and 2) subsequent sediment loading. Non-solar projects generate sediment loading because of sanding and salting operations in winter months. Solar farms do not, in general, sand and salt their roadways. Therefore, a typical solar project is a 40 ha grass field with some gravel roadways (typically 5% of the site by area), ten (10) relatively small transformers and one (1) relatively small substation. In many instances, calculations post- to pre- may indicate that stormwater management is not required. However, based on our experience, stormwater management is generally required, specifically during construction and until the site is fully re-vegetated.

The following is a list of issues that have been identified that, based on our experience, affect the overall volumes and rates of runoff leaving the sites. There is no single guideline developed, to our knowledge, that addresses the calculations and design considerations relating to the issues experienced below:

• Compaction of soils – soils experience significant compaction based on the volume and type of construction activity (drill rigs installing thousands of piles, graders/dozers working the fields, excavators, boom trucks installing racking, numerous trucks, ATVs and other vehicles, etc.); the resulting increase in compaction of soil may cause an increase in runoff and sediment transport until the site is fully re-vegetated.
• Topsoil – The removal of topsoil from a site may result in the loss of vital organic matter required for plant growth. This may result in much less vegetation and/or increased time to re-vegetate the site. On sites where topsoil is not replaced, or is contaminated with subsoil, the lag in full vegetation establishment could extend for a few years. During this time, the bare or partially bare soils may experience erosion and washouts. This may result in the need to re-start the vegetation process: fix the erosion, add topsoil and vegetation (seeding) and/or apply erosion and sediment control measures such as erosion control blankets.
• Soils / depth to bedrock – Often, geotechnical information is provided at the onset of a project, but further studies or investigations may not be conducted. The vast majority of sites are constructed based on soils information from ten test pits or boreholes over a 40 ha site. This may provide only a high level understanding of site soils that may be considered relatively limited information when completing grading, preparing a rock profile for the site or balancing the site based on the cut/fill required. Further complicating designs may be pockets of differing soil types found over a site of this size. The ten test locations may not identify these pockets and modifications in the field may be required.
• Construction methods – Contractors must be careful not to “open up” (remove vegetation and topsoil) an entire site all at once. When severe weather occurs, such a site may experience significant erosion issues and, in some cases, may not possess sufficient erosion and sediment controls to combat the increase in flow from a bare soil surface. The phasing of construction is of great significance with projects of this magnitude and must be addressed during the design stage and implemented during construction.
• Concentrating flow (roadways) – The requirement to access transformers and inverter houses may result in the need to develop an on-site road network. A road network is typically laid out on a plan and each transformer is apportioned a “block” of arrays which make up one-tenth of the area of the project. The road network may not account for the topography, sometimes resulting in roads being located in the least desirable areas, specifically, around the perimeter of the site. This may result in the need to direct runoff via culverts or other means across the roadway and into a ditch or adjacent field with limited opportunity to spread the flow. Sites that have roads that are located on +10% slopes have an increased probability of erosion during major rain/runoff events.
• Concentrated flow (long reaches) – As a function of the work environment and grading activities, relatively long distances (or reaches) of solar developments may be smoothed out to permit the piles/panels to be installed and to promote effective transportation networks. The challenge with this is that the combination of long reaches and the smooth surfaces may result in an increased runoff velocity. Under pre-development conditions, the areas may have had generally similar characteristics, however, without the grading activities, small pockets, depressions, etc. may have existed that would capture runoff, reduce flow velocities, provide opportunity for infiltration and/or ensure that not all runoff left the site. Once smoothed out, runoff may not have had these same opportunities, resulting in more flow running off, collecting and then eroding the soils. Generally speaking, runoff is considered overland sheet flow for up to 30 m (100’), at which point it tends to form shallow concentrated flow. This shallow concentrated flow could extend for several hundred metres and could give rise to issues. It is at this point where runoff could form rills and gullies leading to erosion concerns and sediment transport.

Temporary and permanent measures

As part of the stormwater management design, temporary erosion and sediment control (ESC) measures are required during construction of solar projects, as with any other construction project in Ontario. The design of such measures requires an understanding of construction activities and construction flow. Again, a solar farm differs from other development projects, such as a building, because of the continuous and long-term disturbance of typically un-vegetated ground during construction. This requires unique and more robust ESC measures compared to projects that are more conventional.

These temporary measures become the lifeline of the project. It is clear that any temporary controls should have a site-specific design. That is to say, the designer should be reviewing the flows, volumes and drainage area upstream to ensure the controls are sufficient and will be able to withstand the runoff flow and quality anticipated for the project. A few key examples include:

• Reviewing the sheet flow velocity over bare soils to ensure the need for erosion control matting is or is not required;
• Reviewing flow through rock flow check dams to ensure that runoff overtop is able to be conveyed in a major event (and not washout an adjacent roadway); and
• Reviewing flow through flow spreaders to ensure they are sufficiently wide to distribute the flow.

Permanent SWM and ESC measures may be required to control water and possible sediment transport after full buildout of a solar project. These measures differ from temporary measures as they are designed for the site that has all its panels in place and has been fully re-vegetated. At the design stage, the final SWM and ESC measures that are proposed have generally assumed that the site is fully vegetated. In our experience, the design flows during construction should be increased from the typical post-development conditions to account for the bare or partially bare soils and lack of vegetation within ditches that may increase water velocities.

Construction

Challenges pertaining to SWM and ESC during construction have been encountered at numerous solar projects in Ontario. These challenges may appear early on in the development of sites that are stripped of vegetation (and topsoil) prior to solar panel installation and other site works. Heavy vehicular traffic during construction may exacerbate runoff issues. With the widespread disturbance over the entire area of 40 ha or more, ESC measures may be inadequate. In addition, the desire to complete projects on time may mean that year-round construction takes place, even during the winter and spring thaws.

As noted above, working throughout the winter and spring freshet is possible, and in some instances necessary, to meet tight deadlines. Designs must account for flows over frozen soils or an increase in runoff coefficient during winter/spring thaws. Consequently, standard ESC measures may be inadequate for winter runoff events. Given the scale of the typical solar project, this is an important area of design and enhanced ESC measures such as shot rock roads over existing vegetation and/or sediment traps should be considered.

Seasonal limitations must also be considered. For example, during winter, issues are exacerbated when:

• One cannot install silt fence in the winter into frozen ground (not very easily at least);
• Straw bales are typically not available;
• Straw wattles and mulch silt socks are also not easily installed or readily available.

Understanding that construction during winter months is very difficult to predict and provides a number of challenges, contractors should install specific controls, limit the areas that are opened up, and ensure additional materials (e.g. straw/mulch, erosion control blanket/matting and stone) are available on site to use, if necessary. Although these measures cannot guarantee there will be no issues with erosion and runoff during construction, they may provide a contractor with the necessary means to maintain and stabilize the exposed soils and limit the transport of suspended solids, if such a condition occurred.

Post-Construction

Re-vegetation does not occur immediately following solar panel installation at most sites. In some cases, topsoil is removed and vegetative regrowth may be hindered. In other cases, works are completed late in the year and re-vegetation efforts may not be successful. These conditions may hinder or prevent re-vegetation in, and around, the solar panels. The design conditions for the SWM and ESC measures therefore may not be met. This may lead to challenges with post-construction runoff, even with properly designed and constructed ESC measures in place. The positive impact of a fully vegetated site with properly engineered and constructed SWM measures in controlling runoff and suspended solids movement cannot be overstated. Therefore stripping of vegetation should be avoided wherever possible, and where vegetation must be stripped, the installation of appropriately sized retention/settlement ponds prior to stripping must be considered.

Monitoring

At many sites in Ontario, monitoring of runoff is required under the REA. Typically, total suspended solids (TSS) are required to be tested at locations where water flows off-site. Depending on the discharge location and receiving body, different requirements may apply. In some cases, a fixed limit of 25 mg/L for TSS for water leaving the site (regardless of receiver) may apply. In other cases, TSS in runoff may be limited to a factor increase in turbidity or TSS concentration relative to upstream levels of these parameters.

It should be noted that TSS is not the only compound/contaminant that should be monitored. Depending on the former use of the site, other nutrients (phosphorus, nitrogen etc.) may be present and require testing. Site-specific monitoring criteria should be established.

In all cases where TSS monitoring is conducted, it should be accompanied by a Contingency Plan that provides the Operations and Maintenance staff with a procedure in order to address TSS concerns, should they arise.

Remediation

Following completion of a solar farm, remediation may be required to prevent turbid runoff from leaving the site. This may require new SWM and ESC measures, combined with concerted efforts to revegetate sites. In some cases, new or additional retention ponds may need to be installed and active pumping and water control may be required. Other remediation efforts may include:

• Construction of additional ditching and grading to provide positive drainage from low lying areas;
• Construction of additional piping (culverts, storm sewer) to direct runoff across roads or into storm sewer networks;
• Paving of low level crossings to prevent erosion / granular washouts;
• Addition of topsoil and seeding;
• Placement of erosion control blankets on steep slopes to prevent erosion;
• Application of hydroseed with tackifier to prevent erosion on steep slopes;
• Construction of flow dissipation or flow spreading devices;
• Construction of dry and wet retention ponds;
• Construction of infiltration trenches;
• Placement of fill in low-lying areas to promote positive drainage;
• Construction of roads and roadside ditches to provide safe passage and convey runoff;
• Reconstruction of roadside ditches to ensure subgrade is drained;
• Relocation of arrays of panels;
• Re-alignment of ditches and movement of discharge points from the site;
• Reconnecting existing tile drains destroyed during construction;
• Placing berms to limit adjacent floodwater from entering the site;
• Removing berms to limit concentration of runoff within a site; and
• Use of flocculants to control TSS in runoff water.

Lessons Learned

The identification and correction of issues related to SWM and ESC at solar projects provides a number of “lessons learned” which can be applied to new and existing projects to prevent issues in the future. The following recommendations are presented.

Site selection:

• Avoid sites with steep topography and fine-grained soils;
• Exercise caution with sites adjacent to sensitive surface water receptors (e.g. cold water streams);
• Avoid sites that may be within a floodplain (limits the ability to grade, may result in seasonal flooding);
• Avoid sites with shallow bedrock (if grading is anticipated);
• Avoid ecologically sensitive lands (rare vegetation or species at risk);
• Conduct advanced planning on the front end (advanced biological surveys); and
• Conduct pre-condition surveys – these are essential to ensure the concerns of neighbouring landowners and municipalities may be adequately addressed.

Design:

• Ensure that the design team (electrical design, civil, structural, etc.) communicate to ensure that the design meets all needs, including prevention of erosion and sediment transport;
• Effectively plan the location of SWM measures (e.g. a pond must be located downstream of the development; a pond on top of a hill is not much use);
• Ensure that the design is conservative with respect to runoff potential;
• Provide a reasonable degree of redundancy in designing SWM and ESC measures;
• Ensure that stormwater management design criteria from the MOECC are followed;
• Execute reasonable engineering judgment with respect to the solar farm design; it is not an urban development, municipal road or provincial highway. It should not be designed as such, but it does require a level of sophistication with respect to the collecting and conveying of runoff;
• Review the stormwater design in the context of different storm events (4-hour Chicago storm, 12- and 24-hour SCS storm events); in a number of situations, given the rural nature of most sites, the peak flows may be as a result of the shorter duration events but the SWM measures (ponds) will need to be sized for the volume of the longer events (24-hour);
• Ensure the design takes construction over winter months into account;
• Ensure the design uses “bare soils” calculations to account for runoff during construction;
• Ensure sufficient geotechnical data are available, including depth to groundwater and percolation rates (if designing infiltration trenches);
• Review the designs with operations and maintenance staff to ensure the farms are accessible and operational;
• Provide guidelines to the contractor regarding staging of works to be completed on site;
• Design robust temporary ESC measures that include quantity management and which require continued use and maintenance after commercial operation of the site if it is not fully re-vegetated;
• Emphasize that permanent SWM and ESC measures are based on a fully vegetated site; and
• Ensure the design takes into account the laydown areas and specifically the fuel tanks and refuelling stations to ensure they are in a location which will is not prone to flooding or ponding runoff.

Construction:

• Develop a spill containment and response plan prior to the start of construction;
• Ensure placement of all temporary SWM and ESC measures prior to any construction; ensure regular maintenance of these measures during construction and through to full re-vegetation of the site;
• Ensure regular site inspections (especially during, or immediately after, storm or rapid thaw events) are completed by a civil engineering consultant to review construction, SWM and ESC measures;
• Ensure additional materials for ESC are on site, especially over winter months;
• Ensure contractor implements staged construction process;
• Minimize the removal of vegetation (and topsoil) prior to construction, especially over winter months;
• Provide sufficient detail on the plans to permit a contractor to construct the farm;
• Minimize construction truck traffic, especially over bare soils; and
• Do not construct during inclement weather or during spring thaw (if possible).

Monitoring:

• The vegetation, stormwater management features and outlets of the sites should be monitored throughout the life of the project;
• If TSS/turbidity monitoring are conducted, ensure that a Contingency Plan is prepared and is implemented, if exceedances of limits are observed;
• At a minimum, bi-annual inspections should be performed and the frequency should be increased if issues arise; and

Owner and Operation and Maintenance staff should contact appropriate consultants in the event that issues arise so that a suitable solution may be developed.

Jason Sharp, P.Eng., Adam O’Connor, P.Eng., and Mark Priddle, P.Geo., are with of McIntosh Perry Consulting Engineers Ltd. For more information, visit: www.mcintoshperry.com.

This article appears in ES&E Magazine’s December 2017 issue.