turbid runoff from a solar project during construction ...€¦ · for solar panels. during the...

Lessons learned: Solar projects present unique stormwater management challengesBy Jason Sharp, Adam O’Connor and Mark Priddle

The design and construction of util-ity-scale solar projects in Ontario blossomed with the Green Energy Act passing in 2009. Between 2010 and

2016, more than 100 ten-megawatt (MW) solar farms were constructed in Ontario. Approvals for these sites were issued by the Ontario Ministry of the Environment and Climate Change (MOECC), usually under Renewable Energy Approvals (REA).

A typical 10 MW photovoltaic (PV) installation requires about 40 ha of land for solar panels.

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

SITE SELECTIONSelection of a site for solar develop-

ment is typically based on a number of factors, including land availability and cost, topography, existing site conditions, constraints (water courses and wetlands, agricultural zoning), community accep-tance, and proximity to power grid for connection.

In hindsight, it has become apparent that the selection of sites must place more significance on topography, exist-ing site conditions, and constraints such as nearby watercourses and soil types. All of these factors influence runoff that, if not properly managed, can result in negative impacts downstream.

When a solar farm is constructed on farmland, the land is drastically trans-formed from a site that would see mini-mal 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 consul-tation and regulatory review result in comments from the public, all municipal levels of government and Conservation Authorities. 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 operation.

As it relates to the SWM component

Turbid runoff from a solar project during construction.

continued overleaf…

Environmental Science & Engineering Magazine8 | December 2017

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of the review process, these projects are approved based on a conceptual SWM report. These reports typically provide high-level information such as delinea-tion of watersheds, identification of inter-nal drainage areas, completion of calcu-lations (e.g., rational method for peak flows), confirmation that SWM is, or is not, required for the site, and estimation of the scale/locations to store stormwater.

In some cases, preliminary grading plans are not provided, which may pose the question of whether the proposed drainage areas can be constructed. An engineering, procurement and construc-tion (EPC) contractor is usually retained. EPC contracts must place appropriate emphasis on completing grading, SWM and erosion and sediment controls (ESC) prior to installing panels and cabling, etc. Otherwise, civil work (including grad-ing around piles and panels) would need to be completed. This can be especially challenging as solar projects may have 10,000 piles with live panels.

DESIGN (STORMWATER MANAGEMENT)The design of large solar projects

in Ontario has experienced a learning curve with respect to minimizing SWM issues. A solar farm represents a hard surface with concentrated flow devel-oping during a precipitation event. The hard surface is discontinuous, with solar panels arranged in rows, but with vege-tated surfaces (post-construction) in the driplines. The design of SWM and grad-ing for a solar project is markedly differ-ent from other types of development.

Key issues are the amount of hard surface and subsequent sediment load-ing. A typical solar site is a grass field with panels, some gravel roadways, a few transformers and a small substation. Calculations may indicate that SWM is not required. However, based on our experience, SWM is generally required, specifically during construction and until the site is fully re-vegetated.

The following factors can affect the volumes and rates of runoff leaving the sites:

Compaction of soils – Soils experience compaction from numerous construction vehicles. This may increase runoff and

Erosion beneath solar panels as a result of concentrated flow and limited vegetation.



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sediment transport until the site is fully re-vegetated.

Topsoil – Removal of topsoil from a site results in less vegetation and/or increased time to re-vegetate the site. Where topsoil is not replaced, full re-veg-etation is hindered and bare soils may experience erosion and washouts.

Soils/depth to bedrock – Often geotechnical information is provided at the start of a project and further studies or investigations may not be conducted. The majority of sites are constructed based on limited soils information.

Construction methods – Vegetation and topsoil should not be removed from an entire site all at once. A solar site may experience significant erosion issues as it does not possess sufficient ESC measures to combat the increase in flow from bare soil. Construction phasing of solar proj-ects must be addressed and implemented.

Concentrated flow (roadways) – Roads to access on-site buildings are required and these may not account for topography, resulting in construction in undesirable areas. Direct runoff across roadways and into ditches or adjacent fields with limited opportunity to spread flow may result.

Concentrated flow (long reaches) – Relatively long distances (or reaches) may be smoothed out to permit piles/panels to be installed and build roads. This combination of long reaches and smooth surfaces may result in increased runoff velocity that erodes soil.

There is no single guideline developed, to our knowledge, that addresses the calculations and design considerations relating to these issues.

TEMPORARY AND PERMANENT MEASURES

As part of the SWM design, tempo-rary ESC measures are required during construction. A solar farm differs from other development projects 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 more conventional projects. The designer should be reviewing flows, volumes and drainage area upstream to ensure controls are sufficient and will be able to withstand

Wetland formation beneath solar panels.

the anticipated runoff flow and quality.Permanent SWM and ESC measures

may be required to control water and sediment transport after full build-out of a solar project. These differ from tempo-rary 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 expe-rience, design flows during construction should be increased from the typical post-development conditions to account for bare or partially bare soils that may increase water velocities.

CONSTRUCTIONSWM and ESC challenges at solar proj-

ects appear early on in the development of sites that are stripped of vegetation (and topsoil) prior to site works. Heavy vehic-ular traffic during construction may exac-erbate runoff issues. With the widespread disturbance over the entire area, ESC measures may be inadequate. In addition, year-round construction may take place.

Working throughout the winter and spring seasons is possible, and in some instances necessary, to meet tight dead-lines. 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 and the design should consider enhanced ESC measures. Seasonal limita-tions to implementation of ESC measures must also be considered (e.g., silt fence cannot be installed in frozen ground).

Understanding that construction during winter months provides a number of chal-lenges, contractors should install specific controls, limit the areas that are opened up, and ensure additional ESC materials are available for use. Although these measures cannot guarantee there will be no issues with erosion and runoff during construc-tion, they may provide a contractor with the necessary means to maintain and stabi-lize the exposed soils and limit suspended solids transport.

POST-CONSTRUCTIONRe-vegetation does not occur imme-

diately following solar panel installation at most sites, especially where topsoil has been removed. In other cases, works are completed late in the year. These condi-tions may hinder or prevent re-vegetation and design conditions for the SWM and ESC measures may not be met. This may lead to challenges with post-construction runoff, even with properly designed and


STORMWATER

constructed ESC measures in place.The positive impact of a fully vege-

tated site with properly engineered and constructed SWM measures to control runoff and suspended solids movement cannot be overstated. Stripping of vegeta-tion should be avoided wherever possible, and, where vegetation must be stripped, the installation of appropriately sized retention/settlement ponds prior to strip-ping must be considered.

MONITORINGAt many sites in Ontario, monitoring of

runoff is required under the REA. Typi-cally, total suspended solids (TSS) are required to be tested at locations where water flows off-site. It should be noted that TSS is not the only compound/contami-nant that should be monitored. Depend-ing on the former use of the site, other nutrients (phosphorus, nitrogen, etc.) may be present and require testing.

In all cases where TSS monitoring is conducted, it should be accompanied by

a contingency plan that provides O&M staff with a procedure to address any TSS concerns. Vegetation, SWM features and outlets should be monitored for the life of the project. Bi-annual inspections should be performed and frequency increased if issues arise.

REMEDIATIONFollowing 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 re-veg-etate sites. Other remediation efforts may include:• Construction of additional ditching and grading, and of additional piping;• Paving of low level crossings;• Addition of topsoil and seeding; erosion control blankets and/or hydroseed on steep slopes;• Construction of flow dissipation/spreading devices, retention ponds and infiltration trenches;

• Placement of fill in low-lying areas;• Construction of roads, ditches;• Relocation of panels;• Re-alignment of ditches and movement of discharge points;• Reconnecting damaged/destroyed tile drains;• Placing berms to limit floodwater from entering site or removing them to limit concentration of runoff; and• Use of flocculants to control TSS in runoff water.

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.

DESIGN CONSIDERATIONS• Ensure design includes prevention of erosion and sediment transport.• Effectively plan the location of SWM measures.


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• Ensure design is conservative with respect to runoff potential and SWM design criteria from MOECC are followed.• Provide a reasonable degree of redun-dancy in designing SWM and ESC measures.• Review the stormwater design in the context of different storm events; peak flows from shorter duration events but SWM measures sized for the volume of longer events (24-hour).• Ensure design considers construc-tion over winter months and uses “bare soils” calculations to account for runoff during construction.• Ensure sufficient geotechnical data are available, including depth to ground-water and percolation rates.• Review designs with O&M staff to ensure the farms are accessible and oper-ational.• Provide guidelines to contractor regard-ing staging of works to be completed.• Design temporary ESC measures that include quantity management and provide maintenance plans after opera-tion if it is not fully re-vegetated.• Ensure design provides laydown zones (with fuel tanks/refuelling stations) in areas not prone to flooding.

PROJECT CONSTRUCTION• Develop a spill containment and response plan prior to start of construction.• Ensure placement of all temporary

It has become apparent that the selection of sites must place more significance on topography, existing site conditions, and constraints such as nearby watercourses and soil types.

SWM and ESC measures prior to construction; regularly maintain these measures during construction and until full re-vegetation.• Ensure regular site inspections (during, or immediately after, storm or rapid thaw events) are completed by a civil engineer-ing 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 removal of vegetation (and topsoil) prior to construction.• Provide sufficient detail on plans to permit a contractor to construct the farm.• Minimize construction truck traffic, especially over bare soils.

• Do not construct during inclement weather or during spring thaw (if possi-ble).

This article has been condensed. To read the full-length article, visit: www.esemag.com/stormwater-lessons

Jason Sharp, P.Eng., Adam O’Connor, P.Eng., and Mark Priddle, P.Geo., are with McIntosh Perry Consulting Engineers Ltd. For more information, email: [email protected], [email protected], or [email protected]


STORMWATER

Environmental Science & Engineering Magazine26 | May 2013

Sustainable Ecosystems

Green infrastructure and sus-tainability goals are of in-creasing importance, andachieving them requires tech-

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The West Don Lands, in Toronto, On-tario, is a community that is people fo-cused, family friendly, environmentallysustainable and beautifully designed forliving. It has a Stage 1 LEED ND GOLDcertification under the pilot program es-tablished by the U.S. Green BuildingCouncil.

One notable sustainable component,utilized in the design of the area’s streets,is a soil retaining system called SilvaCells™. Typical urban trees in the citycore die after approximately seven years.However, Silva Cells help extend theirlife spans, thus promoting the growth ofmature street trees.

Although the City of Toronto had pre-viously used Silva Cells as part of astormwater management pilot program inThe Queensway, their use as part of site

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May2013_ES&E_5_2010 13-05-29 9:41 PM Page 26

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