When it rains, we enjoy hearing the sound of raindrops on our roof. After a drought of so long in California, those drops sound more like music to our ears. While enjoying the music, have you thought about where the stormwater goes to? Well, most of it just goes down the sewer, into the creeks and rivers, and eventually out to the ocean. What if that water is not sent away, but reused, such as, turned into beauty in your garden?
Storm water: waste or asset
In the past, stormwater has been treated as something akin to waste in cities, something that is collected and sent out to waterways in nature as soon possible. As people realize now, there are several issues of this.
First, a big chunk of water is lost. Rainwater is freshwater that is basically clean in most circumstances. 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.
“Stormwater could be a significant addition to California’s water supply. Los Angeles estimates that rainfall could provide nearly half a million acre-feet (620 million cubic meters) per year. Steven Moore, a member of the State Water Resources Control Board, said, ‘Stormwater could make a difference, it could see us through seven years of drought instead of five.’”
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 the waterways untreated, the toxic substances can pollute the water and harm birds, fishes and other aquatic life that live there.
One more issue is the loss of deep water infiltration. As the water that falls on impervious surfaces such as roof and concrete is sent right away, water that would otherwise have gone into soil, percolated and recharged the ground water is lost. As you can see, in cities where impervious cover is common, runoff can be as high as 55%, versus 10% with natural ground cover.
It has become clear that rainwater is not a waste, but an asset, a valuable resource of water supply, something that we should capture and reuse. While a common way to do so is using a rain barrel, there is another more direct way – build a rain garden.
What is a rain garden?
According to Wikipedia, “a rain garden is a planted depression or a hole that allows rainwater runoff from impervious urban areas, like roofs, driveways, walkways, parking lots, and compacted lawn areas, the opportunity to be absorbed. This reduces rain runoff by allowing stormwater to soak into the ground (as opposed to flowing into storm drains and surface waters which causes erosion, water pollution, flooding, and diminished groundwater).”
“The purpose of a rain garden is to improve water quality in nearby bodies of water and to ensure that rainwater becomes available for plants as groundwater rather than being sent through stormwater drains straight out to sea. Rain gardens can cut down on the amount of pollution reaching creeks and streams by up to 30%.”
So exactly what is a rain garden? To understand, we just need to turn our eyes to nature.
Imitate the nature
In spring time, when we go to a nature reserve or park, chances are we can see fields and fields of wild flowers. No one ever installs an irrigation system or waters these plants; they just live and keep turning out splendid blossom, year after year. How do they do it?
The answer is, after tens of thousands years, the native plants have adapted to the environment. In California where it rains in winter and gets dry in summer, another area in the world that has Mediterranean climate, plants take in all the water they can get in winter, grow rapidly, and bloom in spring. When summer comes and it becomes dry, they slow their growth or simply go dormant. They stay this way until winter, when the rains come again. As the raindrops come down, they “wake up” from the dormancy, drink up all that water and start to grow and bloom again.
They don’t need any additional watering; they just take all the water there is and live throughout a year. This is what plants in a rain garden will do.
At a rain garden, the depression or ditch will collect the rainwater runoff from a roof. When it rains, water will be collected there. The plants in the garden will absorb the rain water, and grow; When the rain season ends, they can just live on their own. Very little or no additional watering is needed for these plants in most cases. Just like their brothers and sisters in the nature, they can live with just the rainwater.
Compared with water supplied to each household, which is treated with chemicals to comply with the sanitary standards, guess which water the plants like better? Plants watered with rain water can usually grow faster, bigger, and have brighter blossom.
Designing a rain garden
Like so many lawns in California, Larry’s (not his real name) lawn turned brown during the historic drought. Though the drought ended and last winter was one of the wettest on record, the lawn did not come back . The brown lawn had been bothering Larry for a long time, but he was not sure what to do about it, until he heard that his lawn can be built into a beautiful garden; not just any new garden, but a rain garden!
One of the downspouts (the one on the left) is right next to the front yard. When it rains, the rainwater will just flow into the garden. The lawn is on a very slight slope from the house to the sidewalk, so the runoff will go outwards naturally. If a shallow basin is built close to the side of sidewalk, the rainwater can reach there and be stored in it.
That is exactly the design proposed to Larry. A small winding ditch will take the rainwater from downspout, and send it to this shallow basin. Some plants will be planted. After they absorb the rainwater in winter, they may only need a little watering in the remainder of the year, saving a remarkable amount of water.
In addition, since the garden would meet all the requirements of Santa Clara Water District’s Landscape Conversion Rebates program, the garden can apply for the rebate.
Larry liked the proposal. It was a “Go” for the rain garden!
Installing a rain garden
- First, the shape of the garden needs to be defined.
For the safety of the foundation, the rain garden should be some distance away. Usually it is advised that at least 10 feet of space should be left between the basin and the house.
- Next, the shape of the rain garden is outlined.
How big should the garden be? It depends on how much runoff the roof can produce, and design an area that can take much of that runoff.
Suppose the area of the roof is 1000 square feet, with one inch of rain, it can produce about 600 gallons of runoff. If the rain garden is 1 foot deep, to absorb this much water, it needs an area of about 80 square feet. If the roof area is bigger, the rain garden should be larger too.
What shape can a rain garden be? It can be of anything – a circle, a bean, or a peanut. The smooth, curvy lines of these shape not only look appealing, but also reduce the force of runoff and effect of erosion.
- Soil preparation
The bottom of a rain garden needs to be covered with a special type of soil, to help with water infiltration. It is a mixture of organic materials and coarse sand. The bottom of the whole area that water flows by and stays should be covered with the mix.
- Plant selection
Plants in a rain garden should be able to stand both conditions well: wet and dry. Their roots should be able to take moisture for a long time, yet also survive in hot dry summer.
One plant that fits this very well is the Douglas Iris. A tough California native, it can be found close to beaches along the west coast. Hardy, drought tolerant, yet tolerant of wet soil, this is great choice for a rain garden.
The beauty of a rain garden
The garden is done! This is before
The rain garden
When it rains, with a garden like this, the rainwater will be captured, and reused. Something that was sent away before can be turned into so much beauty in our own garden!