storm water

EHH week 16: rain garden excavation

As everyone knows, it rains in the Seattle area.  We get 37 inches in an average year, but once in a while we will get an inch in a single day.  In May 2011, Seattle got 1.78 inches over two days!  All that water has to go somewhere.


The Harvest House site is a wonderful piece of property (see blog 1), but its major defect is that there is no stormwater outlet pipe and the soils do not percolate particularly well.  So the civil engineer, springline design, designed a rain garden to collect and then gradually percolate the rain.


A rain garden is an open pit into which rain flows from the impervious surfaces on the site (see blog 10).  The bottom of the pit is filled with 18 inches of special soil designed to hold water like a sponge.  That soil is then planted with marsh plants.


When it rains, first the “sponge” gets soaked, then water will pool up to the rim of the pit.  Over a few days the water will gradually both evaporate and infiltrate into the ground below the rain garden.  The sloping site at this project required a series of 5 terraced pits, each with a concrete dam to hold back the ponded water.  The lowest pond is expected to never overflow its rim.


The excavation for this rain garden is huge, but the landscaper will spread the dirt pile around the rest of the site so we don’t need to export any off site.

EHH week 30: rain garden

The rain garden is complete.  Its job is to collect all the stormwater from the site (see Week 16 blog) and allow it to gradually disperse.  During a storm, rain enters at the top and as upper pools fill, they spill over to lower ones in a controlled manner through a notch in the top of each concrete wall.  The entire rain garden can hold 90,000 gallons of water, which is a lot of rain.


Once full, the pools then gradually empty by either percolating down into the earth or evaporating up into the sky.  As the wetland plants in the rain garden mature they will help speed the rate of emptying.


The landscape contractor managed to redistribute all the dirt from the rain garden excavation.  So the project has not had to export any dirt, which is a success ecologically and economically.


Rain gardens are becoming more common in the greater Seattle area.  They have been used at shopping center parking lots and along the sides of residential streets.  For sites with restricted areas, buried detention pipes are a better solution.  But for sites with enough footprint area, I really like the way that rain gardens make visible the process of stormwater collection and dispersion.  They are miniature versions of nature’s hydrological cycle.