By Jennifer Meneghelli, senior civil engineer at SRK Consulting.
RENEWABLE energy projects in South Africa are being developed at a time of equally urgent focus on the country’s water resources; this is therefore a good time for developers to apply low impact development (LID) options in their stormwater management plans.
Solar farms that generate energy from arrays of photovoltaic panels present a good example of where the LID approach can add value, according to Jennifer Meneghelli, senior civil engineer at SRK Consulting. Often occupying many hectares of rural land, solar panels create a large, impervious surface area that channels stormwater in a new way. Rainwater runs off these panels in sheets, falling from a height that can create scouring and erosion of soil – leading to contamination of water.
Soft engineering
“One common traditional response to this water flow is hard engineering, using concrete channels to direct water away from the panels to a drain or watercourse,” said Meneghelli. “However, this is more applicable to an industrial site, where the aim is to keep clean water separate from dirty water; it is not an optimal strategy for solar installations.”
Rather than concentrating and collecting the flow of water running off the panels, the objective should be to dissipate and distribute the flow – in similar ways to before the panels were installed. A more sustainable methodology, based on LID, therefore looks to maintain the natural infiltration of stormwater into the soil as far as possible.
Drip-line
PV plants are generally located in arid areas with high numbers of sun days per year, in order to achieve maximum performance. Therefore, rainfall seldom occures, but must be managed as storm events in these areas tend to be brief and intense.
“Solar arrays are normally some distance above the ground, so the ‘drip-line’ where water cascades off the panels is often where most damage or gouging can be caused,” she said. “The lower the dripline can be to the ground, the better to reduce the erosive force; gravel can then be used to dissipate the force of the flow and prevent erosion.”
Scarifying of the ground under the panels, and planting deep-rooted indigenous vegetation underneath the panels, can help maintain the health of the local ecosystem and enhance biodiversity. Rather than just grass, a variety of vegetation can create a meadow-type environment where natural pollination is encouraged.
Construction impacts
“As local practitioners in this country, we have developed sustainable drainage systems for the urban environment – and this approach could be valuably extended to the renewable energy space,” she argued. “The need for LIP guidelines is becoming more pressing as there are likely to be many more of these renewable projects in South Africa in the near future.”
Wind farms
While wind farms create a relatively limited impervious surface – amounting to only about 2% of the project site – there are ways to optimise stormwater management, she said. Again, most of the impact is experienced during the construction stage, when there is considerable soil disturbance for elements such as deep foundations and numerous hardstands.
“It is advisable to create erosion protection measures around the large concrete base of the wind turbine towers. Burying the tower foundation below ground level will also allow a layer of topsoil to be placed over the concrete, and then revegetated to ensure better infiltration of stormwater” she advised.
