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.