With El Nino coming, water management is on everyone’s mind in drought-stricken California. Experts highlight the benefits of green rooftops as one of the best ways to manage storm water runoff, and avoid the type of mudslides that recently hit after California’s first rains of the season.
by Kiona Smith-Strickland, Gizmodo, 10/18/2015, 8:40PM
Water management experts say decentralized techniques like rainwater collection tanks, green rooftops, and even absorbent pavement could be the best way to manage water from storms and prevent the kind of runoff that caused flash floods and mudslides in southern California last week.
The mudslides trapped 200 cars on southern California freeways on Thursday night, and as Gizmodo’s Alissa Walker wrote, “The mudslide is a reminder that it’s not just heavy rain we need to worry about—heavy rain falling on the state’s parched ground will bring disaster.”
And heavy rains are coming to drought-stricken California this winter. It’s true that the precipitation would bring at least some relief from the drought, but – as we saw last week – it could also cause flooding and mudslides in areas ill-prepared to handle water runoff from storms. That leaves California in a difficult position, unable to spare any rainwater but desperately needing to control it.
Coincidentally, a group of water management experts – engineers and urban planners from several California universities and Australia, along with water managers from Orange County Public Works and an engineering consulting firm – just called for better storm runoff management in a review article in the journal Environmental Science and Technology.
Based on research in southeastern Australia, which is currently dealing with its own epic drought, the team recommends adopting what are called “low-impact development technologies,” which include things like rainwater collection tanks and green rooftops. LIDs, as they’re called, help urban rainwater runoff more closely mimic the natural drainage of the area, which can help prevent flooding and mudslides.
Mostly, the Problem Is Pavement
The problem is that modern construction has basically overwhelmed the natural water system. Before the early 20th century, most of California’s natural landscape hadn’t been paved over for buildings, roads, and parking lots. That left plenty of grassland, forest, and desert soil to soak up water after a storm or a heavy snow. Rainwater or snow melt soaked into the ground and percolated through soil and rock into the water table and into streams and lakes. Eventually, water made its way to the ocean.
But today, most of the soil that once soaked up the rainwater is paved or built over, so it holds the water rather than absorbing it – causing worsening urban floods that threaten traffic, homes, and businesses.
Meanwhile, all those paved surfaces redirect a heavy flow of water straight into nearby streams, which causes those streams to flood – again threatening homes and human lives, as well as the natural ecosystem. The sudden inrush of water also causes erosion.
And then there’s everything the water is carrying from city streets into streams and oceans: pollutants, diseases, and loads of sediment. “The massive volumes and pollutants associated with stormwater runoff are a deadly one-two punch for streams and lead to a condition known as ‘urban stream syndrome,’” said UC Irvine graduate student in civil and environmental engineering Asal Askarizadeh, the paper’s lead author.
That means contamination with all the chemicals associated with human cities, along with pathogens from our garbage and our sewage, and it can cause imbalances in the natural chemistry of streams, and even increased water temperatures. That’s bad for the plants and wildlife that life in affected streams – and for everything downstream, all the way to the ocean.
Mimicking Natural Drainage
The solution, according to Askarizadeh and his colleagues, is to capture as much rainwater as possible and then re-use it. That sounds like a good idea anyway, given the drought, but it’s also meant to prevent the floods and erosion that come with uncontrolled stormwater.
According to co-author Megan Rippy, a postdoctoral researcher in civil and environmental engineering at UC Irvine, the team tried to provide a basis for city planners to estimate how much rainwater they should allow to soak into the ground, and how much they should capture and use to keep it from escaping into local streams. She explained in a press release, “The ratio of those two volumes depends on local climate and what the landscape looked like in pre-industrial times.”
Capturing the right amount of rainwater will require big changes in how we think about water management and urban infrastructure, say the authors. The water infrastructure of most cities is based on centralized infrastructure, like aqueducts and treatment plants, but lead author Askarizadeh and his colleagues say that’s not the best way to manage water runoff from storms. Instead, they recommend what they call “distributed infrastructure,” which includes things like rainwater tanks and green rooftops, as a complement to (not, they say, a replacement for) those centralized tools.
“The reason is that in order to protect receiving waters and streams, we need to capture the runoff as close to where it’s generated – for example, your home – as possible,” said co-author Brett Sanders, chair of the Department of Civil and Environmental Engineering at UC Irvine.
That would mean some major changes in how local and state governments think about water management – and in what individuals expect from their city’s water infrastructure. As co-author David Feldman, co-chair of the Department of Planning, Policy, and Design at UC Irvine, explained, “We expect the government to manage our water supply completely, and in some places, it’s even illegal to harvest rainwater locally. Laws and habits are going to have to change if we are to adapt to new climate and urban realities.”
The Future of Flood Control?
What might those changes look like? Imagine having a garden, or even an open grassy space, on the roof of your office, or a rainwater collection tank atop your apartment building. Green rooftops would soak up rainfall and gradually release some of it back into the atmosphere through evaporation. In the process, they would provide some extra insulation for buildings, which is nice bonus for energy conservation (and utility bills).
You could use collected rainwater from your roof to do laundry and flush toilets – tasks that use lots of water but don’t need that water to be treated or drinkable. In fact, 2 million people in Australia are already using rainwater from their roofs to flush their toilets.
“Using drinking water to flush toilets is literally washing our future down the drain,” said senior author Stanley Grant, a professor of civil and environmental engineering at UC Irvine. Toilet water doesn’t need to potable anyway, and using rainwater would conserve drinking water and probably reduce most people’s water bills in the bargain.
Of course, that’s going to require not only legalizing rainwater collection in places where it’s currently illegal, but also building such low-impact development technologies into urban water planning.