From a modern raindrop's perspective, the earth's surface can be divided into two types: slick and squishy. Or if you're an urban planner, impervious and pervious. Impervious surfaces include almost the entire city, from roofs to driveways. Pervious surfaces include kitchen sponges and the squishy floor of the primeval forest.

Here's what a raindrop experiences as it falls in downtown Seattle today:

Wheeee! SPLAT. Wheeee! Holy %&$#!, is that the ocean again? Wheee! SLURP. Ommmmmm.....

All the raindrops hit the ground and immediately make a run for the ocean. Some are slightly detained, spend some time in a puddle. But mostly, there's a surge following the rainfall that frequently overwhelms the stormwater systems, then slows to a trickle.



In Seattle, this means big problems. When there's a storm, the stormwater pipes aren't sized to handle the load. Rather than flood the streets, the city opens up the sewer lines to discharge the rain. This system, called Combined Sewage Overflow, means that after a storm, certain parks smell like an outhouse.

This has led the city to think about gardens differently. Gardening is no longer a hobby. It's going to save the city millions of dollars. That's because gardens, especially those kept in cover crop during the winter, diminish the runoff problem for cities. Soil improvements intended to help plants also improve the soil's capacity to detain water. The slowed water recharges the subsoil, feeds the aquifer, or passes into plant roots and escapes as water vapor through plant leaves. Our gardens can do this even more effectively if we consider how water passes over the garden surface.

This neighbor created an (ugly) berm, trapping water before it hits the street.



Imagine how attractive it could have been, had it been planted and maintained by this gardener:



Here in Seattle, we'll never recreate the primeval forest and gently tread soils that predated the city. But Seattle Public Utilities is trying to engineer little pockets of healthy soil that will absorb some of the runoff. Recently on my street, the city ripped out a piece of the roadway and replaced it with a pocket of highly engineered piece of landscaping.


The landscape “bulb” interrupts the little stream of water that flows down the side of the road on its way to the storm drain. There's one on each side of the street. I saw the excavation for these bulbs, they go several feet deep. In some ways, they're like French drains - a large volume of porous soil temporarily stores water. The water is then slowly allowed to trickle into the surrounding clay. The builders used a number of other little tricks to slow down the water: they built a little dam in the middle. Wetland plants bind the soil and turn some of the water into vapor. There's a flat overflow area like an estuary (remember estuaries?). If any water makes it through this series of obstacles, it passes through a rocky filter that says “where are you going with that dirt?” before the water finally escapes to the storm drain.



Recent lawsuits have given this kind of project a new importance in Seattle. Next week, I'll talk about what happens when Seattle takes a neighborhood with no functioning stormwater system and gives it a natural drainage system that approximates the primeval forest.