When it rains and all the raindrops fall on our roofs, have you thought about where the stormwater go to? Well, most of it just goes down the sewer, into the creeks and rivers, and eventually out to the ocean.
Stormwater – a waste?
Stormwater has been treated as something akin to waste in cities, something that is collected and sent out as soon as possible. A complete infrastructure is in place to get this done: gutters and downspouts to collect rains that fall on the rooftops, drains and catchbasins to gather runoffs from downspouts, streets and parking lots, underground storm sewers will then convey all the runoffs and discharge them to a natural water system such as a creek, river and ocean.
There are a couple issues with this. First, a big chunk of rainwater is lost to runoff. Rainwater is freshwater that is basically clean in most circumstances, which can be used directly for outdoor purposes, as well as indoor with proper filtering and cleaning. It falls right on our roof so no transportation is required to receive that water. However, in the current infrastructure, that much freshwater is sent right away, requiring another huge set of infrastructure to deliver the water we need.
Another issue is pollution. As it flows through the surface of the city, stormwater runoff collects all kinds of pollutants such as motor oil, gas, chemicals, fertilizers, pesticides, etc. As the stormwater is discharged into nature untreated, it pollutes the water system discharged into. Toxic substances from cars and pesticides can harm birds, fishes and other aquatic life. Nutrients from the fertilizers can cause the overgrowth of algae, depleting oxygen in waterways and aquatic habitats.
The third issue is insufficient water infiltration for the soil under the impervious cover and in general. As we can see in the illustration below, in a natural environment (left), 25% of water is infiltrated in shallow surface, and another 25% will percolate deep into the soil. In a city environment (right), those figures drop down to 10% and 5% respectively, so the water that goes into soil reduces from 50% of total to a mere 15%, a 70% reduction. The lack of deep infiltration is a big problem. Without proper recharge, the groundwater is seriously depleted in many places. As we rely on ground water as part of our water supply, this has a big impact for our water safety.
Lastly, waterway erosion and threat of flood. As a huge amount of water is gathered and discharged into waterway, the volume and speed it packs can erode the banks of the stream or river; when the volume is too heavy, it can flood surrounding areas.
Benefits of capturing and reusing rainwater
People’s thinking about rainwater has completely changed. Now, rain water is no longer thought as waste; instead, it is viewed as an asset, something we need to capture and reuse.
- A source of water supply
From 2013-2017, California experienced a historic drought. At its worst point, the water content in the snowpack was only 5% of normal. The drought was so severe, it was one of the worst in the state’s history.
After the drought, everyone realized we could no longer take the water supply as we knew it for granted. With climate change, drought might become more frequent and serious; on another hand, with economic expansion and population growth, our demand for water will just grow. How can we build the reliable water supply that can meet our needs?
“Stormwater could be a significant addition to California’s water supply. While the potential is still unknown in the Bay Area, Los Angeles estimates that rainfall could provide nearly half a million acre-feet (620 million cubic meters) per year, said Steven Moore, a member of the State Water Resources Control Board. This may sound trivial compared to the 33 million acre-feet people use statewide each year, but it’s not. “Stormwater could make a difference,” Moore said. “It could see us through seven years of drought instead of five.”
From a cost perspective, local stormwater capture is one of the cheapest methods for water supply. It is only more costly than urban water conservation, but much cheaper than others like recycling and ocean water desalination.
- Reducing pollution and recharging groundwater
As the importance of rainwater is more thoroughly understood, people have been taking all kinds of steps to keep rainwater instead of letting it flow away. In the cities, permeable surfaces are replacing the impervious ones, and more and more rain gardens have been built, in the streets, around offices and in our gardens.
When it rains, the rainwater can infiltrate the soil from the permeable surfaces and rain gardens. In the process, harmful pollutants in the water can be filtered out; the cleaned water can percolate deep in the soil, replenishing groundwater.
Here is a storm drain at a street corner. The catch basin around it was built into a rain garden, allowing the rain water to sink into the soil.
- Reducing the threat of erosion and flood
Since water is directed away from the runoff, the total runoff volume will reduce, and the speed and energy that it packs up will lessen. As a result, the force to erode will be smaller, and threat of flood lower.
On Oct 6 2017, SB231 was signed into law in California, making it much easier to fund and build rainwater capture projects. The key is the clarification about whether stormwater projects are subject to the exemption of prop 218:
Excerpts from SB231:
The Legislature finds and declares all of the following:
(a) The ongoing, historic drought has made clear that California must invest in a 21st century water management system capable of effectively meeting the economic, social, and environmental needs of the state.
(b) Sufficient and reliable funding to pay for local water projects is necessary to improve the state’s water infrastructure.
(c) Proposition 218 was approved by the voters at the November 5, 1996, statewide general election. Some court interpretations of the law have constrained important tools that local governments need to manage storm water and drainage runoff.
(d) Storm waters are carried off in storm sewers, and careful management is necessary to ensure adequate state water supplies, especially during drought, and to reduce pollution. But a court decision has found storm water subject to the voter-approval provisions of Proposition 218 that apply to property-related fees, preventing many important projects from being built.
(h) Proposition 218 exempts sewer and water services from the voter-approval requirement. Sewer and water services are commonly considered to have a broad reach, encompassing the provision of clean water and then addressing the conveyance and treatment of dirty water, whether that water is rendered unclean by coming into contact with sewage or by flowing over the built-out human environment and becoming urban runoff.
(l) The Legislature reaffirms and reiterates that the definition found in Section 230.5 of the Public Utilities Code is the definition of “sewer” or “sewer service” that should be used in the Proposition 218 Omnibus Implementation Act.
With SB231, it is clear that rainwater capture projects do qualify for the Prop 218 exempts, making them much easier to fund and build.
To summarize, facing the ever increasing demand for water and a future with possibly longer and more frequent drought, we now look at stormwater with a completely new perspective. Gone are the days when we think of it as a waste; instead we know it is a great asset, and will try to capture and reuse it in a way that will benefit us, and the environment the best.