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Monday, April 20, 2009

Fun with Rain Gardens

above: my rain garden experiment

Just under a year from the time we moved into our home, we experienced stormwater in a way we never had before. This also presented an opportunity to learn some lessons in stormwater management that I thought may be of interest.

Let me explain. Runoff from the adjacent street collects in a drainage structure which used to discharge into a small pipe that ran along the northern boundary of our lot into a stream off property.

It was certainly no great feat of engineering but at the time seemed to serve it’s purpose (seemed being the key word). I’m sure whoever designed and installed the system had a basic understanding of the workings of stormwater - something like: water runs down road to low point. Put drain inlet at aforementioned low point. Attach pipe to drain inlet. Run pipe downhill and in a straight line (shortest distance right?). Point outlet to stream and we’re done. This equation works great until additional ingredients work their way into the formula. Leaves + twigs + dirt + plastic grocery sacks + discarded hamburger wrapper + tropical storm = lots of water not where it is supposed to be. We leaned this when Hurricane Ivan paid a visit in Sept of 2004.

Basically there was substantial clogging of our stormwater pipes which caused the system to literally break apart, spewing a geyser of muddy stormwater which severely eroded our yard and then puddled in our basement.

I tried several times to repair the system, but time and again, nature did not comply. I decided to try a different method of stormwater management, which takes some cues from Mother Nature herself.

I began by abandoning the underground pipe in favor of a surface system. The new system would be composed of a series of small rain gardens, terraced down the slope. The contractor who installed the previous system had buried the pipe in a trench with coarse road-base stones ranging in size from a baked potato (hey, I’m from Idaho) to small watermelons. I reused these stone to line my rain garden and armor the stream channel connecting the gardens.

I also placed several drop structures in the system. This helps to dissipate the force of the velocity during heavy runoff – it also creates a great ‘babbling brook’ resonance during rain storms.

above: cascading drop structures in the "Fern Garden"

I have found that the stormwater entering my gardens carry a surprising amount of sediment. The first pond of the system is the deepest (about 10” -12”) and has a wide, rock-lined overspill. This creates a relatively calm pool in rain events and most of the sand, trash and large sediment will drop out very beginning. I clean this out a couple of times a year and use the sediment to fill holes or spread elsewhere in the yard.

above left: street runoff entering upper pool
above right: sand and other sediment deposited in pool

I spent the most amount of time building the channel through the fern garden on the north yard of the house. This channel is stone-lined with several small waterfalls. I also laid a path by recycling broken concrete pieces from demolition (more on that in another post).

above: upper and lower pools in the fern garden

The final pool is the largest and I have found that it does a decent job at settling out clay fines and other small particles. I have started cultivating an iris garden on the margins of the lower ponds. All of the irises (Blue Flag Iris or Iris versicolor for horticulture geeks) are from a single plant (all my project budget would allow) that I’ve been dividing over the past three seasons– they do extremely well in these heavy, moist soils. I created a shelf lined with stone to protect the iris from swifter currents that develop in very heavy rain events. I plan to experiment with introducing some rushes and other plants later this season.

above: iris garden

Other plants in this garden include wild strawberry (provides quick cover), many species of native fern, rhododendron, buttercup (vigorous but also invasive – be careful), violets, a flowering dogwood, and black berry and raspberry canes. There are also some trillium and crocus for early spring interest, as well as several varieties of tulip. Most of these plants were transplanted from elsewhere on my lot, given me by friends, rescued from development sites, or are uninvited guests that haven’t been caught yet.

I have had the rain gardens in for over three years now and have really enjoyed this experiment. They have held up well and provide a lot of interest especially when it rains. I haven’t had water in my basement since the gardens went in. They also prevent several cubic feet of sediment and trash from entering our river systems.

Feel free to contact me if you have any comments or would like to know more.