Bioswale AKA Zen Storm Water Garden




Introduction: Bioswale AKA Zen Storm Water Garden

You would never know now that my tranquil "Zen" garden was once the location of a huge erosion problem. I built this "bioswale" in 2015. This picture was taken during the 2016/17 winter after more than 100 inches of rainfall in the Santa Cruz Mountains. My bioswale handled all of the water pouring off our roof— no problem—with no hillside erosion, and zero maintenance.

According to USDA's Natural Resources Conservation Service website, bioswales are "stormwater runoff conveyance systems." That's exactly what my bioswale does, but this oasis, in what was an underutilized part of the yard, also provides a great spot for meditation.

Step 1: Make Rain Music

Thanks to people sharing resources about bioswales online, I learned how to build one. So I decided to pay it forward by sharing my process for building a very tough, 100+ inch winter rain-tested, beautiful bioswale!

I used to worry about how much of our hillside would be washed away whenever it rained. Now, I relax to the music from my rain chain. What erosion problem?! Thanks to the bioswale, our hillside plantings are safe.

My bioswale design comes with a "rain chain" downspout that is connected to a standard gutter system. This feature adds greatly to the front of our house, and during rains there is nothing better than listening to the rain tinkling down the rain chains. Although this element is not required to build your bio-swale, I do think this feature is worth considering. I ordered my rain chain online from, and it came with extra hardware and parts for future repairs. I promise I don't work for them! But I do want to give them a shout-out for a great product, excellent website shopping experience and quick delivery. In fact, I am listening to one of their rain chain music videos as I type this and it's quite nice!

Step 2: Identify Location of Drainage Problem...

Our high fire danger location in the Santa Cruz Mountains contributed to the decision to use of slate instead of wood for the staircase that spanned the east side of the house. I planted the area with cherry trees, pittosporum, and rockrose. A truck load of compost and planting mix was applied to the planting areas, and I gathered locally sourced rocks to shape a "dry stream bed" that I designed to address drainage issues at the lower end of the stairs. The upper half of the staircase had an existing oak tree and didn't seem as steep, so we weren't so worried about erosion—at first.

Enter California "stormaggen" during the winter of 2014/15. I first noticed the area around the downspout for the rain gutter had flooded the front garden, overflowed onto the patio and formed a large puddle along the house. I looked out at the hillside during this isolated "atmospheric river" event, and the upper hillside looked like a water fall! The next day I surveyed the damage and found a large percentage of my planting mix had washed down the hillside onto the lower parking area, and deep erosion trenches made it clear that the next storm would only continue the damage.

I traced the destruction back to the rain gutter downspout on the northeast corner of our house.

After doing some calculations, I figured out that if it rained an inch-an-hour, then our roof was depositing 1800 gallons of water on the slope during that hour—if this massive gush of water continued, the concrete foundation supporting our staircase would soon be undermined.

Step 3: Build the Swale

I researched erosion control systems online and found lots of information about "bioswales." Because California had been enduring a crippling drought for the past four years, I had read about bioswales and knew that they helped to percolate rain water back into the soil and "recharge" underground aquifers. Planted with specialized plants, that can handle soggy soil all winter, they also help filter storm runoff and the plantings provide important habit for wildlife.

When the soil dried out that spring, we went to work. My 14-year old nephew and twin 14-year old sons helped me to dig five trenches underneath this 6X9' foot area. I used "upside down" landscaping paint to mark the trenches for the boys in this area, which is 8-10 feet down from the rain spout. Due to the constraints of this space, each trench was progressively shorter as it went up the rise; the shortest trench was about 3 feet long.

The location that I picked for the bioswale was wide enough to build a series of trenches that would stop, catch and 'sink' the excess storm water. Each trench varied in length, but the longest trench was located furthest down the hill, away from the rain gutter, and spanned approximately 6 feet. The trenches went across the hillside, like this III (perpendicular to the slope) not down the hillside, like this -- -- -- . This design element is important because the trenches must stop the flow of water, and if the trenches run down the slope (or parallel to slope), then they will channel the water down the hill, and they can actually increased the flow and damage. I also added an outflow channel to the berm side of the bioswale; this outlet is designed to direct excess water down a rocky spillway, away from the stairs, when the pond nears capacity.

Each trench was about 4-6 inches deep and wide, and the digging part took three slightly lazy teen boys about three hours, and that includes lots of stops in the shade to enjoy ice cold lemonade, animated conversation and several attempts to infiltrate the house with muddy shoes. They used a pick ax, trenching shovel and round point shovels to complete the trenching. One, focused adult could probably complete all the trenching in well under two hours.

Step 4: Add Soil Stabilizers and Rocks...

Once the trenches were dug, we filled each one with gravel. Any type of gravel will work, but I used 1-inch gravel. Pea gravel could escape into the soil, so I wouldn't recommend that small, but 3/4 to 1-inch works fine. We used bags of inexpensive gravel purchased at the home building supply store.

After filling the trenches, I moved some soil around to shape a concave, pond area, and I also built a 2-foot high berm on the downslope side, using some heavy clay soil that I loaded up in a wheel barrel from another area of the yard. To finish the berm side, I installed one strip of "commercial" or "professional" grade "bender board." This was the only material that was not available at the home building supply. For this, I had to go to a lumber yard where they sell this stiffer version in 20 foot lengths, but I had them cut it into two, 10-foot lengths. I would recommend making the extra trip to get the higher quality, less flexible, bender board because you need this decorative edging to hold up the base of the berm over time, and that requires more rigidity. I used 10-inch redwood stakes to hold the bender board in place.

Next, we spread commercial grade landscape fabric over the pond area of the bio-swale, and covered that with 23-gauge hardware cloth, which we staked into place with "sod staples." All of these materials were found at the home building supply store. The hardware cloth was the hardest to find. You will locate it, mostly likely, in the wire fencing area. Many people, myself included, use this to keep gophers out of raised bed containers. I bought one roll of the hardware cloth and used the entire roll across this area. I needed wire snippers to fit the pieces together and cover the odd-shaped pond area.

Next, we covered the pond area with a mix of bagged "river rocks" and surrounded the pond with planting mix for the "bio" part of our bio-swale.

Step 5: Connect the Rain Gutter to the Bioswale

I ordered the rain chains online, and be forewarned, they were not cheap, but every day I see them out there, I reassure myself that the investment improves the 'curb appeal' of our home and therefore was a wise investment. They are incredibly easy to install and can handle tough rainstorms with grace. In fact, we have had a series of "atmospheric river" events this winter in the Santa Cruz Mountains and they channel water off our roof with ease. I think they are much easier to maintain than traditional rain gutters, especially on our house that requires a very tall ladder to maintain traditional gutters. For the rain chains, I don't need a ladder. Instead, I use a bamboo stake to poke up through the top and clear the downspout.

Step 6: Place 4-Inch Perforated Drain Pipe

After I installed the rain chains, I dug a shallow trench, sloping gently downhill to ensured the water from the rain chain would flow in the direction of my bioswale.

Since the only open space was located about 10-feet downhill from my rain chain, I connected this shallow trench to a four-inch, perforated drain pipe. I covered the drainpipe with large cobbles to create a dry stream effect and finished this area with the other 10-foot strip of bender board. This strip of bender board separated my planting area from the channel, and protected the soil that bordered the front porch from the drainage area at the base of the rain chains. Our general contractor had placed concrete drainage "spillways," which are available at building supply stores at the base of our downspouts, and I kept those in place, but added more river rock to complete the natural look.

Step 7: Select Sturdy Plants

The "bio" aspect of the bioswale helps to filter stormwater. Since storm runoff often originates in areas that contain contaminants, such as asphalt shingles, asphalt driveways and concrete patios, the plantings do more than just provide a pretty garden space, the plants provide surface filtration services in the swale area and can improve water quality. They can also stabilize the surrounding soil, and, in our case, they created a sturdy garden where there had once been a mess of flood debris and erosion damage. Pick tough plants that can withstand boggy conditions all winter. If you tell your local nursery expert that you are building a bioswale, they should be able to help you. This flag grass looks great even after a really tough winter. Other plants that worked in my area included, "Juncus" Rush, a California Native that I moved from another area of our garden, another native, the "Sword" fern continues to thrive a year later, and a couple of Japanese "ornamental" grasses still look great more than a year later. After surviving this tough cycle of drought and excessive rainfall, I know all four of these plants will continue to grow and fill in the space. The Azalea on the berm has survived, but is not thriving, and several of the grasses I planted along the backside also died, but those most likely did not like the acidic soil from the oak tree—these included, "fiber optic" grass and mondo grass. The Japanese Maple is also doing fine after a tough year.

Step 8: More Than 100 Inches of Rain Later...

The Buddha remains in a peaceful landscape, thanks to the bioswale design. Despite record-breaking rains, the drainage issues on this sloping hillside have been completely resolved. The pond area has filled up a few times, but by morning all of the water has percolated through the soil. The bioswale eliminated all flooding on the front patio, yard and sloping hillside. I have put zero maintenance into this area. If I had a leaf blower, I would probably use it to remove the oak leaf debris, but otherwise, this is a virtually maintenance free solution to what was a major drainage problem. I added a $15 solar accent light to create a night time focal point from the staircase. I spent less than $200 on all materials for the bioswale (not including planting and rain chain), and I am helping to combat the effects of climate change by recharging underground aquifers in California, which feels pretty darn good.

I wish you the best of luck in constructing your own garden of Zen:)

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    DIY Hacks and How Tos

    This looks really nice. I wish that I had something like this in my back yard.


    Reply 5 years ago

    Thanks! It was actually fun to build and very low-tech:)


    5 years ago

    That's amazing and wonderful! I live on the other side of the country, and excessive water flow and erosion are constant problems. I no longer have a yard, but will share your instructable with a relative who is losing half her backyard in rainstorms. It is upsetting to her to see all her carefully nurtured topsoil disappear into a storm sewer, and this is a perfect solution. Thank you for sharing your instructable.


    Reply 5 years ago

    Thank you for this! Great to hear that someone else may benefit from this experience. I know the planet also appreciates you sharing the good word. Keeping topsoil out of storm drains helps everyone!