Friday, October 23rd, 2009 Archives

Woodland Rain Garden








This will be my first winter in WA in two years, and already my instinctive rainfall calculations are off.  My carefully planted woodland garden, intended in part to provide a buffer zone to capture driveway runoff before it reached house, has instead become a minature Mississippi. 

A mature cedar tree under which this area was planted  justified my refusing to add soil to elevate this part of the yard above the side yard that runs to the south side of the house. Once the front yard was done and the roots partly compacted on that side from the excavator back in June, I figured it best to leave the poor tree alone. Cedars have a network of shallow roots that are easily damaged, and I was concerned that the weight of a few more inches of sil might suffocate them.  Instead, I placed about two inches of much lighter mulched wood chips and horse manture  over the area and edged it with a low rock wall to hold the material back off of the side yard, which was about six inches lower.  These few inches, I felt, would absorb any surface runoff.

frontlawn2 6-09

Front yard with excavator, June 2009

However, even before full soil saturation was acheived, a hard overnight rainfall October 17 was enough to flood my little garden. What went wrong?  The amount of water collected over about 70 feet of open driveway at a 0.5% gradient combined with soil compaction and a lack of vegetation probably combined to overwhelm the buffering capacity of my short strip of mulch.  Although I had planted a lace-cap hydrangea, two Pieres ‘Mountain Fireand a few sword ferns, it was not enough to slow the surface flow.  Subsequently, both the volume and velocity of the water was sufficient carve a drainage through the rock wall and down into the back  yard where it disappeared into the septic drainfield.

runoff through side yardAs I stalked about the yard in the pouring rain with my camera, I pondered my options. I had been considering installing a small reflecting pond surrounded by vegetation in the path of the current flood. That, however, would not be sufficient to absorb the flow.  Digging out a small retention pond would be  infeasible if I want to protect the cedar tree. I tend to avoid berms as a way of delaying the inevitable (kind of like pushing rocks uphill, really), so I ruled out shunting the water further along the drive and into woods. Filling the area with vegetation will probably be the only way to baffle the flow and keep it stationary long enough to sink into the ground before it reaches the back yard and drainfield.  I need a rain garden.

According to a University of Rhode Island website on sustainable landscaping (raingarden.htm),  a rain garden “is a natural or dug shallow depression designed to capture and soak up stormwater runoff from your roof or other impervious areas around your home like driveways, walkways, and even compacted lawn areas…The rain garden is planted with suitable trees, shrubs, flowers, and other plants allowing runoff to soak into the ground and protect water quality.” 

Planting dense grasses or multi-stemmed, creeping vegetation should protect the surface from channelization and the give the water time to soak in.  My observations the morning after the heavy rainfall confirmed that the soil quickly absorbed the water after the rain ceased, despite the relatively high clay content.  This may change as winter progresses and the soil reaches saturation, but experimentation with other temporary methods of water baffling, such as pieces of wood, may prove that slowing the flow will be enough.  Some calucations of slope, anticipated runoff volume, and soil absorption capacity based upon composition will aid my work if I feel like playing engineer for a day.  Things are far too wet right now for planting, but I’m already designing a plan for spring.

I think it’s the combination of problem-solving, geology, engineering, botany, and creativity that makes this enterprise sooo much more fun than my previous paying job!

The Rain Garden

  • October 23rd, 2009
  • Posted in Landscaping
  • Comments Off on The Rain Garden